SOME FACTORS WHICH GOVERN PLAQUE FORMATION BY LACTIC STREPTOCOCCAL BACTERIOPHAGE.

Abstract

Summary A study was conducted of factors which may influence the accuracy of the plaque count obtained by the double-agar layer method as applied to lactic streptococcal bacteriophage. Host cell cultural media were skimmilk or hydrolyzed milk protein broth (Sobol's). Streptococcus lactis strains ML3, C10, C2 and Streptococcus cremoris strains C3, KH, R1, US3 and the homologous bacteriophages were used. Plaque formation on Trypticase Soy Agar was generally enhanced by: (a) modification of the cultural media or the seed layer agar by the addition of CaCl2, phenylalanine, proline, riboflavin, niacin, calcium pantothenate, and biotin; (b) using a six-hr culture of host cells rather than cells incubated for 18 or 24 hr; and (c) selection of the proper serial transfer system for reactivating the host cells from a frozen or lyophilized state. Sobol's broth yielded lower plaque counts than skimmilk irrespective of the modifications used. Responses to plaque enhancement factors were influenced by species and strain differences of S. lactis and S. cremoris. In general, strains of S. cremoris had a more exacting requirement for calcium, media enrichment, and length of incubation of host cells than S. lactis strains.