Effect of Penicillin Resistance on the Ability of Some Fermented Milk Starter Bacteria to Develope Titration Acidity and Proteolysis in Milk

Abstract

The aim of this study was to develop penicillin resistance of yoghurt starter bacteria (Lactobacillus delbrueckii spp. bulgaricus and Streptococcus salivarius ssp. thermophilus) by isolating the resistant bacteria from agar plates containing gradual concentrations of penicillin using fluctuation test. Minimum inhibitory concentration (MIC) was used for measurement of resistance level. The effect of penicillin resistance on the ability of bacteria to develop acidity and enhance proteolysis of milk was also studied. Lactobacillus bulgaricus (Lac) was highly sensitive to penicillin (MIC = 0.04 IU/ml). Its resistance was developed to be 1.00 IU/ml. While, Streptococcus thermophilus (Str) was less sensitive to penicillin (MIC = 0.10 IU/ml). Its resistance was developed to be 1.40 IU/ml. Penicillin resistance had no effect on the ability of bacteria to develop titration acidity in penicillin – free milk. Total titration acidity (as % lactic acid) of Lac – and Str – single starters were 1.22 and 0.48%, respectively. Mixed starters (Lac: Str in the ratio of 1: 1) achieved 1.02 – 1.08% after incubation of milk for 120 min in 40C°. When penicillin (0.05 IU/ml) – containing milk was used, sensitive starters failed to develop acidity to acceptable levels. Mixed starters composed of penicillin resistant bacteria elevate milk acidity to 0.91%. Penicillin resistance improved proteolysis (measured as % soluble tyrosine) in milk treated with yoghurt starter. The highest degree of proteolysis (48 Mg soluble tyrosine/ml) was recorded for milk inoculated with mixed starters having penicillin resistant Lac. The degree of proteolysis was decreased in the presence of penicillin in milk, but mixed starters having penicillin resistant Lac still superior.