Swarming motility by Photorhabdus temperata is influenced by environmental conditions and uses the same flagella as that used in swimming motility

Function and Global Regulation of Type III Secretion System and Flagella in Entomopathogenic Nematode Symbiotic Bacteria

Currently, it is widely accepted that the type III secretion system (T3SS) serves as the transport platform for bacterial virulence factors, while flagella act as propulsion motors. However, there remains a noticeable dearth of comparative studies elucidating the functional disparities between these two mechanisms. Entomopathogenic nematode symbiotic bacteria (ENS), including Xenorhabdus and Photorhabdus, are Gram-negative bacteria transported into insect hosts by Steinernema or Heterorhabdus. Flagella are conserved in ENS, but the T3SS is only encoded in Photorhabdus. There are few reports on the function of flagella and the T3SS in ENS, and it is not known what role they play in the infection of ENS. Here, we clarified the function of the T3SS and flagella in ENS infection based on flagellar inactivation in X. stockiae (flhDC deletion), T3SS inactivation in P. luminescens (sctV deletion), and the heterologous synthesis of the T3SS of P. luminescens in X. stockiae. Consistent with the previous results, the swarming movement of the ENS and the formation of biofilms are dominated by the flagella. Both the T3SS and flagella facilitate ENS invasion and colonization within host cells, with minimal impact on secondary metabolite formation and secretion. Unexpectedly, a proteomic analysis reveals a negative feedback loop between the flagella/T3SS assembly and the type VI secretion system (T6SS). RT-PCR testing demonstrates the T3SS's inhibition of flagellar assembly, while flagellin expression promotes T3SS assembly. Furthermore, T3SS expression stimulates ribosome-associated protein expression.

Confinement discerns swarmers from planktonic bacteria

Powered by flagella, many bacterial species exhibit collective motion on a solid surface commonly known as swarming. As a natural example of active matter, swarming is also an essential biological phenotype associated with virulence, chemotaxis, and host pathogenesis. Physical changes like cell elongation and hyper-flagellation have been shown to accompany the swarming phenotype. Less studied, however, are the contrasts of collective motion between the swarming cells and their counterpart planktonic cells of comparable cell density. Here, we show that confining bacterial movement in circular microwells allows distinguishing bacterial swarming from collective swimming. On a soft agar plate, a novel bacterial strain Enterobacter sp. SM3 in swarming and planktonic states exhibited different motion patterns when confined to circular microwells of a specific range of sizes. When the confinement diameter was between 40 μm and 90 μm, swarming SM3 formed a single-swirl motion pattern in the microwells whereas planktonic SM3 formed multiple swirls. Similar differential behavior is observed across several other species of gram-negative bacteria. We also observed 'rafting behavior' of swarming bacteria upon dilution. We hypothesize that the rafting behavior might account for the motion pattern difference. We were able to predict these experimental features via numerical simulations where swarming cells are modeled with stronger cell-cell alignment interaction. Our experimental design using PDMS microchip disk arrays enabled us to observe bacterial swarming on murine intestinal surface, suggesting a new method for characterizing bacterial swarming under complex environments, such as in polymicrobial niches, and for in vivo swarming exploration.

Inactivation of the Major Hemolysin Gene Influences Expression of the Nonribosomal Peptide Synthetase Gene swrA in the Insect Pathogen Serratia sp. Strain SCBI

Hemolysins are important virulence factors for many bacterial pathogens, including Serratia marcescens The role of the major hemolysin gene in the insect pathogen Serratia sp. strain SCBI was investigated using both forward and reverse-genetics approaches. Introduction of the major hemolysin gene into Escherichia coli resulted in a gain of both virulence and hemolytic activity. Inactivation of this hemolysin in Serratia sp. SCBI resulted in a loss of hemolysis but did not attenuate insecticidal activity. Unexpectedly, inactivation of the hemolysin gene in Serratia sp. SCBI resulted in significantly increased motility and increased antimicrobial activity. Reverse transcription-quantitative PCR (qRT-PCR) analysis of mutants with a disrupted hemolysin gene showed a dramatic increase in mRNA levels of a nonribosomal peptide synthetase gene, swrA, which produces the surfactant serrawettin W2. Mutation of the swrA gene in Serratia sp. SCBI resulted in highly varied antibiotic activity, motility, virulence, and hemolysis phenotypes that were dependent on the site of disruption within this 17.75-kb gene. When introduced into E. coli, swrA increases rates of motility and confers antimicrobial activity. While it is unclear how inactivation of the major hemolysin gene influences the expression of swrA, these results suggest that swrA plays an important role in motility and antimicrobial activity in Serratia sp. SCBI.IMPORTANCE The opportunistic Gram-negative bacteria of the genus Serratia are widespread in the environment and can cause human illness. A comparative genomics analysis between Serratia marcescens and a new Serratia species from South Africa, termed Serratia sp. strain SCBI, shows that these two organisms are closely related but differ in pathogenesis. S. marcescens kills Caenorhabditis nematodes, while Serratia sp. SCBI is not harmful and forms a beneficial association with them. This distinction presented the opportunity to investigate potential differences in regulation of common virulence mechanisms between these two species. With the emergence of antibiotic-resistant microorganisms, there is a widespread need to understand the regulation of pathogenesis. The significance of this study is the presentation of evidence for cross-pathway regulation of virulence factors and how the elimination of one mechanism may be compensated for by the upregulation of others.

