Milk and dairy products are considered to be at high risk of bacterial contamination. Escherichia coli and Bacillus cereus are common contaminants of raw and processed milk and their recovery in milk and dairy products may be a health concern because of the possible presence of pathogenic strains. The present project aims to study the prevalence of E. coli and B. cereus in raw milk, and their antimicrobial resistance and biofilm formation ability of isolates.
A total 60 samples of raw milk were collected from a Victorian dairy producer between September 2106 and July 2017. Isolation of E. coli and B. cereus was carried out according to conventional culture methods and confirmed with MALDI-TOF MS. All E. coli and B. cereus isolates were subjected to antimicrobial susceptibility tests based on the Clinical and Laboratory Standards Institute (CLSI) criteria using the disk diffusion method. Biofilm formation was assessed using the colorimetric microplate method.
E. coli and B. cereus were isolated from 90% and 33.3% of raw milk samples, respectively. Antimicrobial susceptibility testing of E.coli showed a high prevalence of antibiotic resistance to Erythromycin, Ampicillin, Streptomycin and Tetracycline. Antimicrobial susceptibility testing of B. cereus showed a high prevalence of antibiotic resistance toward Ceftriaxone, Penicillin and Ampicillin. All isolates were sensitive to Ciprofloxacin, Clindamycin, Gentamicin, Tetracycline and Vancomycin. According to the results, most tested E. coli strains produced biofilm. In contrast, less than 50% of B. cereus produced biofilm. The high level of E. coli and B. cereus contamination coupled with their capacity to produce biofilm can be a concern for dairy producers. The high level of contamination, antimicrobial resistance and biofilm formation ability warrants further investigation of other properties, including genetic diversity and toxigenic potential.