A predator-prey interaction between a marine Pseudoalteromonas sp. and Gram-positive bacteria

Glutamicibacter sp. ZY1 antagonizes pathogenic Vibrio parahaemolyticus via iron competition

Probiotics are prior agents for treating bacterial infection with advantages of inhibiting pathogenic bacteria and improving immune responses of hosts, thus increasing the survival rate of cultured animals. In this study, one Vibrio parahaemolyticus YDE17 pathogenic to shrimp and its antagonist Glutamicibacter sp. ZY1 were screened, and ZY1 showed stable inhibitory effects on diverse Vibrio spp., especially V. parahaemolyticus. ZY1 secreted inhibitory substances into supernatant, and the activity of inhibitory substances did not change after being treated under different temperatures, proteinase K, and pH (6-10), which indicated that the inhibitory substances might be small molecules, which led us to trace the siderophore production. The siderophore production of YDE17 co-incubated with the cell-free supernatant of ZY1 was greater than that of YDE17 alone, which indicated that the cell-free supernatant of ZY1 created iron-limiting conditions for YDE17. This finding was confirmed by iron supplementation assays, in which the inhibitory activity of the cell-free supernatant of ZY1 on YDE17 as well as the siderophore production of YDE17 decreased in the presence of FeCl3. The effect of iron on inhibition was further confirmed by in vivo infection. The relative percent survival of ZY1 to shrimp challenged by YDE17 was 83.3%, but the survival rates of shrimp challenged with YDE17/ZY1/FeCl3 were similar to that of YDE17, both of which were significantly lower than the 70% survival rate of shrimps simultaneously challenged by ZY1/YDE17. Our study offers a new probiotic resource to control vibriosis, which works through iron competition with the opportunistic pathogens of Vibrio spp.IMPORTANCEBacteria belonging to Vibrio spp., especially Vibrio parahaemolyticus, are important opportunistic pathogens infecting a wide range of hosts including fish, shrimp, shellfish, and crab. Antibiotics are effective but show the disadvantages of antibiotic generation, microecology destruction, and biological toxicology; thus, new treatments of Vibrio infection are urgently recommended. In our present study, Glutamicibacter sp. ZY1, belonging to the phylum Actinomycetes, was selected and showed high inhibitory activity to inhibit V. parahaemolyticus pathogenic to shrimp. Glutamicibacter sp. ZY1 antagonized V. parahaemolyticus YDE17 through producing siderophore to compete for iron, based on the results of both in vitro and in vivo experiments under different iron levels. This study offers a new strategy to control Vibrio infection in aquaculture.

Decoding Microbial Plastic Colonisation: Multi-Omic Insights Into the Fast-Evolving Dynamics of Early-Stage Biofilms

Marine plastispheres represent dynamic microhabitats where microorganisms colonise plastic debris and interact. Metaproteomics has provided novel insights into the metabolic processes within these communities; however, the early metabolic interactions driving the plastisphere formation remain unclear. This study utilised metaproteomic and metagenomic approaches to explore early plastisphere formation on low-density polyethylene (LDPE) over 3 (D3) and 7 (D7) days, focusing on microbial diversity, activity and biofilm development. In total, 2948 proteins were analysed, revealing dominant proteomes from Pseudomonas and Marinomonas, with near-complete metagenome-assembled genomes (MAGs). Pseudomonas dominated at D3, whilst at D7, Marinomonas, along with Acinetobacter, Vibrio and other genera became more prevalent. Pseudomonas and Marinomonas showed high expression of reactive oxygen species (ROS) suppression proteins, associated with oxidative stress regulation, whilst granule formation, and alternative carbon utilisation enzymes, also indicated nutrient limitations. Interestingly, 13 alkanes and other xenobiotic degradation enzymes were expressed by five genera. The expression of toxins, several type VI secretion system (TVISS) proteins, and biofilm formation proteins by Pseudomonas indicated their competitive advantage against other taxa. Upregulated metabolic pathways relating to substrate transport also suggested enhanced nutrient cross-feeding within the more diverse biofilm community. These insights enhance our understanding of plastisphere ecology and its potential for biotechnological applications.

Predation on bacterial pathogens by predatory bacteria of sewage origin: three days prey-predator interactions

BACKGROUND

Predatory bacteria are Gram-negative microorganisms that grow within and lyse their bacterial prey. As bacterial predators have potential applications in medicine and biotechnology, the aim of this study was to isolate and identify predators from environmental samples. Therefore, three environmental bacteria belonging to the genus Bdellovibrio were isolated and characterized.

RESULTS

The predator isolates tolerated pH range from 5 to 9 well, and were killed at pH ranges of 2 and 12. They survived best at 4 °C and 29 °C, tolerated to some extent -20 °C and even -80 °C, and were completely killed at 60° C. Furthermore, the host range analysis of the predator isolates was carried out on five Gram-negative (Acinetobacter baumannii, Klebsiella pneumoniae, Pseudomonas aeruginosa, Escherichia coli, Salmonella enterica) and two Gram-positive bacteria (Staphylococcus aureus, and Enterococcus faecalis). The environmental predator isolates inhibited Gram-negative preys more efficiently than Gram-positive. Predators inhibited S. enterica the most (70.48%) and P. aeruginosa the least (3.84%) among Gram-negative preys. The best inhibitory effect among Gram-positive preys was observed for S. aureus (29.83%).

CONCLUSIONS

The isolates showed broad-range predation on diverse preys under various pH and temperature conditions. Therefore, the predator isolates identified in here may be suitable choices for controlling the population of Gram-negative bacteria in different fields.

Saccharomyces cerevisiae strains L7 contribute to flavor and deacidification in Suanyu, a Chinese traditional fermented fish

Saccharomyces cerevisiae L7 was found to be an excellent starter and biological deacidification strain for Suanyu, however, the underlying mechanisms remain poorly understood. This study aimed to investigate the acid inhibition mechanism of S. cerevisiae L7. The strain enhances the sensory and flavor characteristics of Suanyu. The growth of Lactiplantibacillus plantarum is inhibited due to competition for carbon sources, resulting in a decrease in cell count from 9.00 Lg CFU/mL at 48 h to 7.70 Lg CFU/mL in co-culture. The addition of yeast reduces acidity, decreasing it from 5.83 g/kg to 0.82 g/kg at 48 h, while increasing sugar utilization to 94.52%. We found that cell contact was the main method of inhibition between the two microbials. Transcriptome analysis revealed that multiple pathways were affected under co-culture, ultimately leading to a decrease in lactic acid production. These findings provide valuable insights into the microbial interactions involved in biological deacidification.

The type II secretion system as an underappreciated and understudied mediator of interbacterial antagonism

Interbacterial antagonism involves all major phyla, occurs across the full range of ecological niches, and has great significance for the environment, clinical arena, and agricultural and industrial sectors. Though the earliest insight into interbacterial antagonism traces back to the discovery of antibiotics, a paradigm shift happened when it was learned that protein secretion systems (e.g., types VI and IV secretion systems) deliver toxic "effectors" against competitors. However, a link between interbacterial antagonism and the Gram-negative type II secretion system (T2SS), which exists in many pathogens and environmental species, is not evident in prior reviews on bacterial competition or T2SS function. A current examination of the literature revealed four examples of a T2SS or one of its known substrates having a bactericidal activity against a Gram-positive target or another Gram-negative. When further studied, the T2SS effectors proved to be peptidases that target the peptidoglycan of the competitor. There are also reports of various bacteriolytic enzymes occurring in the culture supernatants of some other Gram-negative species, and a link between these bactericidal activities and T2SS is suggested. Thus, a T2SS can be a mediator of interbacterial antagonism, and it is possible that many T2SSs have antibacterial outputs. Yet, at present, the T2SS remains relatively understudied for its role in interbacterial competition. Arguably, there is a need to analyze the T2SSs of a broader range of species for their role in interbacterial antagonism. Such investigation offers, among other things, a possible pathway toward developing new antimicrobials for treating disease.

Diversity in the utilization of different molecular classes of dissolved organic matter by heterotrophic marine bacteria

Heterotrophic marine bacteria utilize and recycle dissolved organic matter (DOM), impacting biogeochemical cycles. It is currently unclear to what extent distinct DOM components can be used by different heterotrophic clades. Here, we ask how a natural microbial community from the Eastern Mediterranean Sea (EMS) responds to different molecular classes of DOM (peptides, amino acids, amino sugars, disaccharides, monosaccharides, and organic acids) comprising much of the biomass of living organisms. Bulk bacterial activity increased after 24 h for all treatments relative to the control, while glucose and ATP uptake decreased or remained unchanged. Moreover, while the per-cell uptake rate of glucose and ATP decreased, that of Leucin significantly increased for amino acids, reflecting their importance as common metabolic currencies in the marine environment. Pseudoalteromonadaceae dominated the peptides treatment, while different Vibrionaceae strains became dominant in response to amino acids and amino sugars. Marinomonadaceae grew well on organic acids, and Alteromonadaseae on disaccharides. A comparison with a recent laboratory-based study reveals similar peptide preferences for Pseudoalteromonadaceae, while Alteromonadaceae, for example, grew well in the lab on many substrates but dominated in seawater samples only when disaccharides were added. We further demonstrate a potential correlation between the genetic capacity for degrading amino sugars and the dominance of specific clades in these treatments. These results highlight the diversity in DOM utilization among heterotrophic bacteria and complexities in the response of natural communities.

