LuxS distribution and AI-2 activity of Campylobacter spp

Aims: This study investigates the distribution of LuxS within Campylobacter (Camp.) species and Autoinducer (AI)‐2 activity of Camp. jejuni NCTC 11168 in food matrices. Methods and Results: LuxS (S‐ribosylhomocysteinase) sequences of different Campylobacter spp. were compared, and AI‐2 activity was measured with an AI‐2 reporter assay. Highest LuxS homologies were shared by Camp. jejuni, Camp. coli and Camp. upsaliensis, and their LuxS sequences had more similarities to the analysed Arcobacter and Vibrio harveyi strains than to all other analysed Campylobacter species. Of 15 analysed species only Camp. lari, Camp. peloridis and Camp. insulaenigrae did not produce AI‐2 molecules. Cultivation of Camp. jejuni NCTC 11168 in chicken juice reduced AI‐2 activity, and this reduction is not because of lower luxS expression or functionality. Conclusion: Not all Campylobacter species encode luxS. Food matrices can reduce AI‐2 activity in a LuxS‐independent manner. Significance and Impact of the Study: Besides, Camp. lari, Camp. peloridis and Camp. insulaenigrae do not show AI‐2 activity. Further investigations should clarify the function of AI‐2 in Campylobacter spp. and how species lacking luxS could overcome this alteration. Furthermore, the impact of food matrices on these functions needs to be determined as we could show that chicken juice reduced AI‐2 activity.     

Campylobacter jejuni as a secondary colonizer of poultry biofilms

Aims: The objective of this study was to determine if survival of culturable Campylobacter jejuni outside the host was increased by entrapment in pre-established biofilms. Methods and Results: Campylobacter jejuni was inoculated into four biofilm populations isolated from poultry environments and cultured at three temperatures. Survival of culturable Camp. jejuni in some pre-established biofilms was extended vs survival of culturable Camp. jejuni in broth. But some biofilms were detrimental to survival of culturable Camp. jejuni. Denaturing gradient gel electrophoresis analysis indicated differences in bacterial profiles depending on initial source and temperature of culturing, which may have had impacts on survival of culturable Camp. jejuni. Further investigation showed no evidence of interspecies cell signalling indicating that secondary colonization was only physical. Conclusions: The results of this study show Camp. jejuni’s attachment to surfaces is facilitated by pre-established biofilms and survival of culturable Camp. jejuni may be extended in some pre-established biofilms, but these biofilms do not fully explain long-term survival of culturable Camp. jejuni outside hosts. Significance and Impact of the Study: This study provides new information concerning survival of culturable Camp. jejuni outside the host and shows biofilms may be important in transmission and prevalence of Camp. jejuni.     

Detection and quantification of Campylobacter jejuni and Campylobacter coli using real-time multiplex PCR

Aims: We describe a real‐time quantitative multiplex polymerase chain reaction (qmPCR) assay to identify and discriminate between isolates of Campylobacter jejuni and Campylobacter coli. Methods and Results: Two novel sets of primers and hydrolysis probes were designed to amplify the unique DNA sequences within the hipO, ccoN and cadF genes that are specific to Camp. jejuni and Camp. coli. Using the designed optimized qmPCR assay conditions, the amplification efficiency is in range from 108 to 116%. These qmPCR assays are highly specific for Camp. jejuni and Camp. coli, as seen through testing of 40 Campylobacter strains and 17 non‐Campylobacter strains. In chicken juice and tap water models spiked with known quantities of Camp. jejuni, qmPCR detected 10²–10³ CFU ml^?1 within 4 h. Conclusions: The qmPCR assays developed in this study provide reliable and simultaneous detection and quantification of Camp. jejuni and Camp. coli, with good amplification reaction parameters. Significance and Impact of the Study: Following further validation, the qmPCR assay reported here has the potential to be applied to various sample types as an alternative and rapid methodology.     

