A dual regulatory role of cyclic adenosine monophosphate receptor protein in various virulence traits of Vibrio vulnificus

The cAMP receptor protein gene contributes to growth, stress resistance, and colonization of Actinobacillus pleuropneumoniae

Porcine infectious pleuropneumonia (PCP) is a severe disease of porcine caused by Actinobacillus pleuropneumoniae (APP). The spread of PCP remains a threat to the porcine farms and has been known to cause severe economic losses. The cAMP receptor protein (CRP) serves as a pivotal player in helping bacteria adapt to shifts in their environment, particularly when facing the challenges posed by bacterial infections. In this study, we investigated the role of CRP in APP. Our results revealed that crp mutant (Δcrp) strains were more sensitive to acidic and osmotic stress resistance and had lower biofilm formation ability than wild-type (WT) strains. Furthermore, the Δcrp strains showed deficiencies in anti-phagocytosis, adhesion, and invasion upon interaction with host cells. Mice infected with the Δcrp strains demonstrated reduced bacterial loads in their lungs compared to those infected with the WT strains. This study reveals the pivotal role of crp gene expression in regulating pleuropneumonia growth, stress resistance, iron utilization, biofilm formation, phagocytosis, adhesion, invasion and colonization. Our discoveries offer novel perspectives on understanding the development and progression of APP infections.

A Class IV Adenylate Cyclase, CyaB, Is Required for Capsule Polysaccharide Production and Biofilm Formation in Vibrio parahaemolyticus

Cyclic AMP (cAMP) receptor protein (CRP), encoded by crp, is a global regulator that is activated by cAMP, a second messenger synthesized by a class I adenylate cyclase (AC-I) encoded by cyaA in Escherichia coli. cAMP-CRP is required for growth on nonpreferred carbon sources and is a global regulator. We constructed in-frame nonpolar deletions of the crp and cyaA homologs in Vibrio parahaemolyticus and found that the Δcrp mutant did not grow in minimal media supplemented with nonpreferred carbon sources, but the ΔcyaA mutant grew similarly to the wild type. Bioinformatics analysis of the V. parahaemolyticus genome identified a 181-amino-acid protein annotated as a class IV adenylate cyclase (AC-IV) named CyaB, a member of the CYTH protein superfamily. AC-IV phylogeny showed that CyaB was present in Gammaproteobacteria and Alphaproteobacteria as well as Planctomycetes and Archaea. Only the bacterial CyaB proteins contained an N-terminal motif, HFxxxxExExK, indicative of adenylyl cyclase activity. Both V. parahaemolyticus cyaA and cyaB genes functionally complemented an E. coli ΔcyaA mutant. The Δcrp and ΔcyaB ΔcyaA mutants showed defects in growth on nonpreferred carbon sources and in swimming and swarming motility, indicating that cAMP-CRP is an activator. The ΔcyaA and ΔcyaB single mutants had no defects in these phenotypes, indicating that AC-IV complements AC-I. Capsule polysaccharide and biofilm production assays showed significant defects in the Δcrp, ΔcyaBΔcyaA, and ΔcyaB mutants, whereas the ΔcyaA strain behaved similarly to the wild type. This is consistent with a role of cAMP-CRP as an activator of these phenotypes and establishes a cellular role for AC-IV in capsule and biofilm formation, which to date has been unestablished. IMPORTANCE Here, we characterized the roles of CRP and CyaA in V. parahaemolyticus, showing that cAMP-CRP is an activator of metabolism, motility, capsule production, and biofilm formation. These results are in contrast to cAMP-CRP in V. cholerae, which represses capsule and biofilm formation. Previously, only an AC-I CyaA had been identified in Vibrio species. Our data showed that an AC-IV CyaB homolog is present in V. parahaemolyticus and is required for optimal growth. The data demonstrated that CyaB is essential for capsule production and biofilm formation, uncovering a physiological role of AC-IV in bacteria. The data showed that the cyaB gene was widespread among Vibrionaceae species and several other Gammaproteobacteria, but in general, its phylogenetic distribution was limited. Our phylogenetic analysis also demonstrated that in some species the cyaB gene was acquired by horizontal gene transfer.

