Investigation of mesophilic Aeromonas: response to hydrogen peroxide and role in false-positive Colilert reaction.
Landre, Julien B. P.
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Mesophilic Aeromonas are opportunistic human pathogens which produce a wide range of virulence factors and have been isolated from both untreated and chlorinated drinking waters. The presence of these microorganisms in the distribution systems suggests that Aeromonas could display an adaptive response to oxidant present during water treatment. This adaptive response of Aeromonas would lead to interference in analysis for faecal coliforms used to determine the quality of potable drinking water, and be a potential source of intestinal disorders. The Colilert defmed substrate technology system was developed as a one-step detection of both coliforms and E. coli while suppressing non-coliform heterotrophic growth. Aeromonas species were previously shown to cause production of falsepositive reaction at high cell densities (Edberg et ai., 1988). Similar results were obtained in our study when using fresh Colilert reagents. However, results obtained during this project showed Aeromonas to mediate false-positive reactions at low cell densities (101 cells/ml in presence of salt, 102 cells/ml in absence of salt) when using Colilert reagents within 4 weeks prior to shelf-life expiry. Increased incidence in falsepositive reactions mediated by Aeromonas were shown to be dependent upon the stability of the Colilert reagent affected with age. Such Aeromonas interference would lead to over-estimation of coliforms and E. coli in potable drinking water supplies. The ability of bacteria to adapt to a wide range of stress factors such as pH, heat shock, oxidants or starvation has been extensively studied. Little is known about the response of Aeromonas to such stress conditions. During this project, it has been demonstrated that mesophilic Aeromonas display an adaptive tolerance response to a lethal oxidative challenge through pre-treatment with a sub-lethal dose of oxidant. The stress adaptation process was demonstrated to occur through synthesis of stress proteins and modulation of pre-existing catalase. Of the species studied, A. sobria was most sensitive, whereas A. caviae and A. hydrophila displayed similar responses to oxidative stress. The hypersensitivity of A. sobria did not impair the adaptive response of the organism. During our investigations, stationary phase Aeromonas cells have been shown to be more resistant than their logarithmic counterpart and suggested that excreted molecules may playa role in protecting the cells. Re-suspension of fresh cells into spent medium from a stationary phase cells revealed a higher resistance of these cells compared to those re-suspended in minimal medium. This resistance was demonstrated to be mediated by non-proteinaceous, thermo-sensitive effector molecule. A potential candidate as the effector molecule, butyryl homo serine lactone, was synthesised and assayed. Preliminary data strongly suggest that this molecule has a role to play in the stress adaptation phenomenon and might be involved in stimulating synthesis of key stress proteins.