American Podiatric Medical Licensing Examination (APMLE) Part 3 Practice Test

Bacteroides are best described by their combination of Gram-negative cell wall structure and strict anaerobic metabolism. They are typically rod-shaped (bacilli) and non-spore-forming, and they reside in the gut as part of the normal anaerobic flora. The Gram-negative status means their cell wall has a thin peptidoglycan layer with an outer membrane containing lipopolysaccharide, which helps explain their staining and some aspects of their biology. Their anaerobic nature means they thrive only in environments without oxygen and can cause infections when they escape the gut or when anaerobic conditions are established at other sites. The other descriptors conflict with these traits: they are not Gram-positive, not cocci, and not aerobic. Hence, they fit best as Gram-negative, anaerobic bacteria.