Therapeutic potential of otilonium bromide against Vibrio vulnificus

New drugs are urgently required for the treatment of infections due to an increasing number of new strains of diseases-causing pathogens and antibiotic-resistant bacteria. A library of drugs approved by Food and Drug Administration was screened for efficacy against Vibrio vulnificus using antimicrobial assays. We found that otilonium bromide showed potent antimicrobial activity against V.vulnificus and had a synergistic effect in combination with antibiotics. Field emission transmission electron microscope images revealed that otilonium bromide caused cell division defects in V.vulnificus. Moreover, it significantly inhibited V.vulnificus swarming motility and adhesion to host cells at concentrations lower than the minimum inhibitory concentration. To investigate its inhibitory action mechanisms, we examined the effect of otilonium bromide on the expression levels of several proteins crucial for V.vulnificus growth, motility, and adhesion. It decreased the protein expression levels of cAMP receptor protein and flagellin B, but not HlyU or OmpU. In addition, otilonium bromide significantly decreased the expression levels of outer membrane protein TolCV1, thus inhibiting RtxA1 toxin secretion and substantially reducing V.vulnificus cytotoxicity to host cells. Collectively, these findings suggest that otilonium bromide may be considered as a promising candidate for treating V.vulnificus infections.

cAMP Receptor Protein Positively Regulates the Expression of Genes Involved in the Biosynthesis of Klebsiella oxytoca Tilivalline Cytotoxin

Klebsiella oxytoca is a resident of the human gut. However, certain K. oxytoca toxigenic strains exist that secrete the nonribosomal peptide tilivalline (TV) cytotoxin. TV is a pyrrolobenzodiazepine that causes antibiotic-associated hemorrhagic colitis (AAHC). The biosynthesis of TV is driven by enzymes encoded by the aroX and NRPS operons. In this study, we determined the effect of environmental signals such as carbon sources, osmolarity, and divalent cations on the transcription of both TV biosynthetic operons. Gene expression was enhanced when bacteria were cultivated in tryptone lactose broth. Glucose, high osmolarity, and depletion of calcium and magnesium diminished gene expression, whereas glycerol increased transcription of both TV biosynthetic operons. The cAMP receptor protein (CRP) is a major transcriptional regulator in bacteria that plays a key role in metabolic regulation. To investigate the role of CRP on the cytotoxicity of K. oxytoca, we compared levels of expression of TV biosynthetic operons and synthesis of TV in wild-type strain MIT 09-7231 and a Δcrp isogenic mutant. In summary, we found that CRP directly activates the transcription of the aroX and NRPS operons and that the absence of CRP reduced cytotoxicity of K. oxytoca on HeLa cells, due to a significant reduction in TV production. This study highlights the importance of the CRP protein in the regulation of virulence genes in enteric bacteria and broadens our knowledge on the regulatory mechanisms of the TV cytotoxin.

Effect of cAMP Receptor Protein Gene on Growth Characteristics and Stress Resistance of Haemophilus parasuis Serovar 5

Haemophilus parasuis (HPS), a member of the family Pasteurellaceae, is a common bacteria in the upper respiratory tract of pigs but under certain circumstances can cause serious systemic disease (Glasser's disease) characterized by severe infection of the upper respiratory tract, fibrinous polyserositis, polyarthritis, and meningitis. cAMP receptor protein (CRP) is among the most important global regulators, playing a vital role in adapting to environmental changes during the process of bacterial infection. In order to investigate the function of the crp gene in the growth characteristics of H. parasuis serovar 5 (HPS5) and its ability to overcome adverse environmental stresses, a crp mutant strain (Δcrp) was constructed and verified. In this study, we found that the crp gene was involved in growth rate, biofilm formation, stress tolerance, serum resistance, and iron utilization. Compared with the wild type, both the growth rate of the crp mutant and its resistance to osmotic pressure decreased significantly. Similar phenomena were also found in biofilm formation and iron utilization. However, the resistance to heat shock and serum complement of the crp mutant were enhanced. This study aimed to reveal the function in growth characteristics and stress resistance of the crp gene in HPS5. Whether it relates to virulence requires additional in-depth research.

The role of Vibrio vulnificus virulence factors and regulators in its infection-induced sepsis

Due to the development of Marine aquaculture, infections caused by Vibrio vulnificus are common all over the world. Symptoms of V. vulnificus infection vary from gastrointestinal illness to septicemia. After infection with V. vulnificus, some patients showed gastrointestinal symptoms, including vomiting, fever, diarrhea, and so on. Others appeared wound infection at the site of contact with bacteria, and even developed sepsis. Once it develops into sepsis, the prognosis of patients is very poor. However, its underlying pathogenic mechanism remains largely undetermined. Growing evidence shows that it can induce primary septicemia mainly via essential virulence factors and regulators. Therefore, it is important to identify the factors that play roles in sepsis. In this review, we systematically expounded the role of V. vulnificus virulence factors and regulators in its infection-induced sepsis in order to provide useful information for the treatment and prevention of V. vulnificus.

