Penicillin G and Oxacillin Resistance in Staphylococcus aureus Strains Isolated from Bovine Subclinical Mastitis

摘要

Sixty-eigth out of 530 different S.aureus field strains isolated from suhclinical cases of bovine mastitis from Germany (n =26),Indonesia (n = 16),Mexico (n = 16)and Brazil (n =9),respectively,were selected to be studied in the present work.The strains were tested phenotypically as well as genotypically for the presence of penicillin G-and oxacillin-resistance.For the primary phenotypical species identification of the 530 S.aureus strains,plasmacoagulase-test and Api 32Staph system was applied.This was confirmed by molecular detection of the S.aureus specific genes encoding 23S rRNA,thermostahle nuclease (nuc),clumping factor (clfA),coagulase (coa)and protein A region Xr (spa).The selection of the 68 strains was carried out by the random selection of one up to two strains per herd; additionally only strains with differences in size polymorphism of coa and Xr region of the.spa gene and different macrorestriction profiles were includecl here.Genotypic resistance to semisynthetic penicillins (methicillin/oxacillin)and penicillin G was studied through the identification of mecA-and blaZ-genes,respectively.The mecA gene was detected in only one S.aureus isolate from Brazil which was not phenotypically resistant against semisynthetic penicillins as shown by the use of standard disc diffusion method.BBL-Chromoagar and MIC-determination by Vitek Ⅱ.In contrast penicillin-resistance of strains based on the presence of the blaZ gene could be observed in 50 (73.5％ )of the investigated strains.However,only 40 (58.8％)of these 50 blaZ-positive strains were phenotypically penicillin-resistant.According to the presented data resistance to semisynthetic penicillins in S.aureus field strains seems to be not a major problem in dairy herds of the investigated countries despite the long-term use of these antiinfective substances in the field.