Prevalence and antimicrobial susceptibility of Listeria spp. isolated from bulk raw milk in north-eastern Poland

Abstract

Purpose of research: The aim of the study was to determine the presence of Listeria spp. strains in the milk samples obtained from dairy farms in north-eastern Poland and to determine the profile of resistance to antibiotics recommended in the treatment of listeriosis. Material and methods: 500 samples of bulk milk were analyzed. Milk samples were obtained from dairy farms located in Warmia and Mazury region in Poland. Chronic mastitis, requiring frequent and long-term use of antibiotics has been documented in these herds. Isolation of Listeria spp. was performed according to the standard procedure PN-EN ISO 11290-1: 2017-07. Antibiotic resistance testing was performed by the disc diffusion method according to the Clinical & Laboratory Standards Institute (CLSI) recommendations. Results: In total, out of 500 samples of pooled milk, based on biochemical properties, 8 isolates were confirmed as belonging to the genus Listeria (1.6%). The further identification of Listeria strains on the basis of MicrobactListeria12L showed that 3 strains (3/8, 37.5%) belonged to L. monocytogenes species and 5 strains (5/8, 62.5%) belonged to L. innocua species. The analysis of sensitivity to commonly used antimicrobial agents showed that all isolates, both belonging to L. monocytogenes and L. innocua species, were sensitive to ampicillin. Multidrug resistance, defined as resistance to at least three classes of antibiotics, was confirmed among four isolates (50%). Research summary: The studies undertaken revealed that raw milk can pose a risk for public health due to the prevalence of pathogenic Listeria spp. among which multidrug resistant strains are present. It is therefore necessary to rationalize the use of antibiotics and to monitor bacterial resistance in the food production environment.