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The influence of the microbial contamination load (xl) and preservative concentration (x2), as predictor variables, on the percentage of vials with growth of B. pumilus spores (y) was studied. The effect of a wide range of spore loads (1‐500,000 spores of B. pumilus per sample) and preservative (chlorhexidine acetate) concentrations (0.03–40 μg/ml) on the percentage of contaminated samples was determined. Each contamination load was studied by adding the specified number of spores to vials containing various concentrations of chlorhexidine dissolved in 2 ml of nutrient broth. Every level of spore and chlorhexidine concentration was represented by 50 or 100 vials. Curves representing the number of contaminated vials against the concentration of chlorhexidine were sigmoidal in shape and the data showed an indirect relationship. Data were analyzed by the regression analysis which revealed a significant correlation (R=0.83) with the regression equation:

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