SODIUM CHLORIDE MEDIA FOR THE SEPARATION OF CERTAIN GRAM-POSITIVE COCCI FROM GRAM-NEGATIVE BACILLI

Abstract

It has been found that pH 6.0 NaCl agars, with salt concentrations of 2-20%, exert marked inhibitory action on the growth of bacilli of the typhoid, paratyphoid, dysentery and colon groups, on species of Proteus, Pseudomonas, and on diphtheroids and Bacillus anthracis. The Gram-positive cocci studied tolerate high salt concentrations, all being positive on transfer from 20% NaCl agar. In pH 6.0 broths, with salt concentrations of 2-25%, the same differential bacteriostasis may be observed, although to a lesser degree than on agar. It has been found that when mixtures of cocci and bacilli in different proportions are cultured on appropriate salt agars, the cocci invariably outgrow the bacilli and may sometimes be recovered in pure culture. Use of 6, 8, 10, and 15% salt agars greatly facilitates isolation of Gram-positive cocci from specimens from mixed infections. Use of such salt agars is therefore suggested for inhibition of Gram-negative bacilli and for isolation of Gram-positive cocci.