Some physical chemical studies of antibiotic sensitive and resistant strains of Staphylococci
Staphylococcus aureus strains sensitive and naturally resistant to methicillin, were studied during this investigation using the technique of particulate electrophoresis. The effects of growth medium on the surface properties of the cell wall were studied. Cells of strains of Staph. aureus which had typical characteristics of methicillin resistant strains, possessed small amounts of surface teichoic acid and showed a minimum in the negative surface charge at pH 4.5. The effect of repeated subculture at 25, 37 or 43 °C in the absence of antibiotic was studied by observing changes in the Minimum Inhibitory Concentration (MIC) and pH-mobility curves. Cells grown repeatedly at °C lost their methicillin-resistance and the ability to produce the enzyme penicillinase. The surface properties were markedly changed, the cells possessed considerable amounts of surface teichoic acid which were comparable to that of methicillin-sensitive strains. The genes controlling methicillin-resistance and penicillinase production are plasmid-borne, they were located on different plasmids, each plasmid being lost independently of the other; this was confirmed by the technique of replica plating. Cells grown repeatedly on nutrient agar at 25 °C became more resistant to methicillin and lacked surface teichoic acid, those grown at 57 °C showed no change in resistance or surface characteristics. Surface changes and loss of methicillin plasmid depended on the concentration of inorganic phosphate in the growth media. A method of preparing growth media containing a low concentration of inorganic phosphate was established.