Antibiotic resistance and biofilm formation in Staphylococcus aureus isolated from dairy cows at the stage of subclinical mastitis in northern Kazakhstan

Abstract. Staphylococcus aureus is an important causative agent of subclinical bovine mastitis worldwide.
The aim of this research was to study the ability of S. aureus to form biofilms.
Additionally, we examined the genes involved in cell resistance and
sensitivity to antibiotics. Samples were collected from December 2020 to May
2021 from Simmental and black-and-white cows. The study was carried out on a
total number of 643 cows, of which 278 (23 %) were in the subclinical
mastitis stage. Finally, 64 S. aureus isolates were isolated and identified. The
highest level of phenotypic resistance was observed to antibiotics of the
tetracycline (tetracycline – 48.4 %, doxycycline – 32.8 %) and
β-lactam (ampicillin – 45.3 %, penicillin – 45.3 %) groups. The
genes encoding antibiotic resistance were characterized with the polymerase
chain reaction method: blaZ in 30 isolates, mecA in 1 isolate, ermC in 15 isolates, aph (3) in 2 isolates, tetK in 19 isolates, tetM in 9 isolates.
The tested S. aureus isolates had the ability to form biofilms in 76.6 % (49/64) of
cases. Of these, 69.4 % were resistant to at least one antibiotic. The
obtained results have shown that S. aureus, identified in cows with subclinical
mastitis, was resistant mainly to tetracycline and β-lactam antibiotics.
In addition, S. aureus isolates expressed resistance genes to the above drugs and had
the ability to form biofilm. This study will help to identify the
extent of antibiotic resistance and monitor S. aureus contamination of raw milk.