Iron and Virulence in Stenotrophomonas Maltophilia: All We Know So Far

Stenotrophomonas maltophilia is a multi-drug-resistant global opportunistic nosocomial pathogen, which possesses a huge number of virulence factors and antibiotics resistance characteristics. Iron has a crucial contribution toward growth and development, cell growth and proliferation, and pathogenicity. The bacterium found to acquire iron for its cellular process through the expression of two iron acquisition systems. Two distinct pathways for iron acquisition are encoded by the S. maltophilia genome-a siderophore-and heme-mediated iron uptake system. The entAFDBEC operon directs the production of the enterobactin siderophore of catecholate in nature, while heme uptake relies on hgbBC and potentially hmuRSTUV operon. Fur and sigma factors are regulators of S. maltophilia under iron-limited condition. Iron potentially act as a signal which plays an important role in biofilm formation, extracellular polymeric substances (EPS), extracellular enzymes production, oxidative stress response, diffusible signal factor (DSF) and siderophore production in S. maltophilia. This review summarizes the current knowledge of iron acquisition in S. maltophilia and the critical role of iron in relation to its pathogenicity.

Identification and Validation of an Antivirulence Agent Targeting HlyU-Regulated Virulence in Vibrio vulnificus

Antimicrobial resistance (AMR) in pathogens is the result of indiscriminate use of antibiotics and consequent metabolic/genetic modulation to evolve survival strategies and clonal-selection in AMR strains. As an alternative to antibiotic treatment, antivirulence strategies are being developed, not only to combat bacterial pathogenesis, but also to avoid emerging antibiotic resistance. Vibrio vulnificus is a foodborne pathogen that causes gastroenteritis, necrotizing wound infections, and sepsis with a high rate of mortality. Here, we developed an inhibitor-screening reporter platform to target HlyU, a master transcriptional regulator of virulence factors in V. vulnificus by assessing rtxA1 transcription under its control. The inhibitor-screening platform includes wild type and ΔhlyU mutant strains of V. vulnificus harboring the reporter construct P_rtxA1::luxCDABE for desired luminescence signal detection and control background luminescence, respectively. Using the inhibitor-screening platform, we identified a small molecule, fursultiamine hydrochloride (FTH), that inhibits the transcription of the highly invasive repeat-in-toxin (rtxA1) and hemolysin (vvhA) along with other HlyU regulated virulence genes. FTH has no cytotoxic effects on either host cells or pathogen at the tested concentrations. FTH rescues host cells from the necrotic cell-death induced by RtxA1 and decreases the hemolytic activity under in vitro conditions. The most important point is that FTH treatment does not induce the antivirulence resistance. Current study validated the antivirulence strategy targeting the HlyU virulence transcription factor and toxin-network of V. vulnificus and demonstrated that FTH, exhibits a potential to inhibit the pathogenesis of deadly, opportunistic human pathogen, V. vulnificus without inducing AMR.

Cyclic AMP Receptor Protein Acts as a Transcription Regulator in Response to Stresses in Deinococcus radiodurans

The cyclic AMP receptor protein family of transcription factors regulates various metabolic pathways in bacteria, and also play roles in response to environmental changes. Here, we identify four homologs of the CRP family in Deinococcus radiodurans, one of which tolerates extremely high levels of oxidative stress and DNA-damaging reagents. Transcriptional levels of CRP were increased under hydrogen peroxide (H₂O₂) treatment during the stationary growth phase, indicating that CRPs function in response to oxidative stress. By constructing all CRP single knockout mutants, we found that the dr0997 mutant showed the lowest tolerance toward H₂O₂, ultraviolet radiation, ionizing radiation, and mitomycin C, while the phenotypes of the dr2362, dr0834, and dr1646 mutants showed slight or no significant differences from those of the wild-type strain. Taking advantage of the conservation of the CRP-binding site in many bacteria, we found that transcription of 18 genes, including genes encoding chromosome-partitioning protein (dr0998), Lon proteases (dr0349 and dr1974), NADH-quinone oxidoreductase (dr1506), thiosulfate sulfurtransferase (dr2531), the DNA repair protein UvsE (dr1819), PprA (dra0346), and RecN (dr1447), are directly regulated by DR0997. Quantitative real-time polymerase chain reaction (qRT-PCR) analyses showed that certain genes involved in anti-oxidative responses, DNA repair, and various cellular pathways are transcriptionally attenuated in the dr0997 mutant. Interestingly, DR0997 also regulate the transcriptional levels of all CRP genes in this bacterium. These data suggest that DR0997 contributes to the extreme stress resistance of D. radiodurans via its regulatory role in multiple cellular pathways, such as anti-oxidation and DNA repair pathways.

