An Example of Bacterial Synergism on Endo Medium.

Abstract

Bacterial Synergism has usually been studied in fluid medium. I wish to report here an example of the association of B. welchii and B. faecadis alkaligenes on an Endo plate with a resulting change of the type of colony formed by the latter. A swab taken from the drainage wound of an acutely infected shoulder, was planted in hormone-dextrose-broth and on blood agar. The broth showed a growth of gram negative bacilli and gram positive cocci in chains. The blood agar showed haemolytic colonies of streptococci and colonies of the coliform type. The broth culture was spread on an Endo plate and after 24 hours incubation this plate showed numerous colonies with red centers and narrow, colorless peripheries. In such isolations only colorless colonies are usually picked for further study, but because of the colorless periphery of these colonies, one was transferred to hormone broth. A smear from a 24 hour growth of this culture showed nothing but gram negative bacilli. Generous portions of the broth culture were transferred to tubes of broth containing dextrose, lactose, saccharose and salicin and to litmus milk. After 24 hours there was fermentation with the production of acid and gas in the tubes with dextrose, lactose and saccharose and the typical “stormy fermentation” of B. welchii in the litmus milk. Gram negative and gram positive bacilli were seen together in all the tubes, except that containing salicin.

The two organisms were isolated in pure culture. The gram negative bacillus fermented none of the carbohydrates and gave an alkaline reaction in litmus niilk. The gram positive bacillus gave the typical reactions of B. welchii in litmus milk and grew well in Veillon agar and on blood agar plates in an anaerobic jar. It was also pathogenic for guinea pigs. The B. faecalis alkaliigenes produced colorless colonies on Endo agar. The B. welchii did not grow on this medium under aerobic conditions. Mixtures of the two planted on Endo agar reproduced the red centered colony with the colorless periphery as seen in the original plate. The anaerobe apparently grows in the interior of the colony of the aerobe and here acts on the lactose and produces acid. Two other examples of this phenomenon were noted later, one from a sample of faeces from a case of suspected typhoid and another from a swab from an infected appendix.