Microbiologic and clinical evidence supporting the role of Aeromonas caviae as a pediatric enteric pathogen

Microbial community origin and fate through a rural wastewater treatment plant

Conventional wastewater treatment relies on a complex microbiota; however, much of this community is still to be characterised. To better understand the origin, dynamics and fate of bacteria within a wastewater treatment plant: untreated primary wastewater, activated sludge, and post-treatment effluent were characterised. From 3,163 Exact Sequence Variants (ESVs), 860 were annotated to species-level. In primary wastewater, 28% of ESVs were putative bacterial species previously associated with humans, 14% with animals and 5% as common to the environment. Differential abundance analysis revealed significant relative reductions in ESVs from potentially humans-associated species from primary wastewater to activated sludge, and significant increases in ESVs from species associated with nutrient cycling. Between primary wastewater and effluent, 51% of ESVs from human-associated species did not significantly differ, and species such as Bacteroides massiliensis and Bacteroides dorei increased. These findings illustrate that activated sludge increased extracellular protease and urease-producing species, ammonia and nitrite oxidizers, denitrifiers and specific phosphorus accumulators. Although many human-associated species declined, some persisted in effluent, including strains of potential health or environmental concern. Species-level microbial assessment may be useful for understanding variation in wastewater treatment efficiency as well as for monitoring the release of microbes into surface water and the wider ecosystem. This article is protected by copyright. All rights reserved.

Heat stress during late gestation disrupts maternal microbial transmission with altered offspring's gut microbial colonization and serum metabolites in a pig model

Heat stress (HS) during gestation has been associated with negative outcomes, such as preterm birth or postnatal metabolic syndromes. The intestinal microbiota is a unique ecosystem playing an essential role in mediating the metabolism and health of mammals. Here we hypothesize late gestational HS alters maternal microbial transmission and structures offspring's intestinal microbiota and serum metabolic profiles. Our results show maternal HS alters bacterial β-diversity and composition in sows and their piglets. In the maternal intestine, genera Ruminococcaceae UCG-005, [Eubacterium] coprostanoligenes group and Halomonas are higher by HS (q < 0.05), whereas the populations of Streptococcus, Bacteroidales RF16 group_norank and Roseburia are decreased (q < 0.05). In the maternal vagina, HS mainly elevates the proportions of phylum Bacteroidetes and Fusobacteria (q < 0.05), whereas reduces the population of Clostridiales Family XI (q < 0.05). In the neonatal intestine, maternal HS promotes the population of Proteobacteria but reduces the relative abundance of Firmicutes (q < 0.05). Moreover, the core Operational taxonomic units (OTU) analysis indicates the proportions of Clostridium sensu stricto 1, Romboutsia and Turicibacter are decreased by maternal HS in the intestinal and vaginal co-transmission, whereas that of phylum Proteobacteria and Epsilonbacteraeota, such as Escherichia-Shigella, Klebsiella, Acinetobacter, and Comamonas are increased in both the intestinal and vaginal co-transmission and the vagina. Additionally, Aeromonas is the only genus that is transmitted from environmental sources. Lastly, we evaluate the importance of neonatal differential OTU for the differential serum metabolites. The results indicate Acinetobacter significantly contributes to the differences in the adrenocorticotropic hormone (ACTH) and glucose levels due to HS (P < 0.05). Further, Stenotrophomonas is the most important variable for Cholesterol, low-density lipoprotein (LDL), diamine oxidase (DAO), blood urea nitrogen (BUN) and 5-hydroxytryptamine (5-HT) (P < 0.10). Overall, our data provides evidence for the maternal HS in establishing the neonatal microbiota via affecting maternal transmission, which in turn affects the maintenance of metabolic health.

Antimicrobial resistance of pathogenic Aeromonas spp. isolated from marketed Pacific abalone (Haliotis discus hannai) in Korea

AIMS

The object of this study was to identify potential health concerns of the Aeromons spp. isolated from marketed Pacific abalone (Haliotis discus hannai) with respect to their virulence and antimicrobial resistance patterns.

METHODS AND RESULTS

We identified 29 strains of aeromonads consisting of five species; Aeromonas hydrophila (n = 9), Aeromonas enteropelogenes (n = 14), Aeromonas veronii (n = 3), Aeromonas salmonicida (n = 2) and Aeromonas sobria (n = 1), by employing series of biochemical tests and gene sequencing. In the phenotypic virulence assays, all isolates showed gelatinase and caseinase activities, while lipase formation (69%), phospholipase production (90%), DNase formation (82%), slime production (49%) and haemolysis activity (α = 18% and β = 82%) were also detected among isolates. Prevalence of virulence genes; aerA (100%), fla (66%), ahyB (73%), act (52%), alt (42%), ast (35%), ser (52%), gcat (69%), ascV (43%), hlyA (83%), lip (52%) and exu (59%) were detected by PCR assays. In disc diffusion test, 100% resistance was detected against ampicillin while cephalothin, rifampicin, oxytetracycline, colistine sulphate, nalidixic acid and piperaciliin were resisted by 86, 73, 42, 35, 28, 20 and 20% of the isolates respectively. Thirteen (45%) of the isolates showed multiple antimicrobial resistance (MAR) indices ≥ 0·2.

CONCLUSIONS

Our findings suggest that the potential health risk posed by the abalone-borne Aeromonas spp. should not be underestimated.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first time to evaluate possible public health risks upon consumption of abalone harbored Aeromonas spp. and also to isolate potential pathogenic and multidrug-resistant Aeromonas spp. from Pacific abalone in Korea.

