Variation in Detection Limits of an ATP Bioluminescence Meter between Bacterial Growth Curve Phases

Introduction: Handheld devices for measuring adenosine triphosphate (ATP) by bioluminescence are increasingly popular for rapid monitoring of surface hygiene. ATP is present in all living cells including bacteria; however, the quantity of ATP per cell is variable between bacterial species and in different growth phases. Therefore, it is not always clear how ATP measurement correlates with the presence of bacteria.

Purpose: The purpose of this study was to determine the detection limit (sensitivity) of the Hygiena SystemSure ATP meter in lag, log, stationary and death phases of the bacterial life cycle for Escherichia coli and Staphylococcus aureus.

Methods: A bacterial growth curve was performed for each organism tested. Samples were taken at 0 hours (lag phase), 4 hours (log phase), 12 hours (stationary phase), and 24 hours (death phase). Samples were diluted and tested by the SystemSure and cultured for colony counts. For each growth phase the colony count (colony forming units/ml or CFU/ml) at which the SystemSure reading fell below the “clean” cutoff of ≤10 relative light units (RLU) was determined.

Results: The limits of detection of the SystemSure in CFU/ml were as follows. For E. coli: lag phase: 2,175,000; log phase: 24,000; stationary phase: 2,860,000; and death phase 1,010,000.  For S. aureus, the detection limits were: lag phase: 1,115,000; log phase: 171,000; stationary phase: 15,300,000; and death phase: 1,110,000.

Significance: The CFU/ml at which the SystemSure read ≤10 RLU indicates the concentration of living organisms present in the sample. The detection limit for the SystemSure varied by 100 fold depending on the bacterial life phase, with the greatest sensitivity during log phase and the least sensitivity during stationary phase. Users of ATP bioluminescence devices must be aware that living bacteria may be present on a surface that is determined to be “clean” by ATP detection.