IMPORTANCE

A major goal of microbial ecology is to predict the dynamics of natural communities based on the identity of the organisms, their physiological traits, and their genomes. Our results show that several clades of heterotrophic bacteria each grow in response to one or more specific classes of organic matter. For some clades, but not others, growth in a complex community is similar to that of isolated strains in laboratory monoculture. Additionally, by measuring how the entire community responds to various classes of organic matter, we show that these results are ecologically relevant, and propose that some of these resources are utilized through common uptake pathways. Tracing the path between different resources to the specific microbes that utilize them, and identifying commonalities and differences between different natural communities and between them and lab cultures, is an important step toward understanding microbial community dynamics and predicting how communities will respond to perturbations.

The resting cyst of dinoflagellate Scrippsiella acuminata host bacterial microbiomes with more diverse trophic strategies under conditions typically observed in marine sediments

Variation in the condition of marine sediments provides selective preservation milieus, which act as a key determinant for the abundance and distribution of dinoflagellate resting cysts in natural sediments. Microbial degradation is an understudied biological factor of potential importance in the processes. However, gaps remain in our knowledge about the fundamental information of the bacterial consortia associated with dinoflagellate resting cysts both in laboratory cultures and in the field. Here we used Scrippsiella acuminata as a representative of cyst-producing dinoflagellates to delineate the diversity and composition of bacterial microbiomes co-existing with the laboratory-cultured resting cysts, and to explore possible impacts of low temperature, darkness, and anoxia (the mock conditions commonly observed in marine sediments) on the associated bacterial consortia. Bacterial microbiome with high diversity were revealed associated with S. acuminata at resting stage. The mock conditions could significantly shift bacterial community structure and exert notably inhibitory effects on growth-promoting bacteria. Resting cysts under conditions typically observed in marine sediments fostered bacterial microbiomes with more diverse trophic strategies, characteristic of prominently enriched anaerobic chemotrophic bacteria generating energy via respiration with several different terminal electron acceptors, which yielded more acidic milieu unfavorable for the preservation of calcareous resting cysts. Our findings suggest that there is complex and dynamic interaction between dinoflagellates resting cysts and the associated bacterial consortia in natural sediments. This intrinsic interaction may influence the maintenance and/or accumulation of dinoflagellate resting cysts with potential of germination and initiation blooms in the field.

Genome-based taxonomic analysis of the genus Pseudoalteromonas reveals heterotypic synonyms

The Pseudoalteromonas genus comprises members that have been demonstrated to play significant ecological roles and produce enzymes, natural products, and activities that are beneficial to the environment and economy. A comprehensive evaluation of the genus revealed that the genomes of several Pseudoalteromonas species are highly similar to each other, exceeding species cutoff values. This evaluation involved determining and comparing the average nucleotide identity, in silico DNA-DNA hybridization, average amino acid identity, and the difference in G + C% between Pseudoalteromonas type strains with publicly available genomes. The genome of the Pseudoalteromonas elyakovii type strain was further assessed through additional sequencing and genomic comparisons to historical sequences. These findings suggest that six Pseudoalteromonas species, namely P. mariniglutinosa, P. donghaensis, P. maricaloris, P. elyakovii, P. profundi, and P. issachenkonii, should be reclassified as later heterotypic synonyms of the following validly published species: P. haloplanktis, P. lipolytica, P. flavipulchra, P. distincta, P. gelatinilytica, and P. tetraodonis. Furthermore, two names without valid standing, 'P. telluritireducens' and 'P. spiralis', should be associated with the validly published Pseudoalteromonas species P. agarivorans and P. tetraodonis, respectively.

Decoupling between the genetic potential and the metabolic regulation and expression in microbial organic matter cleavage across microbiomes

Metagenomics, metatranscriptomics, and metaproteomics are used to explore the microbial capability of enzyme secretion, but the links between protein-encoding genes and corresponding transcripts/proteins across ecosystems are underexplored. By conducting a multi-omics comparison focusing on key enzymes (carbohydrate-active enzymes [CAZymes] and peptidases) cleaving the main biomolecules across distinct microbiomes living in the ocean, soil, and human gut, we show that the community structure, functional diversity, and secretion mechanisms of microbial secretory CAZymes and peptidases vary drastically between microbiomes at metagenomic, metatranscriptomic, and metaproteomic levels. Such variations lead to decoupled relationships between CAZymes and peptidases from genetic potentials to protein expressions due to the different responses of key players toward organic matter sources and concentrations. Our results highlight the need for systematic analysis of the factors shaping patterns of microbial cleavage on organic matter to better link omics data to ecosystem processes.

IMPORTANCE

Omics tools are used to explore adaptive mechanism of microbes in diverse systems, but the advantages and limitations of different omics tools remain skeptical. Here, we reported distinct profiles in microbial secretory enzyme composition revealed by different omics methods. In general, the predicted function from metagenomic analysis decoupled from the expression of corresponding transcripts/proteins. Linking omics results to taxonomic origin, functional capability, substrate specificity, secretion preference, and enzymatic activity measurement suggested the substrate's source, concentration and stoichiometry impose strong filtering on the expression of extracellular enzymes, which may overwrite the genetic potentials. Our results present an integrated perspective on the need for multi-dimensional characterization of microbial adaptation in a changing environment.

Phytoplankton-derived polysaccharides and microbial peptidoglycans are key nutrients for deep-sea microbes in the Mariana Trench

BACKGROUND

The deep sea represents the largest marine ecosystem, driving global-scale biogeochemical cycles. Microorganisms are the most abundant biological entities and play a vital role in the cycling of organic matter in such ecosystems. The primary food source for abyssal biota is the sedimentation of particulate organic polymers. However, our knowledge of the specific biopolymers available to deep-sea microbes remains largely incomplete. One crucial rate-limiting step in organic matter cycling is the depolymerization of particulate organic polymers facilitated by extracellular enzymes (EEs). Therefore, the investigation of active EEs and the microbes responsible for their production is a top priority to better understand the key nutrient sources for deep-sea microbes.

RESULTS

In this study, we conducted analyses of extracellular enzymatic activities (EEAs), metagenomics, and metatranscriptomics from seawater samples of 50-9305 m from the Mariana Trench. While a diverse array of microbial groups was identified throughout the water column, only a few exhibited high levels of transcriptional activities. Notably, microbial populations actively transcribing EE genes involved in biopolymer processing in the abyssopelagic (4700 m) and hadopelagic zones (9305 m) were primarily associated with the class Actinobacteria. These microbes actively transcribed genes coding for enzymes such as cutinase, laccase, and xyloglucanase which are capable of degrading phytoplankton polysaccharides as well as GH23 peptidoglycan lyases and M23 peptidases which have the capacity to break down peptidoglycan. Consequently, corresponding enzyme activities including glycosidases, esterase, and peptidases can be detected in the deep ocean. Furthermore, cell-specific EEAs increased at 9305 m compared to 4700 m, indicating extracellular enzymes play a more significant role in nutrient cycling in the deeper regions of the Mariana Trench.

CONCLUSIONS

Transcriptomic analyses have shed light on the predominant microbial population actively participating in organic matter cycling in the deep-sea environment of the Mariana Trench. The categories of active EEs suggest that the complex phytoplankton polysaccharides (e.g., cutin, lignin, and hemicellulose) and microbial peptidoglycans serve as the primary nutrient sources available to deep-sea microbes. The high cell-specific EEA observed in the hadal zone underscores the robust polymer-degrading capacities of hadal microbes even in the face of the challenging conditions they encounter in this extreme environment. These findings provide valuable new insights into the sources of nutrition, the key microbes, and the EEs crucial for biopolymer degradation in the deep seawater of the Mariana Trench. Video Abstract.

