Determination of the gram type using the reaction between polymyxin B and lipopolysaccharides of the outer cell wall of whole bacteria.

The polymyxin B-dependent formation of protrusions (blebs) of lipopolysaccharides on the cell wall of whole bacteria is suggested as a specific reaction for the determination of the Gram type of newly isolated bacteria. A total of 35 different bacterial species from 27 genera were studied as well as the influence of various test parameters. The reaction was specific: all 13 species of the positive Gram type gave a negative reaction, but all bacteria of the negative Gram type gave a distinct positive reaction.