Ans. C: Escherichia coli Coliforms: Gram-negative, non spore-forming, oxidase-negative, rod-shaped facultative anaerobic bacteria that ferment lactose to acid and gas within 24-48h at 36+-2degC. Not specific indicators of fecal pollution. Thermotolerant coliforms: Coliforms that produce acid and gas from lactose at 44.5+-0.2degC within 24+-2h, also known as fecal coliforms due to their role as fecal indicators. Escherichia coli (E. coli): Thermophilic coliforms that produce indole from tryptophan. Most appropriate group of coliforms to indicate fecal pollution from warm-blooded animals. Fecal streptococci (FS): Gram-positive, catalase-negative cocci from selective media (e.g. azide dextrose broth or m Enterococcus agar) that grow on bile aesculin agar and at 45degC, belonging to the genera Enterococcus and Streptococcus possessing the Lancefield group D antigen. Enterococci: All fecal streptococci that grow at pH 9.6, 10deg and 45degC and in 6.5% NaC1. Nearly all are members of the genus Enterococcus, and also fulfil the following criteria: resistance to 60degC for 30 min and ability to reduce 0.1% methylene blue. The enterococci are a subset of fecal streptococci that grow under the conditions outlined above. Sulphite-reducing clostridia (SRC): Gram-positive, spore-forming, non-motile, strictly anaerobic rods that reduce sulphite to H2S. Clostridium perfringens: As for SRC, but also ferment lactose, sucrose and inositol with the production of gas, produce a stormy clot fermentation with milk, reduce nitrate, hydrolyse gelatin and produce lecithinase and acid phosphatase.