Identification and characterization of Photorhabdus temperata mutants altered in hemolysis and virulence

Photorhabdus temperata is a symbiont of the entomopathogenic nematode Heterorhabditis bacteriophora and an insect pathogen. This bacterium produces a wide variety of virulence factors and hemolytic activity. The goal of this study was to identify hemolysin-defective mutants and test their virulence. A genetic approach was used to identify mutants with altered hemolytic activity by screening a library of 10 000 P. temperata transposon mutants. Three classes of mutants were identified: (i) defective (no hemolytic activity), (ii) delayed (delayed initiation of hemolytic activity), and (iii) early (early initiation of hemolytic activity). The transposon insertion sites for these mutants were identified and used to investigate other physiological properties, including insect pathogenesis and motility. The hemolysin-defective mutants, P10A-C11, P10A-H12, and P79-B5, had inserts in genes involved in RNA turnover (RNase II and 5'-pentaphospho-5'-adenosine pyrophosphohydrolase) and showed reduced virulence and production of extracellular factors. These data support the role of RNA turnover in insect pathogenesis and other physiological functions.

Flagellar Regulation and Virulence in the Entomopathogenic Bacteria-Xenorhabdus nematophila and Photorhabdus luminescens

There is a complex interplay between the regulation of flagellar motility and the expression of virulence factors in many bacterial pathogens. Here, we review the literature on the direct and indirect roles of flagellar motility in mediating the tripartite interaction between entomopathogenic bacteria (Photorhabdus and Xenorhabdus), their nematode hosts, and their insect targets. First, we describe the swimming and swarming motility of insect pathogenic bacteria and its impact on insect colonization. Then, we describe the coupling between the expression of flagellar and virulence genes and the dynamic of expression of the flagellar regulon during invertebrate infection. We show that the flagellar type 3 secretion system (T3SS) is also an export apparatus for virulence proteins in X. nematophila. Finally, we demonstrate that phenotypic variation, a common property of the bacterial symbionts of nematodes, also alters flagellar motility in Photorhabdus and Xenorhabdus. Finally, the so-called phenotypic heterogeneity phenomenon in the flagellar gene expression network will be also discussed. As the main molecular studies were performed in X. nematophila, future perspectives for the study of the interplay between flagellum and invertebrate interactions in Photorhabdus will be discussed.

Elucidation of the Photorhabdus temperata Genome and Generation of a Transposon Mutant Library To Identify Motility Mutants Altered in Pathogenesis

The entomopathogenic nematode Heterorhabditis bacteriophora forms a specific mutualistic association with its bacterial partner Photorhabdus temperata. The microbial symbiont is required for nematode growth and development, and symbiont recognition is strain specific. The aim of this study was to sequence the genome of P. temperata and identify genes that plays a role in the pathogenesis of the Photorhabdus-Heterorhabditis symbiosis. A draft genome sequence of P. temperata strain NC19 was generated. The 5.2-Mb genome was organized into 17 scaffolds and contained 4,808 coding sequences (CDS). A genetic approach was also pursued to identify mutants with altered motility. A bank of 10,000 P. temperata transposon mutants was generated and screened for altered motility patterns. Five classes of motility mutants were identified: (i) nonmotile mutants, (ii) mutants with defective or aberrant swimming motility, (iii) mutant swimmers that do not require NaCl or KCl, (iv) hyperswimmer mutants that swim at an accelerated rate, and (v) hyperswarmer mutants that are able to swarm on the surface of 1.25% agar. The transposon insertion sites for these mutants were identified and used to investigate other physiological properties, including insect pathogenesis. The motility-defective mutant P13-7 had an insertion in the RNase II gene and showed reduced virulence and production of extracellular factors. Genetic complementation of this mutant restored wild-type activity. These results demonstrate a role for RNA turnover in insect pathogenesis and other physiological functions.

