Enhancement of the infectivity of Fusobacterium necrophorum by other bacteria.

Necrobacillosis is caused by Fusobacterium necrophorum (FN), but other organisms are often present in the lesions. Their possible role was studied in experiments made with a virulent FN strain which, by itself, produced fatal necrobacillosis in mice provided that large doses (greater than 10(6) organisms, subcutaneously) were given. Mice were inoculated subcutaneously with FN suspended in sub-lethal doses (0.1 ml) of undiluted or diluted broth cultures of other bacteria. Undiluted culture of a strain of Escherichia coli reduced the infective dose of FN to less than 10 organisms; in the necrobacillosis lesions that developed, fusobacteria greatly outnumbered E. coli. A heat-killed preparation or sterile filtrate of E. coli culture had little if any effect on FN. Citrobacter freundii and comparatively small numbers of Corynebacterium (Actinomyces) pyogenes produced effects similar to that of E. coli. An alpha-haemolytic streptococcus, Pseudomonas aeruginosa, Bacteroides fragilis and Fusobacterium nucleatum also enhanced the infectivity of FN, though less strikingly than E. coli. FN increased the persistence in vivo of the alpha-haemolytic streptococcus and B. fragilis, and enabled the latter to multiply profusely.