Extended Incubation of LST and BGB Tubes and the MPN Estimates of Coliforms

Abstract

Extending the incubation time past the standard 48 h for presumptive lauryl sulfate tryptose (LST) broth and confirmed brilliant green bile (BGB) broth tests for coliforms resulted in increases in most probable number (MPN) estimates. Whereas 40% of the samples showed an increase in the presumptive MPN when the incubation was extended from 48 to 72 h, only 5% showed confirmed MPNs, which exceeded the 95% confidence limits established for the 48-h confirmed MPNs. Extending the incubation of BGB tubes to 72 h resulted in less than 5% of the samples exhibiting increased MPNs, which exceeded the 48-h 95% confidence limits. Some loss in viability of coliforms was observed when LST tubes were incubated beyond 72 h. The study demonstrates the importance of adhering to standard incubation times for interlaboratory comparisons and ensuring regulatory compliance.