Metal Ion And Penicillin Resistance In Staphylococcus Aureus

Abstract

An agar dilution test and a disc diffusion test were developed for determining lead susceptibility of Staphylococcus aureus. An agar medium with 0.008 molar concentrations each of lead acetate and EDTA was found to be reliable for screening lead ion resistant strains of S. aureus. A test using six mm discs of Whatman #3 filter paper impregnated with 20 ul of 0.2 M lead acetate was also found to be adequate for determining lead resistance of S. aureus. We found that in this test the acetate does not precipitate into the medium, yet it gives a definitive test for lead ion resistance. A total of 113 strains of S. aureus were screened for resistance to cadmium, arsenate, arsenite, lead, bismuth, and inorganic mercury ions as well as to penicillin (10 units) by the disc diffusion method. We found that many of these resistances were linked because they frequently appeared together among all the strains of S. aureus. On the other hand, there was significant variability among all the strains. Finally, our data support the existing theories of staphylococcal plasmid evolution.