Mixed bacterial cultures have been applied in commercial plants to treat refractory gold concentrates. The operating conditions of these plants vary and there are differences in the design of the plants and their response to process interruptions, particularly loss of aeration. A review of literature on the effect of operating conditions on the bacterial population has been completed to try and explain the reasons for the differences in plant design and the greater sensitivity to air interruption seen in some commercial operations. Operating parameters covered in the review are temperature, nutrient composition, solution pH, substrate, and dissolved metals. A possible reason for the greater impact of loss of aeration is proposed based on this review. Some of the consequences of the bacterial culture selection on capital and operating cost are also presented.