Which organism is described as anaerobic gram-positive diphtheroids?

The organism described as anaerobic gram-positive diphtheroids is Propionibacterium. This genus comprises anaerobic bacteria that are often found in human skin and various body sites, playing a role in the normal microbiota.

Propionibacterium are characterized as gram-positive and can be distinguished by their rod shape, often resembling diphtheroids due to their pleomorphic nature. These bacteria are also known for their ability to ferment various substrates, leading to the production of propionic acid, which is a distinguishing metabolic feature.

In contrast, Mycobacterium tuberculosis is an aerobic bacterium that causes tuberculosis and is not classified as a diphtheroid. Lactobacillus is another group of gram-positive bacteria, but they are typically classified as lactic acid bacteria and are not typically described with the term diphtheroid, nor are they classified strictly as anaerobes. Fusobacterium is a genus of gram-negative anaerobes and therefore does not fit the description of being gram-positive diphtheroids.

In summary, Propionibacterium stands out due to its anaerobic characteristics, gram-positive nature, and its resemblance to diphtheroids, aligning perfectly with the question's requirements.