relationship between frequency of escherichia coli and prevalence of salmonella and shigella spp. in a natural river

Introduction: microbial contamination prediction through detecting the indicator bacteria in natural waters is the first health and environmental step for preventing the transmission of waterassociated diseases. this study was designed to determine the correlation between escherichia coli as the indicator bacterium, on the one hand, and salmonella and shigella spp. as the pathogenic bacteria, on the other hand.materials and methods: totally, 60 natural water samples were collected from natural rivers in gorgan during summer and autumn of 2016. in order to detect escherichia coli, the membrane filter method with endo agar and imvic test was used and, in order to detect salmonella and shigella, the 9tube fermentation method and biochemical tests with selenite f medium, xld agar, tsi, urea, and sim were utilized. for the statistical analysis, pearson rsquo;s correlation test was used at the significance coefficient of <0.05.results: results showed that all of the analyzed bacteria were detected in the water samples. the frequency of salmonella, shigella, escherichia coli, klebsiellaenterobacter, and citrobacter was 9.5, 22.2, 4.8, 22.2, and 65.1%, respectively. the statistical analysis demonstrated no statistical correlation between escherichia coli, on the one hand, and salmonella and shigella, on the other hand; but the relationship between salmonella and shigella was significant.conclusion: judging the microbial quality of water supplies cannot be sufficient only based on the presence or absence of escherichia coli bacterium. therefore, that the use of other secondary indicator bacteria such as fecal streptococci and supplementary sulphitereducing clostridia will be advised.