Enumeration Method for Pseudomonas aeruginosa in Water with High Bacterial Background Flora

Reliable detection of Pseudomonas aeruginosa (PA) in water is essential for public health water quality assessments, yet culture-based methods remain challenged by high background flora. This study compared the performance of mPA-C agar and the commercial RAPID' P. aeruginosa (RAPID PA) agar in (ozone-) treated wastewater effluent and (spiked) swimming pond water and evaluated how presumptive colony colour definitions and confirmation steps affected performance characteristics. mPA-C showed strong matrix dependence and high observer variability due to a broad colour colony variation (of both PA and non-PA). RAPID PA produced clearer and more consistent pigmentation with relatively limited background growth and performed more consistently across matrices. Expanding the presumptive colony colour definition significantly improved sensitivity of RAPID PA with minimal loss of specificity. A quick subculturing step on RAPID PA for visual confirmation further enhanced specificity and markedly reduced false-positive rates. Extended incubation to 48 h increased performance only for swimming ponds. RAPID PA performed more consistently across matrices. Overall, using RAPID PA with expanded colour criteria and subculturing as a low-effort confirmation step enabled effective culture-based detection of P. aeruginosa in high-background-flora water samples. This medium represents a practical and robust alternative for routine monitoring.