Genomic and Transcriptomic Diversification of Flagellin Genes Provides Insight into Environmental Adaptation and Phylogeographic Characteristics in Aeromonas hydrophila

Aeromonas hydrophila is an opportunistic motile pathogen with a broad host range, infecting both terrestrial and aquatic animals. Environmental and geographical conditions exert selective pressure on both geno- and phenotypes of pathogens. Flagellin, directly exposed to external environments and containing important immunogenic epitopes, may display significant variability in response to external conditions. In this study, we conducted a comparative analysis of ~  150 A. hydrophila genomes, leading to the identification of six subunits of the flagellin gene (fla-1 to fla-4, flaA, and flaB). Individual strains harbored different composition of flagellin subunits and copies. The composition of subunits showed distinct patterns depending on environmental sources. Strains from aquatic environments were mainly comprised of fla-1 to fla-4 subunits, while terrestrial strains predominated in groups harboring flaA and flaB subunits. Each flagellin showed varying levels of expression, with flaA and flaB demonstrating significantly higher expression compared to others. One of the chemotaxis pathways that control flagellin movement through a two-component system was significantly upregulated in flaA(+ 1)/flaB(+ 1) group, whereas flaA and flaB showed different transcriptomic expressions. The genes positively correlated with flaA expression were relevant to biofilm formation and bacterial chemotaxis, but flaB showed a negative correlation with the genes in ABC transporters and quorum sensing pathway. However, the expression patterns of fla-2 to fla-4 were identical. This suggests various types of flagellin subunits may have different biological functions. The composition and expression levels of flagellin subunits could provide valuable insights into the adaptation of A. hydrophila and the differences among strains in response to various external environments.

Identification and characterization of a novel 6'-N-aminoglycoside acetyltransferase AAC(6')-Va from a clinical isolate of Aeromonas hydrophila

Background

Aeromonas species have been identified as agents responsible for various diseases in both humans and animals. Multidrug-resistant Aeromonas strains pose a significant public health threat due to their emergence and spread in clinical settings and the environment. The aim of this study was to determine a novel resistance mechanism against aminoglycoside antimicrobials in a clinical isolate.

Methods

The function of aac(6')-Va was verified by gene cloning and antibiotic susceptibility tests. To explore the in vivo activity of the enzyme, recombinant proteins were expressed, and enzyme kinetics were tested. To determine the molecular background and mechanism of aac(6')-Va, whole-genome sequencing and bioinformatic analysis were performed.

Results

The novel aminoglycoside N-acetyltransferase gene aac(6')-Va confers resistance to several aminoglycosides. Among the antimicrobials tested, ribostamycin showed the highest increase (128-fold) in the minimum inhibitory concentration (MIC) compared with the control strains. According to the MIC results of the cloned aac(6')-Va, AAC(6')-Va also showed the highest catalytic efficiency for ribostamycin [kcat/Km ratio = (3.35 ± 0.17) × 104 M-1 s-1]. Sharing the highest amino acid identity of 54.68% with AAC(6')-VaIc, the novel aminoglycoside N-acetyltransferase constituted a new branch of the AAC(6') family due to its different resistance profiles. The gene context of aac(6')-Va and its close relatives was conserved in the genomes of species of the genus Aeromonas.

Conclusion

The novel resistance gene aac(6')-Va confers resistance to several aminoglycosides, especially ribostamycin. Our finding of a novel resistance gene in clinical A. hydrophila will help us develop more effective treatments for this pathogen's infections.

Comparative genomics analysis of the multidrug-resistant Aeromonas hydrophila MX16A providing insights into antibiotic resistance genes

In this paper, the whole genome of the multidrug-resistant Aeromonas hydrophila MX16A was comprehensively analyzed and compared after sequencing by PacBio RS II. To shed light on the drug resistance mechanism of A. hydrophila MX16A, a Kirby-Bauer disk diffusion method was used to assess the phenotypic drug susceptibility. Importantly, resistance against β-lactam, sulfonamides, rifamycins, macrolides, tetracyclines and chloramphenicols was largely consistent with the prediction analysis results of drug resistance genes in the CARD database. The varied types of resistance genes identified from A. hydrophila MX16A revealed multiple resistance mechanisms, including enzyme inactivation, gene mutation and active effusion. The publicly available complete genomes of 35 Aeromonas hydrophila strains on NCBI, including MX16A, were downloaded for genomic comparison and analysis. The analysis of 33 genomes with ANI greater than 95% showed that the pan-genome consisted of 9556 genes, and the core genes converged to 3485 genes. In summary, the obtained results showed that A. hydrophila exhibited a great genomic diversity as well as diverse metabolic function and it is believed that frequent exchanges between strains lead to the horizontal transfer of drug resistance genes.

