Comparative medical microbiology: Which statement is true for both Mycobacterium tuberculosis and Streptococcus pneumoniae (consider transmission and cell envelope features)?

Introduction / Context:
Different pathogens share overlapping epidemiology yet possess distinct structural features. Comparing Mycobacterium tuberculosis (acid-fast bacillus) with Streptococcus pneumoniae (encapsulated Gram-positive coccus) highlights how similar transmission routes can mask very different cell-envelope compositions and disease patterns.

Given Data / Assumptions:

• We evaluate which single statement is true for both organisms.
• Consider route of acquisition, cell wall composition, presence of capsules, and dissemination potential.
• Assume typical community-acquired settings.

Concept / Approach:
Both organisms are primarily acquired via respiratory droplets/aerosols, meaning inhalation is a shared route. However, their envelopes differ: M. tuberculosis contains abundant mycolic acids (acid-fast), while S. pneumoniae lacks mycolic acids but often has a prominent polysaccharide capsule. Conversely, M. tuberculosis generally lacks a classical polysaccharide capsule. Both can disseminate beyond the lungs (e.g., pneumococcal bacteremia/meningitis; miliary or extrapulmonary TB), so it is inaccurate to say they “rarely enter the bloodstream” collectively.

Step-by-Step Solution:
Identify the shared transmission: inhalation of respiratory particles. Check mycolic acids: present in Mycobacterium, absent in S. pneumoniae. Check capsules: common in S. pneumoniae, not a defining feature of M. tuberculosis. Assess dissemination: both can enter the bloodstream in significant subsets of cases. Select the only statement that applies to both: “are acquired by inhalation.”

Verification / Alternative check:
Epidemiologic descriptions confirm droplet transmission for pneumococcus and aerosolized droplet nuclei for tuberculosis; structural microbiology confirms divergent cell envelope compositions.

Why Other Options Are Wrong:

• High mycolic acids: True for M. tuberculosis, false for S. pneumoniae.
• Polysaccharide capsules: True for many pneumococcal strains, not a defining feature of M. tuberculosis.
• Stay in the lung and rarely enter the bloodstream: False; both can disseminate (pneumococcal bacteremia; miliary TB).
• None of these: Incorrect because the shared inhalational route is correct.

Common Pitfalls:
Assuming that sharing a respiratory route implies similar envelope chemistry or pathogenesis; these organisms differ markedly in staining, structure, and vaccine targets.

Final Answer:
Are acquired by inhalation is the statement that is true for both pathogens.