Transfer of Campylobacter jejuni from raw to cooked chicken via wood and plastic cutting boards

Aims: We quantified Campylobacter jejuni transferred from naturally contaminated raw chicken fillets and skins to similar cooked chicken parts via standard rubberwood (RW) and polyethylene cutting boards (PE). Methods and Results: RW and PE cutting boards (2·5 × 2·5 cm²) were constructed. RW surfaces were smooth and even, whereas PE was uneven. Scoring with scalpel blades produced crevices on RW and flaked patches on the PE boards. Raw chicken breast fillets or skin pieces (10 g) naturally contaminated with Camp. jejuni were used to contaminate the cutting boards (6·25 cm²). These were then briefly covered with pieces of cooked chicken. Campylobacter jejuni on raw chicken, the boards, and cooked chicken pieces were counted using a combined most‐probable‐number (MPN)‐PCR method. The type of cutting board (RW, PE; unscored and scored) and temperature of cooked chicken fillets and skins were examined. Unscored PE and RW boards were not significantly different in regards to the mean transfer of Camp. jejuni from raw samples to the boards. The mean transfer of Camp. jejuni from scored RW was significantly higher than from scored PE. When the chicken fillets were held at room temperature, the mean transfer of Camp. jejuni from scored RW and PE was found to be 44·9 and 40·3%, respectively. Conclusions: RW and PE cutting boards are potential vehicles for Camp. jejuni to contaminate cooked chicken. Although cooked chicken maintained at high temperatures reduced cross‐contamination via contaminated boards, a risk was still present. Significance and Impact of the Study: Contamination of cooked chicken by Camp. jejuni from raw chicken via a cutting board is influenced by features of the board (material, changes caused by scoring) and chicken (types of chicken parts and temperature of the cooked chicken).     

Cyclic diguanylate regulation of Bacillus cereus group biofilm formation

Biofilm formation can be considered a bacterial virulence mechanism. In a range of Gram‐negatives, increased levels of the second messenger cyclic diguanylate (c‐di‐GMP) promotes biofilm formation and reduces motility. Other bacterial processes known to be regulated by c‐di‐GMP include cell division, differentiation and virulence. Among Gram‐positive bacteria, where the function of c‐di‐GMP signalling is less well characterized, c‐di‐GMP was reported to regulate swarming motility in Bacillus subtilis while having very limited or no effect on biofilm formation. In contrast, we show that in the Bacillus cereus group c‐di‐GMP signalling is linked to biofilm formation, and to several other phenotypes important to the lifestyle of these bacteria. The Bacillus thuringiensis 407 genome encodes eleven predicted proteins containing domains (GGDEF/EAL) related to c‐di‐GMP synthesis or breakdown, ten of which are conserved through the majority of clades of the B. cereus group, including Bacillus anthracis. Several of the genes were shown to affect biofilm formation, motility, enterotoxin synthesis and/or sporulation. Among these, cdgF appeared to encode a master diguanylate cyclase essential for biofilm formation in an oxygenated environment. Only two cdg genes (cdgA, cdgJ) had orthologs in B. subtilis, highlighting differences in c‐di‐GMP signalling between B. subtilis and B. cereus group bacteria.     

Synergistic effects of antibiotics and an N-acyl homoserine lactone analog on Porphyromonas gingivalis biofilms

Aims: To investigate the effects of the combined application of an N‐acyl homoserine lactone (HSL) analog and antibiotics on biofilms of Porphyromonas gingivalis, a major pathogen of periodontal disease. Methods and Results: Antibiotics used were cefuroxime, ofloxacin and minocycline. A flow‐cell model was used for biofilm formation. Samples were divided into four groups: control, analog‐treated, antibiotic‐treated and combined application groups. Biofilm cell survival was determined using adenosine triphosphate (ATP) bioluminescence and confocal laser microscopy (CLSM). In the combined application group, the ATP count in biofilm cells was significantly decreased compared with the antibiotic‐treated group (Games–Howell test, P < 0·05). A combination of cefuroxime and the analog was most effective against the P. gingivalis biofilm. CLSM observations revealed that the proportion of dead cells was highest in the combined application group. Conclusions: The combined application of the N‐acyl HSL analog and antibiotics was effective at reducing the viability of P. gingivalis cells in biofilms. Significance and Impact of the Study: The combined application of the N‐acyl HSL analog and antibiotics may be successful for eradicating infections involving bacterial biofilms, such as periodontitis.     

Demonstration of persistent strains of Campylobacter jejuni within broiler farms over a 1‐year period in Lithuania

