On MacConkey agar, colonies are colourless, flat, often swarm slightly and are 2-3mm in diameter (this is specific to Proteus vulgaris and Proteus mirabilis). Other species do not swarm. They are resistant to polymyxin B and colistin. Proteus species can resemble non- motile Salmonella biochemically, and can agglutinate in polyvalent salmonella antisera