Situation of Colicin Receptors in Surface Layers of Bacterial Cells

SUMMARY: The susceptibilities to a series of colicins of Escherichia coli B, two Proteus mirabilis strains and their respective stable L-forms were compared. Certain stable L-forms are known to be completely devoid of their cell walls and consequently are completely phage-resistant. Nevertheless, such L-forms show the same or even higher degrees of susceptibility to colicins than do their parent bacteria; they also display exactly the same specific patterns of susceptibilities as do the corresponding normal bacillary forms. Colicins may become adsorbed directly to the cytoplasmic membranes of L-forms. The attachment of a colicin molecule to an adsorption site localized in the bacterial cell wall therefore cannot be regarded as a compulsory initial step leading to the killing of the cell (and the specificity of the interaction between colicins and bacterial cells cannot be determined by cell-wall receptors). Adsorption of colicins to P. mirabilis (normal and L-forms) is not possible after killing them with formaldehyde. The time of rescue by trypsin of cells which have adsorbed colicin is decreased to zero with L-forms. Obviously, the ‘lethal’ adsorption of colicins is to receptors in the cytoplasmic membrane.