Comparative proteomic analysis reveals new components of the PhoP regulon and highlights a role for PhoP in the regulation of genes encoding the F1F0 ATP synthase in Edwardsiella tarda

Opposing Effects of PhoPQ and PmrAB on the Properties of Salmonella enterica serovar Typhimurium: Implications on Resistance to Antimicrobial Peptides

The increasing number of resistant bacteria is a major threat worldwide, leading to the search for new antibiotic agents. One of the leading strategies is the use of antimicrobial peptides (AMPs), cationic and hydrophobic innate immune defense peptides. A major target of AMPs is the bacterial membrane. Notably, accumulating data suggest that AMPs can activate the two-component systems (TCSs) of Gram-negative bacteria. These include PhoP-PhoQ (PhoPQ) and PmrA-PmrB (PmrAB), responsible for remodeling of the bacterial cell surface. To better understand this mechanism, we utilized bacteria deficient either in one system alone or in both and biophysical tools including fluorescence spectroscopy, single-cell atomic force microscopy, electron microscopy, and mass spectrometry (MoskowitzS. M.;Antimicrob. Agents Chemother.2012, 56, 1019−103022106224; ChengH. Y.;J. Biomed. Sci.2010, 17, 6020653976). Our data suggested that the two systems have opposing effects on the properties of Salmonella enterica. The knockout of PhoPQ made the bacteria more susceptible to AMPs by making the surface less rigid, more polarized, and permeable with a slightly more negatively charged cell wall. In addition, the periplasmic space is thinner. In contrast, the knockout of PmrAB did not affect its susceptibility, while it made the bacterial outer layer very rigid, less polarized, and less permeable than the other two mutants, with a negatively charged cell wall similar to the WT. Overall, the data suggest that the coexistence of systems with opposing effects on the biophysical properties of the bacteria contribute to their membrane flexibility, which, on the one hand, is important to accommodate changing environments and, on the other hand, may inhibit the development of meaningful resistance to AMPs.

How the PhoP/PhoQ System Controls Virulence and Mg2+ Homeostasis: Lessons in Signal Transduction, Pathogenesis, Physiology, and Evolution

The PhoP/PhoQ two-component system governs virulence, Mg2+ homeostasis, and resistance to a variety of antimicrobial agents, including acidic pH and cationic antimicrobial peptides, in several Gram-negative bacterial species. Best understood in Salmonella enterica serovar Typhimurium, the PhoP/PhoQ system consists o-regulated gene products alter PhoP-P amounts, even under constant inducing conditions. PhoP-P controls the abundance of hundreds of proteins both directly, by having transcriptional effects on the corresponding genes, and indirectly, by modifying the abundance, activity, or stability of other transcription factors, regulatory RNAs, protease regulators, and metabolites. The investigation of PhoP/PhoQ has uncovered novel forms of signal transduction and the physiological consequences of regulon evolution.

PepA binds to and negatively regulates esrB to control virulence in the fish pathogen Edwardsiella piscicida

As an important marine fish pathogen, Edwardsiella piscicida infects a broad range of fish species and causes substantial economic losses. The EsrA-EsrB two-component system is essential for the expression of type III and type VI secretion systems (T3/T6SSs), the key virulence determinants in the bacterium. In this study, a pull-down assay with the esrB promoter as bait was performed to identify the upstream regulators of esrB. As a result, PepA, a leucyl aminopeptidase, was identified as a repressor of EsrB and T3/T6SS expression. PepA bound to the esrB promoter region and negatively regulated the production of T3/T6SS proteins in early stages. Moreover, PepA was found to affect the in vivo colonization of E. piscicida in turbot livers through the regulation of EsrB expression. Collectively, our results enhance the understanding of the virulence regulatory network and in vivo colonization mechanism of E. piscicida. One sentence summary: PepA regulates EsrB expression in Edwardsiella piscicida.

EsrB negatively regulates expression of the glutamine sythetase GlnA in the fish pathogen Edwardsiella piscicida

Edwardsiella piscicida is a gram-negative bacterial pathogen invading a wide range of fish species. Response regulator EsrB is essential for the activation of type III and type VI secretion systems (T3/T6SS). In this study, proteomes of the wild-type E. piscicida EIB202 and ΔesrB mutant strains were compared to identify the regulon components of EsrB cultured in DMEM allowing T3/T6SS expression. As a result, 19 proteins showed different expression, which were identified to be associated with T3/T6SS, related to amino acid transport and metabolism, and energy production. Particularly, GlnA, a glutamine synthetase essential for ammonia assimilation and glutamine biosynthesis from glutamate, was found to be regulated negatively by EsrB. Moreover, GlnA affected bacterial growth in vitro and bacterial colonization in vivo. Collectively, our results indicated that EsrB plays important roles in regulating the expression of metabolic pathways and virulence genes, including glutamine biosynthesis in E. piscicida during infection.

