Virulence factors-pathogenicity relationships for Aeromonas species from clinical and food isolates

Outcome of co-infection with opportunistic and multidrug resistant Aeromonas hydrophila and A. veronii in zebrafish: Identification, characterization, pathogenicity and immune responses

Fish can be potentially co-infected by two or more bacterial strains, which can make synergistic influence on the virulence of infection. In this study, two opportunistic and multidrug resistant Aeromonas strains were isolated from wounds of morbid zebrafish with typical deep skin lesions similar to Motile Aeromonas Septicemia. Isolates were genetically identified as A. hydrophila and A. veronii by 16 S rRNA sequencing and phylogenetic analysis. Both isolates were positive for virulent genes (aerA, lip, ser, exu gcaT) and selected phenotypic tests (DNase, protease, gelatinase, lipase, biofilm production and β-haemolysis). A. hydrophila and A. veronii had strong antibiotic resistance against ampicillin, tetracycline, nalidixic acid, kanamycin, erythromycin, clindamycin and trimethoprim-sulfamethoxazole. Histopathological studies revealed that co-infection causes severe necrosis and hypertrophy in the muscles, kidney and liver of zebrafish. Naturally co-infected zebrafish showed highly induced tnf-α, il-1β, il-6, il-12, ifn, ifn-γ, cxcl18 b and ccl34a.4 at transcription level compared to healthy fish, suggesting virulence factors may activate immune and inflammatory responses of zebrafish. Experimentally infected zebrafish showed significantly higher mortality under co-infection with A. hydrohila and A. veronii (87%), followed by individual challenge of A. hydrophila (72%) or A. veronii (67%) suggesting that virulence of A. hydrophila have greater pathogenicity than A. veronii during co-infection.

Characterization of virulence, cell surface characteristics and biofilm-forming ability of Aeromonas spp. isolates from fish and sea water

Members of the genus Aeromonas are emerging human pathogens, causing a variety of extra-intestinal, systemic and gastrointestinal infections in both immunocompetent and immunocompromised persons. Aeromonas virulence is multifaceted and involves surface-associated molecules, motility, biologically active extracellular products and biofilm formation. Aeromonads, isolated from diverse freshwater fish species as well as sea water, were screened for biofilm formation, with varying physicochemical parameters including temperature, agitation and nutrient availability. Motility, cell surface characteristics (auto-aggregation, hydrophobicity and S layer), and extracellular virulence factor production (haemolysis, proteolysis, DNase production) were also assessed to identify potential associations with the biofilm phenotype. Biofilm formation was influenced by environmental conditions, with isolates preferentially forming biofilms in nutrient-rich media at 30 °C, although strong biofilm formation also occurred at 37 °C. Strong biofilm formation was observed for Aeromonas culicicola isolates following exposure to nutrient-rich conditions, while Aeromonas allosaccharophila isolates preferred nutrient-poor conditions for biofilm formation. Source-/species-specific correlations, ranging from weak to strong, were observed between biofilm formation and motility, cell surface characteristics and/or extracellular virulence factor production. Understanding the specific mechanisms by which Aeromonas species adhere to abiotic surfaces may aid in preventing and/or treating disease outbreaks in aquaculture systems and could lead to effective eradication of these fish pathogens.

Potentially pathogenic features of heterotrophic plate count bacteria isolated from treated and untreated drinking water

Heterotrophic plate counts (HPCs) are commonly used to assess the general microbiological quality of drinking water. Drinking water quality specifications worldwide recommend HPC limits from 100 to 500 cfu ml(-1). A number of recent studies revealed evidence that these bacteria may not be as harmless as generally accepted. It appears that immuno-compromised individuals are particularly at risk. This would include the very young and very old patients with diseases such as AIDS and patients on therapy for purposes such as organ transplantation and cancer treatment. In this study, 339 bacterial colonies were isolated at random from selected treated and untreated drinking water in South Africa using routine heterotrophic plate count tests. In a first step to screen for potentially pathogenic properties, 188 (55.5%) of the isolates showed alpha- or beta-haemolysis on human- and horse-blood agar media. Subsequent analysis of the haemolytic isolates for enzymatic properties associated with pathogenicity revealed the presence of chondroitinase in 5.3% of the isolates, coagulase in 16.0%, DNase in 60.6%, elastase in 33.0%, fibrinolysin in 53.7%, gelatinase in 62.2%, hyaluronidase in 21.3%, lecithinase in 47.9%, lipase in 54.8% and proteinase in 64.4%. Fluorescein and pyocyanin were not produced by any of the isolates. Among the haemolytic isolates, 77.7% were resistant to oxacillin 1 microg, 59.6% to penicillin G 2 units, 47.3% to penicillin G 10 units, 54.3% to ampicillin 10 microg and 43.1% to ampicillin 25 microg. Cell culture studies revealed that 96% of haemolytic isolates were cytotoxic to HEp-2 cells, and 98.9% of the 181 cytotoxic isolates adhered to HEp-2 or Caco-2 cells. HEp-2 cells were invaded by 43.6%, and Caco-2 cells by 49.7%, of the 181 cytotoxic isolates. The invasion index on HEp-2 cells ranged from 1.9 x 10(-1) to 8.9 x 10(-6), whereas the invasion index on Caco-2 cells varied between 7.7 x 10(-2) and 8.3 x 10(-6). The most commonly isolated genera with these potentially pathogenic features were Aeromonas, Acinetobacter, Aureobacterium, Bacillus, Chryseobacterium, Corynebacterium, Klebsiella, Moraxella, Pseudomonas, Staphylococcus, Tsukamurella and Vibrio. The results obtained in this study support earlier findings on potentially pathogenic features of bacteria detected by routine HPCs on drinking water. These findings are in agreement with some epidemiological studies, which indicated an association between HPCs in drinking water and the incidence of gastroenteritis in consumers. However, the extent of the health risk concerned needs to be defined in more detail for meaningful revision of quality guidelines for HPCs in drinking water.