The effect of cultural conditions on the susceptibility of staphylococci to killing by the cationic proteins from rabbit polymorphonuclear leucocytes

Abstract

Seventeen strains of Staph. aureus and 12 strains of Staph. epidermidis were tested for their susceptibility to killing by the cationic proteins from rabbit polymorphs, when cultured under different conditions. The resistance of the V strain of Staph. aureus of Adlam, Pearce and Smith (1970a,b) was confirmed and shown to be independent of the conditions of growth. All other strains were more or less susceptible when grown in brain heart infusion broth or proteose peptone broth, with or without lactate, but were highly resistant when grown in these media supplemented with glucose. The resistance was not due to changes in pH. Susceptibility was restored when the organisms were again grown on broth media without glucose. Since preliminary observations indicate that the cationic proteins inhibit the action of NADH oxidase, it is suggested that the organisms are susceptible only when growth depends on aerobic respiration. If the culture contains an energy source such as glucose which can be utilized by an alternative pathway, the organism loses its susceptibility.

After culture in a chemically defined medium, most strains of Staph. aureus were resistant and most strains of Staph. epidermidis were susceptible to the cationic proteins. The resistance of Staph. aureus was not removed by adding vitamins, purine and pyrimidine bases or serum (from clotted cell free plasma) to the growth medium but was abolished by proteose peptone. The active fraction in proteose peptone was partially excluded by Sephadex G-25, but not by G-10. Some of its activity was lost on acid hydrolysis. The cause of the difference in susceptibility between Staph. aureus and Staph. epidermidis after growth in a chemically defined medium or in this medium supplemented with serum is not known. It may be related to the known difference between these organisms in their ability to grow in serum from clotted blood, which contains similar cationic proteins.