Prevalence of Antibiotic Resistance in Streptococcus Thermophilus and Lactobacillus Strains from Dahi, a Traditional Indian Fermented Milk

ABSTRACT This study examined antimicrobial resistance (AMR) in Streptococcus thermophilus and Lactobacillus spp. isolated from traditional Indian Dahi. From 96 traditional Dahi samples, 104 S. thermophilus and 100 Lactobacillus isolates were tested for antibiotic susceptibility. S. thermophilus showed predominant resistance to nalidixic acid and oxacillin, with additional resistance to clindamycin, trimethoprim, ofloxacin, co-trimoxazole, erythromycin, cefuroxime, and cefepime. Lactobacillus isolates were highly resistant to nalidixic acid (97%), norfloxacin, and ofloxacin, followed by trimethoprim, co-trimoxazole, teicoplanin, oxacillin, amikacin, and vancomycin. Multiple antibiotic resistance index ranged from 0.07-0.38 in S. thermophilus and 0.07-0.45 in Lactobacillus. Thirteen S. thermophilus isolates were extended-spectrum β-lactamase producers harboring blaTEM, with high MIC cutoffs against cefepime and cefotaxime. Macrolide resistance genes were found in S. thermophilus, aminoglycoside resistance genes in Lactobacillus, and the multidrug transporter gene (emeA) in 16 S. thermophilus strains. No mobile genetic elements were detected. These findings highlight AMR in traditional fermented milk bacteria, underscoring food-chain risks.