Explain Theory or Principle of Determination of Coliforms?

Coliforms are gram negative, non-spore forming, facultative anaerobic bacteria which produce acid and gas on lactose fermentation at 37°C within 48 hours. It includes E. coli, Klebsiella and Enterobacter and are used as indicator of fecal contamination. Though coliforms commonly inhabit the intestinal tract, their presence on food may be because of non-feacal origin. Therefore, the results of coliform counts should be cautiously interpreted.