Effect of Inoculum Size on In Vitro Susceptibility Testing with Lincomycin

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RESUMO

There is disagreement in the literature as to whether lincomycin is primarily a bacteriostatic or a bactericidal agent against gram-positive cocci and also regarding the levels of activity of this agent against susceptible microorganisms. These questions were examined in a study of the effect of inoculum size on the results of tube dilution susceptibility determinations with lincomycin against 49 clinical isolates of Staphylococcus aureus and 25 strains of streptococci and pneumococci. Lincomycin was both highly active and bactericidal when tested against 40 strains of S. aureus with inocula containing a maximum of 104 cells per ml [median minimal inhibitory concentration (MIC), 0.78 μg/ml; median minimal bactericidal concentration (MBC), 1.56 μg/ml]. With inocula of 105 cells per ml, lincomycin was primarily bacteriostatic (median MIC, 1.56 μg/ml; median MBC, 12.5 μg/ml). There were further decreases in inhibitory levels and significant losses of bactericidal activity when inocula containing more than 107 cells were tested (median MIC, 3.13 μg/ml; median MBC > 100 μg/ml). Similar measurements with streptococci and pneumococci revealed a lesser effect of inoculum size. The mean MBC value for α-hemolytic streptococci increased from 0.40 to 1.05 μg/ml with an increase in inocula from 104 to 106 cells per ml, but without a marked increase in MIC values. Similar results were obtained for β-hemolytic streptococci and pneumococci.

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