Collection of mullet fish (Mugil cephalus) from west coast of India: evaluation of its quality with relation to food safety

The present study focuses on bacterial and selected heavy metal contaminations of economically important mullet fish (Mugil cephalus) collected from West coast of Gujarat, India. Molecular identification using 16S rRNA sequencing revealed the presence of Aeromonas veronii, A. mollusorum, A. cavae, A. bivalvum, and V.alginolyticus in the gill and the intestine along with some other non-pathogenic bacteria. Pathogenicity of different Aeromonas species was confirmed by hemolysin assay. Apart from pathogenicity, multidrug resistance pattern was also reported against some commonly used antibiotics. Heavy metal analysis of different parts such as ventral and dorsal muscles as well as gills of M. cephalus revealed maximum concentration of Pb (24.08 ± 4.40 mg/kg), Cd (8.25 ± 3.04 mg/kg), and Cu (33.67 ± 5.34 mg/kg), which were higher than the permissible limit. To the best of our knowledge, this is the first study analyzing different heavy metals and associated bacteria in M. cephalus fish in India. Further, the distribution of heavy metals in M. cephalus fish from other countries was also compared.

Production of Non-Ribosomal Peptide Synthetase (NRPS)-Dependent Siderophore by Aeromonas Isolates

BACKGROUND

Aeromonas species are Gram-negative ubiquitous bacteria, facultative anaerobic rods that infect both invertebrates and vertebrates. Various fish species develop hemorrhagic disease and furunculosis due to Aeromonas spp. Aeromonas strains generate certain active compounds such as siderophores, which are the final products of non-ribosomal peptide synthetase (NRPS) activity. The present study attempted to investigate the prevalence of Aeromonas isolates in marketed fish sources. We also examined the siderophore production ability of these isolates.

METHODS

Among the molecular tools, 16S rRNA analysis was used to identify Aeromonas species and their epidemiological distributions. The hemolytic activity of the strains and biochemical assays were used to confirm the identity of the isolates. We also determined the chemical nature of siderophores in these strains.

RESULTS

A total of seven Aeromonas isolates obtained from fish were included to determine the siderophore production. Of 7 isolates, 4 produced siderophore, and their chemical nature was also determined. The siderophore produced by Aeromonas was invariably found to be of hydroxamate. Four Aeromonas isolates were selected for PCR identification of NRPS-encoding gene. The conserved sequence was present in all four selected isolates. Furthermore, siderophores were qualitatively tested for their antibacterial activity against pathogenic bacteria and a significant level of inhibitory activity was observed in siderophores from the four isolates.

CONCLUSION

Our results showed the ability of the isolated strains in production of siderophores with a high level of activity against Salmonella paratyphi. These siderophores could find applications in biomedical industries.

Draft Genome Sequence of Aeromonas caviae Strain 429865 INP, Isolated from a Mexican Patient

Aeromonas caviae is an emerging human pathogen. Here, we report the draft genome sequence of Aeromonas caviae strain 429865 INP which shows the presence of various putative virulence-related genes.

Molecular Detection, Quantification, and Toxigenicity Profiling of Aeromonas spp. in Source- and Drinking-Water

Aeromonas is ubiquitous in aquatic environments and has been associated with a number of extra-gastrointestinal and gastrointestinal illnesses. This warrants monitoring of raw and processed water sources for pathogenic and toxigenic species of this human pathogen. In this study, a total of 17 different water samples [9 raw and 8 treated samples including 4 basin water (partial sand filtration) and 4 finished water samples] were screened for Aeromonas using selective culturing and a genus-specific real-time quantitative PCR assay. The selective culturing yielded Aeromonas counts ranging 0 - 2 x 10(3)CFU/ml and 15 Aeromonas isolates from both raw and treated water samples. The qPCR analysis indicated presence of a considerable nonculturable population (3.4 x 10(1) - 2.4 x 10(4) cells/ml) of Aeromonas in drinking water samples. Virulence potential of the Aeromonas isolates was assessed by multiplex/singleplex PCR-based profiling of the hemolysin and enterotoxin genes viz cytotoxic heat-labile enterotoxin (act), heat-labile cytotonic enterotoxin (alt), heat-stable cytotonic enterotoxin (ast), and aerolysin (aerA) genes. The water isolates yielded five distinct toxigenicity profiles, viz. act, alt, act+alt, aerA+alt, and aerA+alt+act. The alt gene showed the highest frequency of occurrence (40%), followed by the aerA (20%), act (13%), and ast (0%) genes. Taken together, the study demonstrated the occurrence of a considerable population of nonculturable Aeromonads in water and prevalence of toxigenic Aeromonas spp. potentially pathogenic to humans. This emphasizes the importance of routine monitoring of both source and drinking water for this human pathogen and role of the developed molecular approaches in improving the Aeromonas monitoring scheme for water.

Quorum sensing activity of Aeromonas caviae strain YL12, a bacterium isolated from compost

Quorum sensing is a well-studied cell-to-cell communication method that involves a cell-density dependent regulation of genes expression mediated by signalling molecules. In this study, a bacterium isolated from a plant material compost pile was found to possess quorum sensing activity based on bioassay screening. Isolate YL12 was identified using matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry and molecular typing using rpoD gene which identified the isolate as Aeromonas caviae. High resolution tandem mass spectrometry was subsequently employed to identify the N-acyl homoserine lactone profile of Aeromonas caviae YL12 and confirmed that this isolate produced two short chain N-acyl homoserine lactones, namely C4-HSL and C6, and the production was observed to be cell density-dependent. Using the thin layer chromatography (TLC) bioassay, both AHLs were found to activate C. violaceum CV026, whereas only C6-HSL was revealed to induce bioluminescence expression of E. coli [pSB401]. The data presented in this study will be the leading steps in understanding the role of quorum sensing in Aeromonas caviae strain YL12.

