During sanitary and bacteriological testing of water with the membrane filter technique there were revealed two red colonies on the membrane filter (Endo medium) through which 500 ml of water was filtered. Calculate the coli index and coli titer of this water:

Answer options

1. 250 and 2
2. 2 and 500
3. 4 and 250
4. 500 and 2
5. 250 and 4

Explanation

The coli titter is determined as the smallest volume of water in which the presence of Escherichia coli is established. The coli index is determined as the number of Escherichia coli cells in 1l. water sample, and it is calculated based on the coli titter value with the equation: Coli titer 500/2 = 250 Coli index = 1000/coli titer 1000/250 = 4