Isolation, Antimicrobial Susceptibility Determination and Genotyping of Sphingomonas paucimobilis from Milk Samples of Cows with Mastitis

Mastitis is one of the most economically significant diseases affecting dairy cattle worldwide, with its subclinical form causing greater production losses than the clinical form. Sphingomonas paucimobilis, a Gram-negative, yellow-pigmented, aerobic bacillus, has recently been recognized as an emerging environmental pathogen in cases of mastitis. This study aimed to isolate S. paucimobilis from milk samples of cows with mastitis in the İzmir and Aydın regions, determine its antimicrobial susceptibility, and perform genotyping. A total of 100 milk samples were collected from cows with mastitis (50 clinical and 50 subclinical cases). Conventional bacteriological methods, VITEK® Compact 2, and PCR were used for identification. Eight (8%) S. Paucimobilis isolates were obtained, including six from subclinical mastitis samples and two from clinical mastitis samples. Antimicrobial susceptibility testing by the Kirby-Bauer disk diffusion method revealed high resistance rates to amoxicillin-clavulanic acid (87.5%), ampicillin-sulbactam (100%), and cefquinome (100%), while all isolates were susceptible to cefoperazone and gentamicin. Genotyping using the (GTG)₅-PCR method showed that all isolates had distinct patterns, with genetic similarity ranging from 32% to 98%. The findings indicate that S. paucimobilis should be considered among potential environmental mastitis pathogens and highlight the need for advanced diagnostic methods and effective antimicrobial stewardship to prevent resistance development.