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Comparison of Plate Counts, Petrifilm, Dipslides, and Adenosine Triphosphate Bioluminescence for Monitoring Bacteria in Cooling-Tower Waters

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Effective bacterial control in cooling-tower systems requires accurate and timely methods to count bacteria. Plate-count methods are difficult to implement on-site, because they are time- and labor-intensive and require sterile techniques. Several field-applicable methods (dipslides, Petrifilm, and adenosine triphosphate [ATP] bioluminescence) were compared with the plate count for two sample matrices—phosphate-buffered saline solution containing a pure culture of Pseudomonas fluorescens and cooling-tower water containing an undefined mixed bacterial culture.

For the pure culture, (1) counts determined on nutrient agar and plate-count agar (PCA) media and expressed as colony-forming units (CFU) per milliliter were equivalent to those on R2A medium ( p = 1.0 and p = 1.0, respectively); (2) Petrifilm counts were not significantly different from R2A plate counts ( p = 0.99); (3) the dipslide counts were up to 2 log units higher than R2A plate counts, but this discrepancy was not statistically significant ( p = 0.06); and (4) a discernable correlation ( r 2 = 0.67) existed between ATP readings and plate counts.

For cooling-tower water samples ( n = 62), (1) bacterial counts using R2A medium were higher (but not significant; p = 0.63) than nutrient agar and significantly higher than tryptone-glucose yeast extract (TGE; p = 0.03) and PCA ( p < 0.001); (2) Petrifilm counts were significantly lower than nutrient agar or R2A ( p = 0.02 and p < 0.001, respectively), but not statistically different from TGE, PCA, and dipslides ( p = 0.55, p = 0.69, and p = 0.91, respectively); (3) the dipslide method yielded bacteria counts 1 to 3 log units lower than nutrient agar and R2A ( p < 0.001), but was not significantly different from Petrifilm ( p = 0.91), PCA ( p = 1.00) or TGE ( p = 0.07); (4) the differences between dipslides and the other methods became greater with a 6-day incubation time; and (5) the correlation between ATP readings and plate counts varied from system to system, was poor ( r 2 values ranged from <0.01 to 0.47), and the ATP method was not sufficiently sensitive to measure counts below approximately 10 4 CFU/mL.
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Keywords: Petrifilm; Pseudomonas fluorescens; adenosine triphosphate (ATP) bioluminescence; bacteria; bacterial enumeration; cooling towers; dipslides; heterotrophic plate count; plate counts

Document Type: Research Article

Publication date: 2009-04-01

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