An Aeromonas caviae genomic island is required for both O-antigen lipopolysaccharide biosynthesis and flagellin glycosylation

Aeromonas caviae Sch3N possesses a small genomic island that is involved in both flagellin glycosylation and lipopolysaccharide (LPS) O-antigen biosynthesis. This island appears to have been laterally acquired as it is flanked by insertion element-like sequences and has a much lower G+C content than the average aeromonad G+C content. Most of the gene products encoded by the island are orthologues of proteins that have been shown to be involved in pseudaminic acid biosynthesis and flagellin glycosylation in both Campylobacter jejuni and Helicobacter pylori. Two of the genes, lst and lsg, are LPS specific as mutation of them results in the loss of only a band for the LPS O-antigen. Lsg encodes a putative Wzx flippase, and mutation of Lsg affects only LPS; this finding supports the notion that flagellin glycosylation occurs within the cell before the flagellins are exported and assembled and not at the surface once the sugar has been exported. The proteins encoded by flmA, flmB, neuA, flmD, and neuB are thought to make up a pseudaminic acid biosynthetic pathway, and mutation of any of these genes resulted in the loss of motility, flagellar expression, and a band for the LPS O-antigen. Furthermore, pseudaminic acid was shown to be present on both flagellin subunits that make up the polar flagellum filament, to be present in the LPS O-antigen of the A. caviae wild-type strain, and to be absent from the A. caviae flmD mutant strain.

Structural characterization of the O-chain polysaccharide of Aeromonas caviae ATCC 15468 lipopolysaccharide

The O-chain polysaccharide produced by a mild acid degradation of Aeromonas caviae ATCC 15468 lipopolysaccharide was found to be composed of L-rhamnose, 2-acetamido-2-deoxy-D-glucose, 2-acetamido-2-deoxy-D-galactose and phosphoglycerol. Subsequent methylation and CE-ESIMS analyses and 1D/2D NMR ((1)H, (13)C and (31)P) spectroscopy showed that the O-chain polysaccharide is a high-molecular-mass acidic branched polymer of tetrasaccharide repeating units with a phosphoglycerol substituent having the following structure: [structure: see text] where Gro represents glycerol and P represents a phosphate group.

Facteurs de virulence d’Aeromonas caviae isolés de cas cliniques

Aeromonas caviae, an ubiquitous aquatic organism, has long been considered to be of low pathogenicity, and its virulence mechanisms are still not clearly understood. Twenty-eight A. caviae isolates of clinical origin, most often monomicrobic, were identified in our university hospital over a four year period. Patients, mostly immunocompromised, were: eight diarrhoeal infants, 13 diarrhoeal adults, seven bacteraemic adults. Adults were frequently suffering from underlying intestinal malignancy, hepatobiliary disease, gastrectomy. Virulence factors were investigated. Adherence, studied by use of tissue culture HEp-2 cells, and staining of characteristic lateral flagella, were observed in diarrhoeal strains. Extracellular hemolytic activity was tested on rabbit erythrocytes suspensions at 25 and 37 degrees C. One blood culture isolate showed an important hemolytic activity at 25 degrees C, but none at 37 degrees C. Treatment with furin activated the aerolysin precursor and resulted in significant hemolysis at 37 degrees C, and fluid accumulation in rabbit ileal loops similar to that of A. hydrophila as control. The presence of the hemolysin gene was confirmed in this strain by PCR. In conclusion, A. caviae was shown to be a pathogen isolated from diarrhoea and bacteraemia in immunocompromised patients with malignancies and low gastric acidity as favouring factors. Virulence including the ability to adhere to cells and the production of lateral flagella was observed in diarrhoeal strains. The expression and the production of extracellular hemolytic activity and enterotoxicity at 37 degrees C depended on the activation of the pore forming toxin aerolysin precursor by furin. In vivo the protoxin is probably processed to its mature form by host proteases.

Niche-specific association of Aeromonas Ribotypes from human and environmental origin

A total of 88 Aeromonas isolates from distinct locations and sources (39 from extraintestinal infections, 31 from diarrhoeic, ten from non-diarrhoeic faeces, all human, and eight from fresh water) were subjected to phenospecies identification, serotyping, ribotyping and detection of some virulence markers. The strains belonged to four different phenospecies marked by 19 O serogroups and 38 ribotypes. No strong correlation between these parameters was found, and no group, as defined by the typing methods, could be characterized with a particular set of virulence markers. There was a clear association of ribotypes with the source of the strains. Cluster analysis allowed the identification of a complex of ribotypes belonging to distinct but related sources, including clinical and environmental isolates. These results suggest that ribotyping may be an epidemiological tool suitable for the study of Aeromonas infections.

Prevalence and resistance to antibiotics for Aeromonas species from retail fish in Malaysia

A total of 87 market fish samples representing five types of fish were evaluated for the presence of Aeromonas spp. Of the samples examined, 69%, 55%, 11.5% and 2.3% harbored Aeromonas spp., A. veronii biovar sobria, A. hydrophila and A. caviae, respectively. The 60 isolated Aeromonas spp. strains were further examined for hemolytic activity, resistance to antimicrobial agents and presence of plasmids. Hemolytic activity varied widely among the isolated strains. Though all the isolates demonstrated resistance to three or more of the antibiotics tested, all were susceptible to ceptazidime. Thirty-four (56.7%) of the sixty isolates harbored plasmids, with sizes ranging from 2.3 to 15.7 kb. These results indicate that hemolytic, multiple antibiotic resistant and genetically diverse aeromonads are easily recovered from fish in this region.

Detection and characterization of the hemolysin genes in Aeromonas hydrophila and Aeromonas sobria by multiplex PCR

A multiplex PCR assay was designed to amplify the Aeromonas hydrophila and A. veronii bv. sobria hemolysin and aerolysin genes. The assay was evaluated by using 121 clinical isolates and 7 reference strains of Aeromonas spp., and these were divided into five genotypes on the basis of the results of the multiplex PCR. The five genotypes were characterized as type 1 for those carrying the ahh1 gene only (36% of isolates), type 2 for those carrying the asa1 gene only (8.5% of isolates), type 3 for those carrying both the ahh1 and the asa1 genes (4% of isolates), type 4 for those carrying the ahh1 gene and the A. hydrophila aerA (aerolysin) gene (37.5% of isolates), and type 5 for those in which no hemolysin genes were detected (14% of isolates). The most common single hemolysin gene carried among all the Aeromonas isolates examined was ahh1, with 99 of 128 (77%) of isolates testing positive for this gene either alone or in combination with other hemolysin genes. Phenotypic expression of toxins was evaluated in a Vero cell culture cytotoxicity assay. These results indicated that there is a statistically significant correlation between the cytotoxin titers and the hemolysin genotype. Isolates belonging to genotype 4 (carrying both the ahh1 gene and the aerolysin and hemolysin aerA genes) expressed higher cytotoxin titers than isolates of the other genotypes (P < 0.001). These isolates were more cytotoxic in cell culture and may have greater clinical significance.

