Enumeration of iron-oxidizing bacteria by the membrane filter technique

This work presents a simple methodology to enumerate ferrous-iron-oxidizing bacteria in solution, easily applicable in bioleaching industrial plants, because it does not require expertise or specific equipment. The enumeratio n is based on bacterial concentration by microfiltration through a membrane filter. The filter containing the bacteria is placed on an agarose plate c ontaining ferrous sulphate for bacterial growth. No difference was observed for the enumeration of Acidithiobacillus ferrooxidans ATCC 19859 when eith er 0.1 or 0.22 mum pore size membrane filters were used. However, when the technique was applied to bacteria present in pregnant leaching solution, th e smaller bacteria present in these solutions passed through the 0.22 mum p ore size membrane. Therefore the number of bacteria could be underestimated if they are monitored and filtered using a filter with pore size greater t han 0.1 mum. The limit of detection of this technique was one ferrous-iron- oxidizing bacterium in the filtered solutions.