Med Micro

• Innate (natural) immunity

  Cells: Phagocytes (Neutrophils and macrophages), Extra-cellular killers (Natural killer cells, eosinophils), Soluble factors (e.g. complement, acute phase proteins, others)
• Normal host defence mechanisms against infection

  • Physical and biochemical barriers
  • Innate (natural) immunity
  • Adaptive (acquired) immunity
• Normal/indigenous flora help maintain environment and prevent infection with other organisms
• Disruption of these barriers allows organisms to bypass the first line of defence
• Disruption in any of these components allows organisms to bypass the second line of defence
• Increased risk of infection, dissemination, severe sepsis, and death in neutropenic patients
• Additional risk factor for infection in neutropenic patients: mucositis
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• Infections in immunocompromised hosts are determined by the type of immune deficit
• Adaptive (acquired) immunity

  Humoral (Due to soluble factors, i.e. antibodies, B cell mediated), Cellular (Due to cellular activity, T cell mediated)
• Haematological malignancies and stem cell transplants are prime examples of neutropenic patients
• Physical and biochemical barriers

  1. pH: saliva, stomach, skin, vagina

  2. Flushing mechanisms: tears, urine flow, cilia

  3. Mucus production: traps organisms

  4. Secreted substances, e.g. lysozyme (tears, nasal & secretions) damages cell wall of Gram positive bacteria

  5. Peristalsis of the gut

  6. Skin: sweat, shedding upper layers
• Physical and biochemical barriers

  • pH: saliva, stomach, skin, vagina
  • Flushing mechanisms: tears, urine flow, cilia
  • Mucus production: traps organisms
  • Secreted substances, e.g. lysozyme (tears, nasal & secretions) damages cell wall of Gram positive bacteria
  • Peristalsis of the gut
  • Skin: sweat, shedding upper layers
• Interaction between two arms leads to a coordinated immune response
• Neutropenia is usually defined as an absolute neutrophil count <1500 cells/microL in an adult
• Speaker: 'Dr Amanda Khumalo'
• Effect of immunosuppression on infection:
• Best for extra-cellular pathogens

  • Cellular activity
  • T cell mediated
• Infection by less virulent organisms leads to opportunistic pathogens
• Less able to prevent/respond to infection
• Virulence: the ability of a microorganism to cause disease in the host
• Infections in immunocompromised hosts are determined by

  • Type of immune deficit
  • Severity of immune defect
  • Duration of immune defect
• Pseudomonas aeruginosa rarely infects healthy tissues
• Greater risk of severe infection, even fatal
• May develop more rapidly
• Opportunistic pathogen: an organism that can only cause disease in situations that compromise the host’s defenses
• Pseudomonas aeruginosa infection can be severe, even life-threatening
• Diagnosis of Aspergillus infections is difficult due to various clinical presentations and challenges with interpretation of lab diagnostics
• Primary pathogen: organism that can cause disease in a host regardless of the host’s resident microbiota or immune system
• As immunity falls, Mycobacteria can lead to disseminated TB and extra-pulmonary disease
• Pseudomonas aeruginosa predominantly causes nosocomial infections
• No flowers in rooms to prevent Aspergillus infections (can have fungal spores)
• Specific defects in the host immune system make the host susceptible to infections due to organisms normally cleared in that manner
• Pseudomonas aeruginosa has multiple virulence factors and mechanisms of resistance
• Mycobacteria, especially M. tuberculosis, can cause newly acquired or re-activated latent disease
• High dose glucocorticoids and other drugs/conditions causing impaired cellular immune response are risk factors for Aspergillus infections
• Opportunistic pathogens

  • Pseudomonas aeruginosa
  • Mycobacteria
  • Aspergillus infections
• Severe and prolonged neutropenia is a risk factor for Aspergillus infections
• Pseudomonas aeruginosa shows persistence in the hospital environment
• Antifungal drugs can be used as prophylaxis or pre-emptive treatment in certain groups of immunocompromised patients