Enchanced accuracy of coliform testing in seawater by a modification of the most-probable-number method

A 1-year study of marine water sample from six beach locations showed that the most-probable-number method failed to recover significant numbers of coli-forms. Modifying this method by transferring, after 48 h, presumptive negatives (growth and no gas production) to confirmed and fecal coliform media significantly improved recovery. Tests which were presumptive negative but confirmed as fecal coliform positive were designated as false negatives. Most-probable-number method false negatives occurred throughout the year, with 143 of 270 samples collected producing false negatives. More than 50% of fecal coliform false-negative isolates were Escherichia coli. Inclusion of false-negative tubes into the coliform most-probable-number method data resulted in increased violation of the California ocean water contact sports standard at all sites. More than 20% of the samples collected were in violation of this standard. These data indicate that modification of the most-probable-number method increases detection of coliform numbers in the marine environment.