[Determination of beta-lactamase production in strains of Staphylococcus aureus isolated from the milk of cows].

Determination of sensitivity to penicillin G by a standard disk assay diffusion method was compared with a iodometric method of test papers to determine beta-lactamase production after Jorgensen in Staphylococcus aureus strains isolated from cow's milk from different farms. Out of 179 test strains, 32 strains 17.8%) were found to be well sensitive in the diffusion test; eight of these strains (25.0%) were demonstrated to produce beta-lactamase. 54 strains (30.2%) were sensitive. 27 strains (50%) of this group produced beta-lactamase. 93 strains (51.9%) were resistant to penicillin G in the diffusion test. 86 strains (92.5%) were found to produce beta-lactamase and seven strains were negative in this test. Using the diffusion test of sensitivity in these cultures, 86 strains were sensitive (48.1%) and 93 strains were resistant (51.9%). Beta-lactamase was produced by 121 strains (67.6%) and no beta-lactamase production was recorded in 58 strains (32.4%). Differences in the results of both tests were manifest mainly in the set of strains qualified as sensitive (inhibition zone diameter 24 to 16 mm) and well sensitive (inhibition zone diameter larger than 25 mm). The results indicate that the currently performed diffusion test of sensitivity to penicillin G should be accompanied by an assay of beta-lactamase production. The iodometric method of test papers is simple, rapid and cheap and can be made in any bacteriological laboratory. The high resistance of Staphylococcus aureus strains to penicillin G documents that this antibiotic is little efficient in the treatment of mastitis of this etiology in the given region.