The mechanism of bacteriostasis

Abstract

The effect of bactericidal agents is often tested by adding these substances to the media on which the organisms are planted; and the assumption is usually made that if the substances, when present in the media, exhibit a selective hostility to bacteria they will exhibit a hostility—selective in the same sense—when added directly to the organisms themselves. This assumption is usually justified by the facts; no single exception to such a parallelism has been met with in the large number of experiments made with gentian violet and allied tri-phenylmethanes. We are in the habit therefore of reasoning from experiments which test bacteriostasis to conclusions as to bactericidal value, at least so far as selective features are concerned.

Proof will here be presented to show that this sort of reasoning is not always justified by the facts. Suppose for example that we plant B. prodigiosus and B. megatherium on acid fuchsin agar and find that B. prodigiosus grows well and B. megatherium not at all. We would then certainly expect that if these two organisms were exposed to acid fuchsin and planted on plain agar the latter would be killed and the former unaffected. As a matter of fact the opposite occurs. A similar result is obtained if the experiment be done with the flavines instead of with acid fuchsin.

If we describe the bacteriostasis which a substance exhibits when bacteria are exposed to it (before being planted on plain agar) as intrinsic bacteriostasis, and that which a substance exhibits when it is present in the media on which the unstained bacteria are planted, as extrinsic bacteriostasis: then we may say that the intrinsic selective bacteriostatic features of a substance so far from necessarily running parallel with its extrinsic selective features may run directly counter to them.