New sequential touch method for determining bacterial contact transfer rate from finger to surface

Abstract

Bacteria can be transferred via surface touch. To evaluate the transfer rate, traditional single-touch methods require measuring the number of bacteria on donor and recipient surfaces, which is typically characterized by high levels of uncertainty. In this study, two concentrations of Staphylococcus aureus ATCC 25923 were inoculated on a clean thumb. For each set of trials, sequential touches were made between the thumb and each of 30 sterile glass slides, and each slide was placed in a sterile petri dish. The transferred bacteria on each slide were directly cultured in situ, and the colony-forming units (CFUs) were counted. The bacterial contact transfer rate was calculated by fitting the series of CFUs with the formula established. The average transfer rate was 12.9% under these conditions. The goodness of fit was compared in terms of the number of slides used in a set of trials and the number of CFUs counted on the slides. The use of more slides in a set of trials allowed more accurate evaluation of the transfer rate. The use of fewer than 20 slides was unacceptable. The high density of CFUs on the slides made counting them difficult, but if fewer than five CFUs were counted in a set of trials, the fit would be significantly influenced. To further evaluate the method, the dermal resident microflora on the thumb were also used to perform contact transfer tests. No statistically significant difference was found in the estimated transfer rate between the standard strain and the resident microflora.\n\nIMPORTANCEDiseases can be transferred indoors via the surface route because bacteria and viruses can be transferred to and from the hands when a fomite is touched. Various methods have been used to estimate the bacterial contact transfer rate between hands and surfaces. Evaluated transfer rates have had significant deviations and varied significantly across studies, partially due to the use of the single hand-surface touch method, inefficient hand/surface sampling, and complicated bacteria culture.\n\nIn this study, the bacterial contact transfer rate was evaluated with a new method involving sequential touches between a donor and a series of recipients. The bacteria on the recipients were cultured in situ without hand/surface sampling, which simplified the process of surface bacteria quantification. The new method significantly reduces experimental complexity, decreases random errors in the data, and provides a new method for understanding microbial transfers between surfaces.