Prevalence of Aeromonas spp. Infection in Pediatric Patients Hospitalized with Gastroenteritis in Latvia between 2020 and 2021

Purpose: Aeromonas species are emerging human enteric pathogens. However, there is no systematic analysis of Aeromonas infection in the pediatric population in Latvia. The aim of the study was to describe potential sources, prevalence of infection, associated virulence factors and antimicrobial resistance of Aeromonas spp. isolated from fecal samples. Methods: Stool samples (n = 1360) were obtained from the Children’s Clinical University Hospital between 2020 and 2021. The target population was pediatric patients, 0 to 18 years of age, with a preliminary diagnosis of gastroenteritis. Identification was performed by Maldi-TOF, antimicrobial resistance by Vitek2 and 9 virulence factors by polymerase chain reaction (PCR). Results: Aeromonas spp. were isolated in 50 stool samples; positive findings made up 3.6% of all study cases and included four species: A. hydrophila, A. caviae, A. veronii, and A. eucrenophila. In 42% of the samples, Aeromonas spp. appeared alongside the other significant pathogens: Campylobacter jejuni, Salmonella Enteritidis, Salmonella Typhimurium, Yersinia enterocolitica, norovirus, adenovirus, and rotavirus. The study population positive for Aeromonas spp. infection contained 28 male (56%) and 22 female (44%) patients; median age was 4.56 years. The most common symptoms were: diarrhea, blood in stool, vomiting, abdominal pain, and fever. Aside from expected natural resistance, no significant antibacterial resistance was detected. The presence of multiple virulence genes was noticed in all isolates. No statistically significant correlation was found between the virulence patterns, bacterial species, and the intensity of clinical symptoms. Discussion: According to the clinical data and the results of this study Aeromonas spp. has an important role in pediatric practice and requires appropriate attention and monitoring.

A novel detection method for the pathogenic Aeromonas hydrophila expressing aerA gene and/or hlyA gene based on dualplex RAA and CRISPR/Cas12a

Aeromonas hydrophila is an emerging waterborne and foodborne pathogen with pathogenicity to humans and warm water fishes, which severely threatens human health, food safety and aquaculture. A novel method for the rapid, accurate, and sensitive detection of pathogenic A. hydrophila is still needed to reduce the impact on human health and aquaculture. In this work, we developed a rapid, accurate, sensitive, and visual detection method (dRAA-CRISPR/Cas12a), without elaborate instruments, integrating the dualplex recombinase-assisted amplification (dRAA) assay and CRISPR/Cas12a system to detect pathogenic A. hydrophila expressing aerA and/or hlyA virulence genes. The dRAA-CRISPR/Cas12a method has high sensitivity, which can rapidly detect (about 45 min) A. hydrophila with the limit of detection in 2 copies of genomic DNA per reaction, and has high specificity for three pathogenic A. hydrophila strains (aerA⁺hlyA⁻, aerA⁻hlyA⁺, and aerA⁺hlyA⁺). Moreover, dRAA-CRISPR/Cas12a method shows satisfactory practicability in the analysis of the spiked human blood and stool and fish samples. These results demonstrate that our developed pathogenic A. hydrophila detection method, dRAA-CRISPR/Cas12a, is a promising potential method for the early diagnosis of human A. hydrophila infection and on-site detection of A. hydrophila in food and aquaculture.

Inhibition of microbial pathogens in farmed fish

Aquaculture, also known as aqua farming, is defined as farming fish, crustaceans, mollusks, aquatic plants, algae, and other marine organisms. It includes cultivating fresh- and saltwater populations under controlled conditions compared to commercial fishing or wild fish harvesting. Worldwide, carp, salmon, tilapia, and catfish are the most common fish species used in fish farming in descending order. Disinfectants prevent and/or treat different infections in aquatic animals. The current review indicates the uses of different disinfectants against some important pathogens in aquaculture, with particular reference to tilapia (Oreochromis niloticus) farming. A single review cannot cover all aspects of disinfection throughout aquaculture, so the procedures and principles of disinfection in tilapia farming/aquaculture have been chosen for illustration purposes.

Ubiquitous waterborne pathogens

Water is one of the most important substances on earth and without it life cannot exist. However, poor water quality in many parts of the world has increased the number of water-related diseases, making it the leading cause of disease and death globally for both young and old. Waterborne pathogens cause diseases in humans through two major exposure pathways: drinking water and recreational waters. This chapter on waterborne pathogens will be starting with an introduction, followed by descriptions on classical waterborne pathogens; bacteria, viruses, protozoans, and helminths placing emphasis on the World Health Organization guidelines. Further to conventional waterborne pathogens, fresh organisms and new strains from already known pathogens are being identified and that present important additional challenges to both the water and public health sectors. Hence later part of the chapter focuses on the potential waterborne pathogens and will conclude with a summary of the content.

