**4.6.2 Approximate assay for coliform bacteria count**

To produce coliform bacteria, water is planted into Durham tubes including 2% peptone, 0,5% sodium taurocholate, 1% lactose and bromthymol blue (or bromeresol purple) as indicator. Amount of the water planted varies between 0,1 cm3 and 50 cm3. Concentrated medium is used if more water will be planted. The least water amount including coliform bacteria is detected by assays and contamination degree of the water is determined. Generally, 20 cm of 15 tubes included medium is taken. 0,1-1 cm3 water is put into 5 tubes; 1 cm3 water is put into 5 tubes; and 10 cm3 water is put into 5 tubes. Tubes are left at 37°C for 48 hours. After 24 and 48 hours, tubes are controlled and acid and gas formation is controlled. If gas is produced, it is examined that such gas has filled 1/10 of the small tube in Durham tube. Formation of gas which will occupy 1/10 of the tube after 24 hours shows possibility of Eschericihia coli reproduction. If no gas has formed within 24 hours or the gas occurred is less than 1/10 of the small tube, tubes are left at 37°C for 48 hours. Any gas formation is considered as suspicious. No gas formation after 48 hours shows no reproduction of Eschericihia coli.

To confirm whether reproduced bacteria are Eschericihia coli, planting is performed from tubes with least amount of water that reproduction was observed to eosine-methylene blue agar by decreasing method. Petri dishes are left at 37°C for 24 hours and examined and it controlled whether the bacteria reproduced is Eschericihia coli.
