Molecular characterization of Lactococcus lactis subspecies in milk and milk products and studying their inhibitory effects on some aerobic and anaerobic bacteria

Background: Lactococcus lactis are Gram-positive ovoid lactic acid bacteria (LAB) widely used as bio-preservatives in milk and dairy products. Objectives: To isolate and identify Lactococcus lactis from milk and dairy products collected in Assiut City over a six-month period (January–June 2024), and to assess the inhibitory effects of its subspecies against selected resistant aerobic and anaerobic bacteria. Methods: A cross-sectional study was conducted on 200 random samples (80 raw milk, 60 yoghurts, 60 cheese) cultured on M17 agar. Biochemical identification involved xylose fermentation and growth in 4% and 6.5% NaCl, while PCR confirmed identification genotypically. Antimicrobial activity was tested using agar well diffusion method. Results: Lc. lactis was detected in 67.5% of milk, 41.7% of cheese, and 33.3% of yoghurt samples. PCR confirmed a 29% overall prevalence: 28.5% Lc. lactis subsp. lactis and 0.5% Lc. lactis subsp. cremoris. The highest frequency of Lc. lactis subsp. lactis occurred in milk. Lc. lactis Subsp. lactis showed strong inhibition of MRSA, CoNS, and C. perfringens but not Gram-negative bacteria, whereas Lc. lactis subsp. cremoris inhibited MRSA and CoNS only. Conclusion: Lactococcus lactis are prevalent in milk and dairy products in Assiut City and exhibit notable antibacterial activity, supporting their potential application as natural bio-preservatives in the dairy industry.