Glycerol inhibition of melanin biosynthesis in the environmental Aeromonas salmonicida 34melT

Investigation of Eumelanin Biosynthesis in Gluconacetobacter tumulisoli FBFS 97: A Novel Insight into a Bacterial Melanin Producer

Acetic acid bacteria (AAB) are a group of bacteria, most of which can produce pigments. However, the mechanism of pigment production by AAB is unclear. A strain of AAB, Gluconacetobacter tumulisoli FBFS 97, which can produce a large amount of brown pigment (BP), was isolated in our previous research. In the current study, it was found that the BP yield of the FBFS 97 strain was enhanced in the presence of tyrosine, and an intermediate of melanin, L-3,4-dihydroxyphenylalanine (L-DOPA), was identified using ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS). The structural properties of BP were analyzed by pyrolysis gas chromatography-mass spectrometry (Py-GC-MS). All these analyses suggest that BP may be eumelanin, a type of melanin. Then, the eumelanin biosynthetic pathway was investigated in the FBFS 97 strain, and three related genes with eumelanin including pheA, yfiH, and phhB in its genome were found and knocked out, respectively. The results showed that eumelanin production increased 1.3-fold in the pheA deletion mutant compared to the wild-type FBFS 97 strain, but when either yfiH or phhB was knocked out, the eumelanin production in the mutants was the same as that in the wild-type FBFS 97 strain. Finally, a possible biosynthetic pathway for eumelanin in the FBFS 97 strain is proposed.

The structural complexity of pyomelanin impacts UV shielding in Pseudomonas species with different lifestyles

Pyomelanin, a polymeric pigment in Pseudomonas, arises mainly from alterations in tyrosine degradation. The chemical structure of pyomelanin remains elusive due to its heterogeneous nature. Here, we report strain-specific differences in pyomelanin structural features across Pseudomonas using PAO1 and PA14 reference strains carrying mutations in hmgA (a gene involved in pyomelanin synthesis), a melanogenic P. aeruginosa clinical isolate (PAM), and a melanogenic P. extremaustralis (PexM). UV spectra showed dual peaks for PAO1 and PA14 mutants and single peaks for PAM and PexM. FTIR phenol : alcohol ratio changes and complex NMR spectra indicated non-linear polymers. UVC radiation survival increased with pyomelanin addition, correlating with pigment absorption attenuation. P. extremaustralis UVC survival varied with melanin source, with PAO1 pyomelanin being the most protective. These findings delineate structure-based pyomelanin subgroups, having distinct physiological effects.

Dark Pigments in Entomopathogenic Fungal Microsclerotia: Preliminary Evidence of a 1,8-Dihydroxynaphthalene-melanin-like Compound in Metarhizium robertsii

Metarhizium robertsii microsclerotia are fungal aggregates composed of compacted, pigmented hyphae. As they are highly tolerant to desiccation and produce infective conidia, they are promising candidates to be formulated as bioinsecticides. Despite this potential, the nature of the pigments within these structures remains unclear. In this study, routine culture media used for the differentiation of M. robertsii microsclerotia were supplemented with four melanin inhibitors, and the resulting propagules were characterized. Inhibitors of the 1,8-dihydroxynaphthalene (DHN)-melanin biosynthetic pathway such as tricyclazole and guaiacol induced significant phenotypic and molecular modifications in the obtained M. robertsii propagules, which exhibited a more spherical shape, reduced size, and increased susceptibility to desiccation, heat, and oxidative stress than microsclerotia obtained without inhibitors. Additionally, genes encoding for a polyketide synthase (Mrpks2) and a putative 1,3,6,8-tetrahydroxynaphthalene reductase (Mrthnr), potentially involved in the DHN-melanin biosynthetic pathway, were upregulated in fungi grown in the inhibitor-added media. In conclusion, M. robertsii microsclerotia contain melanins of type DHN that might play a role in both microsclerotia differentiation and environmental stress tolerance.

Guanine crystal formation by bacteria

BACKGROUND

Guanine crystals are organic biogenic crystals found in many organisms. Due to their exceptionally high refractive index, they contribute to structural color and are responsible for the reflective effect in the skin and visual organs in animals such as fish, reptiles, and spiders. Occurrence of these crystals in animals has been known for many years, and they have also been observed in eukaryotic microorganisms, but not in prokaryotes.

RESULTS

In this work, we report the discovery of extracellular crystals formed by bacteria and reveal that they are composed of guanine monohydrate. This composition differs from that of biogenic guanine crystals found in other organisms, mostly composed of β anhydrous guanine. We demonstrate the formation of these crystals by Aeromonas and other bacteria and investigate the metabolic traits related to their synthesis. In all cases studied, the presence of the bacterial guanine crystals correlates with the absence of guanine deaminase, which could lead to guanine accumulation providing the substrate for crystal formation.

CONCLUSIONS

Our finding of the hitherto unknown guanine crystal occurrence in prokaryotes extends the range of organisms that produce these crystals to a new domain of life. Bacteria constitute a novel and more accessible model to study the process of guanine crystal formation and assembly. This discovery opens countless chemical and biological questions, including those about the functional and adaptive significance of their production in these microorganisms. It also paves the road for the development of simple and convenient processes to obtain biogenic guanine crystals for diverse applications.

