On the Nature of Refractoriness of Certain Gram-Negative Bacilli to Penicillin

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Abstract
The susceptibility of certain strains of E. coli and Salmonella is significantly greater in synthetic medium than in meat infusion broth. Casein hydrolysate, asparagine, glutamic acid to a lesser degree, and possibly some other aminoacids partially antagonize the effect of penicillin upon E. coli in synthetic medium. The antagonism of casein hydrolysate and asparagine can be removed in greater part by methionine. The observations suggest that the refractoriness of Gram-negative bacilli to penicillin is at least to some extent extrinsic in nature.