Effect of penicillin and streptomycin on starter cultures used in the manufacture of pasteurized Palmita-type Venezuelan cheese

The effect of different concentrations of penicillin and streptomycin on the acid production of eleven strains of Enterococcus feacalis and seven strains of Lactobacillus casei used as starters in the manufacture of pasteurized Palmita-type cheese at a pilot plant scale, were studied. All microorganisms were cultured on reconstituted sterile skim milk free of antibiotics and containing known dilutions of penicillin from 0.008 to 1,000 I.U./ml and from 0.007 to 1 mg/ml of streptomycin. The activity of the cultures was measured in terms of titratable acidity after incubation at 35 degrees C for 18 h. A total of 18.2% of the E. faecalis cultures were resistant to concentrations up to 20 I.U./ml of penicillin, being inhibited by 40 I.U./ml, and 63.6% were resistant to concentrations up to 0.07 mg/ml of streptomycin, being inhibited by 0.1 mg/ml. A total of 28.6% of L. casei cultures were resistant to 10 I.U./ml of penicillin, and 42.9% were resistant to 0.05 mg/ml of streptomycin. Lactobacilli were inhibited at lower concentrations than enterococci for both antibiotics. However, some strains of enterococci and lactobacilli were very susceptible to penicillin and streptomycin, 5 I.U./ml and 0.5 I.U./ml of penicillin for enterococci and lactobacilli, respectively, and 0.5 mg/ml and 0.03 mg/ml of streptomycin for enterococci and lactobacilli, respectively. Variations among species and among strains of the same species were noted. The E. faecalis strains Ef8 and Ef9, and the L. casei strains, L1 and L3, are considered the most appropriate to be used as starters for Palmita-type cheese due to their resistance to the antibiotics studied in this work.