Which organisms are commonly involved in opportunistic infections?

• A. Clostridium difficile, Candida albicans
• B. Staphylococcus aureus, Streptococcus pneumoniae
• C. Escherichia coli, Klebsiella pneumoniae
• D. Pseudomonas aeruginosa, Mycobacterium tuberculosis

Answer: (A)

Opportunistic infections occur when host defenses are compromised, allowing organisms that are normally either harmless or kept in check to cause disease. Clostridium difficile fits this pattern because it typically blooms after broad-spectrum antibiotics disrupt the gut flora, leading to antibiotic-associated colitis. Candida albicans is a common commensal fungus on skin and mucous membranes, but when immunity is reduced or flora is disturbed, it can overgrow and invade, causing oral, vaginal, or systemic infections. Together, they exemplify organisms that cause disease primarily in the context of impaired defenses, unlike many pathogens that commonly cause illness in healthy individuals. Other listed organisms can cause disease as well, but their typical role is not as classic for opportunistic infections as these two.