LACTIC STREAK AGAR

The most common microorganisms used in the dairy industry as starter cultures are lactic streptococci. Lactococcus lactis and its subspecies cremoris and diacetylactis belong to this group. These can be differentiated by biochemical tests, the major criteria being arginine hydrolysis and test for diacetyl and acetoin. Reddy’s Differential Agar, Modified (Lactic Streak Agar) recommended for the qualitative and quantitative differentiation of lactic streptococci was originally described by Reddy et al and further modified by Mullan and Walker. This medium is recommended by APHA for the differential enumeration of lactic streptococci.
Lactose fermenters produce acid and form yellow colonies. Lactococcus lactis initially produces acid but later on turns to violet-purple colour due to the release of ammonia from arginine. Lactococcus lactis subspecies diacetylactis produces a more intense purple colour than Lactococcus lactis. The former utilizes the suspended calcium citrate and the citrate degrading colonies exhibit clear zones against a turbid background.