Biofilm and Sediment are Major Reservoirs of Virulent Aeromonas hydrophila (vAh) in Catfish Production Ponds

The genus Aeromonas comprises more than 60 recognized species that include many important fish pathogens such as the causative agents of furunculosis and motile Aeromonas septicemia (MAS). Although MAS is typically considered a secondary infection, a new virulent A. hydrophila (vAh) strain has been causing devastating losses to the catfish industry in Alabama since 2009. The objective of this study was to characterize the spatiotemporal distribution of Aeromonas sp. and, specifically, vAh in a commercial catfish farm in western Alabama. We sampled biofilm, sediment, and water from three ponds during four consecutive months during the growing season. Total aerobic counts were between 8.8 × 10⁵ and 1.5 × 10⁶  CFU/mL but were significantly higher in biofilm and sediment than in water throughout the sampling period. Total Aeromonas counts in water samples significantly increased in all three ponds after the month of August and ranged from 7.8 × 10³ to 4.9 × 10⁴  CFU/mL. A similar trend was observed in biofilm and sediment samples for which total Aeromonas counts increased in samples taken in late summer to early fall. Over time, the concentration of Aeromonas in water samples decreased by one order of magnitude, while there was a significant increase in sediments as temperature dropped. The virulent vAh was detected in 35.4% of biofilm samples and 22.9% of sediment samples, suggesting that both environments serve as the major reservoir for this pathogen. Future monitoring efforts should focus on targeting sediment and biofilms since samples of these appear to naturally enrich for the presence of vAh and other Aeromonas species.

The Social Life of Aeromonas through Biofilm and Quorum Sensing Systems

Bacteria of the genus Aeromonas display multicellular behaviors herein referred to as “social life”. Since the 1990s, interest has grown in cell-to-cell communication through quorum sensing signals and biofilm formation. As they are interconnected, these two self-organizing systems deserve to be considered together for a fresh perspective on the natural history and lifestyles of aeromonads. In this review, we focus on the multicellular behaviors of Aeromonas, i.e., its social life. First, we review and discuss the available knowledge at the molecular and cellular levels for biofilm and quorum sensing. We then discuss the complex, subtle, and nested interconnections between the two systems. Finally, we focus on the aeromonad multicellular coordinated behaviors involved in heterotrophy and virulence that represent technological opportunities and applied research challenges.

Genomic Characterization of the Novel Aeromonas hydrophila Phage Ahp1 Suggests the Derivation of a New Subgroup from phiKMV-Like Family

Aeromonas hydrophila is an opportunistic pathogenic bacterium causing diseases in human and fish. The emergence of multidrug-resistant A. hydrophila isolates has been increasing in recent years. In this study, we have isolated a novel virulent podophage of A. hydrophila, designated as Ahp1, from waste water. Ahp1 has a rapid adsorption (96% adsorbed in 2 min), a latent period of 15 min, and a burst size of 112 PFU per infected cell. At least eighteen Ahp1 virion proteins were visualized in SDS-polyacrylamide gel electrophoresis, with a 36-kDa protein being the predicted major capsid protein. Genome analysis of Ahp1 revealed a linear doubled-stranded DNA genome of 42,167 bp with a G + C content of 58.8%. The genome encodes 46 putative open reading frames, 5 putative phage promoters, and 3 transcriptional terminators. Based on high degrees of similarity in overall genome organization and among most of the corresponding ORFs, as well as phylogenetic relatedness among their DNAP, RNAP and major capsid proteins, we propose a new subgroup, designated Ahp1-like subgroup. This subgroup contains Ahp1 and members previously belonging to phiKMV-like subgroup, phiAS7, phi80-18, GAP227, phiR8-01, and ISAO8. Since Ahp1 has a narrow host range, for effective phage therapy, different phages are needed for preparation of cocktails that are capable of killing the heterogeneous A. hydrophila strains.

Microbial quality and molecular identification of cultivable microorganisms isolated from an urban drinking water distribution system (Limassol, Cyprus)

Microorganisms can survive and multiply in aged urban drinking water distribution systems, leading to potential health risks. The objective of this work was to investigate the microbial quality of tap water and molecularly identify its predominant cultivable microorganisms. Tap water samples collected from 24 different households scattered in the urban area of Limassol, Cyprus, were microbiologically tested following standard protocols for coliforms, E. coli, Pseudomonas spp., Enterococcus spp., and total viable count at 22 and 37 °C. Molecular identification was performed on isolated predominant single colonies using 16SrRNA sequencing. Approximately 85% of the household water samples were contaminated with one or more microorganisms belonging to the genera of Pseudomonas, Corynebacterium, Agrobacterium, Staphylococcus, Bacillus, Delftia, Acinetobacter, Enterococcus, Enterobacter, and Aeromonas. However, all samples tested were free from E. coli. This is the first report in Cyprus molecularly confirming specific genera of relevant microbial communities in tap water.