Transcriptomic dissection of the horizontally acquired response regulator EsrB reveals its global regulatory roles in the physiological adaptation and activation of T3SS and the cognate effector repertoire in Edwardsiella piscicida during infection toward turbot

Edwardsiella piscicida is the leading pathogen threatening worldwide aquaculture industries. The 2-component system (TCS) EsrA-EsrB is essential for the pathogenesis of this bacterium. However, little is known about the regulon and regulatory mechanism of EsrA-EsrB or about the factors that mediate the interaction of TCS with bacterial hosts. Here, our RNA-seq analysis indicated that EsrB strongly induces type III and type VI secretion systems (T3/T6SS) expression and that it modulates the expression of both physiology- and virulence-associated genes in E. piscicida grown in DMEM. EsrB binds directly to a highly conserved 18-bp DNA motif to regulate the expression of T3SS and other genes. EsrB/DMEM-activated genes include 3 known and 6 novel T3SS-dependent effectors. All these effector genes are highly induced by EsrB during the late stage of in vivo infection in fish. Furthermore, although in vivo colonization by the bacterium relies on EsrB and T3/T6SS expression, it does not require the expression of individual effectors other than EseJ. The mutant lacking these 9 effectors showed significant defects in in vivo colonization and virulence toward turbot, and, more importantly, a high level of protection against challenges by wild-type E. piscicida, suggesting that it may represent a promising live attenuated vaccine. Taken together, our data demonstrate that EsrB plays a global regulatory role in controlling physiologic responses and the expression of T3SS and its cognate effector genes. Our findings will facilitate further work on the mechanism of molecular pathogenesis of this bacterium during infection.

Comparative proteomic analysis unravels a role for EsrB in the regulation of reactive oxygen species stress responses in Edwardsiella piscicida

As a leading pathogen, Edwardsiella piscicida can cause hemorrhagic septicemia in fish and gastro-intestinal infections in humans. The two-component regulatory system EsrA-EsrB plays essential roles in pathogenesis through the type III and type VI secretion systems, and hemolysin production in E. piscicida It is unclear whether other virulence- or stress response-associated genes are regulated by EsrA-EsrB. In this study, the proteomes of wild-type E. piscicida EIB202 and esrB mutant strains were compared to reveal EsrB regulon components after growth in Luria-Bertani broth (LB). Overall, the expression levels of nine genes exhibited significant changes, and five of them required the presence of EsrB, while others exhibited higher levels in the esrB mutant. The diverse functions of these proteins were identified, including amino acid metabolism, oxidative stress defense and energy production. Interestingly, superoxidase dismutase and thiol peroxidase were the most significantly down-regulated by EsrB. Furthermore, other reported reactive oxygen species (ROS) resistance-related genes were also down-regulated by EsrB as revealed by quantitative real-time. Compared with the wild-type and the complement strain esrB⁺, ΔesrB displayed a significantly enhanced ROS resistance. These results demonstrated that EsrB plays important roles in the ROS resistance pathway in E. piscicida grown in LB conditions.

Temperature-dependent expression of virulence genes in fish-pathogenic bacteria

Virulence gene expression in pathogenic bacteria is modulated by environmental parameters. A key factor in this expression is temperature. Its effect on virulence gene expression in bacteria infecting warm-blooded hosts is well documented. Transcription of virulence genes in these bacteria is induced upon a shift from low environmental to a higher host temperature (37°C). Interestingly, host temperatures usually correspond to the optimum for growth of these pathogenic bacteria. On the contrary, in ectothermic hosts such as fish, molluscs, and amphibians, infection processes generally occur at a temperature lower than that for the optimal growth of the bacteria. Therefore, regulation of virulence gene expression in response to temperature shift has to be modulated in a different way to that which is found in bacteria infecting warm-blooded hosts. The current understanding of virulence gene expression and its regulation in response to temperature in fish-pathogenic bacteria is limited, but constant extension of our knowledge base is essential to enable a rational approach to the problem of the bacterial fish diseases affecting the aquaculture industry. This is an interesting issue and progress needs to be made in order to diminish the economic losses caused by these diseases. The intention of this review is, for the first time, to compile the scattered results existing in the field in order to lay the groundwork for future research. This article is an overview of those relevant virulence genes that are expressed at temperatures lower than that for optimal bacterial growth in different fish-pathogenic bacteria as well as the principal mechanisms that could be involved in their regulation.

Complete Genome Sequence of Channel Catfish Gastrointestinal Septicemia Isolate Edwardsiella tarda C07-087

Edwardsiella tarda is a Gram-negative facultative anaerobe causing disease in animals and humans. Here, we announce the complete genome sequence of the channel catfish isolate E. tarda strain C07-87, which was isolated from an outbreak of gastrointestinal septicemia on a commercial catfish farm.