EVALUATION OF THREE MICROORGANISM RECOVERY PROCEDURES USED TO DETERMINE HANDWASH EFFICACY
Daryl S. Paulson, PhD., Dairy, Food & Environmental Sanitation
Accurate and reliable determinations of the microbial populations residing on the hands are critical in evaluating the effectiveness of handwash products and methods (Block, 1991; Paulson, 1993). Only when one is sure of reliable hand sampling methods can one attempt to assess the benefits of the handwash procedure in terms of microbial reduction.
In trying to establish the efficacy of various handwash products and handwash methods, a number of food processing plants conduct experiments with their workers to evaluate antimicrobial efficacy. The two hand sampling methods most commonly used in these experiments in measuring the levels of microorganisms remaining on the hands are the "swab" and the "finger press" techniques.
In brief, the swab technique consists of swabbing the palmer surfaces of the hands as well as between the digits with a pre-moistened swab and culturing it on an agar plate. The finger press method is conduced by having test subjects press their palmer surface and/or finger pads lightly onto an agar plate.
There is also a third method which is not widely used in the food industry but is the standard hand sampling method for the evaluation of medical hand disinfection products. It is the "glove juice" method. This method consists of placing surgical gloves over the hands, instilling a surfactant to strip the hands of bacteria, and the plating of aliquots taken from the "glove juice" contained in the gloves (ASTM, 1987).
A study was designed in our laboratory to compare these three methods for their accuracy in estimating known microbial populations on the hands. Four different contamination levels were used to evaluate how the sampling methods responded in terms of accurate population estimates to varying population levels.
The focus of this study was with contaminating microorganisms, not with the normal microrgrnisms residing on the hands. We artificially seeded human volunteers' hands with known levels of the marker bacteria Serratia marcescens. The use of Serratia marcescens was valuable to this study in three ways. First, it is as resistant to mechanical removal from the hands as such pathogens as E coli, Salmonella sp., and Shiegella sp. Second it develops red bacterial colonies on tryptic soy agar which distinguishes it from any other microrganism residing on the hands. This prevents mistaking the artificially contaminated microorganisms with normal or other transient microorganisms, thereby preventing biasing the recovery estimate of the wash procedure. Third, since it is seeded onto the hands in known, equal population levels, the precision and accuracy of the three hand sampling procedures can be compared directly.