Staphylococcus aureus related to bovine mastitis in Switzerland: Clonal diversity, virulence gene profiles, and antimicrobial resistance of isolates collected throughout 2017

摘要

Staphylococcus aureus can be associated with subclinical,acute, chronic, and toxic cases of bovine intramammaryinfections, leading to considerable financiallosses for the dairy industry in Switzerland and worldwide.In addition, milk products are one of the mostcommon food categories implicated in staphylococcalfood poisoning in humans. Detailed information onthe population structure, as well as the virulence andresistance characteristics of Staph. aureus originatingfrom bovine mastitis milk is needed to allow for sourceattribution and risk assessment of Staph. aureus in afood poisoning context and to improve therapeuticapproaches in cattle. Our objective was to assess thepopulation structure, phenotypic resistance patterns,and virulence and resistance gene profiles of Staph. aureusisolates from bovine mastitis milk in Switzerland.To this end, 58 Staph. aureus strains were characterized.The DNA microarray was used to test for thepresence or absence of virulence and resistance genes.In addition, minimum inhibitory concentrations ofvarious antimicrobial agents were determined by microdilution.To assess the population structure of theisolates, we determined clonal complexes (CC) usingDNA microarray hybridization profiles and performedmultilocus sequence typing and spa typing. The strainswere assigned to 7 clonal complexes, 10 sequence types,and 11 spa types, with CC705 (43%), CC97 (33%), andCC20 (12%) representing the most common lineagesand t529 (43%) and t267 (21%) representing the mostcommon spa types. Only 1 isolate was assigned to CC8,a clonal lineage linked to high within-herd prevalenceof mastitis. A total of 14% (n = 8) of strains were classifiedas resistant to penicillin, and 1 strain each wasclassified as oxacillin and pirlimycin resistant. Althoughno clinical breakpoints are available for the combinationof kanamycin/cefalexin, growth of all strains wasinhibited by the lowest combination of kanamycin/cefalexin concentrations tested (4 μg/mL of kanamycinand 0.4 μg/mL of cefalexin). One strain assigned toCC20, ST389, and t2094 exhibited resistance to penicillin,oxacillin, and pirlimycin as well as intermediatesusceptibility to erythromycin and high minimum inhibitoryconcentration for several antimicrobial agents,for which no breakpoints were available.