Temperature Change Induces the Expression of vuuA Encoding Vulnibactin Receptor and crp Encoding Cyclic AMP Receptor Protein in Vibrio vulnificus

Upon entering the human body, Vibrio vulnificus, a gram-negative marine bacterium, must withstand a temperature change (TC) from 25 to 37 °C. This bacterium acquires iron mainly via the vulnibactin receptor (VuuA)-mediated iron uptake system (IUS), which is under the positive control of cyclic AMP receptor protein (CRP), a global regulator responsible for catabolite repression. In this study, we examined the effect of TC on the expression of vuuA and crp, and the reciprocal relation between VuuA-mediated IUS and CRP under iron-limited conditions. Iron limitation increased vuuA expression but decreased crp expression. TC resulted in increased vuuA and crp expression. A crp or vuuA mutation reciprocally decreased vuuA or crp expression. TC could increase vuuA or crp expression even in a crp- or vuuA-mutated background. These results indicate that TC increases the expression of both vuuA and crp by facilitating metabolism under iron-limited conditions, and that CRP and VuuA-mediated IUS interact coordinately toward optimal metabolism in V. vulnificus.

Rethinking the roles of CRP, cAMP, and sugar-mediated global regulation in the Vibrionaceae

Many proteobacteria modulate a suite of catabolic genes using the second messenger cyclic 3', 5'-AMP (cAMP) and the cAMP receptor protein (CRP). Together, the cAMP-CRP complex regulates target promoters, usually by activating transcription. In the canonical model, the phosphotransferase system (PTS), and in particular the EIIA(Glc) component for glucose uptake, provides a mechanistic link that modulates cAMP levels depending on glucose availability, resulting in more cAMP and activation of alternative catabolic pathways when glucose is unavailable. Within the Vibrionaceae, cAMP-CRP appears to play the classical role in modulating metabolic pathways; however, it also controls functions involved in natural competence, bioluminescence, pheromone signaling, and colonization of animal hosts. For this group of marine bacteria, chitin is an ecologically relevant resource, and chitin's monomeric sugar N-acetylglucosamine (NAG) supports robust growth while also triggering regulatory responses. Recent studies with Vibrio fischeri indicate that NAG and glucose uptake share EIIA(Glc), yet the responses of cAMP-CRP to these two carbon sources are starkly different. Moreover, control of cAMP levels appears to be more dominantly controlled by export and degradation. Perhaps more surprisingly, although CRP may require cAMP, its activity can be controlled in response to glucose by a mechanism independent of cAMP levels. Future studies in this area promise to shed new light on the role of cAMP and CRP.

Growth on glucose decreases cAMP-CRP activity while paradoxically increasing intracellular cAMP in the light-organ symbiont Vibrio fischeri

Proteobacteria often co-ordinate responses to carbon sources using CRP and the second messenger cyclic 3', 5'-AMP (cAMP), which combine to control transcription of genes during growth on non-glucose substrates as part of the catabolite-repression response. Here we show that cAMP-CRP is active and important in Vibrio fischeri during colonization of its host squid Euprymna scolopes. Moreover, consistent with a classical role in catabolite repression, a cAMP-CRP-dependent reporter showed lower activity in cells grown in media amended with glucose rather than glycerol. Surprisingly though, intracellular cAMP levels were higher in glucose-grown cells. Mutant analyses were consistent with predictions that CyaA was responsible for cAMP generation, that the EIIA(Glc) component of glucose transport could enhance cAMP production and that the phophodiesterases CpdA and CpdP consumed intracellular and extracellular cAMP respectively. However, the observation of lower cAMP levels in glycerol-grown cells seemed best explained by changes in cAMP export, via an unknown mechanism. Our data also indicated that cAMP-CRP activity decreased during growth on glucose independently of crp's native transcriptional regulation or cAMP levels. We speculate that some unknown mechanism, perhaps carbon-source-dependent post-translational modulation of CRP, may help control cAMP-CRP activity in V.fischeri.

The cyclic-di-GMP signaling pathway in the Lyme disease spirochete, Borrelia burgdorferi

In nature, the Lyme disease spirochete Borrelia burgdorferi cycles between the unrelated environments of the Ixodes tick vector and mammalian host. In order to survive transmission between hosts, B. burgdorferi must be able to not only detect changes in its environment, but also rapidly and appropriately respond to these changes. One manner in which this obligate parasite regulates and adapts to its changing environment is through cyclic-di-GMP (c-di-GMP) signaling. c-di-GMP has been shown to be instrumental in orchestrating the adaptation of B. burgdorferi to the tick environment. B. burgdorferi possesses only one set of c-di-GMP-metabolizing genes (one diguanylate cyclase and two distinct phosphodiesterases) and one c-di-GMP-binding PilZ-domain protein designated as PlzA. While studies in the realm of c-di-GMP signaling in B. burgdorferi have exploded in the last few years, there are still many more questions than answers. Elucidation of the importance of c-di-GMP signaling to B. burgdorferi may lead to the identification of mechanisms that are critical for the survival of B. burgdorferi in the tick phase of the enzootic cycle as well as potentially delineate a role (if any) c-di-GMP may play in the transmission and virulence of B. burgdorferi during the enzootic cycle, thereby enabling the development of effective drugs for the prevention and/or treatment of Lyme disease.