Evaluation of biocide efficacy by microcalorimetric determination of microbial activity in biofilms

The microcalorimetric measurement of microbial activity of biofilm samples allows easy testing of the efficacy of biocides. This has been demonstrated in experiments with biofilm samples consisting of sulfate reducing (SRB) and chemoorganotrophic (COT) bacteria formed in batch culture on mild steel coupons. Additionally, biofilms were produced in continuous culture on the surface of a flow-through gold tubing in a measuring cylinder of the calorimeter. The biofilm samples were treated with the biocides formaldehyde, tetramethylammoniumhydroxide, 1,8-dihydroxyanthraquinone, and a commercial biocide with glutaraldehyde as one of the active compounds at varying concentrations and incubation times. For evaluation of biocide efficacy, microbial activity was monitored and cell counts were determined. All biocides were able to reduce microbial activity, but cell numbers did not decrease correspondingly. Formaldehyde exhibited the best effect. Only 3% of the original microbial activity remained, and a reduction in cell numbers of five orders of magnitude in the case of SRB was measured. In contrast, tetramethylammoniumhydroxide had only a slight effect. Microbial activity was reduced only to 20%, and the cell numbers did not decrease at all. The other biocides exhibited intermediate effects. In general, cell numbers of chemoorganotrophic bacteria in these biofilm samples decreased more than did the numbers of SRB. If the biocide containing medium was substituted by a biocide-free one, regrowth and reactivation of biofilm cells resulted. However, the activity did not reach initial values within the experimental time.