Isolation, identification, and molecular characterization of multidrug-resistant salmonella spp. from eggs and mayonnaise in Bangladesh: Implications for food safety

• Overall Salmonella spp. prevalence was 19.3%, with 18.3% in eggs and 23.3% in mayonnaise • All isolates were MDR (100%), with highest resistance to azithromycin, tetracycline, colistin • Biofilm formation was observed in all isolates with 45% strong and 55% moderate producers. • Isolates showed strong stress tolerance at pH 3.5, pH 1.5, and bile 2–4% for survival. • MDR, biofilm-forming, and stress-tolerant isolates pose a serious public health risk. Salmonella spp. contamination in eggs and egg-based products poses a major food safety concern; however, comprehensive data on molecular characteristics, antimicrobial resistance, biofilm formation, and stress tolerance—particularly in mayonnaise—in Bangladesh remain limited. This study aimed to isolate, identify, and characterize Salmonella spp. from eggs and mayonnaise, focusing on antimicrobial resistance, acid and bile tolerance, and biofilm-forming ability. A total of 150 samples (120 eggs and 30 mayonnaise) were collected from various retail markets and sources. Presumptive Salmonella spp. isolates were identified using cultural and biochemical tests, followed by molecular confirmation through PCR of the invA gene. Antibiotic susceptibility was assessed against 10 antibiotics using disc diffusion method. Acid and bile tolerance were evaluated under simulated gastrointestinal conditions, and biofilm formation was quantified using the microtiter plate assay. Salmonella spp. was detected in 19.3% of samples, including 18.3% of eggs and 23.3% of mayonnaise, with PCR confirming 25 isolates as Salmonella spp . enterica serovar Enteritidis. All 20 tested isolates exhibited multidrug resistance (MDR), showing the highest resistance to azithromycin (80%), tetracycline (75%), and colistin (75%). Biofilm assays revealed that all MDR isolates could form biofilms, with 45% strong and 55% moderate formers. The isolates survived acidic conditions, with reduced growth at pH 1.5 and good growth at pH 3.5, and tolerated up to 4% bile salts, showing optimal growth at 2%. These results indicate that MDR, biofilm-forming, and stress-tolerant Salmonella spp. in eggs and mayonnaise pose a significant public health risk, requiring improved surveillance, hygiene, and antimicrobial control measures.