Chloral hydrate plates for the inhibition of swarming of Proteus.

In routine bacteriology the swarming of Proteus on plate cultures often masks the identity or presence of other bacteria which may be present. Mixed cultures including Proteus are often obtained from open wounds or sinuses following abdominal or chest operations, so that unless some special selective medium is used it is usually impossible to identify or to isolate other bacteria from a culture containing Proteus. Frequently ,B-haemolytic streptococci are, missed in such cultures, and thus one often fails to identify an organism causing some infection which may have become secondarily infected with Proteus. Numerous methods have been published for the inhibition of swarming of Proteus on culture plates, the s-mplest being the use of 6 per cent agar (Hayward and Miles, 1943). This high concentration of agar often leads to technical difficulties in preparation, and if the plates are not sufficiently dried, or if a very moist specimen is inoculated, Proteus is not necessarily prevented from swarming. Many chemicals have been incorporated in agar to make selective media, ihe most popular method being that of adding 1/5,000 sodium azide (Snyder and Lichstein, 1940). Unfortunately, when sodium azide blood-agar plates are used the blood tends to haemolyse and the plates do not keep satisfactorily before use. Jones and Handley (1945) described a satisfactory highly selective medium for the isolation of Salmonellae from material contaminated with Proteus. In 1931 Kraimer and Koch reported the results of their experiments using 14 different types of media for the inhibition of swarming, and showed how the addition of chloral hydrate prevented the swarming of Proteus. In this country the practical use of chloral hydrate plates appears to be little known except in certain laboratories; this paper has therefore been written in order to bring it to the knowledge of everyone as being a practical method.