Recovery of coliform bacteria from rural groundwater supplies using reduced oxygen concentrations during incubation

The effect of decreased oxygen concentration during incubation of M-Endo medium on detection of coliforms from rural groundwater supplies was examined. Incubation oxygen concentrations of 0¿(anaerobic GasPak), 4, 8, and 21% (atmospheric) were examined. Our findings point to several advantages of using anaerobic incubation for the isolation of coliforms: (i) higher verification rates with concomitant decreases in occurrence of false-positive coliforms; (ii) overall reduction in growth of nonsheen colonies; and (iii) reduction in colony size for nonsheen organisms, thereby minimizing crowding effects and facilitating enumeration of coliform colonies. However, these advantages were not sufficient to permit increased recovery of total coliforms as compared with aerobic incubation. In addition, the increased frequency of detecting false-negative coliforms during anaerobic incubation is a disadvantage to this method. While detection of total coliforms was reduced under conditions of anaerobiosis,the detection of fecal coliforms and (or) E. coli was not impeded.