A critical evaluation of sampling methods used for assessing microorganisms on surfaces

Methods used to evaluate the effectiveness of cleaning and disinfection regimes, or of putative antimicrobial surfaces, rely on an estimate of the amount of viable microbial cells remaining on a surface after treatment. In essence, microbial cells are applied to the surface, and the number remaining after a specified time/treatment is assessed by variety of methods. This paper provides a critical commentary on these methods.The most common method relies on removal of the cells from the surface, by swabbing or agitation, plating onto culture media, and counting the number of colonies obtained. However, the surface should always be subsequently examined for residual cells: low numbers of colonies are deemed indicative of effective cleaning (i.e. few cells on the surface), but they could also indicate that cells have not been removed from the surface. Swabbing efficiency can be affected by moisture at the surface, presence of organic material, surface topography and presence of antimicrobial compounds.It is important to be aware of the limitations of a given method for assessing the presence of microorganisms on a surface, as well as of the intended antimicrobial property of the surface or agent applied to the surface.

Research highlights▶ Microorganisms not always removed from surface by swabbing. ▶ More than one method recommended for assessing surface contamination. ▶ Cells attached on surfaces should be visualised microscopically, and/or in situ viability assessed.