Penicillin-tolerant and -sensitive strains of oral streptococci were treated with penicillin to determine the production of a post-antibiotic effect (PAE). No PAE was seen with any of the strains tested but most strains of the other oral streptococcal species produced a PAE. Cultures on nitrocellulose filters treated with penicillin were examined by scanning electronmicroscopy and showed that tolerant and sensitive strains lost the ability to adhere to the filter after application of antibiotic. When the filters were treated with β-lactamase, before processing for microscopy, the tolerant strains but not the sensitive ones recovered and grew in a confluent lawn similar to the control cultures that had not received antibiotic. Transmission electronmicroscopic examination of similarly treated cultures produced comparable results. Bizarre morphological changes were a feature of the tolerant strains of


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