Differential enumeration of Bifidobacterium longum and Lactobacillus acidophilus in the presence of Streptococcus thermophilus

Modified (T and M-MRS) and selective (Bile and LP-MRS) media were assayed to quantify Bifidobacterium longum (BL) and Lactobacillus acidophilus (LAC) in the presence of Streptococcus thermophilus (Str). Microbial suspensions (isolated and mixed) were tested at 37degrees/72 h aerobically with T-MRS, M-MRS and Bile-MRS; all tests using LP-MRS were done anaerobically. LP-MRS was tested under 4 different combinations of lithium chloride and sodium propionate concentrations (0.2; 0.4; 0.5 and 0.6% of LiCl with respectively 0.3; 0.6; 0.75 and 0.9% of sodium propionate). The ability of each medium to inhibit the organisms was also tested. M-MRS allowed the growth of Str. Differential counting of LAC was possible using T and Bile-MRS, aerobically, (the latter condition being more practical and economical). Differential counting of BL was possible using LP-MRS in anaerobiosis, with a combination of 0.6% of LiCl and 0.9% of sodium propionate.