Development of cross-priming amplification assays for rapid and sensitive detection of Aeromonas hydrophila

Aeromonas hydrophila has been increasingly implicated as the aetiologic agent of various human diseases. Therefore, reliable laboratory detection and identification of this bacterium has become clinically and epidemiologically desirable. We developed a nearly instrument-free, simple molecular method for rapid detection of Aer. hydrophila using a cross-priming amplification (CPA) assay with the desA gene as the target. The desA gene is crucial for the survival and growth of Aer. hydrophila under iron starvation. The results can be visualized as colour changes without opening the reaction tubes. No false-positive results were observed for the 33 non-Aer. hydrophila strains tested to evaluate assay specificity. The limit of detection for Aer. hydrophila was approximately 200 copies of desA per reaction (on reference plasmids) and 5 × 10(3)  CFU g(-1) Aer. hydrophila in simulated human stool, which is the same sensitivity as a qPCR assay. The performance of the CPA assay was also evaluated with 100 stool specimens from diarrhoea patients and 40 environmental water samples. In conclusion, the simplicity, cost-effectiveness and nearly instrument-free platform of the CPA assay make it practical for use in primary care facilities and smaller clinical laboratories.

SIGNIFICANCE AND IMPACT OF THE STUDY

Aeromonas hydrophila is a human pathogen that infects via exposed wounds or ingestion of contaminated water and food. In this study, a CPA-based PCR method was developed for specific, rapid, cost-effective detection of Aer. hydrophila, and the test results could be visualized without opening the reaction tubes. This is the first report on the application of the CPA method for the detection of Aer. hydrophila. This novel method could be practical for use in primary care facilities and smaller clinical laboratories.

Common features of opportunistic premise plumbing pathogens

Recently it has been estimated that the annual cost of diseases caused by the waterborne pathogens Legionella pneumonia, Mycobacterium avium, and Pseudomonas aeruginosa is $500 million. For the period 2001-2012, the estimated cost of hospital admissions for nontuberculous mycobacterial pulmonary disease, the majority caused by M. avium, was almost $1 billion. These three waterborne opportunistic pathogens are normal inhabitants of drinking water--not contaminants--that share a number of key characteristics that predispose them to survival, persistence, and growth in drinking water distribution systems and premise plumbing. Herein, I list and describe these shared characteristics that include: disinfectant-resistance, biofilm-formation, growth in amoebae, growth at low organic carbon concentrations (oligotrophic), and growth under conditions of stagnation. This review is intended to increase awareness of OPPPs, identify emerging OPPPs, and challenge the drinking water industry to develop novel approaches toward their control.

Effectiveness of ultrasound, UV-C, and photocatalysis on inactivation kinetics of Aeromonas hydrophila

In this study, bactericidal effects of 24 kHz ultrasound, ultraviolet (UV-C) irradiation, and titanium dioxide (TiO2) photocatalyst were studied on inactivation of Aeromonas hydrophila, an emerging pathogen listed on the US Environmental Protection Agency's (US EPA) candidate contaminant list. Metabolic activity (using the AlamarBlue dye) assays were performed to assess the residual activity of the microbial cells after the disinfection treatments along with culture-based methods. A faster inactivation rate of 1.52 log min(-1) and inactivation of 7.62 log10 was observed within 5 min of ultrasound exposure. Ultrasound treated cells repaired by 1.4 log10 in contrast to 5.3 log10 repair for UV-C treated cells. Ultrasound treatment significantly lowered the reactivation of Aeromonas hydrophila in comparison to UV-C- and UV-C-induced photocatalysis. Ultrasound appeared to be an effective means of inactivating Aeromonas hydrophila and could be used as a potential disinfection method for water and wastewater reuse.

