What organism is responsible for causing "lumpy jaw"?

The organism responsible for causing "lumpy jaw" is Actinomyces israelii. This bacterium is part of the normal flora of the mouth and gastrointestinal tract but can become pathogenic under certain conditions, leading to actinomycosis. The characteristic presentation of "lumpy jaw" involves the formation of abscesses and swellings in the jaw area, which are often associated with the chronic infection caused by this organism.

Actinomyces israelii forms sulfur granules, which can be observed in draining pus and contribute to the diagnosis. The condition is typically seen in animals and humans who have had trauma or poor oral hygiene, which allows this opportunistic pathogen to invade tissue.

In contrast, the other organisms listed are associated with different types of infections. Nocardia brasiliensis is known for causing pulmonary infections and can lead to cutaneous nocardiosis but is not associated with lumpy jaw. Trichophyton rubrum is a dermatophyte commonly responsible for skin and nail infections, while Microsporum canis is primarily known for causing ringworm infections, particularly in cats and dogs. Each of these organisms causes specific diseases that differ significantly from the actinomycosis associated with Actinomyces israelii.