SUMMARY: The growth of on agar media containing bile salts is conditioned by a number of factors. The proportion of total cells able to grow on a medium not containing bile salts rapidly declines at temperatures of incubation above 43°. On a medium containing bile salts and lactose a distinctly inhibitory effect is observed at 37°; with most strains this effect is still more pronounced at 44°. Some brands of bile salts are appreciably more inhibitory than others. Inclusion of phosphate in a bile salts medium introduces a markedly inhibitory factor, the severity of which varies with the strain of organism; some strains are virtually unable to grow on such a medium.

When a culture of is suspended in water containing only small concentrations of inorganic salts an increasing proportion of the population becomes attenuated so that the cells are unable to grow on bile salts lactose agar at 44°. An occasional strain may also exhibit sensitiveness to the presence of neutral red. This attenuating effect may be largely decreased if, before the inoculum is mixed with the bile salts and agar, it is subjected to a short period of incubation with lactose broth. This treatment has been made the basis of a technique for obtaining a colony count of which is applicable to polluted waters.


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