Evaluation of colistin and nalidixic acid in Todd-Hewitt broth for selective isolation of group B streptocci.

Todd-Hewitt broth (THB) containing colistin and nalidixic acid was compared with four other media with respect to efficacy of isolation of group B streptocci. It was effective as plain THB, THB with gentamicin and nalidixic acid and blood, and THB with colistin, nalidixic acid, and blood. THB with gentamicin and nalidixic acid, but without blood, was inhibitory to group B streptocci. The value of THB with colistin and nalidixic acid lies in its ability to successfully inhibit gram-negative organisms and still promote group B streptococcal growth without the addition of blood. This greatly reduces the time and expense of media preparation and permits early determination of bacterial growth, so that other means of rapidly identifying group B streptocci can be applied at the earliest possible time.