Susceptibility of Proteus mirabilis and its Stable L-Forms to Erythromycin and Other Macrolides

Abstract

Proteus spp. and, other Gram-negative rods have been reported to be highly resistant to macrolides¹, but nothing is known about the mechanism of this natural resistance. Using Proteus mirabilis I have compared the susceptibilities of the normal bacillary forms and the so-called stable L-forms. The latter are distinguished by the absence of the cell walls of the parent strain. The absence has been demonstrated by the lack of phage receptors^2,3 and by chemical analysis⁴. In this respect they are similar to the protoplasts of Gram-positive bacteria produced by the action of lysozyme⁵; but, unlike these, they are able to grow well on solid media and may also be adapted to cultivation in broth. Like protoplasts, however, they do not revert to the normal rod forms, but grow indefinitely as L-forms even in the absence of penicillin, which was used for their induction in the first place.