Ideal Method for Detection of Bacteria on Hospital Surfaces Still Elusive, Researchers Say

Zoie A. Aiken, MSc, of the Division of Microbial Diseases at the UCL Eastman Dental Institute in London, and colleagues, assert that there is currently no ideal method for the detection and quantification of bacteria on hospital environmental surfaces. Discussing the issue in a concise communication in the May issue of Infection Control and Hospital Epidemiology, the researchers acknowledge that the detection of pathogenic bacteria on environmental surfaces is critical, and that while ATP bioluminescence is being applied in hospitals to measure surface contamination, they add that further development of new technologies in this area is warranted.

Aiken, et al. (2011) compared commercial luminometers for detecting the number Staphylococcus aureus associated with surfaces in the healthcare environment. As the reserachers explain, "The detection of organisms from hospital surfaces is commonly performed by sampling the surface with either swabs or agar contact methods such as dipslides or contact plates. Adenosine triphosphate (ATP) bioluminescence utilizes the firefly luciferase enzyme to catalyze the conversion of ATP into adenosine monophosphate, resulting in the emission of light. The amount of light emitted is quantified by a luminometer and is directly proportional to the initial amount of ATP in the sample. ATP bioluminescence assays have been used since the 1970s in the food industry, and reports on the use of this method to rapidly assess the efficiency of cleaning regimens are increasing."

The researchers assessed five different methods of quantifying bacteria from a test surface and report that their data showed that the ATP bioluminescence methods tested were not robust enough to generate quantitative data on bacterial numbers, especially at low concentrations. As Aiken, et al. (2011) note, "This work demonstrated that ATP bioluminescence was not suitable for accurately detecting the number of bacteria on a test surface over a range of concentrations. Previous studies have shown that luminometers are unable to detect low numbers of bacteria from a test surface, specifically <103 CFU/cm, and have demonstrated a poor correlation between CFU and RLU outside of a laboratory setting. In this laboratory study, somatic ATP was degraded before the release and quantification of bacterial ATP, and under optimal conditions, the reagent kit could detect the equivalent of as few as 5 bacterial cells. The luminometers that capture the generated light are able to detect lower light signals than those described in other studies, suggesting the overall sensitivity of the assay could be increased. If the aerobic colony count from a hand-touch surface is above 5 CFU/cm2, then there might be an increased risk of infection for a patient in that vicinity, and the methods described using ATP bioluminescence with luminometers would struggle to detect this low-level contamination."

The researchers add, "The viable count based on culture technique proved to be the most suitable method for detecting bacteria on test surfaces because bacteria were always detected, even at low concentrations, and this well-established method is inexpensive to perform. The viable count technique has a proven sensitivity of 110 CFU/cm2 from a wet surface. However, a high variability in the number of bacteria counted is often observed, especially at lower concentrations, and this was mirrored in our study. Thus, the viable count technique also has drawbacks and should not be considered a gold standard."

Reference: Aiken ZA, Wilson M and Pratten J. Evaluation of ATP Bioluminescence Assays for Potential Use in a Hospital Setting. Infect Control Hosp Epidem. Vol. 32, No. 5. May 2011.