Using nitrous acid-modified de Man, Rogosa, and Sharpe medium to selectively isolate and culture lactic acid bacteria from dairy foods

Abstract

Nitrous acid was used to modify traditional de Man, Rogosa, Sharpe medium to evaluate whether the addition of sodium nitrite to MRS medium could improve the rate of growth and density of various lactic acid bacteria and nontarget species. Yogurt and Cheddar cheese were inoculated with individual bacterial species followed by the recovery and enumeration of the species using the pour plate method to compare the sensitivity between nitrous acid-modified MRS (mMRS) and traditional MRS. Lactobacillus delbrueckii ssp. bulgaricus were recovered at significantly higher counts from cheese in nitrous acid mMRS than MRS, whereas no significant difference was observed for other species and food systems. Growth curves were also generated for multiple lactic acid bacteria as well as nonstarters in both mMRS and MRS to measure the selectivity of nitrous acid mMRS. The selectivity evaluation of nitrous acid mMRS demonstrated that 5 of the tested lactic acid bacterial species (Bifidobacterium longum, Streptococcus salivarius, Lactococcus lactis, Lactobacillus acidophilus, and Lactobacillus delbrueckii ssp. bulgaricus) grew to significantly higher densities more rapidly in mMRS broth than in traditional MRS. Nontarget bacteria Enterococcus faecalis and Bacillus cereus revealed a more prolific growth rate and higher optical density readings in traditional MRS compared with mMRS. It was determined that nitrous acid mMRS is a viable alternative medium for culturing selected lactic acid bacteria, and offers an improved formulation of MRS for use in standard evaluation methods and optimization of probiotic and other dairy cultures.