Biosynthetic mechanism of the yellow pigments in the marine bacterium Pseudoalteromonas sp. strain T1lg65

The Pseudoalteromonas genus marine bacteria have attracted increasing interest because of their abilities to produce bioactive metabolites. The pigmented Pseudoalteromonas group encodes more secondary metabolite biosynthetic gene clusters (BGCs) than the non-pigmented group. Here, we report a yellow pigmented bacterium Pseudoalteromonas sp. strain T1lg65, which was isolated from a mangrove forest sediment. We showed that the yellow pigments of T1lg65 belong to the group of lipopeptide alterochromides. Further genetic analyses of the alterochromide BGC revealed that the yellow pigments are biosynthesized by aryl-polyene synthases and nonribosomal peptide synthases. Within the gene cluster, altA encodes a tyrosine ammonia acid lyase, which catalyzes synthesis of the precursor 4-hydroxycinnamic acid (4-HCA) from tyrosine in the alterochromide biosynthetic pathway. In addition, altN, encoding a putative flavin-dependent halogenase, was proven to be responsible for the bromination of alterochromides based on gene deletion, molecular docking, and site mutagenesis analyses. In summary, the biosynthetic pathway, precursor synthesis, and bromination mechanism of the lipopeptide alterochromides were studied in-depth. Our results expand the knowledge on biosynthesis of Pseudoalteromonas pigments and could promote the development of active pigments in the future.IMPORTANCEThe marine bacteria Pseudoalteromonas spp. are important biological resources because they are producers of bioactive natural products, including antibiotics, pigments, enzymes, and antimicrobial peptides. One group of the microbial pigments, alterochromides, holds a great value for their novel lipopeptide structures and antimicrobial activities. Previous studies were limited to the structural characterization of alterochromides and genome mining for the alterochromide biosynthesis. This work focused on the biosynthetic mechanism for alterochromide production, especially revealing functions of two key genes within the gene cluster for the alterochromide biosynthesis. On the one hand, our study provides a target for metabolic engineering of the alterochromide biosynthesis; on the other hand, the 4-HCA synthase AltA and brominase AltN show potential in the biocatalyst industry.

Genomic Island-Encoded Diguanylate Cyclase from Vibrio alginolyticus Regulates Biofilm Formation and Motility in Pseudoalteromonas

Many bacteria use the second messenger c-di-GMP to regulate exopolysaccharide production, biofilm formation, motility, virulence, and other phenotypes. The c-di-GMP level is controlled by the complex network of diguanylate cyclases (DGCs) and phosphodiesterases (PDEs) that synthesize and degrade c-di-GMP. In addition to chromosomally encoded DGCs, increasing numbers of DGCs were found to be located on mobile genetic elements. Whether these mobile genetic element-encoded DGCs can modulate the physiological phenotypes in recipient bacteria after horizontal gene transfer should be investigated. In our previous study, a genomic island encoding three DGC proteins (Dgc137, Dgc139, and Dgc140) was characterized in Vibrio alginolyticus isolated from the gastric cavity of the coral Galaxea fascicularis. Here, the effect of the three DGCs in four Pseudoalteromonas strains isolated from coral Galaxea fascicularis and other marine environments was explored. The results showed that when dgc137 is present rather than the three DGC genes, it obviously modulates biofilm formation and bacterial motility in these Pseudoalteromonas strains. Our findings implied that mobile genetic element-encoded DGC could regulate the physiological status of neighboring bacteria in a microbial community by modulating the c-di-GMP level after horizontal gene transfer.

Staphylococcus aureus sacculus mediates activities of M23 hydrolases

Peptidoglycan, a gigadalton polymer, functions as the scaffold for bacterial cell walls and provides cell integrity. Peptidoglycan is remodelled by a large and diverse group of peptidoglycan hydrolases, which control bacterial cell growth and division. Over the years, many studies have focused on these enzymes, but knowledge on their action within peptidoglycan mesh from a molecular basis is scarce. Here, we provide structural insights into the interaction between short peptidoglycan fragments and the entire sacculus with two evolutionarily related peptidases of the M23 family, lysostaphin and LytM. Through nuclear magnetic resonance, mass spectrometry, information-driven modelling, site-directed mutagenesis and biochemical approaches, we propose a model in which peptidoglycan cross-linking affects the activity, selectivity and specificity of these two structurally related enzymes differently.

Transcriptomic Analysis Followed by the Isolation of Extracellular Bacteriolytic Proteases from Lysobacter capsici VKM B-2533T

The aim of the study was to search for, isolate and characterize new bacteriolytic enzymes that show promising potential for their use in medicine, agriculture and veterinary. Using a transcriptomic analysis, we annotated in Lysobacter capsici VKM B-2533^T the genes of known bacteriolytic and antifungal enzymes, as well as of antibiotics, whose expression levels increased when cultivated on media conducive to the production of antimicrobial agents. The genes of the secreted putative bacteriolytic proteases were also annotated. Two new bacteriolytic proteases, Serp and Serp3, were isolated and characterized. The maximum bacteriolytic activities of Serp and Serp3 were exhibited at low ionic strength of 10 mM Tris-HCl, and high temperatures of, respectively, 80 °C and 70 °C. The pH optimum for Serp was 8.0; for Serp3, it was slightly acidic, at 6.0. Both enzymes hydrolyzed autoclaved cells of Micrococcus luteus Ac-2230^T, Proteus vulgaris H-19, Pseudomonas aeruginosa and Staphylococcus aureus 209P. Serp also digested cells of Bacillus cereus 217. Both enzymes hydrolyzed casein and azofibrin. The newly discovered enzymes are promising for developing proteolytic antimicrobial drugs on their basis.

Pseudomonas ability to utilize different carbon substrates and adaptation influenced by protozoan grazing

Bacteria are major utilizers of dissolved organic matter in aquatic systems. In coastal areas bacteria are supplied with a mixture of food sources, spanning from refractive terrestrial dissolved organic matter to labile marine autochthonous organic matter. Modelling scenarios indicate that in northern coastal areas, the inflow of terrestrial organic matter will increase, and autochthonous production will decrease, thus bacteria will experience a change in the food source composition. How bacteria will cope with such changes is not known. Here, we tested the ability of an isolated bacterium from the northern Baltic Sea coast, Pseudomonas sp., to adapt to varying substrates. We performed a 7-months chemostat experiment, where three different substrates were provided: glucose, representing labile autochthonous organic carbon, sodium benzoate representing refractive organic matter, and acetate - a labile but low energy food source. Growth rate has been pointed out as a key factor for fast adaptation, and since protozoan grazers speed-up the growth rate we added a ciliate to half of the incubations. The results show that the isolated Pseudomonas is adapted to utilize both labile and ring-structured refractive substrates. The growth rate was the highest on the benzoate substrate, and the production increased over time indicating that adaptation did occur. Further, our findings indicate that predation can cause Pseudomonas to change their phenotype to resist and promote survival in various carbon substrates. Genome sequencing reveals different mutations in the genome of adapted populations compared to the native populations, suggesting the adaptation of Pseudomonas sp. To changing environment.

The FilZ Protein Contains a Single PilZ Domain and Facilitates the Swarming Motility of Pseudoalteromonas sp. SM9913

Swarming regulation is complicated in flagellated bacteria, especially those possessing dual flagellar systems. It remains unclear whether and how the movement of the constitutive polar flagellum is regulated during swarming motility of these bacteria. Here, we report the downregulation of polar flagellar motility by the c-di-GMP effector FilZ in the marine sedimentary bacterium Pseudoalteromonas sp. SM9913. Strain SM9913 possesses two flagellar systems, and filZ is located in the lateral flagellar gene cluster. The function of FilZ is negatively controlled by intracellular c-di-GMP. Swarming in strain SM9913 consists of three periods. Deletion and overexpression of filZ revealed that, during the period when strain SM9913 expands quickly, FilZ facilitates swarming. In vitro pull-down and bacterial two-hybrid assays suggested that, in the absence of c-di-GMP, FilZ interacts with the CheW homolog A2230, which may be involved in the chemotactic signal transduction pathway to the polar flagellar motor protein FliM_p, to interfere with polar flagellar motility. When bound to c-di-GMP, FilZ loses its ability to interact with A2230. Bioinformatic investigation indicated that filZ-like genes are present in many bacteria with dual flagellar systems. Our findings demonstrate a novel mode of regulation of bacterial swarming motility.