Aims: The aim of the study was to investigate the flock prevalence of Campylobacter jejuni and Campylobacter coli in broiler farms in Lithuania and to identify possible persistent strains of Camp. jejuni using amplified fragment length polymorphism (AFLP) typing method. Methods and Results: During 1 year, 42 broiler flocks from 9 broiler farms were examined to determine the prevalence of Campylobacter‐positive broiler flocks in Lithuania. Among 42 broiler flocks examined, 31 flocks (73·8%) were positive for Camp. jejuni and 17 flocks (40·48%) for Camp. coli. Campylobacter jejuni isolates were genotyped by AFLP method using BspDI and BglII restriction enzymes. Typing of 190 isolates generated 50 AFLP genotypes with the highest diversity of strains found in the summer season. Each farm showed one or more predominant AFLP types, and one AFLP type (A32) was found in five broiler farms over a 1‐year period. Conclusions: Campylobacter jejuni and Camp. coli are highly prevalent in broiler farms in Lithuania. Farm‐specific genotypes were identified in all farms examined. Type A32 was present and persisted in different broiler farms, and a common source of transmission of Camp. jejuni was suspected. Significance and Impact of the Study: For the first time, Camp. jejuni in broiler flocks has been genetically characterized in Lithuania. Persistent strains of Camp. jejuni were detected over one period at the beginning of broiler meat production chain and, therefore, the identification of contamination source of such strains and the mechanism of their particular ability to persist are crucial to establish effective control measures against Camp. jejuni infection in broiler farms.     

Autoinducer‐2 is produced in saliva‐fed flow conditions relevant to natural oral biofilms

Aims: We evaluated the ability of a dual‐species community of oral bacteria to produce the universal signalling molecule, autoinducer‐2 (AI‐2), in saliva‐fed biofilms. Methods and Results: Streptococcus oralis 34, S. oralis 34 luxS mutant and Actinomyces naeslundii T14V were grown as single‐ and dual‐species biofilms within sorbarods fed with 25% human saliva. AI‐2 concentration in biofilm effluents was determined by the Vibrio harveyi BB170 bioluminescence assay. After homogenizing the sorbarods to release biofilm cells, cell numbers were determined by fluorometric analysis of fluorescent antibody‐labelled cells. After 48 h, dual‐species biofilm communities of interdigitated S. oralis 34 and A. naeslundii T14V contained 3·2 × 10⁹ cells: fivefold more than single‐species biofilms. However, these 48‐h dual‐species biofilms exhibited the lowest concentration ratio of AI‐2 to cell density. Conclusions: Oral bacteria produce AI‐2 in saliva‐fed biofilms. The decrease of more than 10‐fold in concentration ratio seen between 1 and 48 h in S. oralis 34–A. naeslundii T14V biofilms suggests that peak production of AI‐2 occurs early and is followed by a very low steady‐state level. Significance and Impact of the Study: High oral bacterial biofilm densities may be achieved by inter‐species AI‐2 signalling. We propose that low concentrations of AI‐2 contribute to the establishment of oral commensal biofilm communities.     

Ribosomal operon intergenic sequence region (ISR) heterogeneity in Campylobacter coli and Campylobacter jejuni

Aims: The intergenic sequence regions (ISR) between the 16S and 23S genes of Campylobacter jejuni and Campylobacter coli are markedly different for each species. However, in the genomic sequence for Camp. coli RM2228 , two rRNA operons have an ISR that is characteristic of Camp. coli, and the third operon is characteristic of Camp. jejuni. The aim of this study was to determine the prevalence of ISR heterogeneity in these organisms. Methods and Results: PCR primers were designed to yield a 327‐base pair (bp) product for Camp. coli and 166‐bp product for Camp. jejuni. A strain like Camp. coli RM2228 should yield products of both sizes. DNA from a panel of Camp. coli (n = 133) and Camp. jejuni (n = 134) isolates were tested. All of the isolates yielded products of the predicted size for the species. To verify the data for Camp. coli RM2228 , each ribosomal operon from the isolate was individually amplified by PCR and tested with the ISR primer pair. Products of both sizes were produced as predicted. Conclusions: The cross‐species heterogeneity of the ISR seen in Camp. coli RM2228 is uncommon. Significance and Impact of the Study: The heterogeneity must have been caused by horizontal gene transfer at a frequency lower than predicted from housekeeping gene data. Thus, it can be expected that species identification based on the ISR can be confused in rare isolates.     

