Comparison of selective media for the enumeration and identification of fecal streptococci from natural sources

Five media and procedures were compared for their recovery of fecal streptococci on membrane filters. Growth and numbers of a variety of pure cultures were compared, and recoveries from raw sewage and sewage effluent were determined on the five media. A total of 512 isolates were identified and the specificity of each medium was determined from these results. While KE agar recovered significantly higher numbers of organisms, only 81% of the positive colonies were fecal streptococci. Slanetz and Bartley (SB) medium showed similarly low specificity and the mE procedure gave low recoveries. Best overall results were obtained on mEnterococcus (mEnt) agar with high (91%) specificity for enterococci, and very good recovery efficiency with low background counts. This medium was also the only one demonstrating any appreciable recovery of other fecal streptococci with 93% of the background colonies being confirmed as fecal streptococci which most were Streptococcus bovis, S. equinus, or S. salivarius. Pfizer selective enterococcus (PSE) agar was second in overall performance with a high (94%) rate of specificity but this medium had lower recovery efficiency and high background counts. Performance of PSE agar could be considerably improved, without impairing specificity, by using a total count disregarding esculin hydrolysis.