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Testing for Pseudomonas aeruginosa

We all know the saying “an old dog can’t learn new tricks”. I detest it. It promotes a resistance to innovation and change.

I see this same resistance in water hygiene as we favour “traditional” methods instead of advancement and, consequently, fail to benefit from these changes. Take the analysis of water samples for Pseudomonas aeruginosa.

P. aeruginosa is a ubiquitous, gram-negative, oxidase-positive bacteria, responsible for approximately 10% of all nosocomial – or hospital-acquired – infections. Within certain augmented care patient groups, it is associated with fatality rates as high as 50%.

Some studies suggest that around 50% of these infections originate from hospital taps and showers. The Department of Health recommends that water outlets in augmented care units are tested for P. aeruginosa every six months, and remedial works undertaken at outlets that test positive. Inadequacies and delays in sampling procedures or remedial works can increase the risk of nosocomial infections.

The traditional P. aeruginosa analysis method outlined in HTM 04–01 (Appendix 4) has changed little over the years. The water sample is passed through a 0.2µ membrane filter, then placed onto a selective agar plate containing cetyl trimethylammonium bromide and antibiotic nalidixic acid. This is incubated at 37°C for two days and then examined for the presence of bacterial colonies.

Confirmatory analysis of any putative colonies requires inoculation onto milk cetrimide agar plates for an additional 22 ± 4 hours, and the exposure of poorly pigmented colonies to daylight for between two and four hours, before results are confirmed.

Further tests may be required to distinguish non-pigmented P. aeruginosa colonies from Pseudomonas fluorescens. This takes in excess of 4.3 days – excluding the time taken for sample collection, transportation, processing and reporting of results from the laboratory. This is a critical period when positive outlets continue to be used by high-risk patients.

The Department of Heath’s foresight, however, provides hope – the addendum states that “alternative water-testing methods to filtration can be used that show equivalence and/or improvement on the sensitivity and enumeration of P. aeruginosa”.

Innovative, alternative testing methods are now available, and some have proven in independent studies to be even more sensitive than the conventional method.

Additionally, these methods can be easily adopted by laboratories without additional expensive equipment, giving clear and accurate qualitative and quantitative results within just 12-28 hours without the need for additional confirmatory tests.

The simplicity of such tests means that laboratories can process more samples simultaneously without the bottleneck created by the need to filter water samples. Filtering can also be traumatic to some micro-organisms and they may not survive, which can produce false negative or inaccurately low results.

Adoption of these new methods by laboratories would deliver accurate final confirmatory results faster without the cost of additional tests. This would result in quicker reporting times, allowing preventive measures to be put into place earlier and reducing the risks of a nosocomial P. aeruginosa infection.

Some laboratories are starting to abandon the old conventional tests and embrace the newer, faster techniques, so perhaps “wise” old dogs can learn new tricks after all.

Tom Makin is chief executive of Envirocloud

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