Comparison of genotypes and antibiotic resistance of Campylobacter jejuni isolated from humans and slaughtered chickens in Switzerland

Aims: To get an overview of genotypes and antibiotic resistances in Swiss Campylobacter jejuni implicated in human gastroenteritis and to examine the association with isolates from chickens. Methods and Results: Multilocus sequence typing (MLST) and flaB typing were applied to 136 human clinical isolates. Phenotypic resistance to 12 antimicrobials and genotypic resistance to macrolides and quinolones were determined. MLST resulted in 35 known and six new sequence types (ST). The flaB analysis revealed 35 different types, which – in combination with MLST – increased the resolution of the typing approach. Resistance to quinolones, tetracycline and ampicillin was found in 37·5, 33·1 and 8·1% of the isolates, respectively, whereas macrolide resistance was found only once. Genotypic and phenotypic resistance correlated in all cases. A comparison to Camp. jejuni isolated from slaughtered chickens was performed. While 86% of the quinolone‐sensitive human isolates showed overlapping MLST‐flaB types with those of chicken origin, resistant strains showed only 39% of matching types. Conclusion: Mainly quinolone‐sensitive Camp. jejuni strains implicated in human campylobacteriosis showed matching genotypes with isolates originating from chickens. Significance and Impact of the Study: A large proportion of human cases in Switzerland are likely to originate from domestic chickens, confirming that prevention measures in the poultry production are important.     

Helicobacter hepaticus Cytolethal Distending Toxin Causes Cell Death in Intestinal Epithelial Cells via Mitochondrial Apoptotic Pathway

Background: Helicobacter hepaticus, the prototype for enterohepatic Helicobacter species, colonizes the lower intestinal and hepatobiliary tracts of mice and causes typhlocolitis, hepatitis, and hepatocellular carcinoma in susceptible mouse strains. Cytolethal distending toxin (CDT) is the only known virulence factor found in H. hepaticus. CDT of several Gram‐negative bacteria is associated with double‐stranded DNA breaks resulting in cell cycle arrest and death of a wide range of eukaryotic cells in vitro. We previously observed H. hepaticus CDT (HhCDT) mediated apoptosis in INT407 cells. However, the exact mechanism for the induction of the apoptotic pathway by HhCDT is unknown. The objective of this study was to identify the apoptotic signaling pathway induced by HhCDT in INT407 cells. Materials and Methods: INT407 cells were incubated with or without recombinant HhCDT for 0–72 hours. H2AX phosphorylation and apoptotic parameters were analyzed. Results: H2AX was phosphorylated 24 hours postexposure to HhCDT. Expression of pro‐apoptotic Bax protein was upregulated after 24 hours, while Bcl₂ expression decreased. Cytochrome c was released from mitochondria after 12–24 hours of exposure. Concurrently, caspase 3/7 and 9 were activated. However, pretreatment of INT407 cells with caspase inhibitor (Z‐VAD‐FMK) inhibited the activation of caspase 3/7 and 9. Significant activity of caspase 8 was not observed in toxin treated cells. Activation of caspase 3/7 and caspase 9 confirms the involvement of the mitochondrial apoptotic pathway in HhCDT‐treated cells. Conclusion: These findings show, for the first time, the ability of HhCDT to induce apoptosis via the mitochondrial pathway.     

Improving Film Formation and Photovoltage of Highly Efficient Inverted‐Type Perovskite Solar Cells through the Incorporation of New Polymeric Hole Selective Layers

Two chemically tailored new conjugated copolymers, HSL1 and HSL2, were developed and applied as hole selective layers to improve the anode interface of fullerene/perovskite planar heterojunction solar cells. The introduction of polar functional groups on the polymer side chains increases the surface energy of the hole selective layers (HSLs), which promote better wetting with the perovskite films and lead to better films with full coverage and high crystallinity. The deep highest occupied molecular orbital levels of the HSLs align well with the valence band of the perovskite semiconductors, resulted in increase photovoltage. The high lying lowest unoccupied molecule orbital level provides sufficient electron blocking ability to prevent electrons from reaching the anode and reduces the interfacial trap‐assisted recombination at the poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)/perovskite interface, resulting in a longer charge‐recombination lifetime and shorter charge‐extraction time. In the presence of the HSLs, high‐performance CH₃NH₃PbI _x Cl_{3? x} perovskite solar cells with a power conversion efficiency (PCE) of 16.6% (V _oc: 1.07 V) and CH₃NH₃Pb(I_0.3Br_0.7) _x Cl_{3? x} cells with a PCE of 10.3% (V _oc: 1.34 V) can be realized.     

