Activity of methacycline, related tetracyclines, and other antibiotics against various L-forms and their parent bacteria in vitro.

The activity of methacycline against microbial L-forms and their parent bacteria was compared with that of oxytetracycline, chlortetracycline, tetracycline, and demethylchlortetracycline, as well as with that of 22 other antibiotics which included examples of major groups of antibiotics. The L-forms and bacteria used were Streptococcus faecalis, S. faecium, S. faecalis var. zymogenes, Staphylococcus aureus (three strains), Proteus mirabilis (two strains), Pseudomonas aeruginosa, Escherichia coli (two strains), Sarcina flava, Serratia marcescens, and Klebsiella pneumoniae. The five tetracyclines had similar activities and were more active against L-forms than bacterial forms, except that the bacterial form of S. flava was more susceptible than the L-form. In general, other antibiotics (except the penicillins) were more active against L-forms than bacterial forms. There were certain exceptions where the bacterial form was more susceptible than the L-form. These included the effect of polymyxin B and colistin on P. aeruginosa, E. coli, and P. mirabilis, and the effect of gentamicin on P. aeruginosa, E. coli, S. flava, and S. marcescens.