Ocorrência de bactérias do gênero Aeromonas em queijo-de-minas frescal artesanal

Foram analisadas 160 amostras de queijo-de-minas frescal artesanal, adquiridas no comércio varejista dos municípios de Poços de Caldas - MG e Jaboticabal - SP, a fim de verificar a ocorrência de bactérias do gênero Aeromonas no produto. Oitenta e duas (51,2%) encontravam-se contaminadas pelos microrganismos, com populações que variavam de 5,0×10³ a 4,0×10(5) UFC/grama. Foram identificadas as espécies Aeromonas hydrophila, Aeromonas caviae, Aeromonas schubertii, além de cepas consideradas atípicas. Os resultados evidenciam que bactérias do gênero Aeromonas podem ser veiculadas através do queijo tipo minas frescal artesanal e devem servir de alerta aos serviços de saúde pública.

Vector potential of houseflies for the bacterium Aeromonas caviae

Houseflies, Musca domestica Linnaeus (Diptera: Muscidae), have been implicated as vectors or transporters of numerous gastrointestinal pathogens encountered during feeding and ovipositing on faeces. The putative enteropathogen Aeromonas caviae (Proteobacteria: Aeromonadaceae) may be present in faeces of humans and livestock. Recently A. caviae was detected in houseflies by PCR and isolated by culture methods. In this study, we assessed the vector potential of houseflies for A. caviae relative to multiplication and persistence of the bacterium in the fly and to contamination of other flies and food materials. In experimentally fed houseflies, the number of bacteria increased up to 2 days post-ingestion (d PI) and then decreased significantly 3 d PI. A large number of bacteria was detected in the vomitus and faeces of infected flies at 2-3 d PI. The bacteria persisted in flies for up to 8 d PI, but numbers were low. Experimentally infected flies transmitted A. caviae to chicken meat, and transmissibility was directly correlated with exposure time. Flies contaminated the meat for up to 7 d PI; however, a significant decrease in contamination was observed 2-3 d PI. In the fly-to-fly transmission experiments, the transmission of A. caviae was observed and was apparently mediated by flies sharing food. These results support houseflies as potential vectors for A. caviae because the bacterium multiplied, persisted in flies for up to 8 d PI, and could be transmitted to human food items.

Aeromonas species in foods

Aeromonas species have been recognized as potential or emerging foodborne pathogens for more than 20 years. Aeromonads are estuarine bacteria and are ubiquitous in fresh water, fish and shellfish, meats, and fresh vegetables. Actual sourced foodborne outbreaks are few, but epidemiological evidence suggests that the bacterium can cause self-limiting diarrhea, with children being the most susceptible population. Most aeromonads are psychrotrophic and can grow in foods during cold storage. Aeromonads are not resistant to food processing regimes and are readily killed by heat treatment. A host of virulence factors are present, but the exact role of each in human disease has not been fully elucidated.

Lateral flagella of Aeromonas species are essential for epithelial cell adherence and biofilm formation

Mesophilic Aeromonas strains express a single polar flagellum in all culture conditions and produce lateral flagella on solid media. Such hyperflagellated cells demonstrate increased adherence. Nine lateral flagella genes, lafA-U for Aeromonas hydrophila, and four Aeromonas caviae genes, lafA1, lafA2, lafB and fliU, were isolated. Mutant characterization, nucleotide and N-terminal sequencing demonstrated that the A. hydrophila and A. caviae lateral flagellins were almost identical, but were distinct from their polar flagellum counterparts. The aeromonad lateral flagellins exhibited higher molecular masses on SDS-PAGE, and this aberrant migration was thought to result from post-translational modification through glycosylation. Mutation of the Aeromonas lafB, lafS or both A. caviae lateral flagellins caused the loss of lateral flagella and a reduction in adherence and biofilm formation. Mutations in lafA1, lafA2, fliU or lafT resulted in strains that expressed lateral flagella, but had reduced adherence levels. Mutation of the lateral flagella loci did not affect polar flagellum synthesis, but the polarity of the transposon insertions on the A. hydrophila lafTlU genes resulted in non-motility. However, mutations that abolished polar flagellum production also inhibited lateral flagella expression. We conclude that Aeromonas lateral flagella: (i) play a role in adherence and biofilm formation; (ii) are distinct from the polar flagellum; (iii) synthesis is dependent upon the presence of a polar flagellum filament; and (iv) that the motor proteins of the polar and lateral flagella systems appear to be shared.

Infectious causes of chronic diarrhea

Chronic diarrhea can be seen in association with specific pathogens, usually parasites and occasionally some bacteria. This article reviews pathogens causing chronic diarrhea in immunocompetent individuals and provides a rational diagnostic approach.

Laboratory approaches to infectious diarrhea

Future applications of advanced molecular diagnostics in clinical laboratories will enhance significantly capabilities to diagnose bacterial, parasitic, and viral agents in the early course of disease through enhanced assay sensitivities and specificities and improved turnaround times, theoretically leading to more timely and directed therapeutic intervention. Until such time, clinicians must continue to rely on clinical judgment and the diverse battery of traditional culture techniques, direct examination (including light microscopy and electron microscopy), and immunoassays that are available. Cost considerations and the ever-increasing array of infectious agents responsible for infectious gastroenteritis will continue to drive the development of practice guidelines to assist practitioners with reasoned and reasonable approaches to management of diarrheal illnesses.

