The motile mesophilic Aeromonas are ubiquitous to a wide variety of aquatic environments including drinking water distribution systems. Concern over the presence of mesophilic Aeromonas in public drinking water supplies has been expressed in recent years as it has been regarded as a pathogenic organism of importance in gastroenteritis. A major drinking water distribution system in north-east Scotland was monitored over a 12 month period to determine the prevalence of mesophilic Aeromonas. These data were examined in relation to chlorine concentration, pH, temperature, rainfall and the standard bacteriological indicators of water quality. Aeromonas were isolated to varying degrees from 21 of the 31 reservoirs investigated. The maximum recovery observed during the study was 605 cfu in 300 ml. The probability of isolation generally decreased with increasing levels of chlorination, although this oxidant was found to be ineffective in many reservoirs. Certain reservoirs with poor chlorination profiles yielded very few isolates, whereas some highly chlorinated sites liberated Aeromonas frequently and in relatively high numbers. A seasonal pattern in the incidence of Aeromonas emerged with infrequent isolation during the winter period increasing to a peak during the summer, with most isolates recovered when water temperature was > 12 degrees C. An association was demonstrated between the pattern of Aeromonas isolations and that of rainfall. No relationship was apparent between incidence of Aeromonas and total heterotrophic plate counts.

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 pro®le where pre-exposure of a population to sub-lethal levels of H 2 O 2 stimulates onset of tolerance to further exposure. Acquisition of tolerance critically requires nascent protein synthesis. Further analysis demonstrated population growth phase in¯uences the degree of sensitivity of the organism.Late stationary phase cultures demonstrate a decreased sensitivity compared with younger populations. Signi®cantly, it was also determined that stock culture age in¯uenced the level of sensitivity of the derived experimental culture, where an increased stock culture age corresponded with enhanced resistance to H 2 O 2 . These data show that Aer. hydrophila population phenotype is in¯uenced by the phenotype of the donor stock culture.