Isolation and characterization of coliforms : opportunistic pathogens and standard plate count bacteria from groundwater

Abstract

The bacterial flora of groundwater obtained from wells and distribution sites was investigated from both an ecological and a public health perspective. A majority of the isolates were oxidase positive, non-fermentative, gramnegative bacilli. Extensive heterogeneity of groundwater microflora, as determined by biochemical characterization and antibiograms, was observed both within and among welldistribution (N-D) systems. Of the unique isolates (sorts) found in the W-D systems, 75% were resistant to 2 or more antibiotics at clinically significant concentrations. Community diversity within W-D systems was evaluated by rarefaction which failed to reveal a general trend. Standard plate counts in 2 of 3 wells were significantly higher on Standard Methods Agar diluted ten-fold than on the same medium at the standard concentration. Pseudomonas aeruginosa, Flavobacterium sp., Aeromonas hydrophila and Yersinia enterocolitica were detected in 21, 18, 7 and 3% respectively of water samples analyzed over a 12 month period. A selective medium was developed for the isolation of Flavobacterium sp. and was effective in suppressing 98% of the background flora when used in a membrane filtration (MF) procedure. Yersinia Selective Agar was employed in an MF procedure for the isolation of Y. enterocolitica. The use of anaerobic incubation in this procedure effectively suppressed background growth both in the presence and absence of an antimicrobic supplement. Anaerobic incubation of m-endo LES Agar (AN-MF) was effective in suppressing non-coliform growth in the total coliform MF test and markedly reduced the frequency of overgrown plates. The AN-MF appeared to obviate the need for selective chemical agents and thus has potential value in the isolation of stressed coliforms.