Differentiation of Fast and Slow Milk-Coagulating Isolates in Strains of Lactic Streptococci

Abstract

Differentiation of fast and slow milk-coagulating lactic streptococci was achieved on a modified glycerophosphate milk agar. The new medium contained 0.1% litmus, which improved colony visualization and provided a qualitative measure of acid production. Aerobic incubation of strains on this milk-based medium led to autoinhibition in many strains because of accumulation of hydrogen peroxide. This effect was minimized by addition of catalase or pyruvate. Incubated anaerobically at 30.degree. C, this medium allowed recognition of fast milk-coagulating isolates in 28 single strains and 7 commercial frozen cheese starter concentrates, and differentiation of both fast and slow isolates in strains containing both cell types. Genetically characterized slow isolates could be placed into 1 of 2 distinct groups by colony appearance. The 1st group fermented lactose but was proteinase-negative, and the 2nd group included lactose-negative colonies that were either proteinase positive or negative.