ANAEROBIC GRAM-NEGATIVE, IRREGULAR RODS

Of the many bacteria within this group only the genera Fusobacterium, Bacteroides, Prevotella and Dichelobacter are commonly associated with specific (Table 1) or nonspecific (abscesses in a variety of organs and tissues, dental and oral lesions, chronic pleuropneumonia, and chronic sinusitis) diseases in livestock and humans. Fusobacterium necrophorum is the most important pathogenic species in this group, and, apart from the diseases with which it is primarily associated it may secondarily infect and complicate lesions of infectious diseases in which the mucous membranes of the gastrointestinal tract are involved, such as foot-and-mouth disease and orf. Other important species include Dichelobacter (Bacteroides) nodosus, the agent of ovine foot rot, and Prevotella melaninogenica (Bacteroides melaninogenicus), a contributing pathogen of foot lesions in cattle, primarily interdigital necrobacillosis.

These bacteria are obligate anaerobic, Gram-negative, non-sporeforming bacteria that commonly occur on mucous membranes of the mouth, the upper respiratory tract, and the gastrointestinal and urogenital tracts of healthy animals and humans. They often occur in lesions in association with aerobic or other facultative anaerobic bacteria, e.g. Trueperella (formerly Arcanobacter) pyogenes. Fusobacterium necrophorum and Bacteroides spp. appear, in many instances, to depend on these bacteria to reduce the redox potential in the affected tissues to a level that allows their growth and subsequent tissue invasion.^{7, 23, 24, 29, 36, 37}

Before the advent of the newer methods used in molecular genetics, these bacteria were very difficult to study because they are fastidious in their nutritive and atmospheric requirements, and they are usually found in association with other microorganisms, such as T. pyogenes, Pasteurella multocida, Escherichia coli, streptococci and others, and therefore may be overlooked.^{6, 49} In these mixed infections it does, however, seem likely that anaerobic species such as Bacteroides act synergistically with the other bacteria in the formation of suppurative and necrotizing lesions.⁶ This emphasizes the importance that bacteriological examinations of exudates should include both aerobic and strict anaerobic culturing techniques.

Table 1 Most common specific diseases in livestock in which Fusobacterium necrophorum, Dichelobacter nodosus or Prevotella melaninogenica are the primary causative agent. Bacteria that may play a secondary role are indicated in brackets.

One of the hallmarks of infection by anaerobic bacteria, including Bacteroides, is an exudate with a rather characteristic putrid odour, although the absence of such an odour does not preclude an anaerobic infection.^{6, 48} The odour is primarily the result of metabolic end-products, such as volatile amines, short-chain fatty acids and organic acids.⁴⁸

Fusobacterium necrophorum

Fusobacterium necrophorum is an obligate anaerobic, Gram-negative, non-sporeforming, non-motile and pleomorphic bacterium that varies in shape, ranging from small cocci (0.5–1.75 µm in diameter) to filaments greater than 100 µm long. The bacteria have irregular swellings along their length, and blunt or tapering ends.³¹

On blood agar, colonies of F. necrophorum are convex, translucent to opaque, 1 to 2 mm in diameter, and have a circular outline with scalloped to eroded edges. The colonies are often ridged or uneven.

Fusobacterium necrophorum produces haemolysin and indole, but does not reduce nitrate. Most strains cause either alpha or beta haemolysis on rabbit blood agar. Generally, beta-haemolytic strains are lipase-positive, whereas alpha-haemolytic and non-haemolytic strains are lipase-negative. No lecithinase is produced.²⁶

There is considerable doubt about the validity of conclusions concerning the involvement of F. necrophorum in diseases that appear in publications prior to 1970, because the organism was referred to by a variety of names (e.g. Fusiformis, Bacteroides, Sphaerophorus).²⁶ In 1970 a subcommittee of the International Committee on Nomenclature of Bacteria published a report recommending that members of the genera Sphaerophorus and Fusobacterium be incorporated into a single genus Fusobacterium and that the genus Bacteroides should be retained as a separate...