food and bioproducts processing 88 (2010) 335–340 Contents lists available at ScienceDirect Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp A critical evaluation of sampling methods used for assessing microorganisms on surfaces J. Verran * , J.Redfern, L.A. Smith, K.A. Whitehead School of Biology, Chemistry and Health Science, Manchester Metropolitan University, Chester St., Manchester M1 5GD, United Kingdom abstract 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. © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Microbiology; Surface hygiene; Cleaning 1. Introduction With high numbers of microorganisms potentially present on surfaces, accurate methods for enumeration are essential. Numerous sampling techniques are available to microbiol- ogists, including swabs, agar contact plates, wipes, tapes, hygiene monitors, dust and bulk sampling as well as microscopy of the surface. Swabbing is a widely used sampling method, but it lacks the standardisation required to provide the level of reproducibility required. The efficiency of swab- bing is reliant on the efficiency of the individual carrying out the procedure in three areas: the removal of bacteria from the surface; the removal of bacteria from the swab, and cultivation of bacteria (Moore and Griffith, 2007). In addition, the proper- ties of the surface (topography, wettability, porosity, etc.), and the presence of organic material on the surface can affect the efficiency of swabbing. It should perhaps also be routine to check the surface after swabbing for residual microorganisms. Alternative or supplementary methods are many, but all have their limitations. Contact plates (agar plates pressed ∗ Corresponding author. Tel.: +44 161 247 1206; fax: +44 161 247 6831. E-mail address: j.verran@mmu.ac.uk (J. Verran). Received 8 June 2010; Received in revised form 14 September 2010; Accepted 17 September 2010 directly onto the test surface) are more successful if selective culture media are used for particular indicator microorgan- isms on a surface: if surfaces are rough, or wet, then the sampling is inaccurate, or resultant growth on the agar may be confluent (Egington et al., 1995). Indicators for surface hygiene include ATP biolumines- cence sampling (Holah et al., 1989), which requires swabbing, and ultra-violet light irradiation, which enables a simple visual assessment of gross soiling. Neither method discrim- inates between soil and microorganisms, but the presence of microorganisms raises ATP readings considerably (Whitehead et al., 2008). It is highly likely that – in the food engineering plant – microorganisms will be present on the surface alongside organic material. This material can affect the efficiency of cleaning and disinfection protocols, and can also provide nutrient, or protection for microorganisms (Verran et al., 2008). However, studies on the effect of organic soil on methods for detecting microorganisms are few. In our laboratories, the use of a differential fluorescent stain accompanied with image 0960-3085/$ – see front matter © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fbp.2010.09.011