Edwardsiella tarda Ivy, a lysozyme inhibitor that blocks the lytic effect of lysozyme and facilitates host infection in a manner that is dependent on the conserved cysteine residue

Edwardsiella tarda TraT is an anti-complement factor and a cellular infection promoter

Edwardsiella tarda is a well-known bacterial pathogen with a broad range of host, including fish, amphibians, and mammals. One eminent virulence feature of E. tarda is its strong ability to resist the killing of host serum complement, but the involving mechanism is unclear. In this report, we identified E. tarda TraT as a key player in both complement resistance and cellular invasion. TraT, a surface-localized protein, bound and recruited complement factor H onto E. tarda, whereby inhibiting complement activation via the alternative pathway. TraT also interacted with host CD46 in a specific complement control protein domain-dependent manner, whereby facilitating the cellular infection and tissue dissemination of E. tarda. Thus, by acting as an anti-complement factor and a cellular infection promoter, TraT makes an important contribution to the complement evasion and systemic infection of E. tarda. These results add insights into the pathogen-host interaction mechanism during E. tarda infection.

Edwardsiella tarda TraT promotes cellular infection and serves as an anti-complement factor, shedding light on the mechanisms of E. tarda’s strong evasion of killing by the host.

Inhibition of lysozyme lytic activity by Ivy derived from Photobacterium damselae subsp. piscicida

A pseudotuberculosis pathogen, Photobacterium damselae subsp. piscicida (Pdp), has caused enormous economic damage to yellowtail aquaculture in Japan. The Ivy gene has been discovered in plasmid of Pdp, and it has been proposed that it may help bacteria evade lysozyme-mediated lysis during interaction with an animal host. However, the lysozyme-inhibiting activity of Pdp-derived Ivy (Ivy-Pdp) is unknown, and it is unclear whether it acts as a virulence factor for host biophylaxis. In this study, the inhibitory effect of Ivy-Pdp on lysozyme was evaluated by expressing and purifying the recombinant Ivy-Pdp protein (rIvy-Pdp). The rIvy-Pdp protein inhibited hen egg white lysozyme activity in an rIvy-Pdp-concentration-dependent manner, and its inhibitory effect was similar under different temperature and pH conditions. The serum and skin mucus of the yellowtail (which is the host species of Pdp), Japanese flounder, and Nile tilapia showed bacteriolytic activity. In contrast, the addition of rIvy-Pdp inhibited the lytic activity in the serum of these fish species. In particular, it significantly inhibited lytic activity in the serum and skin mucus of Nile tilapia. On the basis of these results, we suggest that Ivy-Pdp is a temperature- and pH-stable lysozyme inhibitor. Additionally, Ivy-Pdp inhibited the lytic activity of lysozyme, which is involved in host biophylaxis. In summary, we inferred that Ivy-Pdp is an important factor that diminishes the sterilization ability of C-type lysozyme when Pdp infects the host.

Complement C3 and Activated Fragment C3a Are Involved in Complement Activation and Anti-Bacterial Immunity

In the complement system, C3 is a central component in complement activation, immune defense and immune regulation. In all pathways of complement activation, the pivotal step is conversion of the component C3 to C3b and C3a, which is responsible to eliminate the pathogen and opsonization. In this study, we examined the immunological properties of C3 and its activated fragment C3a from Japanese flounder (Paralichthys olivaceus) (PoC3 and PoC3a), a teleost species with important economic value. PoC3 is composed of 1655 amino acid residues, contains the six domains and highly conserved GCGEQ sequence of the C3 family. We found that PoC3 expression occurred in nine different tissues and was upregulated by bacterial challenge. In serum, PoC3 was able to bind to a broad-spectrum of bacteria, and purified native PoC3 could directly kill specific pathogen. When PoC3 expression in Japanese flounder was knocked down by siRNA, serum complement activity was significantly decreased, and bacterial replication in fish tissues was significantly increased. Recombinant PoC3a (rPoC3a) exhibited apparent binding capacities to bacteria and Japanese flounder peripheral blood leukocytes (PBL) and induce chemotaxis of PBL. Japanese flounder administered rPoC3a exhibited enhanced resistance against bacterial infection. Taken together, these results indicate that PoC3 is likely a key factor of complement activation, and PoC3 and PoC3a are required for optimal defense against bacterial infection in teleost.

