Sheen formation and growth response of groundwater bacteria to reduced oxygen concentrations during incubation of M-Endo medium.

In vitro pure-culture studies were conducted to assess growth and sheen formation of groundwater bacteria on M-Endo medium incubated under reduced oxygen concentrations (0, 4, 8, 12, and 16%). Coliform and noncoliform bacteria were isolated from 17 untreated, rural groundwater supplies on M-Endo medium. All 16 coliform isolates tested were capable of sheen formation at oxygen concentrations of 4% or greater, yet some of these same isolates (Enterobacter aerogenes, Enterobacter cloacae, and Hafnia alvei) were either unable to grow or failed to produce a metallic sheen when incubated under strict anaerobiosis. Approximately 70% of the 21 noncoliform isolates examined exhibited growth inhibition at oxygen concentrations of 8% or less. The growth of a false-positive coliform isolate of Serratia fonticola was inhibited when incubated under reduced oxygen concentrations of 16% or less. Our findings suggest that the selectivity of M-Endo medium, and resultant inhibition of noncoliforms and false-positive coliforms, is enhanced by incubation in the absence of oxygen. However, the failure of strict anaerobiosis to permit detection of total coliforms such as Hafnia and Enterobacter spp. may compromise the reliability of this technique for evaluating the sanitary quality of some waters. On the other hand, oxygen concentrations of 4, 8, 12, and 16% permitted adequate sheen development of all coliforms tested while inhibiting some noncoliforms.