Functional genomic characterization of virulence factors from necrotizing fasciitis-causing strains of Aeromonas hydrophila

The genomes of 10 Aeromonas isolates identified and designated Aeromonas hydrophila WI, Riv3, and NF1 to NF4; A. dhakensis SSU; A. jandaei Riv2; and A. caviae NM22 and NM33 were sequenced and annotated. Isolates NF1 to NF4 were from a patient with necrotizing fasciitis (NF). Two environmental isolates (Riv2 and -3) were from the river water from which the NF patient acquired the infection. While isolates NF2 to NF4 were clonal, NF1 was genetically distinct. Outside the conserved core genomes of these 10 isolates, several unique genomic features were identified. The most virulent strains possessed one of the following four virulence factors or a combination of them: cytotoxic enterotoxin, exotoxin A, and type 3 and 6 secretion system effectors AexU and Hcp. In a septicemic-mouse model, SSU, NF1, and Riv2 were the most virulent, while NF2 was moderately virulent. These data correlated with high motility and biofilm formation by the former three isolates. Conversely, in a mouse model of intramuscular infection, NF2 was much more virulent than NF1. Isolates NF2, SSU, and Riv2 disseminated in high numbers from the muscular tissue to the visceral organs of mice, while NF1 reached the liver and spleen in relatively lower numbers on the basis of colony counting and tracking of bioluminescent strains in real time by in vivo imaging. Histopathologically, degeneration of myofibers with significant infiltration of polymorphonuclear cells due to the highly virulent strains was noted. Functional genomic analysis provided data that allowed us to correlate the highly infectious nature of Aeromonas pathotypes belonging to several different species with virulence signatures and their potential ability to cause NF.

Aeromonas spp. clinical microbiology and disease

Members of the genus Aeromonas inhabit various aquatic environments and are responsible for, and are implicated in, a number of intestinal and extra-intestinal infections in humans as well as other animals. This review focuses on invasive human infection and disease and summarizes available findings regarding the microbiology and detection of Aeromonas spp., with emphasis on successful identification and diagnosis, and the control of disease in the population. Antimicrobial resistance and therapy of Aeromonas spp. is also discussed.

Water microbiology. Bacterial pathogens and water

Water is essential to life, but many people do not have access to clean and safe drinking water and many die of waterborne bacterial infections. In this review a general characterization of the most important bacterial diseases transmitted through water—cholera, typhoid fever and bacillary dysentery—is presented, focusing on the biology and ecology of the causal agents and on the diseases’ characteristics and their life cycles in the environment. The importance of pathogenic Escherichia coli strains and emerging pathogens in drinking water-transmitted diseases is also briefly discussed. Microbiological water analysis is mainly based on the concept of fecal indicator bacteria. The main bacteria present in human and animal feces (focusing on their behavior in their hosts and in the environment) and the most important fecal indicator bacteria are presented and discussed (focusing on the advantages and limitations of their use as markers). Important sources of bacterial fecal pollution of environmental waters are also briefly indicated. In the last topic it is discussed which indicators of fecal pollution should be used in current drinking water microbiological analysis. It was concluded that safe drinking water for all is one of the major challenges of the 21st century and that microbiological control of drinking water should be the norm everywhere. Routine basic microbiological analysis of drinking water should be carried out by assaying the presence of Escherichia coli by culture methods. Whenever financial resources are available, fecal coliform determinations should be complemented with the quantification of enterococci. More studies are needed in order to check if ammonia is reliable for a preliminary screening for emergency fecal pollution outbreaks. Financial resources should be devoted to a better understanding of the ecology and behavior of human and animal fecal bacteria in environmental waters.

PCR-DGGE-based methodologies to assess diversity and dynamics of Aeromonas communities

AIMS

Aeromonas is ubiquitous in aquatic environments and may cause infectious diseases in fish and humans. However, reliable and specific methods to evaluate the diversity and dynamics of Aeromonas populations are currently unavailable. This study aimed to develop PCR-DGGE methodologies for culture-independent analysis of Aeromonas populations in water systems.

METHODS AND RESULTS

Three primer sets were designed to amplify selected sections of genes gyrB, rpoD and sodB from Aeromonas. Their specificity was confirmed by in silico analysis and by PCR on DNA from pure cultures. Estuarine water samples were analyzed by PCR-DGGE using those primers. DGGE patterns clearly clustered according to seasonal factors, and Aeromonas communities were surprisingly stable along a salinity gradient. Sequences of cloned amplicons affiliated to sequences belonging to seven Aeromonas species previously isolated from the same environment.

CONCLUSIONS

The three systems used showed to be useful to describe the diversity of Aeromonas communities. However, the combined use of more than one primer set is advisable.

SIGNIFICANCE AND IMPACT OF THE STUDY

The methods presented here can be applied to understand the natural pool of Aeromonas and also to monitor and control these bacteria in aquatic reservoirs.