Microbiome and Physicochemical Features Associated with Differential Listeria monocytogenes Growth in Soft, Surface-Ripened Cheeses

Soft-ripened cheeses (SRCs) are at a higher risk for the growth of the foodborne pathogen Listeria monocytogenes due to favorable moisture content and pH compared to other cheeses. L. monocytogenes growth is not consistent across SRCs, however, and may be affected by physicochemical and/or microbiome characteristics of the cheeses. Therefore, the purpose of this study was to investigate how the physicochemical and microbiome profiles of SRCs may affect L. monocytogenes growth. Forty-three SRCs produced from raw (n = 12) or pasteurized (n = 31) milk were inoculated with L. monocytogenes (103 CFU/g), and the pathogen growth was monitored over 12 days at 8°C. In parallel, the pH, water activity (aw), microbial plate counts, and organic acid content of cheeses were measured, and the taxonomic profiles of the cheese microbiomes were measured using 16S rRNA gene targeted amplicon sequencing and shotgun metagenomic sequencing. L. monocytogenes growth differed significantly between cheeses (analysis of variance [ANOVA]; P < 0.001), with increases ranging from 0 to 5.4 log CFU (mean of 2.5 ± 1.2 log CFU), and was negatively correlated with aw. Raw milk cheeses showed significantly lower L. monocytogenes growth than pasteurized-milk cheeses (t test; P = 0.008), possibly due to an increase in microbial competition. L. monocytogenes growth in cheeses was positively correlated with the relative abundance of Streptococcus thermophilus (Spearman correlation; P < 0.0001) and negatively correlated with the relative abundances of Brevibacterium aurantiacum (Spearman correlation; P = 0.0002) and two Lactococcus spp. (Spearman correlation; P < 0.01). These results suggest that the cheese microbiome may influence the food safety in SRCs. IMPORTANCE Previous studies have identified differences in L. monocytogenes growth between SRCs, but no clear mechanism has yet been elucidated. To the best of our knowledge, this is the first study to collect a wide range of SRCs from retail sources and attempt to identify key factors associated with pathogen growth. A key finding in this research was the positive correlation between the relative abundance of S. thermophilus and the growth of L. monocytogenes. The inclusion of S. thermophilus as a starter culture is more common in industrialized SRC production, suggesting that industrial production of SRC may increase the risk of L. monocytogenes growth. Overall, the results of this study further our understanding of the impact of aw and the cheese microbiome on the growth of L. monocytogenes in SRCs, hopefully leading toward the development of SRC starter/ripening cultures that can prevent L. monocytogenes growth.

Chemical and genomic characterization of a potential probiotic treatment for stony coral tissue loss disease

Considered one of the most devastating coral disease outbreaks in history, stony coral tissue loss disease (SCTLD) is currently spreading throughout Florida's coral reefs and the greater Caribbean. SCTLD affects at least two dozen different coral species and has been implicated in extensive losses of coral cover. Here we show Pseudoalteromonas sp. strain McH1-7 has broad-spectrum antibacterial activity against SCTLD-associated bacterial isolates. Chemical analyses indicated McH1-7 produces at least two potential antibacterials, korormicin and tetrabromopyrrole, while genomic analysis identified the genes potentially encoding an L-amino acid oxidase and multiple antibacterial metalloproteases (pseudoalterins). During laboratory trials, McH1-7 arrested or slowed disease progression on 68.2% of diseased Montastraea cavernosa fragments treated (n = 22), and it prevented disease transmission by 100% (n = 12). McH1-7 is the most chemically characterized coral probiotic that is an effective prophylactic and direct treatment for the destructive SCTLD as well as a potential alternative to antibiotic use.

Novel D-glutamate catabolic pathway in marine Proteobacteria and halophilic archaea

D-glutamate (D-Glu) is an essential component of bacterial peptidoglycans, representing an important, yet overlooked, pool of organic matter in global oceans. However, little is known on D-Glu catabolism by marine microorganisms. Here, a novel catabolic pathway for D-Glu was identified using the marine bacterium Pseudoalteromonas sp. CF6-2 as the model. Two novel enzymes (DgcN, DgcA), together with a transcriptional regulator DgcR, are crucial for D-Glu catabolism in strain CF6-2. Genetic and biochemical data confirm that DgcN is a N-acetyltransferase which catalyzes the formation of N-acetyl-D-Glu from D-Glu. DgcA is a racemase that converts N-acetyl-D-Glu to N-acetyl-L-Glu, which is further hydrolyzed to L-Glu. DgcR positively regulates the transcription of dgcN and dgcA. Structural and biochemical analyses suggested that DgcN and its homologs, which use D-Glu as the acyl receptor, represent a new group of the general control non-repressible 5 (GCN5)-related N-acetyltransferases (GNAT) superfamily. DgcA and DgcN occur widely in marine bacteria (particularly Rhodobacterales) and halophilic archaea (Halobacteria) and are abundant in marine and hypersaline metagenome datasets. Thus, this study reveals a novel D-Glu catabolic pathway in ecologically important marine bacteria and halophilic archaea and helps better understand the catabolism and recycling of D-Glu in these ecosystems.

Dietary Effects of a Short-Term Administration of Microalgae Blend on Growth Performance, Tissue Fatty Acids, and Predominant Intestinal Microbiota in Sparus aurata

Given the potential of microalgae as new aquafeed ingredients, this study focuses on using a blend of microalgae, Tisochrysis lutea, Nannochloropsis gaditana, and Scenedesmus almeriensis, as a dietary ingredient for feeding Sparus aurata juveniles. The growth performance, carcass composition, tissue fatty acid profile, and intestinal microbiota were evaluated after a 30 day-feeding period. A microalgae-free diet was used as control, and three experimental diets were formulated containing 5%, 15%, and 25% of the microalgae blend (MB-5%, MB-15%, and MB-25%, respectively). After 7, 15, and 30 days of feeding experimental diets, biological samples were taken. Growth performance and nutrient utilization were not significantly modified at the end of the experiment. Microalgae inclusion tended to decrease body lipids and affected the fatty acid profile, especially MB-25 diet increased DHA levels. Diet MB-25 promoted appropriate microbial diversity, favoring the presence of probiotic bacteria, such as Lactobacillus, and significantly influencing the fatty acid composition and lipid metabolism in fish. In conclusion, using a short pulse of dietary administration of 25% microalgal blend in S. aurata modulates the intestinal microbiota and lipid composition while maintaining growth performance.

Multi-Costimulatory Pathways Drive the Antagonistic Pseudoalteromonas piscicida against the Dominant Pathogenic Vibrio harveyi in Mariculture: Insights from Proteomics and Metabolomics

Vibrio harveyi is the dominant pathogen in mariculture, and biocontrol of this pathogen using antagonistic probiotics is a long-standing biological challenge. Here, Pseudoalteromonas piscicida WCPW15003 as a probiotic effectively antagonized dominant pathogenic V. harveyi in a mariculture, with a growth-of-inhibition ratio of 6.3 h-1. The antagonistic activities of cells and intracellular components of WCPW15003 made a greater contribution to the antagonistic process than did extracellular metabolites and caused the dominance of WCPW15003 during the antagonistic process in vitro. WCPW15003 was safe for the pearl gentian grouper (♀ Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatus) and, as a consequence of the antagonistic effect on V. harveyi, protected the fish from an immune response in vivo. A comprehensive combined proteomics and metabolomics analysis of antagonistic WCPW15003 and pathogenic V. harveyi in a coculture compared to a monoculture was performed to investigate the antagonistic molecular mechanisms. The results showed that during the antagonistic process, WCPW15003 in a coculture had significantly downregulated metabolic pathways for histidine metabolism, arginine biosynthesis, and phenylalanine metabolism, and upregulated glycerophospholipid metabolism, leading to a competitive advantage against the co-occurring species, V. harveyi. This defined a mechanism by which multi-costimulatory pathways drove P. piscicida WCPW15003 against V. harveyi. IMPORTANCE V. harveyi as a dominant pathogen has become a major hazard in mariculture development and seafood safety, and biocontrol of this pathogen using antagonistic probiotic agents is a long-standing biological challenge. P. piscicida WCPW15003 has promise as a novel, safe, and effective bioagent for specifically inhibiting dominant pathogenic V. harveyi and protects mariculture animals from infection by this pathogen by moderating the host immune response, which is heavily driven by multi-costimulatory pathways in a coculture of WCPW15003 and V. harveyi. This work identified a direction for comprehensively elucidating the molecular mechanism of WCPW15003 antagonism against the dominant pathogen in mariculture using modern molecular biology techniques and provided deep insights into the advantages and potential of this antagonistic probiotic against V. harveyi for the construction of an environmentally friendly, recirculating mariculture system.

Structural and Functional Characterization of β-lytic Protease from Lysobacter capsici VKM B-2533T

The crystal structure of the Lysobacter capsici VKM B-2533^T β-lytic protease (Blp), a medicinally promising antimicrobial enzyme, was first solved. Blp was established to possess a folding characteristic of the M23 protease family. The groove of the Blp active site, as compared with that of the LasA structural homologue from Pseudomonas aeruginosa, was found to have amino acid differences. Biochemical analysis revealed no differences in the optimal reaction conditions for manifesting Blp and LasA bacteriolytic activities. At the same time, Blp had a broader range of action against living and autoclaved target cells. The results suggest that the distinction in the geometry of the active site and the charge of amino acid residues that form the active site groove can be important for the hydrolysis of different peptidoglycan types in target cells.

Stringent Starvation Protein SspA and Iron Starvation Sigma Factor PvdS Coordinately Regulate Iron Uptake and Prodiginine Biosynthesis in Pseudoalteromonas sp. R3

Both deficiency and excess of intracellular iron can be harmful, and thus, the iron homeostasis needs to be tightly regulated in organisms. At present, the ferric uptake regulator (Fur) is the best-characterized regulator involved in bacterial iron homeostasis, while other regulators of iron homeostasis remain to be further explored.

