Electron microscopic observations on the effects of penicillin on the morphology of Chlamydia psittaci.

L cells were infected at high multiplicity with meningopneumonitis organisms and incubated in medium containing 200 units per ml of penicillin. At intervals up to 48 hr after infection, cells were removed and thin sections were prepared for electron microscopic studies on the morphology of the developing organism. Penicillin had no effect on the initial reorganization of the infecting elementary body to form the developmental reticulate body (RB), and, up to 12 hr after infection, the treated and untreated cultures were identical. After that time, however, penicillin-treated organisms showed striking differences in that binary fission was prevented, large abnormal RB forms were seen in great numbers, masses of RB cytoplasmic membranes and envelopes were formed within and outside the RB itself, and large numbers of empty or partially filled small vesicles were pinched off the RB. After 36 hr immature nucleoids were formed within the RB. Throughout all of this period, both the outer cell envelope and the cytoplasmic membrane of these RB were recognized. When infected cells were transferred into penicillin-free medium, the abnormal RB showed recovery to form normal RB both by a budding-like process and by internal fragmentation or subdivision rather like endosporulation. We have concluded that penicillin inhibits binary fission and prevents the synthesis of certain components essential for the formation of the elementary body envelope.