Interactions of Cryptosporidium parvum, Giardia lamblia, vaccinal poliovirus type 1, and bacteriophages phiX174 and MS2 with a drinking water biofilm and a wastewater biofilm

Biofilms colonizing surfaces inside drinking water distribution networks may provide a habitat and shelter to pathogenic viruses and parasites. If released from biofilms, these pathogens may disseminate in the water distribution system and cause waterborne diseases. Our study aimed to investigate the interactions of protozoan parasites (Cryptosporidium parvum and Giardia lamblia [oo]cysts) and viruses (vaccinal poliovirus type 1, phiX174, and MS2) with two contrasting biofilms. First, attachment, persistence, and detachment of the protozoan parasites and the viruses were assessed with a drinking water biofilm. This biofilm was allowed to develop inside a rotating annular reactor fed with tap water for 7 months prior to the inoculation. Our results show that viable parasites and infectious viruses attached to the drinking water biofilm within 1 h and persisted within the biofilm. Indeed, infectious viruses were detected in the drinking water biofilm up to 6 days after the inoculation, while viral genome and viable parasites were still detected at day 34, corresponding to the last day of the monitoring period. Since viral genome was detected much longer than infectious particles, our results raise the question of the significance of detecting viral genomes in biofilms. A transfer of viable parasites and viruses from the biofilm to the water phase was observed after the flow velocity was increased but also with a constant laminar flow rate. Similar results regarding parasite and virus attachment and detachment were obtained using a treated wastewater biofilm, suggesting that our observations might be extrapolated to a wide range of environmental biofilms and confirming that biofilms can be considered a potential secondary source of contamination.

Molecular characterization of a functional type VI secretion system from a clinical isolate of Aeromonas hydrophila

Our laboratory recently molecularly characterized the type II secretion system (T2SS)-associated cytotoxic enterotoxin (Act) and the T3SS-secreted AexU effector from a diarrheal isolate SSU of Aeromonas hydrophila. The role of these toxin proteins in the pathogenesis of A. hydrophila infections was subsequently delineated in in vitro and in vivo models. In this study, we characterized the new type VI secretion system (T6SS) from isolate SSU of A. hydrophila and demonstrated its role in bacterial virulence. Study of the role of T6SS in bacterial virulence is in its infancy, and there are, accordingly, only limited, recent reports directed toward a better understanding its role in bacterial pathogenesis. We have provided evidence that the virulence-associated secretion (vas) genes vasH (Sigma 54-dependent transcriptional regulator) and vasK (encoding protein of unknown function) are essential for expression of the genes encoding the T6SS and/or they constituted important components of the T6SS. Deletion of the vasH gene prevented expression of the potential translocon hemolysin coregulated protein (Hcp) encoding gene from bacteria, while the vasK gene deletion prevented secretion but not translocation of Hcp into host cells. The secretion of Hcp was independent of the T3SS and the flagellar system. We demonstrated that secreted Hcp could bind to the murine RAW 264.7 macrophages from outside, in addition to its ability to be translocated into host cells. Further, the vasH and vasK mutants were less toxic to murine macrophages and human epithelial HeLa cells, and these mutants were more efficiently phagocytosed by macrophages. We also provided evidence that the expression of the hcp gene in the HeLa cell resulted in apoptosis of the host cells. Finally, the vasH and vasK mutants of A. hydrophila were less virulent in a septicemic mouse model of infection, and animals immunized with recombinant Hcp were protected from subsequent challenge with the wild-type (WT) bacterium. In addition, mice infected with the WT A. hydrophila had circulating antibodies to Hcp, indicating an important role of T6SS in the pathogenesis of A. hydrophila infections. Taken together, we have characterized the T6SS from Aeromonas for the first time and provided new features of this secretion system not yet known for other pathogens.

Characterization of Aeromonas virulence using an immunocompromised mouse model

An immunocompromised mouse model was used to characterize Aeromonas strains for their ability to cause opportunistic, extraintestinal infections. A total of 34 isolates of Aeromonas (A. hydrophila [n = 12]), A. veronii biotype sobria [n = 7], A. caviae [n = 4], A. enchelia [n = 4], A. allosaccharophila [n = 2], A. salmonicida (n = 4), and A. bestiarum [n = 1]) were introduced by intraperitoneal injection into immunocompetent or chemically compromised (using cyclophosphamide) mice. The ability of each isolate to persist in the liver and spleen tissue was monitored at 24 hours after exposure. A majority ofA. hydrophila and A veronii v. sobria strains, but none of the isolates of other Aeromonas species, were capable of persistent colonization (<300 cells/mg spleen and liver tissue at 24 hours). The presence or absence of several putative virulence factors (cytotoxicity to HEp-2, lipase activity, elastase activity, and hemolysis) were determined for each isolate using in vitro tests. There were no correlations between the presence or absence of biochemical test results for putative virulence factors and persistence of the isolate in spleen and liver tissue at 24 hours.