Diaminopimelic Acid Metabolism by Pseudomonadota in the Ocean

Diaminopimelic acid (DAP) is a unique component of the cell wall of Gram-negative bacteria. It is also an important component of organic matter and is widely utilized by microbes in the world's oceans. However, neither DAP concentrations nor marine DAP-utilizing microbes have been investigated. Here, DAP concentrations in seawater were measured and the diversity of marine DAP-utilizing bacteria and the mechanisms for their DAP metabolism were investigated. Free DAP concentrations in seawater, from surface to a 5,000 m depth, were found to be between 0.61 μM and 0.96 μM in the western Pacific Ocean. DAP-utilizing bacteria from 20 families in 4 phyla were recovered from the western Pacific seawater and 14 strains were further isolated, in which Pseudomonadota bacteria were dominant. Based on genomic and transcriptomic analyses combined with gene deletion and in vitro activity detection, DAP decarboxylase (LysA), which catalyzes the decarboxylation of DAP to form lysine, was found to be a key and specific enzyme involved in DAP metabolism in the isolated Pseudomonadota strains. Interrogation of the Tara Oceans database found that most LysA-like sequences (92%) are from Pseudomonadota, which are widely distributed in multiple habitats. This study provides an insight into DAP metabolism by marine bacteria in the ocean and contributes to our understanding of the mineralization and recycling of DAP by marine bacteria. IMPORTANCE DAP is a unique component of peptidoglycan in Gram-negative bacterial cell walls. Due to the large number of marine Gram-negative bacteria, DAP is an important component of marine organic matter. However, it remains unclear how DAP is metabolized by marine microbes. This study investigated marine DAP-utilizing bacteria by cultivation and bioinformational analysis and examined the mechanism of DAP metabolism used by marine bacteria. The results demonstrate that Pseudomonadota bacteria are likely to be an important DAP-utilizing group in the ocean and that DAP decarboxylase is a key enzyme involved in DAP metabolism. This study also sheds light on the mineralization and recycling of DAP driven by bacteria.

One fold, many functions-M23 family of peptidoglycan hydrolases

Bacterial cell walls are the guards of cell integrity. They are composed of peptidoglycan that provides rigidity to sustain internal turgor and ensures isolation from the external environment. In addition, they harbor the enzymatic machinery to secure cell wall modulations needed throughout the bacterial lifespan. The main players in this process are peptidoglycan hydrolases, a large group of enzymes with diverse specificities and different mechanisms of action. They are commonly, but not exclusively, found in prokaryotes. Although in most cases, these enzymes share the same molecular function, namely peptidoglycan hydrolysis, they are leveraged to perform a variety of physiological roles. A well-investigated family of peptidoglycan hydrolases is M23 peptidases, which display a very conserved fold, but their spectrum of lytic action is broad and includes both Gram- positive and Gram- negative bacteria. In this review, we summarize the structural, biochemical, and functional studies concerning the M23 family of peptidases based on literature and complement this knowledge by performing large-scale analyses of available protein sequences. This review has led us to gain new insight into the role of surface charge in the activity of this group of enzymes. We present relevant conclusions drawn from the analysis of available structures and indicate the main structural features that play a crucial role in specificity determination and mechanisms of latency. Our work systematizes the knowledge of the M23 family enzymes in the context of their unique antimicrobial potential against drug-resistant pathogens and presents possibilities to modulate and engineer their features to develop perfect antibacterial weapons.

Antibacterial activity of peptaibols from Trichoderma longibrachiatum SMF2 against gram-negative Xanthomonas oryzae pv. oryzae, the causal agent of bacterial leaf blight on rice

Bacterial leaf blight caused by Gram-negative pathogen Xanthomonas oryzae pv. oryzae (Xoo) is one of the most destructive bacterial diseases on rice. Due to the resistance, toxicity and environmental issues of chemical bactericides, new biological strategies are still in need. Although peptaibols produced by Trichoderma spp. can inhibit the growth of several Gram-positive bacteria and plant fungal pathogens, it still remains unclear whether peptaibols have anti-Xoo activity to control bacterial leaf blight on rice. In this study, we evaluated the antibacterial effects of Trichokonins A (TKA), peptaibols produced by Trichoderma longibrachiatum SMF2, against Xoo. The in vitro antibacterial activity analysis showed that the growth of Xoo was significantly inhibited by TKA, with a minimum inhibitory concentration of 54 μg/mL and that the three TKs in TKA all had remarkable anti-Xoo activity. Further inhibitory mechanism analyses revealed that TKA treatments resulted in the damage of Xoo cell morphology and the release of intracellular substances, such as proteins and nucleic acids, from Xoo cells, suggesting the damage of the permeability of Xoo cell membrane by TKA. Pathogenicity analyses showed that the lesion length on rice leaf was significantly reduced by 82.2% when treated with 27 μg/mL TKA. This study represents the first report of the antibacterial activity of peptaibols against a Gram-negative bacterium. Thus, TKA can be of a promising agent in controlling bacterial leaf blight on rice.

The First Homologous Expression System for the β-Lytic Protease of Lysobacter capsici VKM B-2533T, a Promising Antimicrobial Agent

A successful homologous expression system based on Lysobacter capsici VKM B-2533^T and the plasmid pBBR1-MCS5 was first developed for a promising bacteriolytic enzyme of this bacterium, β-lytic protease (Blp). In the expression strains, blp gene expression under the regulation of the GroEL(A) and T5 promoters increased by 247- and 667-fold, respectively, as compared with the wild-type strain. After the cultivation of the expression strains L. capsici P_GroEL(A)-blp and L. capsici P_T5-blp, the Blp yield increased by 6.7- and 8.5-fold, respectively, with respect to the wild-type strain. The cultivation of the expression strain L. capsici P_T5-blp was successfully scaled up. Under fermentation conditions the yield of the enzyme increased by 1.6-fold. The developed homologous system was used to express the gene of the bacteriolytic serine protease (Serp) of L. capsici VKM B-2533^T. The expression of the serp gene in L. capsici P_T5-serp increased by 585-fold. The developed homologous system for the gene expression of bacteriolytic Lysobacter enzymes is potentially biotechnologically valuable, and is promising for creating highly efficient expression strains.

Response of Prokaryotic Communities to Freshwater Salinization

Each year, millions of tons of sodium chloride are dumped on roads, contributing to the salinization of freshwater environments. Thus, we sought to understand the effect of sodium chloride (NaCl) on freshwater lake prokaryotic communities, an important and understudied component of food webs. Using mesocosms with 0.01–2.74 ppt NaCl (0.27–1110.86 mg/L Cl−), we evaluated the effect generated on the diversity and absolute abundance of prokaryotic populations after three and six weeks. A positive relationship between Cl− values and absolute bacterial abundance was found after three weeks. The influence of eukaryotic diversity variation was observed as well. Significant differentiation of bacterial communities starting at 420 mg/L Cl− was observed after three weeks, levels lower than the Canadian and US recommendations for acute chloride exposure. The partial recovery of a “pre-disturbance” community was observed following a drop in salinity at the threshold level of 420 mg/L Cl−. A gradual transition of dominance from Betaproteobacteria and Actinobacteria to Bacteroidia and Alphaproteobacteria was observed and is overall similar to the natural transition observed in estuaries.

Immunomodulation and skin microbiota perturbations during an episode of chronic stress in gilthead seabream

Fish stress is a major concern in the aquaculture industry. Many stressors coming from routine practices can predispose fish to compromised growth, immunity and overall health. This study focuses on the characterization of the skin microbiota using next generation sequencing (NGS) platform by targeting a genomic marker 16S and to determine growth performance and immune status of gilthead seabream (Sparus aurata) during an episode of chronic stress. Two groups were established: control group and chronically stressed group. Stressed fish were subjected to 1 min air exposure twice a week for 4 weeks. Results showed that stress negatively affected fish growth performance. Cellular and humoral systemic immunity remained unaffected while local immunity in skin was positively stimulated (total IgM and peroxidase). Skin mucus microbial composition showed significant differences especially after 14 days. Stressed fish showed a decrease in the abundance of the genera Acinetobacter, NS3a_marine_group and Pseudomonas, while Pseudoalteromonas and Marinagarivorans increased significantly. In conclusion, air exposure stress was associated with alterations in skin mucosal immunity and microbial composition that may have been beneficial to the host favoring adaptation to stress.

d-Alanine Metabolism via d-Ala Aminotransferase by a Marine Gammaproteobacterium, Pseudoalteromonas sp. Strain CF6-2

