Isolation of mutants ofLactobacillus bacterias, grown in skim milk, that resist fatty acid inhibition

The effect of saturated fatty acids from 6∶0 to 16∶0 and oleic acid was determined onLactobacillus leichmannii growing in skim milk. The growth of this strain was markedly inhibited by fatty acids from 8∶0 to 12∶0 but not by straight chain fatty acids greater than 13∶0 or less than 7∶0 and oleate. Laurate was the fatty acid with the highest bactericidal effect. Similar results, with little changes depending on strains, were obtained withL. casei, L. plantarum, L. bulgaricus, L. lactis, L. helveticus. Mutants from theseLactobacillus organisms, resistant to fatty acid inhibition, were isolated by a recycling selection procedure. These mutants exhibited high levels of oxidation for laurate. The presence of 2 mM of this compound in the skim milk culture increased the fatty acid oxidation activity four to ten times higher than was exhibited by the parent strains. The practical implications of these observations are discussed.