Assessing the effects of β-triketone herbicides on HPPD from environmental bacteria using a combination of in silico and microbiological approaches

4-hydroxyphenylpyruvate dioxygenase (HPPD) is the molecular target of β-triketone herbicides in plants. This enzyme, involved in the tyrosine pathway, is also present in a wide range of living organisms, including microorganisms. Previous studies, focusing on a few strains and using high herbicide concentrations, showed that β-triketones are able to inhibit microbial HPPD. Here, we measured the effect of agronomical doses of β-triketone herbicides on soil bacterial strains. The HPPD activity of six bacterial strains was tested with 1× or 10× the recommended field dose of the herbicide sulcotrione. The selected strains were tested with 0.01× to 15× the recommended field dose of sulcotrione, mesotrione, and tembotrione. Molecular docking was also used to measure and model the binding mode of the three herbicides with the different bacterial HPPD. Our results show that responses to herbicides are strain-dependent with Pseudomonas fluorescens F113 HPPD activity not inhibited by any of the herbicide tested, when all three β-triketone herbicides inhibited HPPD in Bacillus cereus ATCC14579 and Shewanella oneidensis MR-1. These responses are also molecule-dependent with tembotrione harboring the strongest inhibitory effect. Molecular docking also reveals different binding potentials. This is the first time that the inhibitory effect of β-triketone herbicides is tested on environmental strains at agronomical doses, showing a potential effect of these molecules on the HPPD enzymatic activity of non-target microorganisms. The whole-cell assay developed in this study, coupled with molecular docking analysis, appears as an interesting way to have a first idea of the effect of herbicides on microbial communities, prior to setting up microcosm or even field experiments. This methodology could then largely be applied to other family of pesticides also targeting an enzyme present in microorganisms.

Easily purified baculovirus/insect-system-expressed recombinant hepatitis B virus surface antigen fused to the N- or C-terminus of polyhedrin

Baculoviruses are circular double-stranded DNA viruses that infect insects and are widely used as the baculoviral expression vectors (BEVs), which provide a eukaryotic milieu for heterologous expression. The most frequently used vector is based on Autographa californica multiple nucleopolyhedrovirus (AcMNPV). However, purification of recombinant proteins produced using BEVs is laborious, time-consuming, and often expensive. Numerous strategies have been explored to facilitate purification of heterologous proteins, such as fusion with occlusion body (OBs)-forming proteins like polyhedrin (Polh). Baculoviruses produce OBs in the late stages of infection to protect the virion in the cellular environment, and the main protein responsible for OB formation is Polh. In this study, we investigated the effect of fusing the gene that encodes the surface antigen (S-HBsAg) of hepatitis B virus (HBV) to either the N- or C-terminus of the AcMNPV Polh. The production of recombinant viruses and recombinant proteins was confirmed, and the ability to form chimeric S-HBsAg-containing OBs was accessed by light and scanning electron microscopy of infected cells. The fusion was found to affect the shape and size of the OBs when compared to wild-type OBs, with the N-terminal fusion producing less-amorphous OBs than the C-terminal construct. In addition, the N-terminal construct gave higher levels of expression than the C-terminal construct. Quantitative and qualitative immunoassays with human serum or plasma antibodies against HBsAg showed that the two forms of the antigen reacted differently. Although both reacted with the antibody, the N-terminal fusion protein reacted with more sensitivity (2.27-fold) and is therefore more suitable for quantitative assays than the C-terminal version. In summary, the BEVs represents a promising tool for the production of reagents for the diagnosis of HBV infection.

Bacterial genome editing by coupling Cre-lox and CRISPR-Cas9 systems

The past decade has been a golden age for microbiology, marked by the discovery of an unprecedented increase in the number of novel bacterial species. Yet gaining biological knowledge of those organisms has not kept pace with sequencing efforts. To unlock this genetic potential there is an urgent need for generic (i.e. non-species specific) genetic toolboxes. Recently, we developed a method, termed chassis-independent recombinase-assisted genome engineering (CRAGE), enabling the integration and expression of large complex gene clusters directly into the chromosomes of diverse bacteria. Here we expand upon this technology by incorporating CRISPR-Cas9 allowing precise genome editing across multiple bacterial species. To do that we have developed a landing pad that carries one wild-type and two mutant lox sites to allow integration of foreign DNA at two locations through Cre-lox recombinase-mediated cassette exchange (RMCE). The first RMCE event is to integrate the Cas9 and the DNA repair protein genes RecET, and the second RMCE event enables the integration of customized sgRNA and a repair template. Following this workflow, we achieved precise genome editing in four different gammaproteobacterial species. We also show that the inserted landing pad and the entire editing machinery can be removed scarlessly after editing. We report here the construction of a single landing pad transposon and demonstrate its functionality across multiple species. The modular design of the landing pad and accessory vectors allows design and assembly of genome editing platforms for other organisms in a similar way. We believe this approach will greatly expand the list of bacteria amenable to genetic manipulation and provides the means to advance our understanding of the microbial world.

Melanin biosynthesis in bacteria, regulation and production perspectives

The production of black pigments in bacteria was discovered more than a century ago and related to tyrosine metabolism. However, their diverse biological roles and the control of melanin synthesis in different bacteria have only recently been investigated. The broad distribution of these pigments suggests that they have an important role in a variety of organisms. Melanins protect microorganisms from many environmental stress conditions, ranging from ultraviolet radiation and toxic heavy metals to oxidative stress. Melanins can also affect bacterial interactions with other organisms and are important in pathogenesis and survival in many environments. Bacteria produce several types of melanin through dedicated pathways or as a result of enzymatic imbalances in altered metabolic routes. The control of the melanin synthesis in bacteria involves metabolic and transcriptional regulation, but many aspects remain still largely unknown. The diverse properties of melanins have spurred a large number of applications, and recent efforts have been done to produce the pigment at biotechnologically relevant scales.