Summary: Naturally occurring methicillin-resistant strains of isolated in hospitals in Britain, France and Denmark were studied. All strains belonged to a few closely related bacteriophage types and all behaved similarly in the presence of methicillin. The minimum inhibitory concentration of methicillin for the strains ranged from 5 to 100 μg./ml., but on ordinary nutrient agar growth in the presence of concentrations well below this was much less luxuriant than on control plates without antibiotic and tended to be confined to the site of heavy inoculum. Gramstained films from these cultures showed great irregularity in the size and staining of the cocci and many swollen forms were seen, suggesting that methicillin might be inhibiting cell wall synthesis without preventing multiplication. Further work showed that with addition of an excess of electrolytes (5% NaCl or 7·5% (NH)SO) or decrease in agar concentration, growth in the presence of methicillin was almost equal to that on control plates. The addition of uracil to a partially defined medium had no significant effect. Initial incubation under anaerobic conditions also improved growth of these strains in the presence of methicillin.


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