Resistance to peptidoglycan-degrading enzymes

The spread of bacterial strains resistant to commonly used antibiotics urges the development of novel antibacterial compounds. Ideally, these novel antimicrobials should be less prone to the development of resistance. Peptidoglycan-degrading enzymes are a promising class of compounds with a fundamentally different mode of action compared to traditionally used antibiotics. The difference in the mechanism of action implies differences both in the mechanisms of resistance and the chances of its emergence. To critically assess the potential of resistance development to peptidoglycan-degrading enzymes, we review the available evidence for the development of resistance to these enzymes in vitro, along with the known mechanisms of resistance to lysozyme, bacteriocins, autolysins, and phage endolysins. We conclude that genetic determinants of resistance to peptidoglycan-degrading enzymes are unlikely to readily emerge de novo. However, resistance to these enzymes would probably spread by the horizontal transfer between intrinsically resistant and susceptible species. Finally, we speculate that the higher cost of the therapeutics based on peptidoglycan degrading enzymes compared to classical antibiotics might result in less misuse, which in turn would lead to lower selective pressure, making these antibacterials less prone to resistance development.

Characterization, expression, and antimicrobial activity of histones from Japanese flounder Paralichthys olivaceus

Histone proteins are not only structurally important for chromosomal DNA packaging but also involved in the regulation of gene expression and the immune response of host against pathogens. Japanese flounder (Paralichthys olivaceus) as one of the most important marine flatfish, suffered from widespread outbreaks of diseases, and its immunological functioning remained to be elucidated. In the present study, we reported the expression patterns of four histones (H1, H2A, H3, and H3.3) and functional characterization of the histone H3.3 from flounder. Quantitative real time RT-PCR (RT-qPCR) analysis showed that expression of the four histones occurred in multiple tissues, but their levels of expression were relatively high in immune organs, and inducible in response to pathogens infection. Infection with extracellular and intracellular bacterial pathogens and viral pathogen regulated the expression of histones in a manner that depended on tissue type, pathogen, and infection stage. Specifically, H1 expression was highly induced by intracellular viral pathogens; H2AX and H3 expressions were highly induced by intracellular bacterial pathogen; dissimilarly, H3.3 expression was slightly induced by extracellular bacterial pathogen, but was inhibited by intracellular bacterial and viral pathogens. To further investigate H3.3 function, recombinant H3.3 (rH3.3) was obtained, and in vitro experiments showed rH3.3 possessed the capability of binding to both Gram-negative and Gram-positive bacteria and inhibiting the growth of some target bacteria. Consistently, In vivo results showed that overexpression of H3.3 promoted the host defense against invading pathogenic microorganism and regulated the expressions of several cytokines. These results suggested that flounder histones exhibit different expression patterns in response to the infection of different microbial pathogens, and H3.3 serves as an immune-related protein and plays an important role in antimicrobial immunity of Japanese flounder. Taken together, this study is the first report about the expression profile of different histones upon different kind of pathogens and anti-infectious immunity of H3.3 in teleost, which offered new insights into the immunological function of histones in teleost.

HutZ is required for biofilm formation and contributes to the pathogenicity of Edwardsiella piscicida

Edwardsiella piscicida is a severe fish pathogen. Haem utilization systems play an important role in bacterial adversity adaptation and pathogenicity. In this study, a speculative haem utilization protein, HutZ_Ep, was characterized in E. piscicida. hutZ_Ep is encoded with two other genes, hutW and hutX, in an operon that is similar to the haem utilization operon hutWXZ identified in V. cholerae. However, protein activity analysis showed that HutZ_Ep is probably not related to hemin utilization. To explore the biological role of HutZ_Ep, a markerless hutZ_Ep in-frame mutant strain, TX01ΔhutZ, was constructed. Deletion of hutZ_Ep did not significantly affect bacterial growth in normal medium, in iron-deficient conditions, or in the presence of haem but significantly retarded bacterial biofilm growth. The expression of known genes related to biofilm growth was not affected by hutZ_Ep deletion, which indicated that HutZ_Ep was probably a novel factor promoting biofilm formation in E. piscicida. Compared to the wild-type TX01, TX01ΔhutZ exhibited markedly compromised tolerance to acid stress and host serum stress. Pathogenicity analysis showed that inactivation of hutZ_Ep significantly impaired the ability of E. piscicida to invade and reproduce in host cells and to infect host tissue. In contrast to TX01, TX01ΔhutZ was defective in blocking host macrophage activation. The expression of hutZ_Ep was directly regulated by the ferric uptake regulator Fur. This study is the first functional characterization of HutZ in a fish pathogen, and these findings suggested that HutZ_Ep is essential for E. piscicida biofilm formation and contributes to host infection.

