Immunity to Microbes and Infectious Diseases

Bacteria are categorized by their cell wall structure:

1. Gram-positive: Possess a thick cell wall that retains crystal violet, staining dark purple or blue.
2. Gram-negative: Possess a thin cell wall that takes up the safranin counterstain, appearing pink or red.
3. Atypical: Lack a cell wall entirely and do not stain well with standard methods.

Once the organism is identified via culture, a Culture and Susceptibility (C&S) report is generated, typically within 24–72 hours. A critical component of this report is the Minimum Inhibitory Concentration (MIC), which is the lowest concentration of an antibiotic required to inhibit bacterial growth.

Clinicians interpret these results using three categories:

• Susceptible (S): The drug is expected to be effective; this should be the preferred choice.
• Intermediate (I): May work under specific conditions (e.g., higher doses), but is generally not the first choice.
• Resistant (R): The drug should not be used for treatment.

Clinical Manifestations of Infection

Pneumonia

Pneumonia is an infection of the lung parenchyma (the alveoli) that results in inflammation and the accumulation of inflammatory exudate. It is classified based on where it was acquired:

• Community-acquired pneumonia (CAP): Contracted outside of healthcare facilities; S. pneumoniae is a common culprit.
• Hospital-acquired (nosocomial) pneumonia: Occurs 48 hours or more after hospital admission. Nosocomial infections are particularly dangerous because the organisms involved are often resistant to multiple antibiotics.

The lungs have robust defense mechanisms. Mechanical filtration occurs via turbulence in the nasal passages, and the mucociliary escalator uses beating cilia to move mucus upward toward the larynx. At the alveolar level, macrophages use pattern recognition receptors (like Toll-like receptors) to detect pathogens, triggering a cytokine cascade (e.g., TNF-$\alpha$ and IL-1) that recruits neutrophils and lymphocytes.

Infectious Diarrhea

Infectious diarrhea is a major cause of global mortality, especially in children. Death is primarily caused by the loss of electrolytes and intravascular volume, leading to cardiovascular failure. Pathogenesis follows two main routes:

1. Toxin-mediated: For example, Vibrio cholerae produces toxins that disrupt fluid balance.
2. Invasive: For example, Salmonella invades the intestinal lining.

HIV and AIDS

The Human Immunodeficiency Virus (HIV) is an obligate intracellular parasite that targets the immune system. It specifically depletes CD4 T lymphocytes. When the immune function is critically impaired, the patient progresses to AIDS (Acquired Immunodeficiency Syndrome), characterized by specific AIDS-defining conditions.

Clinical Note: Patients with HIV are at significantly higher risk for both pneumonia and opportunistic infections like Candida or Aspergillus due to their depleted CD4 counts.