Selective culture medium for the enumeration of Lactobacillus plantarum in the presence of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus

Abstract

Selective enumeration of Lactobacillus plantarum in a mixed culture with yogurt bacteria is difficult, due to their similar growth conditions. We evaluated several culture media to determine their suitability for selective determination. The following media were tested to enumerate the strains of Lactobacillus plantarum, Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus: M17, MRS pH 5.2; 5.6 and 6.2, and modified MRS where glucose was replaced by maltose, sorbitol, mannitol, sucrose or galactose. Further, the addition of vancomycin (20 mg/l) and the use of different cultivation conditions – aerobic or anaerobic, and different cultivation temperatures 30 °C, 37 °C and 45 °C, were chosen to improve selectivity. MRS medium with vancomycin (pH 5.6), and cultivation under aerobic conditions at 30 °C was successfully used for selective enumeration of L. plantarum in a mixed culture with yogurt bacteria after cultivation both in broth and in milk. To distinguish between the remaining bacteria, M17 medium under aerobic conditions at 45 °C and MRS pH 5.2 under anaerobic conditions under 45 °C were recommended for counting of S. thermophilus and L. delbrueckii subsp. bulgaricus, respectively. No statistical differences were found between pour plate and surface plate methods of inoculation (P > 0.05).