A novel calreticulin-related molecule that interacts with bacteria and enhances host resistance against bacterial infection in black rockfish, Sebastes schlegeli

Calreticulin (CRT) is a highly conserved and multi-functional protein with diverse localizations. CRT has lectin-like properties and possesses important immunological activities in mammalian. In teleost, very limited studies on CRT immunologic function have been documented. In the present study, a CRT homologue (SsCRT) was cloned, identified and characterized from black rockfish, Sebastes schlegeli, an important aquaculture species in East Asia. The full length of SsCRT cDNA is 2180 bp and encoded a polypeptide of 425 amino acids. SsCRT contains a signal peptide, three distinct structural and functional domains (N-, P- and C-domains), and an endoplasmic reticulum (ER) retrieval signal sequence (KDEL). The deduced amino acid sequence of SsCRT shares 89-92% overall sequence identities with the CRT proteins of several fish species. SsCRT was distributed ubiquitously in all the detected tissues and was highly expressed in the spleen, muscle and liver. After the infection of fish extracellular bacterial pathogen Vibrio anguillarum and intracellular bacterial pathogen Edwardsiella tarda, the mRNA transcripts of SsCRT in spleen, liver, and head kidney were significantly up-regulated. The expression patterns were time-dependent and tissue-dependent. Recombinant SsCRT (rSsCRT) exhibited apparent binding activities against different bacteria and PAMPs. In vivo studies showed that the expressions of multiple immune-related genes such as TNF13B, IL-1β, IL-8, SAA, Hsp70, and ISG15 in head kidney were significantly enhanced when black rockfish were treated with rSsCRT. Furthermore, rSsCRT reduced pathogen dissemination and replication in fish kidney and spleen. These results indicated that SsCRT served as an immune receptor to recognize and eliminate the invading pathogens, which played a vital role in the immune response of Sebastes schlegeli. These findings provide new insights into understanding the roles of CRT proteins in immune response and pathogen infection in teleost.

A First Study of the Virulence Potential of a Bacillus subtilis Isolate From Deep-Sea Hydrothermal Vent

Bacillus subtilis is the best studied Gram-positive bacterium, primarily as a model of cell differentiation and industrial exploitation. To date, little is known about the virulence of B. subtilis. In this study, we examined the virulence potential of a B. subtilis strain (G7) isolated from the Iheya North hydrothermal field of Okinawa Trough. G7 is aerobic, motile, endospore-forming, and requires NaCl for growth. The genome of G7 is composed of one circular chromosome of 4,216,133 base pairs with an average GC content of 43.72%. G7 contains 4,416 coding genes, 27.5% of which could not be annotated, and the remaining 72.5% were annotated with known or predicted functions in 25 different COG categories. Ten sets of 23S, 5S, and 16S ribosomal RNA operons, 86 tRNA and 14 sRNA genes, 50 tandem repeats, 41 mini-satellites, one microsatellite, and 42 transposons were identified in G7. Comparing to the genome of the B. subtilis wild type strain NCIB 3610^T, G7 genome contains many genomic translocations, inversions, and insertions, and twice the amount of genomic Islands (GIs), with 42.5% of GI genes encoding hypothetical proteins. G7 possesses abundant putative virulence genes associated with adhesion, invasion, dissemination, anti-phagocytosis, and intracellular survival. Experimental studies showed that G7 was able to cause mortality in fish and mice following intramuscular/intraperitoneal injection, resist the killing effect of serum complement, and replicate in mouse macrophages and fish peripheral blood leukocytes. Taken together, our study indicates that G7 is a B. subtilis isolate with unique genetic features and can be lethal to vertebrate animals once being introduced into the animals by artificial means. These results provide the first insight into the potential harmfulness of deep-sea B. subtilis.

