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Selective Plating Medium for Leuconostoc in Mixed Lactic Cultures

Dairy Products Laboratory, Eastern Utilization Research and Development Division, USDA, Washington, D. C.

ABSTRACT

Thirty-six strains of starter organisms (Leuconostoc and Streptococcus) were tested for their susceptibility to various antibiotics and sulfa drugs. Tetracycline hydrochloride was found to have less inhibitory effect on leuconostocs than on streptococci. Consequently, a simple selective plating medium containing tetracycline (0.15 µg/ml) in a tomato juice agar base was developed. Five experimentally mixed starters of known composition and seven commercial multiple-strain lactic starters were used to evaluate the effectiveness of the medium. The mixed starters were plated in control and in the experimental agar and incubated at 30 C for up to 72 hr. Representative colonies were isolated from the tetracycline agar after 48 hr and after 72 hr and were identified by growth in litmus milk and by biochemical tests. Results showed an incubation period of 48 hr to be optimum for best selectivity of leuconostocs, because a few strains of Streptococcus lactis were not inhibited on further incubation.