Which bacterium is primarily responsible for the condition known as "lumpy jaw"?

The bacterium primarily responsible for the condition known as "lumpy jaw" is Actinomyces israelii. This organism is a type of filamentous anaerobic bacterium that is part of the normal flora of the oral cavity and gastrointestinal tract. When it breaches the usual defenses, often following trauma or dental work, it can invade tissues and lead to the characteristic suppurative (pus-producing) infections associated with lumpy jaw, medically referred to as actinomycosis.

The condition presents with swelling of the jaw, which can develop abscesses and sinus tract formation, creating the "lumpy" appearance. Treatment typically involves high doses of antibiotics and sometimes surgical intervention to drain the abscesses.

Other options do not correlate with lumpy jaw as closely. For instance, while Staphylococcus aureus is known for a variety of infections, including skin abscesses, it is not the causative agent of lumpy jaw. Similarly, Streptococcus pyogenes typically causes conditions such as strep throat and skin infections but is not linked to actinomycosis. Escherichia coli primarily causes gastrointestinal disturbances and infections but does not cause lumpy jaw either.