Motility and the polar flagellum are required for Aeromonas caviae adherence to HEp-2 cells

Aeromonas caviae is increasingly being recognized as a cause of gastroenteritis, especially among the young. The adherence of aeromonads to human epithelial cells in vitro has been correlated with enteropathogenicity, but the mechanism is far from well understood. Initial investigations demonstrated that adherence of A. caviae to HEp-2 cells was significantly reduced by either pretreating bacterial cells with an antipolar flagellin antibody or by pretreating HEp-2 cells with partially purified flagella. To precisely define the role of the polar flagellum in aeromonad adherence, we isolated the A. caviae polar flagellin locus and identified five polar flagellar genes, in the order flaA, flaB, flaG, flaH, and flaJ. Each gene was inactivated using a kanamycin resistance cartridge that ensures the transcription of downstream genes, and the resulting mutants were tested for motility, flagellin expression, and adherence to HEp-2 cells. N-terminal amino acid sequencing, mutant analysis, and Western blotting demonstrated that A. caviae has a complex flagellum filament composed of two flagellin subunits encoded by flaA and flaB. The predicted molecular mass of both flagellins was approximately 31,700 Da; however, their molecular mass estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was approximately 35,500 Da. This aberrant migration was thought to be due to their glycosylation, since the proteins were reactive in glycosyl group detection assays. Single mutations in either flaA or flaB did not result in loss of flagella but did result in decreased motility and adherence by approximately 50%. Mutation of flaH, flaJ, or both flagellin genes resulted in the complete loss of motility, flagellin expression, and adherence. However, mutation of flaG did not affect motility but did significantly reduce the level of adherence. Centrifugation of the flagellate mutants (flaA, flaB, and flaG) onto the cell monolayers did not increase adherence, whereas centrifugation of the aflagellate mutants (flaH, flaJ, and flaA flaB) increased adherence slightly. We conclude that maximum adherence of A. caviae to human epithelial cells in vitro requires motility and optimal flagellar function.

Virulence factors of clinical isolates of Aeromonas caviae

The mechanism of pathogenicity of 13 Aeromonas caviae strains isolated from fecal specimens of children with diarrhea was examined. Isolates possessed several virulence agents. Six of 13 isolates produced cholera-like toxin, all strains exhibited adhesive ability and secreted catecholate siderophores. Six isolates were able to grow in heat-inactivated serum. Production of hemolysins was not detected. The isolates had relatively high values of LD50 in a mouse model, which suggests that there is no linkage between pathogenicity to mice and the diarrheagenic ability of the isolates.

Role of flm locus in mesophilic Aeromonas species adherence

The adherence mechanism of Aeromonas caviae Sch3N to HEp-2 cells was initially investigated through four mini-Tn5 mutants that showed a 10-fold decrease in adherence. These mutants lost motility, flagella, and their lipopolysaccharide (LPS) O antigen (O-Ag). Three genes, flmB-neuA-flmD, were found to be interrupted by the transposon insertions; additionally, two other genes, one lying upstream (flmA) and one downstream (neuB), were found to be clustered in the same operon. While the flmA and flmB genes were present in all mesophilic Aeromonas spp. (A. hydrophila, A. caviae, A. veronii bv. veronii, and A. veronii bv. sobria) tested, this was not the case for the neuA-flmD-neuB genes. Construction and characterization of flmB insertion mutants in five other mesophilic Aeromonas strains revealed the loss of motility, flagella, and adherence but did not alter the LPS composition of these strains. Taking the above findings into consideration, we conclude (i) that flagella and possibly the LPS O-Ag are involved in the adherence of the mesophilic Aeromonas to human epithelial cells; (ii) flmA and flmB are genes widely distributed in the mesophilic Aeromonas and are involved in flagella assembly, and thus adherence; and (iii) in A. caviae Sch3N the flmA and flmB genes are found in a putative operon together with neuA, flmD, and neuB and are involved in LPS O-Ag biosynthesis and probably have a role in flagellum assembly.

The response of Aeromonas hydrophila to oxidative stress induced by exposure to hydrogen peroxide

Aeromonas hydrophila, an opportunist human pathogen of low virulence, was shown to display a high degree of sensitivity upon exposure to hydrogen peroxide. As with other species, Aer. hydrophila is able to develop the capacity to resist loss of viability induced by such oxidative stress. Development of stress resistance follows the archetypal profile where pre-exposure of a population to sub-lethal levels of H2O2 stimulates onset of tolerance to further exposure. Acquisition of tolerance critically requires nascent protein synthesis. Further analysis demonstrated population growth phase influences the degree of sensitivity of the organism. Late stationary phase cultures demonstrate a decreased sensitivity compared with younger populations. Significantly, it was also determined that stock culture age influenced the level of sensitivity of the derived experimental culture, where an increased stock culture age corresponded with enhanced resistance to H2O2. These data show that Aer. hydrophila population phenotype is influenced by the phenotype of the donor stock culture.

Hemolytic activity and siderophore production in different Aeromonas species isolated from fish

The hemolytic activity and siderophore production of several strains of motile aeromonads were determined. The hemolytic activity of Aeromonas caviae and Aeromonas eucrenophila was enhanced after trypsinization of the samples. The enhancement of hemolysis was observed in strains that carried an aerolysin-like gene, detected by a PCR procedure. Siderophore production was demonstrated in all but one strain of Aeromonas jandaei. No apparent relationship was observed between the presence of plasmid DNA and hemolysis or siderophore production.

