Gentamicin-Based Medium for the Isolation of Group D Streptococci and Application of the Medium to Water Analysis †

L. Scott Donnelly, Paul A. Hartman
1978 Applied and Environmental Microbiology  
Gentamicin-thallous-carbonate (GTC) medium contained (per liter): 40.0 g of Trypticase soy agar, 5.0 g of KH2PO4, 2.0 g of NaHCO2, 1.0 g of glucose, 1.0 g of esculin, 0.5 g of thallous acetate (TA), 0.5 g of ferric citrate, 0.75 ml of Tween 80, and 2.5 mg of gentamicin sulfate. The NaHCO3 (20 ml of a 10% solution that had been heated to boiling) was added after sterilization of the basal medium. The spread plate technique was used to compare GTC agar with Pfizer selective enterococcus, TA, and
more » ... erococcus, TA, and KF agars by using sewage as well as bovine and swine fecal samples. Significantly greater numbers of group D streptococci were recovered on GTC agar than on Pfizer selective enterococcus or KF agars, within and over all samples. Higher counts also were obtained on GTC than on TA agar, but the differences were not statistically significant. The percentage of false positives was about the same for all four media. Samples of riverwater also were plated on GTC, TA, and KF agars, and significantly higher recoveries were obtained with GTC agar. GTC agar was superior to the other media examined primarily because of increased recoveries of Streptococcus bovis and S. equinus; other advantages of GTC agar were large colony size and short (24-h) incubation period. The percentage of false positives from riverwater was 13% for GTC agar and 0% for TA and KF agars; therefore, confirmation would be necessary when GTC agar is used with some types of environmental samples. S. avium A and B were obtained from Robert H. Deibel, Department of Bacteriology, University of Wisconsin, Madison. Stock cultures were kept on Trypticase soy agar (TSA) plates stored at 4°C in plastic vegetable crispers. Working cultures were made 576 on May 5, 2020 by guest Downloaded from on May 5, 2020 by guest Downloaded from
doi:10.1128/aem.35.3.576-581.1978 fatcat:gbvbu2nwcjh7ldtgs4br7dzygq