Which of the following bacteria is anaerobic?

Actinomyces is a genus of bacteria that is classified as anaerobic, meaning it thrives in environments devoid of oxygen. This species is part of the normal flora of the human mouth and can occasionally cause infections, particularly when the barrier of mucosal tissues is compromised. Its anaerobic nature is a significant factor in its pathogenicity, as it can survive and grow in oxygen-limited environments, such as abscesses.

In contrast, Nocardia, Mycobacterium, and Bordetella are not strictly anaerobic. Nocardia is known to be aerobic, requiring oxygen for its growth. Similarly, Mycobacterium, which includes the pathogen responsible for tuberculosis, also needs oxygen to survive and grow. Bordetella, notorious for causing whooping cough, is also aerobic and relies on oxygen in its metabolic processes. Thus, Actinomyces stands out as the bacteria that is specifically adapted to anaerobic conditions, making it the correct choice in this context.