SUMMARY: The development of resistance to ampicillin (aminophenylacetamidopenicillanic acid) and penicillin G was investigated by selecting resistant variants of strains of Resistance occurred in stepwise manner. The parent strains contained minute amounts of penicillinase and production of the enzyme by resistant variants increased in relation to increase in resistance. Evidence was obtained that production of penicillinase was responsible for this increase in resistance. By using resistant variants and naturally occurring ampicillin-resistant organisms, it was found that production of only small amounts of penicillinase was sufficient to confer a high degree of resistance to the Gram-negative bacteria examined. In contrast, Gram-positive bacteria required much greater quantities of penicillinase for high grade resistance. It is suggested that an important reason for this difference is the complex lipid-containing cell wall of Gram-negative bacteria, which slows penetration by penicillin and enables small amounts of penicillinase to protect the cell. The penicillinase of all the Gram-negative organisms investigated was found to be a -lactamase and to be more active against penicillin G than against ampicillin. This provides one explanation for the greater activity of ampicillin against Gram-negative bacteria.


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