Enumeration of<i>Streptococcus faecalis</i>, with particular reference to polluted waters

Abstract

Factors affecting the growth of Streptococcus faecalis on glucose-yeast extract-sodium azide agar have been studied. Both the high temperature of incubation used (45° C.) and the presence of azide reduced the proportion of cells able to form colonies, the inhibitory action being much more marked with cultures which had become attenuated, either through age or through prolonged immersion in water, than with comparatively young and vigorous cultures. This inhibitory action was found to be largely overcome if the inoculum was subjected to a preliminary period of ‘resuscitation’, by incubating it with double-strength glucose broth before adding the azide-agar portion of the medium and allowing the mixture to set. Neutral red was so inhibitory to some strains of Str. faecalis that it could not be included in the medium. Phosphate, as the potassium salt at a concentration of 0.7%, if autoclaved with the remaining constituents of the medium, exerted a depressing effect on the counts. Added separately it showed no inhibitory action. The spinning-bottle technique (Allen et al. 1952) was adapted for Str. faecalis . When used for samples containing a mixed flora the method, described in the Appendix, permitted the growth only of Str. faecalis . This paper is published by permission of the Department of Scientific and Industrial Research.