ANTIBIOTIC RESISTANCE OF LACTIC ACID BACTERIA: EFFECTS ON YOGURT FERMENTATION

Antibiotic residues (ARs) in milk are a growing concern for food safety and public health worldwide. Beyond their potential adverse effects on human health, the presence of ARs can inhibit the activity of starter cultures in dairy fermentation processes, such as yogurt production, resulting in significant economic losses for the industry. The present study aimed to evaluate the resistance of two commercial lactic cultures to three classes of antibiotics commonly detected in milk. A completely randomized 2×3×2 factorial design was conducted, testing two commercial cultures (LYOFAST - Y 450 B and SACCO, with Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus), three antibiotics (tetracycline, sulfamethazine, and penicillin), and two concentrations (Maximum Residue Limit - C1 and twice the MRL- C2), plus a negative control (without antibiotics). The experiment was performed in triplicate. UHT milk was heated to 45°C, inoculated with the cultures, and incubated at 45°C. The pH of the samples was monitored every 20 minutes until reaching the isoelectric point of casein (pH 4.6). The monitoring time ranged from 180 to 380 minutes, reflecting distinct acidification patterns among treatments. All treatments reached the coagulation pH (~4.6) required for yogurt production, while controls exhibited typical fermentation kinetics, reaching pH 4.6 in approximately 180 minutes. No statistically significant differences were observed between the cultures used (p > 0.05), both demonstrating resistance to the three antibiotics tested at the evaluated concentrations. Among the antibiotics tested, tetracycline showed the greatest inhibitory effect, with concentration C2 significantly prolonging fermentation time, especially in culture LYOFAST - Y 450 B, which continued acidifying until 380 minutes. Sulfamethazine showed a dose-dependent effect, with greater inhibition at the higher concentration (C2) for both cultures. Penicillin had minimal impact, with a pH profile similar to that of the control. These results confirm the intrinsic and acquired resistance of lactic acid bacteria to the tested antibiotics. However, despite this resistance, some antibiotics may delay the fermentation process. This variability in resistance constitutes a crucial factor to be considered in the selection of starter cultures by the dairy industry to minimize the impact of antibiotic residues and ensure the efficiency of fermentation processes.