DIVERSITY, PERSISTENCE, AND VIRULENCE OF AEROMONAS STRAINS ISOLATED FROM DRINKING-WATER DISTRIBUTION-SYSTEMS IN SWEDEN

The Aeromonas populations in 13 Swedish drinking water distribution sy stems, representing different treatments, were investigated, From each system, water samples were collected four times during the period fro m May to September 1994 from raw water and water after treatment and a t two to five sites within the distribution system, In total, 220 wate r samples were collected. From samples containing presumptive Aeromona s, up to 32 colonies were analyzed by the PhenePlate Aeromonas (PhP-AE ) system, which is a highly discriminating biochemical fingerprinting method. Selected isolates from different phenotypes (PhP types) were f urther identified by the API 20 NE system and by gas-liquid chromatogr aphy analysis of fatty acid methyl esters (FAMEs). Selected isolates w ere also assayed for their potential to produce hemolysin and cytotoxi n and for their ability to adhere to human intestinal cells. In total, 117 water samples (53%) contained presumptive Aeromonas which numbere d up to 10(6) CFU/100 mi in raw water and up to 750 CFU/100 mi in tap water. Among the 2,117 isolates that were subjected to typing by the P hP-AE system, more than 300 distinct PhP types were found, of which th e majority occurred only sporadically. Raw (surface) water samples usu ally contained many different PhP types, showing high diversity indice s (Di) (median Di = 0.95). The Aeromonas populations in samples collec ted from within the distribution systems were less diverse (median Di = 0.58) and were often dominated by one major PhP type that was found on several sampling occasions, Seventeen such major PhP types could be found and were represented in 1,037 isolates (49%). Identification by API 20 NE and FAME analysis revealed that most of the major PhP types were Aeromonas hydrophila or belonged to unidentified Aeromonas speci es, Hemolysin and cytotoxin production was observed in most major PhP types (representing 87 and 54% of the assayed isolates, respectively), and adherence was found in 89% of the isolates that produced cytotoxi n, Thus, the data presented here show that although raw water may cont ain very diverse Aeromonas populations, the populations seemed to be r emarkably stable within the studied water distribution systems, and th at some potentially pathogenic Aeromonas strains could persist for sev eral months in drinking water.