Interplay between Peptidoglycan Biology and Virulence in Gram-Negative Pathogens

The clinical and epidemiological threat of the growing antimicrobial resistance in Gram-negative pathogens, particularly for β-lactams, the most frequently used and relevant antibiotics, urges research to find new therapeutic weapons to combat the infections caused by these microorganisms. An essential previous step in the development of these therapeutic solutions is to identify their potential targets in the biology of the pathogen. This is precisely what we sought to do in this review specifically regarding the barely exploited field analyzing the interplay among the biology of the peptidoglycan and related processes, such as β-lactamase regulation and virulence. Hence, here we gather, analyze, and integrate the knowledge derived from published works that provide information on the topic, starting with those dealing with the historically neglected essential role of the Gram-negative peptidoglycan in virulence, including structural, biogenesis, remodeling, and recycling aspects, in addition to proinflammatory and other interactions with the host. We also review the complex link between intrinsic β-lactamase production and peptidoglycan metabolism, as well as the biological costs potentially associated with the expression of horizontally acquired β-lactamases. Finally, we analyze the existing evidence from multiple perspectives to provide useful clues for identifying targets enabling the future development of therapeutic options attacking the peptidoglycan-virulence interconnection as a key weak point of the Gram-negative pathogens to be used, if not to kill the bacteria, to mitigate their capacity to produce severe infections.

Neisseria gonorrhoeae employs two protein inhibitors to evade killing by human lysozyme

The bacterial pathogen Neisseria gonorrhoeae (Gc) infects mucosal sites rich in antimicrobial proteins, including the bacterial cell wall-degrading enzyme lysozyme. Certain Gram-negative bacteria produce protein inhibitors that bind to and inhibit lysozyme. Here, we identify Ng_1063 as a new inhibitor of lysozyme in Gc, and we define its functions in light of a second, recently identified lysozyme inhibitor, Ng_1981. In silico analyses indicated that Ng_1063 bears sequence and structural homology to MliC-type inhibitors of lysozyme. Recombinant Ng_1063 inhibited lysozyme-mediated killing of a susceptible mutant of Gc and the lysozyme-sensitive bacterium Micrococcus luteus. This inhibitory activity was dependent on serine 83 and lysine 103 of Ng_1063, which are predicted to interact with lysozyme’s active site residues. Lysozyme co-immunoprecipitated with Ng_1063 and Ng_1981 from intact Gc. Ng_1063 and Ng_1981 protein levels were also increased in Gc exposed to lysozyme. Gc lacking both ng1063 and ng1981 was significantly more sensitive to killing by lysozyme than wild-type or single mutant bacteria. When exposed to human tears or saliva, in which lysozyme is abundant, survival of Δ1981Δ1063 Gc was significantly reduced compared to wild-type, and survival was restored upon addition of recombinant Ng_1981. Δ1981Δ1063 mutant Gc survival was additionally reduced in the presence of human neutrophils, which produce lysozyme. We found that while Ng_1063 was exposed on the surface of Gc, Ng_1981 was both in an intracellular pool and extracellularly released from the bacteria, suggesting that Gc employs these two proteins at multiple spatial barriers to fully neutralize lysozyme activity. Together, these findings identify Ng_1063 and Ng_1981 as critical components for Gc defense against lysozyme. These proteins may be attractive targets for antimicrobial therapy aimed to render Gc susceptible to host defenses and/or for vaccine development, both of which are urgently needed against drug-resistant gonorrhea.

