Staphylococcus microti Strains Isolated from an Italian Mediterranean Buffalo Herd

Abstract

S. microti is a new species among non-aureus staphylococci (NAS) frequently found in bovine milk samples and associated with subclinical mastitis (SCM). The aim of this study was to analyze the presence of S. microti in 200 composite milk samples and 104 milking parlor surface swabs collected at a buffalo farm in Southern Italy to define its presence in milk and a milking parlor environment. The samples were inoculated onto different agar plates, and the isolates were identified by MALDI-TOF MS. The strains identified as S. microti (54/304 samples, 17.8%) were collected, and their purified genomic DNA was subjected to PCR amplification and whole 16S rRNA gene sequencing. Furthermore, their phenotypic resistance profiles were evaluated by a disk diffusion method, and the genotypic characterization of the tetracycline resistance was performed for the tetM and tetK genes by multiplex PCR. Four and forty-seven S. microti isolates from milk samples of lactating animals with subclinical mastitis (SCM) and intramammary infection (IMI), respectively, and three isolates from milking parlor surfaces were recovered. The genomic DNA was purified from the bacterial isolates, and the amplification and sequencing of the 16S gene further supported the proteomic identification as S. microti. No clinical mastitis was detected in the herd during the study period. The antimicrobial susceptibility testing revealed a worrisome 100% resistance to tetracyclines, genotypically mediated by the tetM gene for all strains. This study highlights that S. microti may be commonly isolated from dairy buffalo milk and milking parlor equipment. Its association with SCM or IMI remains to be established.

Keywords: Staphylococcus microti; buffalo milk; intramammary infection; milking parlor surfaces; subclinical mastitis.

Figure 1

Antibiotic resistance profiles of 54 S. microti isolates. Tested antibiotics: amoxicillin–clavulanate (AMC), amikacin (AK), cefoxitin (FOX), clindamycin (CD), ciprofloxacin (CIP), erythromycin (E), gentamicin (CN), kanamycin (K), oxacillin (OX), penicillin (P), sulfamethoxazole–trimethoprim (SXT), tetracycline (TE), oxytetracycline (T) and streptomycin (S).