Which type of bacterium is commonly found in the gut flora but can cause infections if it escapes the intestinal tract?

The correct choice is Bacteroides fragilis, which is a member of the normal gut flora and plays a significant role in the microbiome's ecosystem. While it helps in digestion and maintains a healthy gut environment, it is notably an opportunistic pathogen. If Bacteroides fragilis escapes the confines of the intestinal tract—such as occurring during surgical procedures, trauma, or diseases that compromise the intestinal barrier—it can lead to infections. These infections may include abscesses and other complications, particularly in the abdominal cavity.

In contrast, Escherichia coli, while also part of the gut flora and capable of causing infections, particularly certain pathogenic strains, is not the only bacterium in this context that can escape and lead to disease. Lactobacillus species are primarily non-pathogenic and often considered beneficial for gut health, typically not associated with infections. Streptococcus pneumoniae is primarily a respiratory pathogen and is not associated with gut flora. Thus, Bacteroides fragilis stands out specifically for its context within the gut’s normal flora and its potential to cause infections outside of it.