The mucosal pathogen Neisseria gonorrhoeae has acquired resistance to almost all recommended antibiotics, and no gonorrhea vaccine currently exists. Attractive targets for therapeutic discovery include bacterial factors that, when inactivated, enhance bacterial susceptibility to host-derived antimicrobial components. The bacterial cell wall-degrading enzyme lysozyme is abundant in mucosal secretions and innate immune cells. To resist killing by lysozyme, some bacteria produce proteins that bind to and directly inhibit the activity of lysozyme. Here, we demonstrate lysozyme inhibitory activity in the N. gonorrhoeae protein Ng_1063. We found that both Ng_1063 and a second, recently described lysozyme inhibitor, Ng_1981, contribute to full resistance of N. gonorrhoeae to lysozyme, including resistance to lysozyme-rich mucosal secretions and human neutrophils. Although Ng_1063 and Ng_1981 are both inhibitors of lysozyme, they are distinct in their sequences, biological activities, and cellular localizations. Because both Ng_1063 and Ng_1981 are extracellular, we propose they can be targeted for vaccines and drugs that sensitize Gc to human antimicrobial defenses.

Pseudomonas fluorescens Filamentous Hemagglutinin, an Iron-Regulated Protein, Is an Important Virulence Factor that Modulates Bacterial Pathogenicity

Pseudomonas fluorescens is a common bacterial pathogen to a wide range of aquaculture animals including various species of fish. In this study, we employed proteomic analysis and identified filamentous hemagglutinin (FHA) as an iron-responsive protein secreted by TSS, a pathogenic P. fluorescens isolate. In vitro study showed that compared to the wild type, the fha mutant TSSfha (i) exhibited a largely similar vegetative growth profile but significantly retarded in the ability of biofilm growth and producing extracellular matrix, (ii) displayed no apparent flagella and motility, (iii) was defective in the attachment to host cells and unable to form self-aggregation, (iv) displayed markedly reduced capacity of hemagglutination and surviving in host serum. In vivo infection analysis revealed that TSSfha was significantly attenuated in the ability of dissemination in fish tissues and inducing host mortality, and that antibody blocking of the natural FHA produced by the wild type TSS impaired the infectivity of the pathogen. Furthermore, when introduced into turbot as a subunit vaccine, recombinant FHA elicited a significant protection against lethal TSS challenge. Taken together, these results indicate for the first time that P. fluorescens FHA is a key virulence factor essential to multiple biological processes associated with pathogenicity.

A CCL21 chemokine of tongue sole (Cynoglossus semilaevis) promotes host resistance against bacterial infection

Chemokines are a large family of chemotactic cytokines. Based on the arrangement of the first two cysteine residues, chemokines are divided into four groups, one of which is the CC chemokine group. In this study, we characterized a CC chemokine, CsCCL21, from half-smooth tongue sole (Cynoglossus semilaevis), and analyzed its activity. CsCCL21 contains two conserved N-terminal cysteine residues in a NCCL motif and is phylogenetically related to the CCL19/21/25 subgroup of CC chemokines. CsCCL21 was constitutively expressed in nine tissues and significantly upregulated by bacterial and viral infection. The recombinant CsCCL21 (rCsCCL21) induced migration of peripheral blood leukocytes. When the two conserved cysteine residues in the NCCL motif were mutated, the chemotactic activity of rCsCCL21 was abolished. rCsCCL21 enhanced the resistance of tongue sole against bacterial infection, but the mutant protein with NCCL mutation lacked this antibacterial effect. Taken together, these results suggest that CsCCL21 is a functional CC chemokine with the ability to recruit leukocytes and is involved in antibacterial immunity in a manner that requires the conserved NCCL motif.

Recombinant sialidase NanA (rNanA) cleaves α2-3 linked sialic acid of host cell surface N-linked glycoprotein to promote Edwardsiella tarda infection

