Obligate anaerobic gram-negative bacilli that do not form spores and grow well in bile are most likely which organism?

The correct choice is Bacteroides spp. because these organisms are well-known obligate anaerobes that thrive in environments lacking oxygen, such as in the human gut. They are gram-negative, non-spore-forming bacilli that can grow in the presence of bile, making them particularly adept at colonizing the gastrointestinal tract. Bacteroides species are significant contributors to the complex microbiota in the intestines and play a vital role in the digestion of polysaccharides.

In contrast, while the other options—Porphyromonas, Fusobacterium, and Prevotella—are also anaerobic and gram-negative, they either have specific growth requirements or are not as commonly associated with a bile-rich environment. For example, Porphyromonas is more often found in oral and periodontal sites, and Prevotella may not consistently exhibit bile tolerance to the same extent as Bacteroides. Fusobacterium, although it is an anaerobe, is more associated with human infections rather than as a predominant member of gut microbiota. Thus, the traits of Bacteroides make it the most likely candidate for the conditions described in the question.