Alanine:Cycloserine Antagonism: I. Significance of Phenomenon in Testing Susceptibility to Cycloserine

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Abstract
Non-acid-fast bacteria are usually resistant to cycloserine by conventional susceptibility testing. Yet, a wide variety of infections in humans caused by non-acid-fast bacteria are favorably influenced by therapy with cycloserine. This paradox was resolved by determination of the susceptibility of cycloserine of 293 clinical isolates of bacteria in 2 synthetic broth media which differed only in the presence or absence of alanine. Escherichia coli, Providence bacilli, enterococcal group bacteria and Staphylococcus aureus were significantly more often susceptible to cycloserine in the absence of alanine. Cycloserine was of low to moderate potency in effect on E. coli and Providence bacilli; of little potency against enterococcal group bacteria; of high to moderate antistaphylococcal potency.