Objective To explore patterns and variations of antibiotic resistance of lactic acid bacteria isolated from yoghurt after serial subcultivation with antibiotics at various concentrations.

Methods Previously isolated strains of lactic acid bacteria (Lactobacillus bulgaricus L. bulgaricus and Streptococcus thermophilus S. thermophilus) were cultivated under the different concentration of antibiotics. Then, the sensitivity to the antibiotics of the strains were evaluated by detecting inhibition zone diameters.

Results Both stains of L. bulgaricus and S. thermophilus exhibited resistance to antibiotics after incubation with low concentration (1.0 × 10 ^–4 – 1.0 × 10 ^–10 mg/mL) of ampicillin, gentamicin sulfate and chloramphenicol. The diameter of the inhibition zone decreased significantly with the increment of serial subcultivation for the strain of L. bulgaricus incubated with 1.0 × 10 ^–2 mg/mL ampicillin and the strain of S. thermophilus with 1.0 × 10 ^–6 mg/mL ampicillin (r_{(L. b)} = – 0.84, r_{(S. t)} = – 0.98, both P < 0.05). However, for both L. bulgaricus and S. thermophilus incubated with low concentration of ampicillin sodium, the diameter of the inhibition zone increased with the increment of serial subcultivation (r_{(L. b)} = 0.96, r_{(S. t)} = 0.82, P < 0.05). While, the diameter of the inhibition zone demonstrated no significant variation with the increment of serial subcultivation for the strains of L. bulgaricus and S. thermophilus incubated with gentamicin sulfate and chloramphenicol (both P > 0.05).

Conclusion Long term exposed to low concentration antibiotics will lead to antibiotic resistance in L. bulgaricus and S. thermophilus. Therefore, monitoring on antibiotic residues in raw milk and antibiotic resistance of lactic acid bacteria should be strengthened in the production of yoghurt.