Persistence of Staphylococcus aureus in Penicillin in vitro

Abstract

Summary: After exposure of Staphylococcus aureus strain Oxford (h) to penicillin in vitro under conditions leading to the death of at least 99·9% of the exposed population, the offspring of surviving cocci (‘persisters’) showed the same sensitivity as the original population. Successive exposure of the offspring of persisters did not increase the proportion of survivors nor their resistance. Persisters were not the most resistant cocci in the original population; their offspring showed the same distribution of cocci resistance as the parent culture. The number of survivors, about 1 in 2500 cocci, was proportional to the original number when exposed to penicillin in a small volume (10–20 ml.), but not in a larger volume (40-200 ml.). Inocula of less than 10 staphylococci allowed to multiply to 103/ml. before addition of penicillin yielded persisters. Populations greater than about 2 x 108/ml. were not acted upon by penicillin, even when the majority of the cocci were only 4 hr old, unless the mixtures were shaken. The proportion of survivors at 37° from equal volumes of penicillin broth containing 105–108 organisms/ml. was not affected by the age and growth phase of the exposed bacteria, conditions of previous storage of the inoculum, initial exposure at low temperature, type of container, aeration by shaking, clumping of cocci, a 1000-fold increase in the dose of penicillin, or addition of streptomycin. When removed from penicillin, persisters and their offspring multiplied at the same rate as the parent culture without requiring a recovery period. The persisters were neither spheroplasts nor L forms. Indirect evidence indicates that persisters survived because at the time of first contact with the penicillin they were in a state unfavourable to initiation of division or cell wall synthesis.