Veriication of Plate Count Agar (PCA) and Reasoner’s 2A (R2A) Agar for Enumeration of Heterotrophic Bacteria in Water and Ice

Abstract

       Pour plate technique is well-recognized as a standard method for enumerating heterotrophic bacteria in water and ice. Either high-nutrient medium such as plate count agar (PCA) or low-nutrient medium such as Reasoner’s 2A (R2A) agar can be used for this standard method. It has been recommended that the low-nutrient medium containing essential substances for recovering injured cells will increase bacterial counts as compared to the high-nutrient medium without any essential substances. This study was aimed to verify the culture medium by comparing the number of heterotrophic bacteria in water and ice grown on PCA and R2A agar. A total of 360 samples of water and ice received at Bureau of Quality and Safety of Food during 2012 - 2014 were divided into 4 types of waters including 60 samples of bottled drinking water, 60 samples of drinking water, 60 samples of potable water, 120 iltrated water samples and 60 samples of ice. All samples were tested by pour plate method using PCA and R2A agar. After incubation at 35°Celsius (°C) for 48 hours, growing colonies on culture plates were counted and categorized into 3 levels: 1-1,000 CFU/ml, 1,001-100,000 CFU/ml and more than 100,000 CFU/ml. The results showed that the mean values of bacterial counts of all types of water samples calculating from the R2A agar were signiicantly higher than those from the PCA agar (P<0.05) at all 3 levels. Comparing the distribution of test results from both media by coeicient of variation (CV), it was found that the R2A agar showed lower CV results than the PCA agar. For ice samples, the mean values and CV results of bacterial counts at the levels greater than 1,000 CFU/ml on the R2A agar were signiicantly higher and lower than those of the PCA agar (P<0.05), respectively. On the contrary, the mean value and CV result of bacterial counts, at the level of 1 - 1,000 CFU/ml, obtained from the R2A agar were not signiicantly diferent from the PCA agar. Taken together, it can be concluded that, for higher productivity, the R2A agar could be used for determining heterotrophic bacteria in water and ice by pour plate technique at 35°C for 48 hours.