[A simple wash technic for the recovery of test pathogens in the evaluation of surface disinfection procedures]

Abstract

Methods currently recommended for quantitative bacteriological sampling of surfaces usually suffer from either unreliable results (plain swabbing techniques and contact cultures) or high expenses in terms of material, equipment and/or labour (rinse techniques using sampling fluids). Therefore, a modified swab-rinse technique (SRT) was devised as a reasonably precise and simple alternative: A small amount of sampling fluid (1.5 ml), which can contain neutralisers, is transferred onto the flat surface under investigation; with this fluid and a pre-moistened small cotton-swab an area of 3 cm diameter is thoroughly washed for 15 s. Then 0.1 ml- and 0.5 ml-amounts of the washliquid are collected with automatic microliter pipettes and transferred and spread onto Casein-Soy-Agar for enumeration. In parallel experiments with contact cultures (rodac plates), the new SRT up to 3000 cfu per sample exhibited a linear answer to increasing inocula of E. coli on tiles (Fig. 1, rings). Rodac-plates proved to possess a rather limited span of reliable counts: above 100 colonies, increasing numbers of bacteria were prevented from forming distinguishable colonies (Fig. 1, dots and +es); thus, colony counts higher than 100 on rodac plates cannot be expected to be true estimates. In addition, in the higher count range the results of a common, simplified counting technique differed markedly from results of true counts (Fig. 2). Both methods were equally sensitive in detecting low counts of bacteria (Fig. 1, Fig. 3). Shake-rinse techniques using high volumes of sampling fluid provide lower sensitivity than contact cultures or the new SRT: The use of 100 ml of sampling fluid (15) and plating of 0.1 ml- or 1.0 ml-aliquots of sampling fluid keeps the threshold for detection of bacteria as high as 1000 and 100 per sample, respectively (Fig.3); nevertheless, sensitivity of shake-rinse techniques can be increased by filtration of the whole sample. Thus, the new swab-rinse technique combines several advantages: wide span of true estimates since washliquid can be diluted for enumeration of high counts; high sensitivity ( = ability to detect small numbers of testbacteria in sample) since about one half of the sample is plated; good recovery of testbacteria from both smooth and coarse surfaces; simplicity; the new swab-rinse technique requires basic laboratory equipment and ordinary media and no shaking- or filtration devices; option for immediate and strong neutralisation of disinfectant residues; the sampling fluid can contain any neutraliser; option for automated colony counts since any kind of petri-dishes can be used for culture.