Edwardsiella tarda is one of the major pathogenic bacteria affecting both marine and freshwater fish species. Sialidase NanA expressed endogenously in E. tarda is glycosidase removing sialic acids from glycoconjugates. Recently, the relationship of NanA sialidase activity to E. tarda infection has been reported, however, the mechanism with which sialidase NanA aids the pathogenicity of E. tarda remained unclear. Here, we comprehensively determined the biochemical properties of NanA towards various substrates in vitro to provide novel insights on the potential NanA target molecule at the host cell. GAKS cell pretreated with recombinant NanA showed increased susceptibility to E. tarda infection. Moreover, sialidase inhibitor treated E. tarda showed a significantly reduced ability to infect GAKS cells. These results indicate that NanA-induced desialylation of cell surface glycoconjugates is essential for the initial step of E. tarda infection. Among the natural substrates, NanA exhibited the highest activity towards 3-sialyllactose, α2-3 linked sialic acid carrying sialoglycoconjugates. Supporting this finding, intact GAKS cell membrane exposed to recombinant NanA showed changes of glycoconjugates only in α2-3 sialo-linked glycoproteins, but not in glycolipids and α2-6 sialo-linked glycoproteins. Lectin staining of cell surface glycoprotein provided further evidence that α2-3 sialo-linkage of the N-linked glycoproteins was the most plausible target of NanA sialidase. To confirm the significance of α2-3 sialo-linkage desialylation for E. tarda infection, HeLa cells which possessed lower amount of α2-3 sialo-linkage glycoprotein were used for infection experiment along with GAKS cells. As a result, infection of HeLa cells by E. tarda was significantly reduced when compared to GAKS cells. Furthermore, E. tarda infection was significantly inhibited by mannose pretreatment suggesting that the bacterium potentially recognizes and binds to mannose or mannose containing chains following desialylation. Together, these results suggest that E. tarda may employ endogenous NanA to desialylate α2-3 glycoproteins on host cells, thus revealing one of the potential binding molecules during infection.

Pseudomonas fluorescens: identification of Fur-regulated proteins and evaluation of their contribution to pathogenesis

Pseudomonas fluorescens is a Gram-negative bacterium and a common pathogen to a wide range of farmed fish. In a previous study, we found that the ferric uptake regulator gene (fur) is essential to the infectivity of a pathogenic fish isolate of P. fluorescens (wild-type strain TSS). In the present work, we conducted comparative proteomic analysis to examine the global protein profiles of TSS and the P. fluorescens fur knockout mutant TFM. Twenty-eight differentially produced proteins were identified, which belong to different functional categories. Four of these proteins, viz. TssP (a type VI secretion protein), PspA (a serine protease), OprF (an outer membrane porin), and ClpP (the proteolytic subunit of an ATP-dependent Clp protease), were assessed for virulence participation in a model of turbot Scophthalmus maximus. The results showed that the oprF and clpP knockouts exhibited significantly reduced capacities in (1) resistance against the bactericidal effect of host serum, (2) dissemination into and colonization of host tissues, and (3) inducing host mortality. In contrast, mutation of tssP and pspA had no apparent effect on the pathogenicity of TSS. Purified recombinant OprF, when used as a subunit vaccine, induced production of specific serum antibodies in immunized fish and elicited significant protection against lethal TSS challenge. Antibody blocking of the OprF in TSS significantly impaired the ability of the bacteria to invade host tissues. Taken together, these results indicate for the first time that in pathogenic P. fluorescens, Fur regulates the expression of diverse proteins, some of which are required for optimal infection.

Edwardsiella tarda evades serum killing by preventing complement activation via the alternative pathway

Edwardsiella tarda is a Gram-negative bacterium with a broad host range that includes a wide variety of farmed fish as well as humans. E. tarda has long been known to be able to survive in host serum, but the relevant mechanism is unclear. In this study, we investigated the fundamental question, i.e. whether E. tarda activated serum complement or not. We found that (i) when incubated with flounder serum, E. tarda exhibited a high survival rate (87.6%), which was slightly but significantly reduced in the presence of Mg(2+); (ii) E. tarda-incubated serum possessed strong hemolytic activity and bactericidal activity, (iii) compared to the serum incubated with a complement-sensitive laboratory Escherichia coli strain, E. tarda-incubated serum exhibited much less chemotactic activity, (iv) in contrast to the serum incubated with live E. tarda, the serum incubated with heat-inactivated E. tarda exhibited no apparent hemolytic capacity. Taken together, these results indicate for the first time that E. tarda circumvents serum attack by preventing, to a large extent, complement activation via the alternative pathway, and that heat-labile surface structures likely play an essential role in the complement evasion of E. tarda.