PCR detection, characterization, and distribution of virulence genes in Aeromonas spp

We found 73.1 to 96.9% similarity by aligning the cytolytic enterotoxin gene of Aeromonas hydrophila SSU (AHCYTOEN; GenBank accession no. M84709) against aerolysin genes of Aeromonas spp., suggesting the possibility of selecting common primers. Identities of 90 to 100% were found among the eight selected primers from those genes. Amplicons obtained from Aeromonas sp. reference strains by using specific primers for each gene or a cocktail of primers were 232 bp long. Of hybridization group 4/5A/5B (HG4/5A/5B), HG9, and HG12 or non-Aeromonas reference strains, none were positive. PCR-restriction fragment length polymorphism (PCR-RFLP) with HpaII yielded three types of patterns. PCR-RFLP 1 contained two fragments (66 and 166 bp) found in HG6, HG7, HG8, HG10, and HG11. PCR-RFLP 2 contained three fragments (18, 66, and 148 bp) found in HG1, HG2, HG3, and HG11. PCR-RFLP 3, with four fragments (7, 20, 66, and 139 bp), was observed only in HG13. PCR-amplicon sequence analysis (PCR-ASA) revealed three main types. PCR-ASA 1 had 76 to 78% homology with AHCYTOEN and included strains in HG6, HG7, HG8, HG10, and HG11. PCR-ASA 2, with 82% homology, was found only in HG13. PCR-ASA 3, with 91 to 99% homology, contained the strains in HG1, HG2, HG3, and HG11. This method indicated that 37 (61%) of the 61 reference strains were positive with the primer cocktail master mixture, and 34 (58%) of 59 environmental isolates, 93 (66%) of 141 food isolates, and 100 (67%) of 150 clinical isolates from around the world carried a virulence factor when primers AHCF1 and AHCR1 were used. In conclusion, this PCR-based method is rapid, sensitive, and specific for the detection of virulence factors of Aeromonas spp. It overcomes the handicap of time-consuming biochemical and other DNA-based methods.

The prevalence of Aeromonas species in feces of horses with diarrhea

Feces collected from 40 horses with diarrhea and 34 horses without diarrhea were examined to determine if an association existed between isolation of Aeromonas spp. and diarrhea. Samples were also examined for Salmonella spp., and identification of viruses and parasite ova. Neither Salmonella spp. nor Aeromonas spp. were isolated from the feces of 34 control horses. Aeromonas spp. were isolated from feces of 22 of 40 (55%) horses with diarrhea. Salmonella spp. were isolated from feces of 8 (20%) horses, and of these, 5 (12.5%) were also positive for Aeromonas spp. Twenty-nine isolates of Aeromonas spp. were recovered from the feces of 22 diarrheic horses. Of these isolates, more than 80% were susceptible on in vitro testing to amikacin, ceftiofur, chloramphenicol, and gentamicin. All isolates were susceptible to enrofloxacin. Diarrheic horses positive for Aeromonas were significantly (P = .04) older than diarrheic horses negative for Aeromonas spp. A significantly greater number of fecal samples were positive for Aeromonas spp. during March through August than samples examined in other months (P = .014). Results of this study indicate that Aeromonas spp. should be considered as a cause of diarrhea in horses.

The prevalence of Aeromonas species in feces of horses with diarrhea

Feces collected from 40 horses with diarrhea and 34 horses without diarrhea were examined to determine if an association existed between isolation of Aeromonas spp. and diarrhea. Samples were also examined for Salmonella spp., and identification of viruses and parasite ova. Neither Salmonella spp. nor Aeromonas spp. were isolated from the feces of 34 control horses. Aeromonas spp. were isolated from feces of 22 of 40 (55%) horses with diarrhea. Salmonella spp. were isolated from feces of 8 (20%) horses, and of these, 5 (12.5%) were also positive for Aeromonas spp. Twenty-nine isolates of Aeromonas spp. were recovered from the feces of 22 diarrheic horses. Of these isolates, more than 80% were susceptible on in vitro testing to amikacin, ceftiofur, chloramphenicol, and gentamicin. All isolates were susceptible to enrofloxacin. Diarrheic horses positive for Aeromonas were significantly (P = .04) older than diarrheic horses negative for Aeromonas spp. A significantly greater number of fecal samples were positive for Aeromonas spp. during March through August than samples examined in other months (P = .014). Results of this study indicate that Aeromonas spp. should be considered as a cause of diarrhea in horses.

Purification and characterization of an Aeromonas caviae metalloprotease that is related to the Vibrio cholerae hemagglutinin/protease

A zinc metalloprotease (AP34) from Aeromonas caviae was purified by ammonium sulfate precipitation and subsequent gel filtration through Sephadex G-100 and Sephadex G-50 Superfine. The molecular mass was estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to be 34 kDa. The protease showed maximum activity at pH 7.0 and was stable at 60 degrees C. AP34 was completely inactivated by EDTA and Zincov. The N-terminal amino acid sequence of AP34 showed a high degree of homology with a range of proteases within the family Vibrionaceae, including the hemagglutinin/protease (HA/P) of Vibrio cholerae. Immunologic relatedness of AP34 and HA/P was demonstrated by Western blotting. AP34-like protease was widely distributed among the aeromonad strains.

Molecular analysis and expression of the extracellular lipase of Aeromonas hydrophila MCC-2

The structural gene encoding the extracellular lipase of Aeromonas hydrophila MCC-2 was cloned and found to be expressed in Escherichia coli using its own promoter. When the cloned gene (lip) was expressed in E. coli minicells, an 80 kDa protein was identified. Subcellular fractionation of E. coli carrying the lip gene indicated that the Lip protein was mainly associated with the membrane fraction. Nucleotide sequence analysis revealed that the gene is 2253 bp long, coding for a 79-9 kDa protein with an estimated pl of 10.36. The deduced protein contains two putative signal peptide cleavage sites: one is a typical signal peptidase cleavage site and the other bears a strong resemblance to known lipoprotein leader sequences. Radioactivity from [3H]palmitate was incorporated into the Lip protein when expressed in E. coli. The deduced protein contains a sequence of VHFLGHSLGA which is very well conserved among lipases. It shows 67% and 65% overall identity to the amino acid sequences of lipase from A. hydrophila strains H3 and JMP636, respectively, but shows little homology to those of other lipases. The Lip protein was purified to homogeneity from both A. hydrophila and recombinant E. coli. In hydrolysis of p-nitrophenyl esters and triacylglycerols, using purified enzyme, the optimum chain lengths for the acyl moiety on the substrate were C10 to C12 for ester hydrolysis and C8 to C10 for triacylglycerol hydrolysis.

