EFFECT OF PENCILLIN ON FREE OR IMMOBILIZED LACTOCOCCI: MILK ACIDIFICATION AND RESIDUAL ANTIBIOTIC LEVEL

Five lactic acid cultures were added to milk containing penicillin, and the effect of immobilization on acidification and residual penicillin concentration were determined. Cells of Lactococcus lactis subsp . cremoris CRA-1 immobilized in calcium alginate beads were less sensitive to penicillin than free cells, and inhibition occurred later in the fermentation. The lactococci were less sensitive to penicillin when high cell densities were inoculated (10⁹ CFU/mL). There was a significant reduction of penicillin (up to 0.3 IU/mL) when milk was inoculated with high populations of Lac. cremoris and incubated at 30C for 6 h. The drop in penicillin concentration was not related to adsorption to the alginate or to the presence of β-lactamase on the cultures. When free cells of Lac. cremoris, Streptococcus salivarius subsp . thermophilus or Lactobacillus delbrueckii subsp . bulgaricus were added to milk containing antibiotics, and incubated at 4 or 10C for 48 h, significant reduction of penicillin levels did not occur in the contaminated milk. Milks containing more than 4 × 10⁶ lactococci CFU/mL can give false positive results in the antibiotic disk assay using Bacillus subtilis.