WRRCTR No.168 Recovery of False Positive Fecal Streptococcus on KF Agar from Marine Recreational Waters

Abstract

Fecal streptococcus (FS) is the most often used alternative to fecal coliforms to assess the quality of recreational water. Because the reliability of KF agar to recover FS bacteria has been reported to approach 100%, this medium was used to test the water quality of Hanauma Bay which was suspected as the source of disease transmission to a group of swimmers using this beach park. Marine water samples from Hanauma Bay, Oahu, were characterized by low concentrations of fecal coliforms and Clostridium perfringens but unusually high concentrations of presumptive FS when KF agar was used. Most of the presumptive FS colonies on KF agar could not be verified and was therefore concluded to be “false positive”. At least two types of catalase-positive bacteria were determined to be responsible for the formation of false-positive colonies on KF agar: one, a gram-positive coccus; the other, a gram-negative, NaCl-requiring bacillus. These false-positive, FS-like bacteria were present in marine recreational waters obtained from 15 other sites, although at lower concentrations than in Hanauma Bay. The presumptive FS counts on KF agar ranged from 53 to 1205/100 ml for the 15 marine sites. In contrast, less than 20 presumptive enterococcus (EC) colonies were recovered and readily confirmed as true enterococci when mEnterococcus agar was used to assay these samples. Thus in tropical climates such as Hawaii, KF agar and its recommended technique should not be used to assay marine waters for FS. Three modifications in the KF agar technique could however prevent the bacteria present in marine waters from producing false-positive FS colonies on KF agar: delete NaCl from KF agar, increase sodium azide concentrations in KF agar, or incubate KF agar anaerobically rather than aerobically.