Which pair of organisms is commonly associated with opportunistic infections?

• A. Clostridium difficile and Candida albicans
• B. Staphylococcus aureus and Escherichia coli
• C. Pseudomonas aeruginosa and Klebsiella pneumoniae
• D. Streptococcus pneumoniae and Haemophilus influenzae

Answer: (A)

Opportunistic infections arise when the host’s defenses are weakened or the normal microbial balance is disrupted, allowing organisms that are usually harmless or limited to specific niches to cause disease.

Candida albicans is a classic opportunistic pathogen: it normally lives in the mouth, gut, and vagina without causing problems, but when antibiotics disrupt the normal flora or the immune system is compromised, it can overgrow and lead to infections ranging from oral thrush to invasive candidiasis.

Clostridium difficile is another prime example, typically a gut resident that becomes pathogenic after broad-spectrum antibiotics wipe out competing bacteria. This overgrowth produces toxins that inflame the colon, causing antibiotic-associated diarrhea and colitis.

These organisms exemplify how changes in host defenses or microbiome balance permit disease, which is the essence of opportunistic infections.

The other pairs include organisms that commonly cause disease in healthy people or in broad contexts, not primarily as opportunists due to disrupted flora or weakened immunity, so they don’t illustrate the concept as clearly.