The effect of inoculum volume on the microbiologic detection of naturally occurring Staphylococcus aureus intramammary infections

Abstract

Currently no standard definitions for the diagnosis of Staphylococcus aureus intramammary infection (IMI) exist. As a result, criteria applied in research to diagnose S. aureus IMIs have varied making comparisons between published works difficult. The goal of the current study was to define the optimal inoculum volume used in the diagnosis of naturally occurring S. aureus IMIs. Microbiologic results from 2 field studies examining S. aureus IMIs were used to examine the effects of inoculum volume on the microbiologic detection of S. aureus. A total of 1,583 milk samples were included in the analysis, and the results of using a 0.01-ml and a 0.1-ml inoculum are presented. Using a 0.01-ml inoculum resulted in a sensitivity of 91% (95% confidence interval [CI]: 88.6-93%) and a specificity of 99.4% (95% CI: 98.6-99.8%). Using the larger 0.1-ml inoculum resulted in a sensitivity of 96.8% (95% CI: 95.2-97.9%) and a specificity of 99.3% (95% CI: 98.4-99.7%). All false-positive samples were from S. aureus-negative quarters in S. aureus-positive cows. There were no false-positive cultures from S. aureus-negative cows. Of the false-negative samples, the majority (77%) were from 6 of the 34 S. aureus-positive quarters. Results from the current study of naturally occurring S. aureus IMIs support the hypothesis that, when using quarter level milk samples, a S. aureus IMI is most accurately diagnosed using a 0.1-ml inoculum. Regardless of inoculum volume, a single quarter sample culture that is positive with S. aureus (≥1 colony-forming unit) is sufficient to diagnose a S. aureus IMI.