Quantitation of Staphylococcus aureus in seawater using CHROMagar SA

Abstract

A microbiological algorithm has been developed to analyze beach water samples for the determination of viable colony forming units (CFU) of Staphylococcus aureus (S. aureus). Membrane filtration enumeration of S. aureus from recreational beach waters using the chromogenic media CHROMagar SA alone yields a positive predictive value (PPV) of 70%. Presumptive CHROMagar SA colonies were confirmed as S. aureus by 24-hour tube coagulase test. Combined, these two tests yield a PPV of 100%. This algorithm enables accurate quantitation of S. aureus in seawater in 72 hours and could support risk-prediction processes for recreational waters. A more rapid protocol, utilizing a 4-hour tube coagulase confirmatory test, enables a 48-hour turnaround time with a modest false negative rate of less than 10%.

Figure 1

Algorithm for Identification of S. aureus by CHROMagar™ SA and Tube Coagulase Colonies are picked from a filter placed on CSA, then gridded to a secondary CSA plate. Mauve-like (including mauve) candidate colonies are subjected to confirmatory testing by tube coagulase. The gram-stained microscopy and species confirmation discussed in the text were methodological validation tests, and are not considered part of the quantitative algorithm.