Protecting Gram-negative bacterial cell envelopes from human lysozyme: Interactions with Ivy inhibitor proteins from Escherichia coli and Pseudomonas aeruginosa

Lysozymes play an important role in host defense by degrading peptidoglycan in the cell envelopes of pathogenic bacteria. Several Gram-negative bacteria can evade this mechanism by producing periplasmic proteins that inhibit the enzymatic activity of lysozyme. The Escherichia coli inhibitor of vertebrate lysozyme, Ivyc and its Pseudomonas aeruginosa homolog, Ivyp1 have been shown to be potent inhibitors of hen egg white lysozyme (HEWL). Since human lysozyme (HL) plays an important role in the innate immune response, we have examined the binding of HL to Ivyc and Ivyp1. Our results show that Ivyp1 is a weaker inhibitor of HL than Ivyc even though they inhibit HEWL with similar potency. Calorimetry experiments confirm that Ivyp1 interacts more weakly with HL than HEWL. Analytical ultracentrifugation studies revealed that Ivyp1 in solution is a monomer and forms a 30kDa heterodimer with both HL and HEWL, while Ivyc is a homodimer that forms a tetramer with both enzymes. The interaction of Ivyp1 with HL was further characterized by NMR chemical shift perturbation experiments. In addition to the characteristic His-containing Ivy inhibitory loop that binds into the active site of lysozyme, an extended loop (P2) between the final two beta-strands also participates in forming protein-protein interactions. The P2 loop is not conserved in Ivyc and it constitutes a flexible region in Ivyp1 that becomes more rigid in the complex with HL. We conclude that differences in the electrostatic interactions at the binding interface between Ivy inhibitors and distinct lysozymes determine the strength of this interaction. This article is part of a Special Issue entitled: Bacterial Resistance to Antimicrobial Peptides.

Edwardsiella tarda MliC, a lysozyme inhibitor that participates in pathogenesis in a manner that parallels Ivy

Edwardsiella tarda, a bacterial pathogen to farmed fish as well as humans, possesses the genes of two lysozyme inhibitors, ivy and mliC (ivy(Et) and mliC(Et)). We recently studied IvyEt and found it to be implicated in E. tarda virulence. In the present study, we characterized MliC(Et) in comparison with Ivy(Et) in a turbot model. MliC(Et) contains the FWSKG motif and two cysteines (C33 and C98) that are highly conserved in subgroup 1 MliCs but are of unknown functional importance. To examine the essentialness of these conserved structural features, recombinant MliC(Et) (rMliC) and its mutants bearing C33S and W79A (of the FWSKG motif) substitutions were prepared. Subsequent analysis showed that rMliC (i) inhibited lysozyme-induced lysis of a Gram-positive bacterium, (ii) reduced serum-facilitated lysozyme killing of E. tarda, and (iii) when introduced into turbot, promoted bacterial dissemination in fish tissues. The C33S mutation had no influence on the activity of rMliC, while the W79A mutation slightly but significantly enhanced the activity of rMliC. Knockout strains of either mliC(Et) or ivy(Et) were severely attenuated for the ability of tissue invasion, host lethality, serum survival, and intracellular replication. The lost virulence of the mliC transformant (TXΔmliC) was restored by complementation with an introduced mliC(Et) gene. Compared to the Δivy(Et) or ΔmliC(Et) single-knockout strains, the ΔmliC(Et) Δivy(Et) double-knockout strain was significantly impaired in most of the virulence features. Together, these results provide the first evidence that the conserved cysteine is functionally dispensable to a subgroup 1 MliC and that as a virulence factor, MliC(Et) most likely works in a concerted and parallel manner with Ivy.

Pseudomonas fluorescens: iron-responsive proteins and their involvement in host infection