As the most abundant d-amino acid (DAA) in the ocean, d-alanine (d-Ala) is a key component of peptidoglycan in the bacterial cell wall. However, the underlying mechanisms of bacterial metabolization of d-Ala through the microbial food web remain largely unknown. In this study, the metabolism of d-Ala by marine bacterium Pseudoalteromonas sp. strain CF6-2 was investigated. Based on genomic, transcriptional, and biochemical analyses combined with gene knockout, d-Ala aminotransferase was found to be indispensable for the catabolism of d-Ala in strain CF6-2. Investigation on other marine bacteria also showed that d-Ala aminotransferase gene is a reliable indicator for their ability to utilize d-Ala. Bioinformatic investigation revealed that d-Ala aminotransferase sequences are prevalent in genomes of marine bacteria and metagenomes, especially in seawater samples, and Gammaproteobacteria represents the predominant group containing d-Ala aminotransferase. Thus, Gammaproteobacteria is likely the dominant group to utilize d-Ala via d-Ala aminotransferase to drive the recycling and mineralization of d-Ala in the ocean. IMPORTANCE As the most abundant d-amino acid in the ocean, d-Ala is a component of the marine DON (dissolved organic nitrogen) pool. However, the underlying mechanism of bacterial metabolization of d-Ala to drive the recycling and mineralization of d-Ala in the ocean is still largely unknown. The results in this study showed that d-Ala aminotransferase is specific and indispensable for d-Ala catabolism in marine bacteria and that marine bacteria containing d-Ala aminotransferase genes are predominantly Gammaproteobacteria widely distributed in global oceans. This study reveals marine d-Ala-utilizing bacteria and the mechanism of their metabolization of d-Ala. The results shed light on the mechanisms of recycling and mineralization of d-Ala driven by bacteria in the ocean, which are helpful in understanding oceanic microbial-mediated nitrogen cycle.

Ocular Delivery of Predatory Bacteria with Cryomicroneedles Against Eye Infection

The development of potent antibiotic alternatives with rapid bactericidal properties is of great importance in addressing the current antibiotic crisis. One representative example is the topical delivery of predatory bacteria to treat ocular bacterial infections. However, there is a lack of suitable methods for the delivery of predatory bacteria into ocular tissue. This work introduces cryomicroneedles (cryoMN) for the ocular delivery of predatory Bdellovibrio bacteriovorus (B. bacteriovorus) bacteria. The cryoMN patches are prepared by freezing B. bacteriovorus containing a cryoprotectant medium in a microneedle template. The viability of B. bacteriovorus in cryoMNs remains above 80% as found in long-term storage studies, and they successfully impede the growth of gram-negative bacteria in vitro or in a rodent eye infection model. The infection is significantly relieved by nearly six times through 2.5 days of treatment without substantial effects on the cornea thickness and morphology. This approach represents the safe and efficient delivery of new class of antimicrobial armamentarium to otherwise impermeable ocular surface and opens up new avenues for the treatment of ocular surface disorders.

Self-Assembling Peptide Dendron Nanoparticles with High Stability and a Multimodal Antimicrobial Mechanism of Action

Self-assembling nanometer-scale structured peptide polymers and peptide dendrimers have shown promise in biomedical applications due to their versatile properties and easy availability. Herein, self-assembling peptide dendron nanoparticles (SPDNs) with potent antimicrobial activity against a range of bacteria were developed based on the nanoscale self-assembly of an arginine-proline repeat branched peptide dendron bearing a hexadecanoic acid chain. The SPDNs are biocompatible, and our most active peptide dendron nanoparticle, C₁₆-3RP, was found to have negligible toxicity after both in vitro and in vivo studies. Furthermore, the C₁₆-3RP nanoparticles showed excellent stability under physiological concentrations of salt ions and against serum and protease degradation, resulting in highly effective treatment in a mouse acute peritonitis model. Comprehensive analyses using a series of biofluorescence, microscopy, and transcriptome sequencing techniques revealed that C₁₆-3RP nanoparticles kill Gram-negative bacteria by increasing bacterial membrane permeability, inducing cytoplasmic membrane depolarization and drastic membrane disruption, inhibiting ribosome biogenesis, and influencing energy generation and other processes. Collectively, C₁₆-3RP nanoparticles show promising biocompatibility and in vivo therapeutic efficacy without apparent resistance development. These advancements may facilitate the development of peptide-based antibiotics in clinical settings.

Lack of N-Terminal Segment of the Flagellin Protein Results in the Production of a Shortened Polar Flagellum in the Deep-Sea Sedimentary Bacterium Pseudoalteromonas sp. Strain SM9913

Bacterial polar flagella, comprised of flagellin, are essential for bacterial motility. Pseudoalteromonas sp. strain SM9913 is a bacterium isolated from deep-sea sediments. Unlike other Pseudoalteromonas strains that have a long polar flagellum, strain SM9913 has an abnormally short polar flagellum. Here, we investigated the underlying reason for the short flagellum and found that a single-base mutation was responsible for the altered flagellar assembly. This mutation leads to the fragmentation of the flagellin gene into two genes, PSM_A2281, encoding the core segment and the C-terminal segment, and PSM_A2282, encoding the N-terminal segment, and only gene PSM_A2281 is involved in the production of the short polar flagellum. When a chimeric gene of PSM_A2281 and PSM_A2282 encoding an intact flagellin, A2281::82, was expressed, a long polar flagellum was produced, indicating that the N-terminal segment of flagellin contributes to the production of a polar flagellum of a normal length. Analyses of the simulated structures of A2281 and A2281::82 and that of the flagellar filament assembled with A2281::82 indicate that due to the lack of two α-helices, the core of the flagellar filament assembled with A2281 is incomplete and is likely too weak to support the stability and movement of a long flagellum. This mutation in strain SM9913 had little effect on its growth and only a small effect on its swimming motility, implying that strain SM9913 can live well with this mutation in natural sedimentary environments. This study provides a better understanding of the assembly and production of bacterial flagella. IMPORTANCE Polar flagella, which are essential organelles for bacterial motility, are comprised of multiple flagellin subunits. A flagellin molecule contains an N-terminal segment, a core segment, and a C-terminal segment. The results of this investigation of the deep-sea sedimentary bacterium Pseudoalteromonas sp. strain SM9913 demonstrate that a single-base mutation in the flagellin gene leads to the production of an incomplete flagellin without the N-terminal segment and that the loss of the N-terminal segment of the flagellin protein results in the production of a shortened polar flagellar filament. Our results shed light on the important function of the N-terminal segment of flagellin in the assembly and stability of bacterial flagellar filament.

Quinolone Signals Related to Pseudomonas Quinolone Signal-Quorum Sensing Inhibits the Predatory Activity of Bdellovibrio bacteriovorus

Bdellovibrio bacteriovorus is one of the predatory bacteria; therefore, it can act as a novel “living antibiotic,” unlike the current antibiotics. Here the predation of Escherichia coli by B. bacteriovorus was inhibited in the presence of Pseudomonas aeruginosa. This study investigated whether P. aeruginosa-induced predation inhibition is associated with bacterial quorum sensing (QS). Each las, rhl, or pqs QS mutant in P. aeruginosa was used to check the predatory activity of E. coli cells using B. bacteriovorus. As a result, the predatory activity of B. bacteriovorus increased in a mutant pqs QS system, whereas wild-type PA14 inhibited the predatory activity. Moreover, the addition of 4-hydroxy-2-heptylquinoline (HHQ) or the analog triggered the low predatory activity of B. bacteriovorus and killed B. bacteriovorus cells. Therefore, a defensive action of P. aeruginosa against B. bacteriovorus is activated by the pqs QS system, which produces some quinolone compounds such as HHQ.

Draft Genome Sequencing of Pseudoalteromonas tetraodonis Strain kknpp56, a Potent Biofilm-Forming Bacterium Isolated from Early-Stage Marine Biofilm

Pseudoalteromonas tetraodonis strain kknpp56 is an exopolysaccharide (EPS)-producing marine bacterium that forms potent biofilm. To determine the biosynthesis pathways involved in the EPS production of this bacterium, whole-genome sequencing was performed. The complete genome comes from one chromosome containing 3.72 Mbp of DNA with a G+C content of 41%.

Population structure, activity potential and ecotype partitioning of Pseudoalteromonas along the vertical water column of the New Britain Trench

Microbial degradation of organic matter along the vertical profile of the water column is a major process driving the carbon cycle in the ocean. Pseudoalteromonas has been identified as a dominant genus in pelagic marine environments worldwide, playing important roles in the remineralization of organic carbon. However, the current understanding of Pseudoalteromonas was mainly based on shallow water populations or cultivated species. This study analyzed for the first time the structure, activity potential and ecotypes differentiation of Pseudoalteromonas in the water column of the New Britain Trench (NBT) down to 6000 m. Analysis on diversities of the 16S rRNA gene and their transcripts showed that Pseudoalteromonas was greatly enriched in deep-sea waters and showed high activity potentials. The deep-sea Pseudoalteromonas were significantly different from their shallow-water counterparts, suggesting an obvious ecotype division along with the vertical profile. Phylogenetic analysis on the 16S rRNA gene and hsp60 gene of 219 Pseudoalteromonas strains isolated from different depths further showed that the vertical ecotype division could even occur at the strain level, which might be a result of long-term adaptation to environmental conditions at different depths. The discovered depth-specific strains provide valuable models for further studies on adaptation, evolution and functions of the deep-sea Pseudoalteromonas.

