4 Biochemical and Serological Characteristics of Aeromonas

Performance and mechanism of a novel woodchip embedded biofilm electrochemical reactor (WBER) for nitrate-contaminated wastewater treatment

In this study, a woodchip biofilm electrode reactor (WBER) with woodchips embedded anode and cathode was developed, and its denitrification mechanism was analyzed by investigating the denitrification performance, organic matter change, redox environment and microbial community. The results show that the WBER with a carbon rod as anode (C-WBER) had a higher denitrification efficiency (2.58 mg NO- 3-N/(L·h)) and lower energy consumption (0.012 kWh/g NO- 3-N) at 350 mA/m². By reducing the hydroxyl radical and dissolved oxygen concentrations, anode embedding technology effectively decreased the inhibition on microorganisms. Lignin decomposition, nitrification and aerobic denitrification were carried out in anode. Additionally, hydrogen autotrophic denitrification and heterotrophic denitrification were occurred in cathode. The WBER effectively removed nitrate and reduced the cost, providing a theoretical basis and direction for further develop BERs.

Potential pathogenicity ofAeromonas hydrophilacomplex strains isolated from clinical, food, and environmental sources

Aeromonas are autochthonous inhabitants of aquatic environments, including chlorinated and polluted waters, although they can also be isolated from a wide variety of environmental and clinical sources. They cause infections in vertebrates and invertebrates and are considered to be an emerging pathogen in humans, producing intestinal and extra-intestinal diseases. Most of the clinical isolates correspond to A. hydrophila, A. caviae, and A. veronii bv. Sobria, which are described as the causative agents of wound infections, septicaemia, and meningitis in immunocompromised people, and diarrhoea and dysenteric infections in the elderly and children. The pathogenic factors associated with Aeromonas are multifactorial and involve structural components, siderophores, quorum-sensing mechanisms, secretion systems, extracellular enzymes, and exotoxins. In this study, we analysed a representative number of clinical and environmental strains belonging to the A. hydrophila species complex to evaluate their potential pathogenicity. We thereby detected their enzymatic activities and antibiotic susceptibility pattern and the presence of virulence genes (aer, alt, ast, and ascV). The notably high prevalence of these virulence factors, even in environmental strains, indicated a potential pathogenic capacity. Additionally, we determined the adhesion capacity and cytopathic effects of this group of strains in Caco-2 cells. Most of the strains exhibited adherence and caused complete lysis.

The effects of temperature, water activity and pH on the growth of Aeromonas hydrophila and on its subsequent survival in microcosm water

AIMS

The influence of temperature, water activity and pH on the growth of Aeromonas hydrophila, and on its survival after transfer in nutrient-poor water were assessed.

METHODS AND RESULTS

Experiments were carried out according to a Box-Behnken matrix at 10-30 degrees C, 0.95-0.99 water activity (aw) and pH 5-9. The effect of each factor on the kinetic parameters of growth (i.e. the maximal specific growth rate, mumax, and the lag time, lambda) and on the decline of the bacteria in microcosm water (time to obtain a reduction of 5 log, T5 log) were studied by applying central composite design.

CONCLUSIONS

The major effect of temperature and water activity on the growth of A. hydrophila was highlighted, whereas the effect of pH in these experimental conditions was not significant. Models describing the effect of environmental parameters on the growth of A. hydrophila were proposed. The effect of the growth environment, and particularly the incubation temperature, have an influence on the survival ability of the bacteria in nutrient-poor water.

SIGNIFICANCE AND IMPACT OF THE STUDY

The Box-Behnken design was well suited to determine the influence of environmental factors on the growth of A. hydrophila and to investigate the effect of previous growth conditions on its survival in microcosm water.

Biochemical identification and numerical taxonomy of Aeromonas spp. isolated from environmental and clinical samples in Spain

AIMS

To study the phenotypic characteristics of Aeromonas spp. from environmental and clinical samples in Spain and to cluster these strains by numerical taxonomy.

METHODS AND RESULTS

A collection of 202 Aeromonas strains isolated from bivalve molluscs, water and clinical samples was tested for 64 phenotypic properties; 91% of these isolates were identified at species level. Aeromonas caviae was predominant in bivalve molluscs and Aerom. bestiarum in freshwater samples. Cluster analyses revealed eight different phena: three containing more than one DNA-DNA hybridization group but including strains that belong to the same phenospecies complex (Aerom. hydrophila, Aerom. sobria and Aerom. caviae), Aerom. encheleia, Aerom. trota and three containing unidentified Aeromonas strains isolated from bivalve molluscs.

CONCLUSIONS

Aeromonas spp. are widely distributed in environmental and clinical sources. A selection of 16 of the phenotypical tests chosen allowed the identification of most isolates (91%), although some strains remain unidentified, mainly isolates from bivalve molluscs, suggesting the presence of new Aeromonas species. Numerical taxonomy was not in total concordance with the identification of the studied strains.

SIGNIFICANCE AND IMPACT OF THE STUDY

Numerical taxonomy of Aeromonas strains isolated from different sources revealed the presence of potentially pathogenic Aeromonas spp., especially in bivalve molluscs, and phena with unidentified strains that suggest new Aeromonas species.

