Committee on Microbiology and Extraneous Materials

Abstract

Study Director Gun L. Wirtanen (see Recommendations, 46). Sampling of environmental surfaces is of the utmost importance, since these surface organisms can readily be transferred to the food being processed. There are several sources of these surface organisms: workers’ hands, the raw materials themselves, insects, poor sanitation practices, and niches in the equipment, allowing the growth of a variety of organisms (1). These organisms can proliferate to the extent that a biofilm is formed, especially on irregular surfaces which are difficult to clean. Moreover, these biofilms may contain pathogenic, as well as nonpathogenic, microorganisms. The sampling of environmental surfaces can be difficult because microorganisms contained in these biofilms can adhere strongly to the environmental surfaces. If these organisms are removed forcefully, they may be damaged to the extent that they would not be detectable by methods based on multiplication of microorganisms in agar. Environmental surfaces are normally sampled by the Replicate Organism Direct Agar Contact (RODAC) plate method (1) or by swabbing. The principle of the Hygicult TPC dipslide (Orion Diagnostica, Finland) method is similar to that of the RODAC method in that both methods allow the surface of a growth medium to come in direct contact with the environmental surface to be sampled. With the swabbing method, however, it is necessary to transfer the organisms from the swab to the cultivation medium. These contact methods are also based on the gentle detachment of the surface-bound organisms, a limiting factor with the swab method (2). A collaborative study was conducted to validate the Hygicult TPC dipslide method for enumerating total aerobic microflora on surfaces artificially contaminated with bacterial mixtures at 3 levels. The dipslide was compared with the contact plate method and the swabbing method for surface sampling. Twelve laboratories participated in the study. The total number of collaborative samples was 108. The microbial level in each sample was determined in triplicate with each of these 3 methods at each of 3 different incubation conditions (at 25 ± 1 C for 48 and 72 h, as well as 30 1 C for 48 h). Surface sampling methods detached 25–30% at the lowest (theoretical yield 1.4 cfu/cm), 18–20% at the middle (theoretical yield 10.7 cfu/cm), and 16–21% at the highest (theoretical yield 43.6 cfu/cm) levels of bacteria from the stainless steel surfaces. The percentage of acceptable results after removing outliers was 89%. Repeatability standard deviations, RSDr, ranged from 27.2–74.6%, and reproducibility standard deviations, RSDR, ranged from 42.1–97.5%. There were no significant differences obtained either in incubation temperatures (25 and 30 C) or incubation times (48 and 72 h) in all 3 methods. The effect of incubation temperature and time on precision parameters was based on an examination of the estimated precision parameters. The 3 methods gave similar results for all 3 levels tested: 0.35–0.43 cfu/cm at the lowest level, 1.9–2.2 cfu/cm at the middle level, and 7.1–9.1 cfu/cm at the highest level. On the basis of these results, the Study Director has recommended that this method be adopted First Action. The General Referee has recommended that the collaborative study manuscript be revised.