For pathogenic bacteria, the ability to acquire iron is vital to survival in the host. In consequence, many genes involved in iron acquisition are associated with bacterial virulence. Pseudomonas fluorescens is a bacterial pathogen to a variety of farmed fish. However, the global regulatory function of iron in pathogenic P. fluorescens is essentially unknown. In this study, in order to identify proteins affected by iron condition at the expression level, we performed proteomic analysis to compare the global protein profiles of P. fluorescens strain TSS, a fish pathogen, cultured under iron-replete and iron-deplete conditions. Twenty-two differentially expressed proteins were identified, most of which were confirmed to be regulated by iron at the mRNA level. To investigate their potential involvement in virulence, the genes encoding four of the 22 proteins, i.e. HemO (heme oxygenase), PspB (serine protease), Sod (superoxide dismutase), and TfeR (TonB-dependent outermembrane ferric enterobactin receptor), were knocked out, and the pathogenicity of the mutants was examined in a model of turbot (Scophthalmus maximus). The results showed that compared to the wild type, the hemO, pspB, and tfeR knockouts were significantly impaired in the ability to survive in host serum, to invade host tissues, and to cause host mortality. Immunization of turbot with recombinant TfeR (rTfeR) and PspB induced production of specific serum antibodies and significant protections against lethal TSS challenge. Further analysis showed that rTfeR antibodies recognized and bound to TSS, and that treatment of TSS with rTfeR antibodies significantly impaired the infectivity of TSS to fish cells. Taken together, these results indicate for the first time that in pathogenic P. fluorescens, iron affects the expression of a large number of proteins including those that are involved in host infection.

A Moraxella catarrhalis two-component signal transduction system necessary for growth in liquid media affects production of two lysozyme inhibitors

There are a paucity of data concerning gene products that could contribute to the ability of Moraxella catarrhalis to colonize the human nasopharynx. Inactivation of a gene (mesR) encoding a predicted response regulator of a two-component signal transduction system in M. catarrhalis yielded a mutant unable to grow in liquid media. This mesR mutant also exhibited increased sensitivity to certain stressors, including polymyxin B, SDS, and hydrogen peroxide. Inactivation of the gene (mesS) encoding the predicted cognate sensor (histidine) kinase yielded a mutant with the same inability to grow in liquid media as the mesR mutant. DNA microarray and real-time reverse transcriptase PCR analyses indicated that several genes previously shown to be involved in the ability of M. catarrhalis to persist in the chinchilla nasopharynx were upregulated in the mesR mutant. Two other open reading frames upregulated in the mesR mutant were shown to encode small proteins (LipA and LipB) that had amino acid sequence homology to bacterial adhesins and structural homology to bacterial lysozyme inhibitors. Inactivation of both lipA and lipB did not affect the ability of M. catarrhalis O35E to attach to a human bronchial epithelial cell line in vitro. Purified recombinant LipA and LipB fusion proteins were each shown to inhibit human lysozyme activity in vitro and in saliva. A lipA lipB deletion mutant was more sensitive than the wild-type parent strain to killing by human lysozyme in the presence of human apolactoferrin. This is the first report of the production of lysozyme inhibitors by M. catarrhalis.

Modified lysozymes as novel broad spectrum natural antimicrobial agents in foods

In recent years much attention and interest have been directed toward application of natural antimicrobial agents in foods. Some naturally occurring proteins such as lactoperoxidase, lactoferrin, and lysozyme have received considerable attention and are being considered as potential antimicrobial agents in foods. Lysozyme kills bacteria by hydrolyzing the peptidoglycan layer of the cell wall of certain bacterial species, hence its application as a natural antimicrobial agent has been suggested. However, limitations in the action of lysozyme against only Gram-positive bacteria have prompted scientists to extend the antimicrobial effects of lysozyme by several types of chemical modifications. During the last 2 decades extensive research has been directed toward modification of lysozyme in order to improve its antimicrobial properties. This review will report on the latest information available on lysozyme modifications and examine the applicability of the modified lysozymes in controlling growth of Gram-positive and Gram-negative bacteria in foods. The results of modifications of lysozyme using its conjugation with different small molecule, polysaccharides, as well as modifications using proteolytic enzymes will be reviewed. These types of modifications have not only increased the functional properties of lysozyme (such as solubility and heat stability) but also extended the antimicrobial activity of lysozyme. Many examples will be given to show that modification can decrease the count of Gram-negative bacteria in bacterial culture and in foods by as much as 5 log CFU/mL and in some cases essentially eliminated Escherichia coli. In conclusion this review demonstrates that modified lysozymes are excellent natural food preservatives, which can be used in food industry.

PRACTICAL APPLICATION

The subject described in this review article can lead to the development of methods to produce new broad-spectrum natural antimicrobial agents, based on modification of chicken egg white lysozyme, which might potentially replace the currently used synthetic food preservatives.