Phenotypic methods for speciating clinical Aeromonas isolates

AIMS

To establish the suitability of currently available phenotypic methods for speciation of clinical Aeromonas isolates in diagnostic microbiology laboratories.

METHODS

Using 62 Aeromonas spp, three schemes based on biochemical reactions were compared: a series of conventional tests; a system based on the suicide phenomenon, comprising two tubes in total; and a commercially available test, API 20 NE, augmented with a plate assay for beta haemolysin production. The whole cell and outer membrane protein (OMP) profiles of strains were examined by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS PAGE), according to the results of the above schemes, to determine the intra-species homogeneity.

RESULTS

Ninety per cent of strains were identified satisfactorily according to conventional criteria. For these strains, agreement was obtained using the suicide phenomenon and API schemes in 93% and 88% of cases, respectively. The three schemes concurred for 82% of strains. Whole cell protein profiles were unsuitable for comparing strains within a species. However, OMP patterns were similar for 89% of A caviae and 63% of A hydrophila.

CONCLUSION

Phenospeciation of clinical Aeromonas isolates by the scheme based on the suicide phenomenon is simple to perform and accurate, and suitable for use in the diagnostic laboratory. OMP profiles are potentially useful for confirming the identity of A caviae and most A hydrophila, but not A sobria.

Detection of Aeromonas salmonicida from fish by using polymerase chain reaction amplification of the virulence surface array protein gene

A DNA-based assay was developed to detect Aeromonas salmonicida from infected fish by analyzing tissues, feces, and the tank water in which the infected fish were held. This analysis was done both by direct detection from samples and after a bacterial outgrowth step. Polymerase chain reaction (PCR) amplification of a 421-bp sequence from the 3' region of the surface array protein gene (vapA) of A. salmonicida provided a specific and sensitive method for the detection and identification of this important fish pathogen. The sensitivity of PCR detection of A. salmonicida directly from tissues was less than 10 CFU/mg. Furthermore, a detection level of 5 fg, equivalent to approximately 1 cell, was obtained by using purified chromosomal DNA as the template. This highly reproducible assay, which requires 45 min to complete, is therefore sensitive enough to be used as a noninvasive method for monitoring fish populations for the presence of carrier fish. Because the surface protein array (A-layer) is a virulence factor of A. salmonicida, PCR analysis with oligonucleotide primers directed at vapA can also be used to provide information on the potential virulence of a strain.

Antigenic diversity of the S-layer proteins from pathogenic strains of Aeromonas hydrophila and Aeromonas veronii biotype sobria

The antigenic relatedness of paracrystalline surface array proteins with subunit molecular weights of approximately 52,000 from isolates of Aeromonas hydrophila and Aeromonas veronii biotype sobria belonging to a single heat-stable serogroup was examined. Enzyme-linked immunosorbent assay and immunoblotting with two different polyclonal antisera against surface exposed and non-surface-exposed epitopes of the S-layer protein from A. hydrophila TF7 showed that the S-layer proteins of the mesophilic aeromonads were antigenically diverse. NH2-terminal amino acid sequence analysis of four antigenically different proteins showed that while the proteins were structurally related, they differed in primary sequence. Absorption experiments with heterologous live cells showed that cross-reactive epitopes were in non-surface-exposed regions of the S-layer proteins, while absorption with homologous live cells showed that the immunodominant epitopes of the S-layer protein of strain TF7 were strain specific and exposed on the surface of the native, tetragonal array produced by this strain. Proteolytic digestion of the TF7 S-layer protein with trypsin, chymotrypsin, or endoproteinase Glu-C produced an amino-terminal peptide of approximate Mr 38,000 which was refractile to further proteolytic cleavage under nondenaturing conditions. This peptide carried the immunodominant surface-exposed region of the protein, and chemical cleavage with cyanogen bromide further mapped the portion of these surface-exposed epitopes to a peptide of approximate Mr 26,000, part of which maps within the Mr 38,000 protease-resistant NH2-terminal peptide.

Transient intestinal colonization by multiple phenotypes of Aeromonas species during the first week of life

The intestinal colonization rate of Aeromonas spp. was determined for 52 cesarean-born Peruvian neonates. Rectal swabs were obtained daily from newborns during their postdelivery hospitalization (mean = 5.5 days), and the gross appearances of their feces (blind determinations) were recorded. Aeromonas spp. were recovered from rectal swabs of 12 of 52 (23.1%) infants during their first week of life; the isolates were obtained from 5 of 9 (55.6%) infants with at least one stool with a watery consistency and from 7 of 43 (16.3%) neonates with no watery stools (P = 0.022). None of the infected infants became clinically ill. No other commonly recognized enteropathogens were detected in watery stools. An environmental survey indicated that hospital water was the probable source of infection. These and other data indicated that Aeromonas colonization occurs transiently at a very early age in Peruvian neonates and that in some instances, initial infection may be followed several days later by one or more watery stools of normal volume.