Final answer:
Pathogenic bacteria that are part of the normal flora can cause disease when they overcome immune protection, commonly using virulence factors like adhesins, capsules, and toxins. Examples include Staphylococcus epidermidis causing endocarditis, and E. coli acquiring factors leading to diarrheal disease.
Step-by-step explanation:
Normal flora, also known as the normal microbiota, consists of various microorganisms, including pathogens, that reside in different parts of the human body without causing disease under normal circumstances. Certain pathogenic bacteria can be part of the normal microbiota of the respiratory tract. These include opportunistic pathogens, which can overcome the immune protection of the respiratory tract using various virulence factors.
Some bacteria within the normal flora can become opportunistic pathogens under specific conditions. For instance, Staphylococcus epidermidis, which is avirulent on the skin, can cause endocarditis when introduced into the bloodstream. Moreover, Escherichia coli, while usually a harmless commensal organism in the colon, can acquire virulence factors such as toxins that cause severe disease. Respiratory pathogens may employ adhesins and polysaccharide capsules to evade immune defenses, with Clostridium difficile being an example of a pathogen that can become problematic when normal gut flora is disrupted by antibiotic use.