Confirmatory tests were made in 4807 positive tubes of MacConkey broth. It was found that when an equal volume of seawater was added to a double strength MacConkey broth only 88% of the tubes contained coliform organisms (12% false-positive reactions); when, however, the ratio of the volume of seawater to broth was 1/5 or less, coliform organisms were found in 93·5% of the positive tubes (6·5% false-positive reactions). Glutamate medium gives a higher rate of false reactions than MacConkey broth when an equal volume of seawater is added to double strength medium. When the volume ratio of seawater to glutamate medium is equal to or less than 1/5, then the minerals-modified lactose glutamate medium gives better results than MacConkey broth in the coliform and especially in the Escherichia coli count. The difference is statistically significant.
It is thus concluded that for the bacteriological examination of seawater the volume ratio of water to medium should be 1/5 or less and also that glutamate medium is the medium of choice.