Biological characterization of a new type III secretion system effector from a clinical isolate of Aeromonas hydrophila-part II

We recently identified a novel type III secretion system (T3SS) effector, AexU, from a diarrheal isolate SSU of Aeromonas hydrophila, and demonstrated that mice infected with the DeltaaexU mutant were significantly protected from mortality. Although the NH(2)-terminal domain of this toxin exhibits homology to AexT of A. salmonicida, a fish pathogen, and ExoT/S of Pseudomonas aeruginosa, the COOH-terminal domain of AexU is unique, with no homology to any known proteins in the NCBI database. In this study, we purified the full-length AexU and its NH(2)-terminal (amino acid residues 1-231) and COOH-terminal (amino acid residues 232-512) domains after expression of their corresponding genes in Escherichia coli as histidine-tag fusion proteins using the bacteriophage T7 RNA polymerase/promoter-based pET-30a vector system. The full-length and NH(2)- and COOH-terminal domains of AexU exhibited ADP-ribosyltransferase activity, with the former two exhibiting much higher activity than the latter. These different forms of AexU were also successfully expressed and produced in the HeLa Tet-Off cell system using a pBI-EGFP vector, as demonstrated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, Western blot analysis, and intracellular staining of the toxin using flow cytometric analysis. Production of AexU in HeLa cells resulted in possible actin reorganization and cell rounding, as determined by phalloidin staining and confocal microscopy. Based on electron microscopy, the toxin also caused chromatin condensation, which is indicative of apoptosis. Apoptosis of HeLa cells expressing and producing AexU was confirmed by 7-amino actinomycin D (7-AAD) and MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrasodium bromide] assays, by detection of cytoplasmic histone-associated DNA fragments, and by activation of caspases 3 and 9. These effects were much more pronounced in host cells that expressed and produced the full-length or NH(2)-terminal domain of AexU, compared to those that expressed and produced the COOH-terminal domain or the vector alone. This study represents the first characterization of this novel T3SS effector.

Biostability analysis for drinking water distribution systems

The ability to limit regrowth in drinking water is referred to as biological stability and depends on the concentration of disinfectant residual and on the concentration of substrate required for the growth of microorganisms. The biostability curve, based on this fundamental concept of biological stability, is a graphical approach to study the two competing effects that determine bacterial regrowth in a distribution system: inactivation due to the presence of a disinfectant, and growth due to the presence of a substrate. Biostability curves are a practical, system specific approach for addressing the problem of bacterial regrowth in distribution systems. This paper presents a standardized algorithm for generating biostability curves and this will enable water utilities to incorporate this approach for their site-specific needs. Using data from pilot scale studies, it was found that this algorithm was applicable to control regrowth of HPC in chlorinated systems where AOC is the growth limiting substrate, and growth of AOB in chloraminated systems, where ammonia is the growth limiting substrate.

Seasonal and tissue distribution of Laribacter hongkongensis, a novel bacterium associated with gastroenteritis, in retail freshwater fish in Hong Kong

Laribacter hongkongensis, a recently discovered bacterium associated with community-acquired gastroenteritis, has been found in the intestines of freshwater fish. To better understand the epidemiology and ecology of the bacterium, we carried out a surveillance study to investigate possible seasonal variation in the recovery of L. hongkongensis and its distribution in various organs in retail freshwater fish in Hong Kong. Forty whole freshwater fish of two species (20 grass carps and 20 bighead carps), and intestines from 120 grass carps were sampled during a one-year period. L. hongkongensis was isolated from 11 (55%) of the 20 grass carps and 6 (30%) of the 20 bighead carps; and the intestines of 49 (41%) of 120 grass carps. Seasonal variation in the recovery of L. hongkongensis from both whole fish and intestines was observed, with higher isolation rates in spring and summer than in fall and winter. There was also positive correlation between temperature and the isolation rates. When L. hongkongensis was cultured in vitro at different temperatures, shorter lag time and higher growth rate were observed at higher temperatures, with 37 degrees C being optimal among the tested temperatures. L. hongkongensis was commonly found in the gills, stomachs and intestines in both grass carps and bighead carps, and on the skin surface of one fish, but not in other organs. Proper handling of freshwater fish for cooking, especially the gills and gut, is recommended to prevent acquisition of L. hongkongensis, and other freshwater fish related infections.

