The susceptibility of 80 StaphyIococcus aureus isolated to oxacillin was investigated using microtitre, agar dilution and Stokes' disc diffusion methods. There was a bimodal distribution of the isolates according to the oxacillin minimum inhibitory concentration (MIC) values. For the sensitive isolates, the agar dilution method generally gave lower MIC values than the microtitre method, while for the resistant isolates the agar dilution method gave comparable to slightly lower MIC values than the microtitre method. The Stokes disc diffusion method yielding the best results when performed on Mueller-Hinton agar incubated at 30°C for 18 hours; however local strains grew poorly when incubated at 30 C for 18 hours. The next best medium which provided clear disc diffusion results plus good growth was Mueller-Hinton agar incubated at 35°C for 18 hours, on which 10 % of the sensitive isolates appeared intermediate in susceptibility, and none resistant, while all the resistant isolates (microtitre MIC >8mg/1) appeared resistant. Oxacillin resistance among strains of Staphylococcus aureus tested by Stokes' disc diffusion method correlated best with gentamicin resistance, and less often with tetracycline resistance. Therefore gentamicin- or tetracycline-resistance may indicate oxacillin resistance in Staphylococcus aureus. / Thesis (M.Med.)-University of Natal, Durban, 1988.

Bovine mastitis is the most important economic disease to the dairy industry with losses estimated at 2 billion dollars per year in the United States. Staphylococcus aureus (.§.. aureus) is the primary cause of contagious mastitis. Conventional culture methods (National Mastitis Council) were used as a basis for comparing the ability of the enzyme linked immunosorbent assay. ProStaph Iâ ¢, to identify s. aureus. The test had an accuracy of 96%, with a sensitivity of 90% and a specificity of 97%. Results indicated that rinsing teat-cup liners with a 25 ppm iodophor or 100 ppm chlorine solution reduced the presence of S. aureus on the liners by 97%. ProStaph I was used to rapidly screen DHIA preserved milk samples in 10 Virginia cooperator herds. Herds were classified as high (>10% infected) or low prevalence (<10% infected). There were six high prevalence herds after the first test. Average prevalence of cows scoring Ab +2 and +3 was 11.9% ± 7.9. Over the seven month study, prevalence of positive cows declined significantly (P<.OI), but somatic cell count remained relatively unchanged (P>.lO). Four herds continued to have >10% of the animals infected. Incidence of new infection averaged 3.6% ± 2.8 from the first to the last test. Chronic cows averaged 6.9% ± 4.8 over the seven month study. Analysis of variance showed significant (P<.Ol) effects of herd on ProStaph I score J milk yield, and see. Elevated ProStaph I scores were highly correlated (P <.01) with increases in lactation number. ProStaph I changed quadratically (P<.Ol) with increasing SCC. Somatic cell count increased (P<.OI) as ProStaph I score increased. / Master of Science