Characterization of cytotoxic, hemolytic Aeromonas caviae clinical isolates and their identification by determining presence of a unique hemolysin gene

Aeromonas caviae has recently been recognized as an important enteropathogen and its hemolysin is purported to be one of the virulence factors, In this study, a total of 80 clinical isolates of Aeromonas spp. were investigated by PCR with synthetic oligonucleotides targeting a cloned hemolysin-encoding sequence from an A. caviae isolate of clinical origin. Of the 35 clinical A. caviae isolates tested, only 6 contained the target sequence.

Aeromonads in acute diarrhoea and asymptomatic infections in Nigerian children

Stool samples of 616 asymptomatic and 296 diarrhoeic school children were compared for the recovery rate of Aeromonas spp. on ampicillin (10 micrograms/ml) sheep blood agar. Culture filtrates of isolates were tested for haemolysin production with 1% freshly washed rabbit erythrocytes. Stools of 9 (3.0%) diarrhoeic children yielded five strains of A. hydrophila and four of A. veronii (two each of biotypes sobria and veronii), compared to 12 (1.9%) (p > 0.01) asymptomatic children who harbored seven A. hydrophila and five A. caviae strains. Isolates from-diarrhoeic stools were exclusively from children < or = 5 years, while all infected asymptomatic children were > or = 6 years. Culture filtrates of all nine diarrhoeic strains were uniformly enterotoxigenic (intestinal weight ratio > 0.083) and produced haemolysin titres > 128. These phenotypes where variable in carriage strains of A. hydrophila but were not detected in A. caviae. The recovery of A. hydrophila, and A. veronii biotypes from diarrhoeic stools of children < or = 5 years may suggest their involvement in diarrhoea causation in the absence of other diarrhoeagenic agents.

Diversity of Aeromonas sp. in Flemish drinking water production plants as determined by gas-liquid chromatographic analysis of cellular fatty acid methyl esters (FAMEs)

Gas-liquid chromatography of cellular fatty acid methyl esters (FAMEs) was used to determine the phenotypic and genotypic diversity among 489 presumptive Aeromonas strains isolated from five Flemish drinking water production plants. FAME profiles were compared with the predetermined library profiles of a representative database, AER48C, which contains the mean FAME data of all 14 currently established hybridization groups (HGs) or genospecies within Aeromonas. Using AER48C, more than 93% (457 strains) of all presumptive aeromonads isolated on ampicillin-dextrin agar were unequivocally identified as belonging to this genus. Moreover, 85.5% and 73.5% of these strains could be assigned to a particular phenospecies or HG, respectively. Raw and treated surface water samples were dominated by members of the Aer. hydrophila complex (38.8%, comprising HGs 1-3), followed by the Aer. caviae complex (22.7%, comprising HGs 4-6) and the Aer. sobria complex (16.7%, comprising HGs 7-9). HGs 3, 5A/B and 8 were the most prominent genospecies in this type of water. On the other hand, it was found that raw and treated phreatic groundwater samples displayed a much more limited species diversity since these were almost entirely dominated (95.8%) by strains belonging to HGs 2 and 3 of the Aer. hydrophila complex. In general, flocculation-decantation and sand filtration were not shown to influence the overall species distribution in any of the plants examined.

Proteolytic activity of Aeromonas caviae

Aeromonas strains produce a variety of virulence factors including proteases. Studies on the kinetics of growth of Aeromonas caviae NRRL B-966 and its proteases suggest that the proteolytic activities are produced throughout the growth phase, with peak level occurring at stationary phase. A. caviae synthesize both intracellular and extracellular proteases with the latter account for major portion of the total activity. Optimum pH for the A. caviae proteolytic activity is at 7.0. A. caviae produces a thermoresistant protease, whose activity is dependent on Mg++ and Ca++ ions. Inhibition of proteolytic activity by phenyl methyl sulfonyl fluoride suggest the presence of a serine protease in A. caviae. Nitrogenous compounds enhance the proteolytic activity while carbohydrates tested in this study inhibit the activity.

Toxin production, adherence and protein expression by clinical Aeromonas spp. isolates in broth and human pooled ileostomy fluid

The physiological behaviour of clinical Aeromonas spp. isolates was compared following culture in a conventional broth and human pooled ileostomy fluid (PIF). Protein expression was markedly affected by the growth medium, with an overall reduction in whole cell proteins in bacteria grown in ileostomy fluid. In addition, novel outer membrane proteins were produced in PIF but not in broth. The majority of A. hydrophila and A. sobria isolates produced toxin in both broth and PIF, whereas no cytotoxin positive A. caviae were found. Toxin titres were at least two doubling dilutions higher in 40% and 21% of A. hydrophila and A. sobria isolates, respectively, following culture in brain heart infusion broth compared with PIF. Bacterial adherence to Vero and A-549 cells was significantly more common in A. hydrophila (53%) and A. sobria (64%) than in A. caviae (15%) (P < 0.01). We observed increased adherence by 6 aeromonas strains previously classified as adherence-positive, but not by 6 non-adherers, in PIF compared with brain heart infusion broth. The influence of growth medium on the expression of potential virulence determinants by Aeromonas spp. provides a rationale for the use of human ileostomy fluid in future in vitro studies, in order to simulate the nutrient conditions found in vivo.

Aeromonas species exhibit aggregative adherence to HEp-2 cells

Clinical and environmental isolates of Aeromonas species (five A. hydrophila isolates, three A. caviae isolates, and two A. sobria isolates) were tested for their adherence to HEp-2 cells. Clinical isolates of A. hydrophila and A. sobria exhibited aggregative adherence similar to that presented by enteroadherent-aggregative Escherichia coli. Bacterial aggregates adhered to cells with a typical "stacked-brick" appearance. In contrast, A. caviae strains showed a diffuse adherence pattern.