Molecular and functional characterization of a ToxR-regulated lipoprotein from a clinical isolate of Aeromonas hydrophila

Human diseases caused by species of Aeromonas have been classified into two major groups: septicemia and gastroenteritis. In this study, we reported the molecular and functional characterization of a new virulence factor, ToxR-regulated lipoprotein, or TagA, from a diarrheal isolate, SSU, of Aeromonas hydrophila. The tagA gene of A. hydrophila exhibited 60% identity with that of a recently identified stcE gene from Escherichia coli O157:H7, which encoded a protein (StcE) that provided serum resistance to the bacterium and prevented erythrocyte lysis by controlling classical pathway of complement activation by cleaving the complement C1-esterase inhibitor (C1-INH). We purified A. hydrophila TagA as a histidine-tagged fusion protein (rTagA) from E. coli DE3 strain using a T7 promoter-based pET30 expression vector and nickel affinity column chromatography. rTagA cleaved C1-INH in a time-dependent manner. The tagA isogenic mutant of A. hydrophila, unlike its corresponding wild-type (WT) or the complemented strain, was unable to cleave C1-INH, which is required to potentiate the C1-INH-mediated lysis of host and bacterial cells. We indeed demonstrated colocalization of C1-INH and TagA on the bacterial surface by confocal fluorescence microscopy, which ultimately resulted in increased serum resistance of the WT bacterium. Likewise, we delineated the role of TagA in contributing to the enhanced ability of C1-INH to inhibit the classical complement-mediated lysis of erythrocytes. Importantly, we provided evidence that the tagA mutant was significantly less virulent in a mouse model of infection (60%) than the WT bacterium at two 50% lethal doses, which resulted in 100% mortality within 48 h. Taken together, our data provided new information on the role of TagA as a virulence factor in bacterial pathogenesis. This is the first report of TagA characterization from any species of Aeromonas.

Adaptational changes in cellular fatty acid branching and unsaturation of Aeromonas species as a response to growth temperature and salinity

The effects of growth temperature and salinity on the cellular fatty acids were investigated on Aeromonas caviae, Aeromonas hydrophila and Aeromonas sobria. Under optimal growth conditions, fatty acids patterns were dominated by even-numbered chains C(16:0), C(16:1cis9), C(18:1cis11), C(12:0) and C(14:0). Growth temperature modifications induced, in the three Aeromonas species, important changes in fatty acid (i) unsaturation, (ii) branching and (iii) chain length. An important decrease in the C(18:1cis11) fatty acid content was observed for the three species below 15 degrees C and above 25 degrees C. The evolution of C(18:1cis11) and C(16:0) showed a mirror image for the three Aeromonas species. Low NaCl concentrations did not elicit significant changes in the fatty acids content of the three Aeromonas species. However, for high NaCl concentration in the medium, the growth ability was related to an important decrease of the unsaturated to saturated fatty acids ratio indicating a membrane rigidification. Thermal and salinity adaptations were branched fatty acid-dependent for A. caviae, whereas this phenomenon was less significant for A. hydrophila and A. sobria.

Contamination of drinking water by coliforms from biofilms grown on rubber-coated valves

In water samples from drinking water distribution systems, coliform bacteria (predominantly Citrobacter species) were repeatedly detected. Disinfection and flushing of the systems did not erase the problem. The pattern of the coliform occurrences indicated contamination originating from biofilms. After inspection of internal surfaces of the systems, no significant biofilm growth was observed on pipe surfaces, but in a number of cases, visible biofilms were detected on rubber-coated valves which harboured the same coliform species as those found in the drinking water samples. In these cases, the rubber-coated valves seemed to act as point sources for the contamination of water.

Tolerance and starvation induced cross-protection against different stresses in Aeromonas hydrophila

Aeromonas hydrophila is sometimes considered as a controversial human pathogen and reported to be susceptible to food processing procedures and environmental stresses. In this study, we have shown that early stationary phase cells of A. hydrophila were readily killed during up shifts in temperature (in the range 50-70 degrees C), the course of drying (at relative humidity, temperature and brightness of the laboratory) and after 5 min exposure to 20%, 30% and 40% v/v ethanol. However, this bacterium was found moderately susceptible to down shift to 4 degrees C in nutrient poor water, sodium chloride stresses (1.5 and 2 M) and to 12% and 15% v/v ethanol stresses. Tolerance against 1 M NaCl and 10% v/v ethanol was observed. At ambient temperature (24.5 degrees C), this microorganism exhibited a starvation survival state, which was largely independent of the initial cell concentrations (8.82, 7.71 and 6.76 log units). The cross-protection experiments showed that cells starved for short (1 day) or prolonged (50 days) periods developed increased resistance to down shift at 4 degrees C and ethanol stress. This may be of concern to the food-processing industry from the public health perspective.

Environmental isolates of Aeromonas spp. harboring the cagA-like gene of Helicobacter pylori

We investigated the presence of cagA-like gene of Helicobacter pylori in environmental isolates of Aeromonas spp. from different water samples of Calcutta, India, by colony hybridization using a cagA-specific DNA probe and by PCR with cagA-specific primers. Nucleotide sequencing of five PCR products revealed 97 to 98% homology to canonical cagA of H. pylori 26695 as well as to four clinical H. pylori strains from Calcutta. The cagA-like gene of the environmental isolates was unstable in laboratory conditions and tended to be lost upon subculturing.