Antibiotic-resistance profile of Staphylococcus aureus strains isolated from milk of high yield cows in central Russia

The development of microbial resistance to antibiotics determines the effectiveness of the veterinary measures. The study and monitoring, well-designed treatment schemes are those measures that help to reduce the risk of spread of pathogenic and conditional-pathogenic microorganisms. One of the problems in the dairy farming is mastitis of high producing cows which is mainly caused by Staphylococcus aureus. Here we first report findings on the development of resistance in S. aureus strains isolated from milk of clinically healthy dairy cows during treatment with antibiotics of different classes. The study was carried out using high yield holsteinized black-and-white cows ( n = 1321, Kaluga region, 2016) which were vaccinated twice with anti-mastitis drug Mastivak («Ovejero», Spain). Milk was sampled during the lactation period individually from each cow. For the species identification of S. aureus isolates we used the following criteria: (i) morphology and microscopy of the colonies grown on the Baird Parker Agar medium («HiMedia Laboratories Pvt., Ltd», India); (ii) the presence of hemolysis zones on the Azide Blood Agar Pronadisa medium («Conda», Spain) supplemented with 5 % defibrinated sheep blood; (iii) a coagulation of dry citrated rabbit plasma (CJSC «EKOlab», Russia); (iv) biochemical characterization with API 20 Staph panel («BioMerieux», France). S. aureus ATCC 25923 was a reference strain. The sensitivity of the isolated microorganisms to ten antibiotics («Pharmacotherapy Research Center», Russia) including penicillin (PEN, 10, ED), oxacillin (OX, 10 μg), gentamicin (GN, 10 μg), erythromycin (ER, 15 μg), lincomycin (LN, 15 μg), rifampicin (RF, 5 μg), ciprofloxacin (CP, 5 μg), vancomycin (VA, 30 μg), fuzidin (FZ, 10 μg) and novobiocin (NB, 5 μg) was determined by disk diffusion method. A total of 104 strains among 155 isolates (67.1 %) showed resistance to one or more antibiotics. The majority of the strains were resistant to novobiocin (49.7 %) while the smallest part (2.6 %) exhibited resistant to vancomycin. Importantly, the antibiotic resistance to next-generation antibacterial agents (novobiocin and vancomycin) which are currently widely used in medicine should be noted. Gentamicin, rifampicin and vancomycin had the highest efficiency among the tested antibiotics (81.9 %, 86.5 % and 97.4 %, respectively). In order to evaluated the effectiveness of antibiotic treatment strategy, the cows ( n = 87) with pre-detected S. aureus were divided into four groups. Gentamicin was administrated in group I ( n = 26), erythromycin was administrated in group II ( n = 22), rifampicin was administrated in group III ( n = 12), and penicillin and amoxicillin were administrated in group IV ( n = 27). The animals were considered cured when no S. aureus after the treatment. Rifampicin application was the most effective (91.7 %) whereas gentamicin showed the lowest effectiveness (53.8 %). Moreover, the number of multidrug resistant strains reached 55.6-61.5 % after the treatment comparing to 33.3-43.8 % before treatment, thus a tendency of developing multiple drug resistance has been shown.