Assessment of Listeria monocytogenes contamination and antibiotic resistance in raw milk samples collected from Colombo district Sri Lanka

Background: Listeria monocytogenes, a foodborne pathogen, poses a significant risk to public health due to its potential to cause severe infections termed "listeriosis". Raw milk and dairy products are common vehicles for L. monocytogenes transmission, amplifying the risk to consumers. Additionally, the emergence of antibiotic resistance in Listeria strains heightens concerns in human and veterinary medicine. Aim: This study aimed to assess the prevalence of L. monocytogenes in raw milk samples from Sri Lanka's Colombo district and to characterize the antibiotic resistance patterns of positive isolates. Methods: One hundred raw milk samples were collected from various milk collection centers. Samples were sourced from locations including Awissawella, Kahanthota, Meepe, Homagama, and Udahamulla. Antibiotic resistance was evaluated using standard methods like disk diffusion and well diffusion, testing ampicillin, penicillin, streptomycin, and tetracycline. Results: Of the 100 samples, L. monocytogenes was detected by conventional PCR in 83, with Awissawella having the highest occurrence (100%), followed by Kahanthota (81.81%), Meepe (81.48%), Homagama (77.27%), and Udahamulla (44.44%). Among the positive samples, 72 showed varying degrees of antibiotic resistance. The highest level of resistance was observed for penicillin (83.1%), followed by ampicillin (65%), streptomycin (32.53%), and tetracycline (10.84%). Sixty isolates (60%) displayed resistance to more than one antibiotic studied. Findings also revealed the widespread manual milking, predominant water usage over soap for hygiene, and common daily bathing of cows, highlighting the need for improved hygiene practices to minimize contamination risks in dairy farming. Conclusion: Raw milk contaminated with L. monocytogenes pose a serious public health risk. The study highlights the need for stringent hygiene measures in milk production and processing to safeguard consumers. Additionally, the high prevalence of antibiotic-resistant strains underscores the importance of judicious antibiotic use in both human and veterinary medicine to mitigate the further emergence and spread of resistant Listeria strains. [J Microbiol Infect Dis 2024; 14(2.000): 67-79]