Characterisation of haemolytic activity from Aeromonas caviae

Aeromonas caviae, an enteropathogen associated with gastroenteritis, displays several virulence characteristics. Studies on the kinetics of growth of A. caviae and expression of beta-haemolytic toxin revealed that A. caviae produced maximum haemolytic activity extracellularly during the stationary phase. Preliminary studies on the properties of A. caviae haemolysin suggested that divalent cations (Mg2+ and Ca2+) and thiol compounds, dithiothreitol and mercaptoethanol enhanced the haemolytic activity. Addition of L-cysteine, glutathione and EDTA reduced the haemolytic activity. The iron chelator, 2-2' bipyridyl, significantly inhibited the growth of A. caviae possibly by iron limitation, with parallel enhancement of haemolysin production compared to A. caviae grown in excess of iron. These results suggest that A. caviae produces only beta-haemolysin, which resembles the haemolysins reported for several other bacteria and the activity might be regulated by environmental factors especially iron.

Factors influencing beta-galactosidase activity of Aeromonas caviae

Aeromonas caviae, often reported to be associated with diarrhoeal patients, elaborates several virulence factors as well as catabolic enzymes such as xylanase and beta-galactosidase. Studies on the kinetics of growth of A. caviae and synthesis of beta-galactosidase suggested that the activity was cell associated and reached a peak during the late logarithmic phase of growth. The optimum pH for beta-galactosidase activity was 7.0 and required Ca2+ and glutathione for enhancement of its activity; IPTG also slightly improved the activity. Aerobic cultivation of A. caviae in LB containing glucose, fructose, maltose and sucrose completely inhibited the activity possibly due to acetic acid production. Addition of 100 mM cAMP to the media containing glucose (0.25%, w/v) restored the relative activity by 8.8%; however, the final pH of the media remained acidic. Aerobic growth of A. caviae with other carbon sources did not affect beta-galactosidase activity, probably as there was no acid production and thereby the final pH of the media unaltered. Arabinose, xylose and galactose induced the A. caviae beta-galactosidase activity by several folds and lactose moderately enhanced its activity.

The public health significance of Aeromonas spp. in foods

There is now evidence that some strains of Aeromonas species are enteropathogens. Such strains possess virulence properties, such as the ability to produce enterotoxins, cytotoxins, haemolysins and/or the ability to invade epithelial cells. Strains with these properties are common contaminants of drinking water and a wide range of foods. Contact or consumption of contaminated water, especially in summer, is a major risk factor in Aeromonas-associated gastroenteritis. Aeromonas-contaminated foods may also be vehicles of infection. Given the properties of strains that have been described in foods it has been suggested that food-borne illness could result not only from colonization and in vivo expression of virulence factors, but possibly also by intoxication following ingestion of foods that have been stored for a period of time, even under refrigeration. This paper reviews what is known about Aeromonas spp. in foods, their expression of virulence determinants, particularly at refrigeration temperatures, and the questions remaining to be answered to evaluate the risk they pose, so that an appropriate public health response can be determined.

Adherence to HEp-2 cells and enteropathogenic potential of Aeromonas spp

Aeromonas strains (total = 60) of clinical, water and food origin were tested for adherence to HEp-2 cells. Environmental strains were selected (except for A. caviae) to include primarily those expressing other virulence-associated properties. Adhesion was markedly species-dependent (A. veronii biotype sobria, 15 of 26 [58%]. A caviae, 4 of 12 [33%] and A. hydrophila, 2 of 8 [11%]). A. veronii biotype sobria were adhesive, irrespective of source (62 and 54% for clinical and environmental strains, respectively). Adherent strains of this species were enterotoxin-positive and most (13 of 15) grew at 43 degrees C. A. caviae isolated from clinical specimens contained a higher proportion (75%) of adherent strains than environmental strains (13%). Virulent subsets of A. veronii biotype sobria and A. caviae are adherent to HEp-2 cells. The HEp-2 assay is a useful model for investigating mechanisms of adherence and enteropathogenicity of virulent Aeromonas species.

Aeromonas spp as a potential cause of diarrhoea in children

AIMS

To determine the prevalence of Aeromonas spp in the faeces of children and the association with symptoms of gastroenteritis.

METHODS

Faecal specimens (n = 1026) were cultured for Aeromonas spp using three selective media and an enrichment broth at both 30 degrees C and 37 degrees C. The isolation of Aeromonas spp was correlated with symptoms of gastroenteritis, previous antibiotic use, and environmental temperature.

RESULTS

Aeromonas spp (n = 28) from 26 (2.5%) patients were recovered. Bile salt, Irgasan, and brilliant green agar was the most efficient selective culture medium. Eleven of the patients had symptoms of gastroenteritis, usually mild diarrhoea of two to three days' duration, in the absence of other recognised enteropathogens. A caviae was a particularly frequent isolate (nine out of 11 cases) in symptomatic individuals. Only one out of seven Aeromonas spp recovered by enrichment culture alone was possibly associated with symptoms of gastroenteritis. There was a close correlation between the environmental temperature and isolation of Aeromonas spp.

CONCLUSIONS

Aeromonas spp and particularly A caviae may cause gastroenteritis in children, most often during warmer months of the year. Culture for these potential enteropathogens could be confined to summer and autumn months. Bile salt, Irgasan, and brilliant green selective agar, but not alkaline peptone water enrichment, is an efficient culture medium for recovering possibly clinically important isolates.

Typing of Aeromonas strains from patients with diarrhoea and from drinking water

Aeromonas strains (187) from human diarrhoeal stools and from drinking water (263) in The Netherlands were typed by three different methods. Biotyping alone was found to be of little value for epidemiological studies because 84% of all strains belonged to only 10 biotypes. Common biotypes could be further differentiated by serotyping. Gas-liquid chromatography of cell wall fatty acid methyl esters (FAME) was useful for species identification as well as for typing: 86% of all strains could be identified to the species level, and within this group 92% of all identifications corresponded with the biotype. Cluster analysis and principal component analysis of FAME profiles could be used for comparison of strains from different sources and gave the same general conclusions as bio- and serotyping. There was little overall similarity between Aeromonas strains from human (diarrhoeal) faeces and from drinking water, differences being most pronounced for Aeromonas caviae and least for A. sobria.