Identification and analysis of the biological activity of the new strain of Pseudoalteromonas piscicida isolated from the hemal fluid of the bivalve Modiolus kurilensis (F. R. Bernard, 1983)

A cultivated form of bacteria (strain 2202) was isolated from the hemal fluid of the bivalve mollusk Modiolus kurilensis. Based on the set of data collected by genetic and physiological/biochemical analyses, the strain was identified as the species Pseudoalteromonas piscicida. Strain 2202 exhibits antimicrobial activity against Staphylococcus aureus, Candida albicans, and Bacillus subtilis but not against Escherichia coli and Pseudomonas aeruginosa. These activities characterize the behavior of strain 2202 as predator-like and classify it as a facultative predator. Being part of the normal microflora in the hemolymph of M. kurilensis, when external conditions change, strain 2202 shows features of opportunistic microflora. The strain 2202 exhibits selective toxicity towards larvae of various invertebrates: it impairs the early development of Mytilus edulis, but not of Strongylocentrotus nudus. Thus, the selective manner in which P. piscicida strains interact with various species of microorganisms and eukaryotes should be taken into consideration when using their biotechnological potential as a probiotic in aquaculture, source of antimicrobial substances, and factors that prevent fouling.

A Streptomyces globisporus strain kills Microcystis aeruginosa via cell-to-cell contact

Cyanobacterial harmful algal blooms (CyanoHABs) bring economic loss, damage aquatic ecosystems, and produce cyanobacterial toxins that threaten human health. Algicidal bacteria as pathogens can expediate the decline of CyanoHABs. In this study, a Streptomyces globisporus strain (designated G9), isolated from soil near a eutrophic pond, showed high algicidal activity against Microcystis aeruginosa. Experimental results show that G9 preyed on Microcystis through cell-to-cell contact: (1) the hyphae of G9 killed cyanobacterial cells by twining around them, while cells beyond the reach of G9 hyphae were in normal shapes; (2) No algicides were detectable in the supernatant of G9 cultures or G9-Microcystis cocultures. The algicidal ratio of G9 to M. aeruginosa reached 96.7% after 6 days. G9 selectively killed the tested cyanobacterial strains, while it had only minor impacts on the growth of tested Chlorophyceae. Differential gene expression studies show that G9 affected the expression of key genes of M. aeruginosa involved in photosynthesis, microcystin synthesis and cellular emergency responses. Further, the microcystin-LR content decreased gradually with G9 treatment. As the first reported Streptomyces sp. with algicidal (predation) activity requiring cell-to-cell contact with target prey, G9 is a good candidate for the exploration of additional cyanobacteria-bacteria interactions and the development of novel strategies to control CyanoHABs.

Halolysin R4 of Haloferax mediterranei confers its host antagonistic and defensive activities

Halolysins, which are subtilisin-like serine proteases of haloarchaea, are usually secreted into the extracellular matrix via the twin-arginine translocation pathway. A small number of activated molecules can greatly affect cell growth owing to their proteolytic activity. It is, however, unclear as to whether this proteolysis-based growth inhibition by halolysins conveys antagonistic or defensive effects against other resident abd potentially competitive microorganisms. Here, we report that halolysin R4 (HlyR4), encoded by the hlyR4 gene, is the key enzyme in the initial steps of extracellular protein utilization in Haloferax mediterranei HlyR4 shows significant antagonistic activity against other haloarchaeal strains. Deletion of hlyR4 completely halts the inhibition activity of Hfx. mediterranei towards other haloarchaea, while correspondingly, complementation of hlyR4 almost completely restores the inhibition activity. Furthermore, Hfx. mediterranei strains containing hlyR4 showed a certain amount of resistance to halocins and halolysins in milieu, and this function of hlyR4 is reproducible in Haloarcula hispanica The versatility of HlyR4 enables its host to outcompete other haloarchaea living in the same hypersaline environment. Intriguingly, unlike the growth phase-dependent halolysins SptA and Nep, it is likely that HlyR4 may be secreted independent of growth phase. This study provides a new peptide antibiotics candidate in haloarchaea, as well as new insight towards a better understanding of the ecological roles of halolysins.Importance: This study shows that halolysin R4 from Haloferax mediterranei provides its host antagonistic and defensive activities against other haloarchaea, which expands our knowledge on the traditional function of haloarchaeal extracellular proteases. Haloarchaeal extracellular serine proteases have been previously discussed as growth-phase-dependent proteins, whereas our study reports constitutive expression of halolysin R4. This work also clearly reveals a hidden diversity of extracellular proteases from haloarchaea. Studies on multifunctional halolysins reveal that they play an important ecological role in shaping microbial community composition and provide a new perspective towards understanding the intricate interactions between haloarchaeal cells in hypersaline environments. HlyR4 can lyse competing cells living in the same environment, and the cell debris may probably be utilized as nutrients, which may constitute an important part of nutrient cycling in extremely hypersaline environments.

Complete Genome Sequence of Pseudoalteromonas sp. Strain LC2018020214, a Bacterium Isolated from Natural Seawater

Pseudoalteromonas is a genus widely distributed in the ocean and displays antibacterial and antifouling activities. We isolated a Pseudoalteromonas sp. strain (LC2018020214) from coastal water of Qingdao, China, and assembled its complete genome. The genome consists of two circular chromosomes with lengths of 3,700,777 bp and 817,517 bp, respectively, and 3,866 coding sequences.

Pseudoalteromonas is a genus widely distributed in the ocean and displays antibacterial and antifouling activities. We isolated a Pseudoalteromonas sp. strain (LC2018020214) from coastal water of Qingdao, China, and assembled its complete genome. The genome consists of two circular chromosomes with lengths of 3,700,777 bp and 817,517 bp, respectively, and 3,866 coding sequences.

Characterization and Diversity Analysis of the Extracellular Proteases of Thermophilic Anoxybacillus caldiproteolyticus 1A02591 From Deep-Sea Hydrothermal Vent Sediment

Protease-producing bacteria play key roles in the degradation of marine organic nitrogen. Although some deep-sea bacteria are found to produce proteases, there has been no report on protease-secreting Anoxybacillus from marine hydrothermal vent regions. Here, we analyzed the diversity and functions of the proteases, especially the extracellular proteases, of Anoxybacillus caldiproteolyticus 1A02591, a protease-secreting strain isolated from a deep-sea hydrothermal vent sediment of the East Pacific Ocean. Strain 1A02591 is a thermophilic bacterium with a strong protease-secreting ability, which displayed the maximum growth rate (0.139 h^–1) and extracellular protease production (307.99 U/mL) at 55°C. Strain 1A02591 contains 75 putative proteases, including 65 intracellular proteases and 10 extracellular proteases according to signal peptide prediction. When strain 1A02591 was cultured with casein, 12 proteases were identified in the secretome, in which metalloproteases (6/12) and serine proteases (4/12) accounted for the majority, and a thermolysin-like protease of the M4 family was the most abundant, suggesting that strain 1A02591 mainly secreted a thermophilic metalloprotease. Correspondingly, the secreted proteases of strain 1A02591 showed the highest activity at the temperature as high as 70°C, and was inhibited 70% by metalloprotease inhibitor o-phenanthroline and 50% by serine protease inhibitor phenylmethylsulfonyl fluoride. The secreted proteases could degrade different proteins, suggesting the role of strain 1A02591 in organic nitrogen degradation in deep-sea hydrothermal ecosystem. These results provide the first insight into the proteases of an Anoxybacillus strain from deep-sea hydrothermal ecosystem, which is helpful in understanding the function of Anoxybacillus in the marine biogeochemical cycle.

PBP4 Is Likely Involved in Cell Division of the Longitudinally Dividing Bacterium Candidatus Thiosymbion Oneisti

Peptidoglycan (PG) is essential for bacterial survival and maintaining cell shape. The rod-shaped model bacterium Escherichia coli has a set of seven endopeptidases that remodel the PG during cell growth. The gamma proteobacterium Candidatus Thiosymbion oneisti is also rod-shaped and attaches to the cuticle of its nematode host by one pole. It widens and divides by longitudinal fission using the canonical proteins MreB and FtsZ. The PG layer of Ca. T. oneisti has an unusually high peptide cross-linkage of 67% but relatively short glycan chains with an average length of 12 disaccharides. Curiously, it has only two predicted endopeptidases, MepA and PBP4. Cellular localization of symbiont PBP4 by fluorescently labeled antibodies reveals its polar localization and its accumulation at the constriction sites, suggesting that PBP4 is involved in PG biosynthesis during septum formation. Isolated symbiont PBP4 protein shows a different selectivity for β-lactams compared to its homologue from E. coli. Bocillin-FL binding by PBP4 is activated by some β-lactams, suggesting the presence of an allosteric binding site. Overall, our data point to a role of PBP4 in PG cleavage during the longitudinal cell division and to a PG that might have been adapted to the symbiotic lifestyle.

