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Appl Environ Microbiol. 1968 November; 16(11): 1663-1668
Copyright © 1968 American Society for Microbiology. All Rights Reserved.

Effect of Inoculum Size on In Vitro Susceptibility Testing with Lincomycin

Division of Allergy and Infectious Diseases, Department of Medicine, Medical College of Virginia, Virginia Commonwealth University, Richmond, Virginia 23219

ABSTRACT

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 -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 ß-hemolytic streptococci and pneumococci.