Prevention of intestinal Campylobacter jejuni colonization in broilers by combinations of in‐feed organic acids

Aim: We have tested the effect of various combinations of formic acid and sorbate on Campylobacter jejuni colonization in broiler chickens to reduce the colonization of this zoonotic pathogen in broiler chicken flocks. Methods and Results: Chickens were offered feed supplemented with different concentrations and combinations of formic acid and/or potassium sorbate. We found little or no effect on the Camp. jejuni colonization levels in chickens that were given feed supplemented with formic acid alone. A combination of 1·5% formic acid and 0·1% sorbate reduced the colonization of Camp. jejuni significantly, while a concentration of 2·0% formic acid in combination with 0·1% sorbate prevented Camp. jejuni colonization in chickens. This inhibition was replicated in two independent trials with a combination of three different Camp. jejuni strains. Conclusions: Our results show a novel and promising intervention strategy to reduce the incidence of Camp. jejuni in poultry products and to obtain safer food. Significance and Impact of the Study: To ensure food safety, a reduction of the carcass contamination with Camp. jejuni through reduced colonization of this pathogen in broiler chicken flocks is important. A range of organic acids as additives in feed and drinking water have already been evaluated for this purpose. However, no studies have yet shown a complete inhibition of Camp. jejuni colonization in broiler chickens.     

Caecal transcriptome analysis of colonized and non-colonized chickens within two genetic lines that differ in caecal colonization by Campylobacter jejuni

Campylobacter jejuni is one of the most common causes of human bacterial enteritis worldwide. The molecular mechanisms of the host responses of chickens to C. jejuni colonization are not well understood. We have previously found differences in C. jejuni colonization at 7‐days post‐inoculation (pi) between two genetic broiler lines. However, within each line, not all birds were colonized by C. jejuni (27.5% colonized in line A, and 70% in line B). Therefore, the objective of the present experiments was to further define the differences in host gene expression between colonized and non‐colonized chickens within each genetic line. RNA isolated from ceca of colonized and non‐colonized birds within each line was applied to a chicken 44K Agilent microarray for the pair comparison. There were differences in the mechanisms of host resistant to C. jejuni colonization between line A and line B. Ten times more differentially expressed genes were observed between colonized and non‐colonized chickens within line B than those within line A. Our study supports the fact that the MAPK pathway is important in host response to C. jejuni colonization in line B, but not in line A. The data indicate that inhibition of small GTPase‐mediated signal transduction could enhance the resistance of chickens to C. jejuni colonization and that the tumour necrosis factor receptor superfamily genes play important roles in determining C. jejuni non‐colonization in broilers.     

Selecting for development of fluoroquinolone resistance in a Campylobacter jejuni strain 81116 in chickens using various enrofloxacin treatment protocols

Aims: To determine the effect of various enrofloxacin dose regimes on the colonization and selection of resistance in Campylobacter jejuni strain 81116P in experimentally colonized chickens. Methods and Results: Two experiments were undertaken, in which 14‐day‐old chickens were colonized with 1 × 10⁷–1 × 10⁹ CFU g^?1Camp. jejuni strain 81116P and then treated with enrofloxacin at 12–500 ppm in drinking water for various times. Caecal colonization levels were determined at various time‐points after start‐of‐treatment, and the susceptibility of recovered isolates to ciprofloxacin was monitored. Resistance was indicated by growth on agar containing 4 μg ml^?1 ciprofloxacin, MICs of 16 μg ml^?1 and the Thr86Ile mutation in gyrA. Enrofloxacin at doses of 12–250 ppm reduced Camp. jejuni colonization over the first 48–72 h after start‐of‐treatment. The degree of reduction in colonization was dose, but not treatment time, dependent. In all cases, maximal colonization was re‐established within 4–6 days. Fluoroquinolone‐resistant organisms were recoverable within 48 h of start‐of‐treatment; after a further 24 h all recovered isolates were resistant. In contrast, a dose of 500 ppm enrofloxacin reduced colonization to undetectable levels within 48 h, and the treated birds remained Campylobacter negative throughout the remaining experimental period. By high pressure liquid chromatography, for all doses, the maximum concentrations of enrofloxacin and ciprofloxacin in the caecal contents were detected at the point of treatment completion. Thereafter, levels declined to undetectable by 7 days post‐treatment withdrawal. Conclusions: In a model using chickens maximally colonized with Camp. jejuni 81116P, treatment with enrofloxacin, at doses of 12–250 ppm in drinking water, enables the selection, and clonal expansion, of fluoroquinolone‐resistant organisms. However, this is preventable by treatment with 500 ppm of enrofloxacin. Significance and impact of the study: Treatment of chickens with enrofloxacin selects for resistance in Camp. jejuni in highly pre‐colonized birds. However, a dose of 500 ppm enrofloxacin prevented the selection of resistant campylobacters.     

