The effect of exposure of staphylococcus aureus to penicillin on susceptibility to the bactericidal activity of human leukocytes

The effect of brief exposure of staphylococci to penicillin upon susceptibility to leukocidal activity was studied. Antibiotic pretreated staphylococci were incubated with normal human leukocytes and viability of the bacteria was determined. Penicillin pretreated staphylococci were more susceptible to killing by leukocytes than untreated control bacteria. The extent of sensitization varied between different strains of staphylococci. When staphylococci were exposed simultaneously to leukocytes and penicillin, the bacteria were protected from the lethal action of penicillin. This study demonstrates that phagocytosed staphylococci are protected from penicillin, but prior exposure to the antibiotic increases their susceptibility to the activity of leukocytes.