Absence of the paradoxical effect when determining the bactericidal activity of penicillin against penicillin-susceptible strains of viridans group streptococci

J. A. Moody, S. Stewart-Wigglesworth, B. F. Woolfrey

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1 Scopus citations

Abstract

Several reports based on broth dilution plate count data have linked the paradoxical effect with tolerance. In this study, bactericidal time-kill patterns for 15 strains of Streptococcus sanguis and four strains of Streptococcus mitis against penicillin were determined by the agar dilution plate count method. All 19 strains were susceptible to penicillin. Twenty-four-hour killing curve patterns were obtained for each strain, and sequential killing curve patterns representing penicillin action times of 6, 12, and 24 hours were obtained for selected strains. The 19 strains ranged from some that were slowly killed to others that were rapidly killed. The paradoxical effect was not demonstrable for 17 of 19 strains during the killing sequence of the strain. In two strains, the paradoxical effect and slow bactericidal response were not linked phenomena; they were independent and strain-dependent characteristics. Three of four slowly killed tolerant strains did not exhibit the paradoxical effect. For most strains, penicillin had a relatively constant bactericidal activity with increasing concentrations above the minimum inhibitory concentration. The reduction of viable streptococci represented as survivor percentage reflects more accurately the bactericidal effect of penicillin than using arbritary criterion to divide isolates into nontolerant and tolerant isolates.

Original languageEnglish
Pages (from-to)763-772
Number of pages10
JournalCurrent Therapeutic Research - Clinical and Experimental
Volume51
Issue number5
StatePublished - May 25 1992
Externally publishedYes

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