Chitin Degradation Machinery and Secondary Metabolite Profiles in the Marine Bacterium Pseudoalteromonas rubra S4059

Genome mining of pigmented Pseudoalteromonas has revealed a large potential for the production of bioactive compounds and hydrolytic enzymes. The purpose of the present study was to explore this bioactivity potential in a potent antibiotic and enzyme producer, Pseudoalteromonas rubra strain S4059. Proteomic analyses (data are available via ProteomeXchange with identifier PXD023249) indicated that a highly efficient chitin degradation machinery was present in the red-pigmented P. rubra S4059 when grown on chitin. Four GH18 chitinases and two GH20 hexosaminidases were significantly upregulated under these conditions. GH19 chitinases, which are not common in bacteria, are consistently found in pigmented Pseudoalteromonas, and in S4059, GH19 was only detected when the bacterium was grown on chitin. To explore the possible role of GH19 in pigmented Pseudoalteromonas, we developed a protocol for genetic manipulation of S4059 and deleted the GH19 chitinase, and compared phenotypes of the mutant and wild type. However, none of the chitin degrading ability, secondary metabolite profile, or biofilm-forming capacity was affected by GH19 deletion. In conclusion, we developed a genetic manipulation protocol that can be used to unravel the bioactive potential of pigmented pseudoalteromonads. An efficient chitinolytic enzyme cocktail was identified in S4059, suggesting that this strain could be a candidate with industrial potential.

Evolutionary Trajectory of the Replication Mode of Bacterial Replicons

Chromosome replication is an essential process for cell division. The mode of chromosome replication has important impacts on the structure of the chromosome and replication speed.

As typical bacterial replicons, circular chromosomes replicate bidirectionally and circular plasmids replicate either bidirectionally or unidirectionally. Whereas the finding of chromids (plasmid-derived chromosomes) in multiple bacterial lineages provides circumstantial evidence that chromosomes likely evolved from plasmids, all experimentally assayed chromids were shown to use bidirectional replication. Here, we employed a model system, the marine bacterial genus Pseudoalteromonas, members of which consistently carry a chromosome and a chromid. We provide experimental and bioinformatic evidence that while chromids in a few strains replicate bidirectionally, most replicate unidirectionally. This is the first experimental demonstration of the unidirectional replication mode in bacterial chromids. Phylogenomic and comparative genomic analyses showed that the bidirectional replication evolved only once from a unidirectional ancestor and that this transition was associated with insertions of exogenous DNA and relocation of the replication terminus region (ter2) from near the origin site (ori2) to a position roughly opposite it. This process enables a plasmid-derived chromosome to increase its size and expand the bacterium’s metabolic versatility while keeping its replication synchronized with that of the main chromosome. A major implication of our study is that the uni- and bidirectionally replicating chromids may represent two stages on the evolutionary trajectory from unidirectionally replicating plasmids to bidirectionally replicating chromosomes in bacteria. Further bioinformatic analyses predicted unidirectionally replicating chromids in several unrelated bacterial phyla, suggesting that evolution from unidirectionally to bidirectionally replicating replicons occurred multiple times in bacteria.

Mechanisms for Induction of Microbial Extracellular Proteases in Response to Exterior Proteins

Proteins are a main organic nitrogen source for microorganisms. Many heterotrophic microorganisms secrete extracellular proteases (ex-proteases) to efficiently decompose proteins into oligopeptides and amino acids when exterior proteins are required for growth. These ex-proteases not only play important roles in microbial nutrient acquisition or host infection but also contribute greatly to the global recycling of carbon and nitrogen. Moreover, may microbial ex-proteases have important applications in industrial, medical, and biotechnological areas. Therefore, uncovering the mechanisms by which microorganisms initiate the expression of ex-protease genes in response to exterior proteins is of great significance. In this review, the progress made in understanding the induction mechanisms of microbial ex-proteases in response to exterior proteins is summarized, with a focus on the inducer molecules, membrane sensors, and downstream pathways. Problems to be solved for better understanding of the induction mechanisms of microbial ex-proteases are also discussed.

Bradymonabacteria, a novel bacterial predator group with versatile survival strategies in saline environments

BACKGROUND

Bacterial predation is an important selective force in microbial community structure and dynamics. However, only a limited number of predatory bacteria have been reported, and their predatory strategies and evolutionary adaptations remain elusive. We recently isolated a novel group of bacterial predators, Bradymonabacteria, representative of the novel order Bradymonadales in δ-Proteobacteria. Compared with those of other bacterial predators (e.g., Myxococcales and Bdellovibrionales), the predatory and living strategies of Bradymonadales are still largely unknown.

RESULTS

Based on individual coculture of Bradymonabacteria with 281 prey bacteria, Bradymonabacteria preyed on diverse bacteria but had a high preference for Bacteroidetes. Genomic analysis of 13 recently sequenced Bradymonabacteria indicated that these bacteria had conspicuous metabolic deficiencies, but they could synthesize many polymers, such as polyphosphate and polyhydroxyalkanoates. Dual transcriptome analysis of cocultures of Bradymonabacteria and prey suggested a potential contact-dependent predation mechanism. Comparative genomic analysis with 24 other bacterial predators indicated that Bradymonabacteria had different predatory and living strategies. Furthermore, we identified Bradymonadales from 1552 publicly available 16S rRNA amplicon sequencing samples, indicating that Bradymonadales was widely distributed and highly abundant in saline environments. Phylogenetic analysis showed that there may be six subgroups in this order; each subgroup occupied a different habitat.

CONCLUSIONS

Bradymonabacteria have unique living strategies that are transitional between the "obligate" and the so-called facultative predators. Thus, we propose a framework to categorize the current bacterial predators into 3 groups: (i) obligate predators (completely prey-dependent), (ii) facultative predators (facultatively prey-dependent), and (iii) opportunistic predators (prey-independent). Our findings provide an ecological and evolutionary framework for Bradymonadales and highlight their potential ecological roles in saline environments. Video abstract.

Meta-analysis of cheese microbiomes highlights contributions to multiple aspects of quality

A detailed understanding of the cheese microbiome is key to the optimization of flavour, appearance, quality and safety. Accordingly, we conducted a high-resolution meta-analysis of cheese microbiomes and corresponding volatilomes. Using 77 new samples from 55 artisanal cheeses from 27 Irish producers combined with 107 publicly available cheese metagenomes, we recovered 328 metagenome-assembled genomes, including 47 putative new species that could influence taste or colour through the secretion of volatiles or biosynthesis of pigments. Additionally, from a subset of samples, we found that differences in the abundances of strains corresponded with levels of volatiles. Genes encoding bacteriocins and other antimicrobials, such as pseudoalterin, were common, potentially contributing to the control of undesirable microorganisms. Although antibiotic-resistance genes were detected, evidence suggested they are not of major concern with respect to dissemination to other microbiomes. Phages, a potential cause of fermentation failure, were abundant and evidence for phage-mediated gene transfer was detected. The anti-phage defence mechanism CRISPR was widespread and analysis thereof, and of anti-CRISPR proteins, revealed a complex interaction between phages and bacteria. Overall, our results provide new and substantial technological and ecological insights into the cheese microbiome that can be applied to further improve cheese production.

Identification of A Putative T6SS Immunity Islet in Salmonella Typhi

Typhoid fever is a major global health problem and is the result of systemic infections caused by the human-adapted bacterial pathogen Salmonella enterica serovar Typhi (S. Typhi). The pathology underlying S. Typhi infections significantly differ from infections caused by broad host range serovars of the same species, which are a common cause of gastroenteritis. Accordingly, identifying S. Typhi genetic factors that impart functionality absent from broad host range serovars offers insights into its unique biology. Here, we used an in-silico approach to explore the function of an uncharacterized 14-gene S. Typhi genomic islet. Our results indicated that this islet was specific to the S. enterica species, where it was encoded by the Typhi and Paratyphi A serovars, but was generally absent from non-typhoidal serovars. Evidence was gathered using comparative genomics and sequence analysis tools, and indicated that this islet was comprised of Type VI secretion system (T6SS) and contact-dependent growth inhibition (CDI) genes, the majority of which appeared to encode orphan immunity proteins that protected against the activities of effectors and toxins absent from the S. Typhi genome. We herein propose that this islet represents an immune system that protects S. Typhi against competing bacteria within the human gut.