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Enumerating Surface Contamination-An Impossible Challenge?

Published 16 August 2016

Hygiena International continues with its series of weekly articles that discuss monitoring microbiological contamination on surfaces. This week features its third publication, which carries on the topic point by querying, "Enumerating Surface Contamination-An Impossible Challenge?"

Although the concept of environmental monitoring by testing surface swabs is well recognised, the problems and challenges are frequently overlooked or conveniently forgotten for the benefit of simplicity, convenience and convention. Despite advances in swab materials and wetting agents, little has changed since 1917 when the principle of ‘swab and rinse’ was introduced.

In their review, Moore and Griffiths (2007) state that swabbing efficiencies are often poor with recovery rates ranging from 1% to 25%. The inability to control the reproducibility and repeatability of swabbing techniques can result in extreme variability of results obtained. They concluded that “traditional microbiological methods should neither be presumed to be the gold standard nor the optimum means to assess the efficiency of a company’s sanitation program and swabs should not be relied upon to give an accurate indication as to the level of micro-organisms present”.

What are the issues? There is no universally accepted swabbing protocol. Several researchers suggest that sampling methods and components should be chosen based on the type and species of bacteria rather than by the most bacteria recovered. There are many factors affecting swab sampling and the subsequent results obtained. Ideally the sample should be tested immediately after collection to prevent deterioration or abuse of the sample but this is usually impractical.

Delays between sample collection and testing are often inevitable. Protracted delays during transport need to be mitigated by controlled storage and transport. Removing bacteria from surfaces is considered a primary objective but their survival on the swab material and/or in the wetting agent, and their subsequent release into a diluent are equally important. Similarly the surface area covered and pressure applied between the swab bud and surface will influence results. Thereafter the resuspended sample is subject to the inherent variability of the detection method itself.

There are no agreed standards for clean surfaces. There are common proposal from several different industries that quantitative aerobic colony counts should be <2.5 per cm2, or <250 bacteria in a 10 x 10cm square. This is equivalent to a detection limit of 1 part in 100 million and is comparable to trying to collect, detect and quantify a single ant on a tennis court. This contamination level is also the limit of quantitation of the aerobic plate count when using the standard swab and rinse procedure with 10ml diluent.

Counting <25 colonies on a plate gives imprecise and variable results that should be considered semiquantitative or indicative. Grouping contamination levels as bands (or in bins) is a statistically more relevant and pragmatic approach which yields a better representation and correlation of low numbers of microorganisms on surfaces. Under these circumstances, the concept of Pass/ Caution/Fail is more realistic, meaningful and easy to understand where Pass is <250 and Fail is >1000.

The ideal solution would be to have a swab detection system that collected a representative sample to allow maximum recovery from surface without losses due to resuspension, dilution or time to initiate the test, and deliver results in the shortest possible time for trend analysis. This can be achieved if the sample collection device is also the detection device where 100% of the sample can be tested immediately.

MicroSnap Surface Express is able to deliver these attributes but like all modern rapid methods relying on metabolic activity, the challenge is how to compare these different approaches to the unique and variable colony count method, particularly at very low contamination levels.

As Einstein said “we cannot solve our problems with the same thinking we used when we created them”.

We need to acknowledge the reality of the problem and adopt a different perspective.





Source: Company Press Release