Strategies for maintaining Na⁺ balance in zebrafish (Danio rerio) during prolonged exposure to acidic water

Teleost fish store lipids among several tissues primarily as triacylglycerol (TG). Upon metabolic demand, stored TGs are hydrolyzed by hormone-sensitive lipase (HSL). In this study, two distinct cDNAs encoding HSL were isolated, cloned, and sequenced from adipose tissue of rainbow trout. The full-length cDNAs, designated HSL1 and HSL2, were 2562-bp and 2887-bp in length, respectively, and share 82% nucleotide identity. Phylogentic analysis suggests that the two HSLs derive from paralogous genes that may have arisen during a teleost-specific genome duplication event. Quantitative real-time PCR revealed that HSL1 and HSL2 were differentially expressed, both in terms of distribution among tissues as well as in terms of abundance within selected tissues of juvenile trout. HSL1 and HSL2 mRNAs were detected in the brain, spleen, pancreas, kidney, gill, intestine, heart, and white muscle, but were most abundant in the red muscle, liver, and adipose tissue. HSL1 mRNA was more abundant than HSL2 mRNA in the adipose tissue, whereas HSL2 mRNA was more abundant than HSL1 mRNA in the liver. Short term fasting (4 weeks) increased HSL1 and HSL2 mRNA expression in the adipose tissue, but only HSL1 mRNA levels increased in the liver and the red muscle. During a prolonged fast (6 weeks), there was continued elevation of HSL1 and HSL2 mRNA levels in the liver and muscle; HSL mRNA expression in mesenteric fat declined, coincident with depletion of mesenteric fat mass. Refeeding fish reduced HSL expression to levels seen in continuously fed fish. These findings indicate that the pattern of HSL expression is consistent with the diverse lipid storage pattern of fish and suggest that distinct mechanisms serve to regulate differential expression of the two HSLs in tissues and during a progressive fast.     

Survival mechanisms and culturability of <Emphasis Type="Italic">Campylobacter jejuni</Emphasis> under stress conditions

Culture-based isolation and enumeration of bacterial human pathogens from environmental and human food samples has significant limitations. Many pathogens enter a viable but non-culturable (VBNC) state in response to stress, and cannot be detected via culturing methods. Favourable growth conditions with a source of energy and an ideal stoichiometric ratio of carbon to inorganic elements can reverse this VBNC state. This review will focus on the bacterium Campylobacter jejuni which is a leading cause of food borne illness in the developed world. C. jejuni can enter a VBNC state in response to extremes in: pH, moisture content, temperature, nutrient content and salinity. Once in a VBNC state, the organism must maintain an energy balance from substrate oxidation through respiration to grow, divide and remain viable. The goal of this review is a greater understanding of how abiotic stress and thermodynamics influence the viability of C. jejuni.     

空肠弯曲杆菌生物膜形成和调控机理研究进展北大核心CSCD

空肠弯曲杆菌(Campylobacter jejuni)分泌胞外多糖和各种胞外蛋白和核酸等相互交联在一起构成生物膜,可增强其在不利环境下的生存率,尤其是对各种洗涤剂、抗生素和消毒剂的耐受力。本文从介质表面性质、温度、气体环境、以及基因的调控等多方面阐述了空肠弯曲杆菌生物膜结构及形成调控机制,同时对各种去除生物膜的实际应用做了分析和展望,为探寻生物膜的控制方法提供参考。     

Intercellular and intracellular signalling systems that globally control the expression of virulence genes in plant pathogenic bacteria

Plant pathogenic bacteria utilize complex signalling systems to control the expression of virulence genes at the cellular level and within populations. Quorum sensing (QS), an important intercellular communication mechanism, is mediated by different types of small molecules, including N‐acyl homoserine lactones (AHLs), fatty acids and small proteins. AHL‐mediated signalling systems dependent on the LuxI and LuxR family proteins play critical roles in the virulence of a wide range of Gram‐negative plant pathogenic bacteria belonging to the Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria. Xanthomonas spp. and Xylella fastidiosa, members of the Gammaproteobacteria, however, possess QS systems that are mediated by fatty acid‐type diffusible signal factors (DSFs). Recent studies have demonstrated that Ax21, a 194‐amino‐acid protein in Xanthomonas oryzae pv. oryzae, plays dual functions in activating a rice innate immune pathway through binding to the rice XA21 pattern recognition receptor and in regulating bacterial virulence and biofilm formation as a QS signal molecule. In xanthomonads, DSF‐mediated QS systems are connected with the signalling pathways mediated by cyclic diguanosine monophosphate (c‐di‐GMP), which functions as a second messenger for the control of virulence gene expression in these bacterial pathogens.