Microbiological Quality Assessment and Identification of Antibiotic Resistant Bacteria at Different Stages of the Milk Supply Chain in Dhaka City of Bangladesh

Aims: Milk works as an excellent medium for bacterial growth and can turn into a fatal source of food borne diseases when consumed without pasteurization. This study was carried out to examine the microbiological quality of milk from three different points of milk supply chain to investigate whether the dairy stakeholders are maintaining the consumer safety or not.

Study Design: A cross sectional study

Place and Duration: The study took place at the Food Microbiology lab of Institute of Nutrition and Food Science, University of Dhaka from November 2019 to February 2020.

Methodology: A total of 60 samples were studied including raw milk from collection centers, unpackaged pasteurized milk from processing plants and packaged pasteurized milks from retail shops. After carrying out the microbiological analysis the samples were examined for determining the total bacterial count (TBC) and total coliform count (TCC). Antibiotic susceptibility test was done using disk diffusion assay and detection of virulent gene in Salmonella spp. was done by Polymerase Chain Reaction (PCR) using specific invA primer.

Results: The results revealed that all raw milk samples were substandard in terms of TBC and TCC and pasteurized milks from processing plants maintained the standard quality. Importantly, packaged pasteurized milk samples from retail shops had high TBC (>4.0× 104 CFU/mL) and TCC (1.2×103 CFU/mL) containing Pseudomonas, Micrococcus, Streptococcus, Salmonella, Proteus, Staphylococcus, Bacillus and E. coli. Bacteria like Salmonella (75%), Proteus (62.5%) and Vibrio (62.5%) possessed high Multiple Antibiotic Resistance (MAR) index and showed resistance towards antibiotics namely Ampicillin, Amoxicillin, Erythromycin and Colistin. Through further molecular analysis we detected invA virulent gene one of the Salmonella isolates which was collected from the pasteurized milk samples of the retail shops.

Conclusion: High bacterial load in raw milk and packaged pasteurized milk indicate that the milk we consume is substandard in microbiological quality. Precautionary measurements and careful processing of milk may reduce the prevalence of microbiological contamination in the milk supply chain.