The aims of this study were to determine the prevalence, antimicrobial resistance and the biofilm-forming ability of Listeria spp. in bulk raw milk in East-Azerbaijan province, Iran.

A total of 192 bulk raw milk samples were collected from six distinct parts of East-Azerbaijan province. Selective enrichment and isolation were done by using United States Department of Agriculture (USDA) method, then isolates were identified by biochemical tests and confirmed by polymerase chain reaction. Susceptibilities of isolates to different antibiotics were determined by using the disk diffusion assay. Two phenotypic methods were used for investigation of biofilm production: Congo red agar (CRA) and microtiter plate (MTP).

The prevalence of Listeria monocytogenes and other Listeria spp. were 11.97% and 0%, respectively. The highest prevalence rate was found in one of regions located in the center of province (30.43%) and northeast of province had the lowest prevalence rate (3.12%). All isolates were susceptible to vancomycin, erythromycin, chloramphenicol, kanamycin, gentamicin, tetracycline, streptomycin, amoxicillin-clavulanic acid and rifampicin. Only one isolates had intermediate susceptibility to ciprofloxacin. There was the highest resistance to nalidixic acid (100%), followed by to ampicillin (17.39%), and penicillin (13.04%), and the lowest resistance to clindamycin (8.6%) was observed. Based on MTP, 91.30% of isolates were weak biofilm formers. Biofilm production of L. monocytogenes using CRA plates showed that 34.78% and 52.17% of isolates were positive and intermediate biofilm producers, respectively.

The results indicate that prevalence rate of L. monocytogenes is relatively high and there is a potential risk for consumers of raw and unpasteurized milk.