IMPORTANCE

The relationship between Photorhabdus and entomopathogenic nematode Heterorhabditis represents a well-known mutualistic system that has potential as a biological control agent. The elucidation of the genome of the bacterial partner and role that RNase II plays in its life cycle has provided a greater understanding of Photorhabdus as both an insect pathogen and a nematode symbiont.

Molecular characterization of protease activity in Serratia sp. strain SCBI and its importance in cytotoxicity and virulence

A newly recognized Serratia species, termed South African Caenorhabditis briggsae isolate (SCBI), is both a mutualist of the nematode Caenorhabditis briggsae KT0001 and a pathogen of lepidopteran insects. Serratia sp. strain SCBI displays high proteolytic activity, and because secreted proteases are known virulence factors for many pathogens, the purpose of this study was to identify genes essential for extracellular protease activity in Serratia sp. strain SCBI and to determine what role proteases play in insect pathogenesis and cytotoxicity. A bank of 2,100 transposon mutants was generated, and six SCBI mutants with defective proteolytic activity were identified. These mutants were also defective in cytotoxicity. The mutants were found defective in genes encoding the following proteins: alkaline metalloprotease secretion protein AprE, a BglB family transcriptional antiterminator, an inosine/xanthosine triphosphatase, GidA, a methyl-accepting chemotaxis protein, and a PIN domain protein. Gene expression analysis on these six mutants showed significant downregulation in mRNA levels of several different types of predicted protease genes. In addition, transcriptome sequencing (RNA-seq) analysis provided insight into how inactivation of AprE, GidA, and a PIN domain protein influences motility and virulence, as well as protease activity. Using quantitative reverse transcription-PCR (qRT-PCR) to further characterize expression of predicted protease genes in wild-type Serratia sp. SCBI, the highest mRNA levels for the alkaline metalloprotease genes (termed prtA1 to prtA4) occurred following the death of an insect host, while two serine protease and two metalloprotease genes had their highest mRNA levels during active infection. Overall, these results indicate that proteolytic activity is essential for cytotoxicity in Serratia sp. SCBI and that its regulation appears to be highly complex.

Nanohybrids of silver particles on clay platelets delaminate Pseudomonas biofilms

Aim: To evaluate the effectiveness of novel nanohybrids, composed of silver nanoparticles and nanoscale silicate platelets, to clear Pseudomonas aeruginosa biofilms. Materials & methods: The nanohybrids were manufactured from an in situ reduction of silver salts in the silicate platelet dispersion, and then applied to biofilms in vitro and in vivo. Results: In reference to the biocidal effects of gentamycin, the nanohybrids mitigated the spreading of the biofilms, and initiated robust cell death and exfoliation from the superficial layers of the biofilms in vitro. In vivo, the nanohybrids exhibited significant therapeutic effects by eliminating established biofilms from infected corneas and promoting the recovery of corneal integrity. Conclusion: All of the evaluations indicate the high potency of the newly developed silver nanoparticle/nanoscale silicate platelet nanohybrids for eliminating biofilms.

Original submitted 6 June 2012; revised submitted 13 February 2013

Influence of temperature on the physiology and virulence of the insect pathogen Serratia sp. Strain SCBI

The physiology of a newly recognized Serratia species, termed South African Caenorhabditis briggsae Isolate (SCBI), which is both a nematode mutualist and an insect pathogen, was investigated and compared to that of Serratia marcescens Db11, a broad-host-range pathogen. The two Serratia strains had comparable levels of virulence for Manduca sexta and similar cytotoxic activity patterns, but motility and lipase and hemolytic activities differed significantly between them.

Theory of microbe motion in a poisoned environment

The motility of a microorganism which tries to avoid a poisoned environment by chemotaxis is studied within a simple model which couples its velocity to the concentration field of the poison. The latter is time independent but inhomogeneous in space. The presence of the poison is assumed to irreversibly reduce the propulsion speed. The model is solved analytically for different couplings of the total poison dose experienced by the microbe to the propulsion mechanism. In a stationary poison field resulting from a constant emission of a fixed point source, we find a power law for the distance traveled by the microbe as a function of time with a nonuniversal exponent which depends on the coupling in the model. With an inverted sign in the couplings, the acceleration of microbe motion induced by a food field can also be described.