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

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.