Gentamicin-thallous-carbonate medium for isolation of fecal streptococci from foods.

Gentamicin-thallous-carbonate (GTC) agar was formulated by Donnelly and Hartman (Appl. Environ. Microbiol. 35:576-581, 1978) to select for fecal streptococci in sewage and water samples. The present study was conducted to determine the usefulness of GTC agar for the enumeration of fecal streptococci in foods. Comparisons were made with KF streptococcal (KF), Pfizer selective enterococcus (PSE), and thallous acetate (TA) agars. Samples of ground beef pork sausage, frozen broccoli, frozen fish, and ice cream were examined. Presumptive streptococcal counts obtained on GTC agar were significantly higher than those obtained on KF and PSE agars and were comparable to those obtained on TA agar. GTC was more sensitive than KF or PSE agars primarily because of the recovery of greater numbers of Streptococcus bovis and Streptococcus equinus strains. Percentages of confirmed fecal streptococci obtained on GTC, KF, PSE, and TA agars were 70, 95, 80, and 74, respectively. Differences between these percentages were not statistically significant, but they indicated that selectivity of GTC agar could be improved. Advantages of using GTC agar to isolate fecal streptococci from foods include a short incubation time (16 to 18 h) and large, distinct colonies that facilitate rapid enumeration and subsequent confirmation.