Appraisal of media and methods for assay of bacteriophages of lactic streptococci.

To assess the relative merits of tryptone yeast extract agar, the same medium unbuffered, and medium M17 for the assay of nine bacteriophages of lactic streptococci, comparative plaque counts were made with an overlay of 3 or 9 ml. Four of the phages exhibited no significant difference in plating efficiency between media. The effect of overlay volume varied from strain to strain and was different for different media. The 3-ml overlay created suboptimal atmospheric conditions for those strains which had a special requirement for CO(2). The use of a 9-ml overlay obviated the need to incubate plates under CO(2) and overcame the problems related to special calcium requirements when tryptone yeast extract agar was used. The organic buffer (disodium beta-glycerophosphate) was inhibitory to Streptococcus cremoris ML1 and showed no advantage over the inorganic phosphate buffer (K(2)HPO(4)) for most other strains.