Microbiology_8_Disease

MPharm Programme: Disease and Pathogenesis

• Learning Objectives:
  • Sources of disease, including the different stages of disease processes.
  • Pathogenic virulence factors, including examples of toxin producers and biofilms.
  • How diseases spread and how to stop them.

Microbial Infections as a Public Health Problem

• Infectious diseases remain a major global health concern.
• Lower respiratory diseases and diarrheal diseases were significant causes of death in 2016.
• In developed countries, 5-12% of hospital patients experience nosocomial infections (i.e., infections acquired in a hospital).
• Antimicrobial resistance is a major issue in hospitals.
• Individual susceptibility to infections is determined by multiple factors (genes, previous exposures, age, chronic illnesses, and medications).

Top Causes of Deaths: High and Low-Income Countries

• (Data shown in bar graphs for 2016)
• Different causes of death are prevalent in high-income vs low-income countries

Scale of Disease

• Endemic: A persistently present infection within a population.
  • Chickenpox is an example.
• Epidemic: A rapidly spreading infection within a population.
  • Ebola outbreaks (particularly the 2014 one).
• Pandemic: An epidemic that spreads across a wide geographic area (e.g., continents).
  • Historically, significant pandemics include the Black Death and Spanish Flu.
  • The 2009 H1N1 influenza pandemic is a more recent example.

Terminology

• Pathogenicity: A pathogen's ability to cause disease in an organism.
• Virulence: The degree of damage done by a pathogen.
• Virulence/Pathogenicity factor: Microbial factors (products or strategies) aiding pathogenicity, including:
  • Colonization in host tissues (including cell attachment).
  • Evasion of the host's immune response.
  • Immunosuppression.
  • Entry/exit into or from host cells.
  • Obtaining nutrients from the host.

Sources of Disease

• People: The primary source of many highly infectious diseases (e.g., HIV, smallpox, diphtheria).
• Obligate pathogens: Microorganisms that need to cause disease to survive. Examples include Mycobacterium tuberculosis.
• Opportunistic pathogens: Normal commensals that cause disease when host immunity is low or compromised. Examples include Candida albicans, Staphylococcus aureus, and Pseudomonas aeruginosa.
• Commensal organisms: Part of the normal flora, but usually nonpathogenic, unless factors change (e.g. immunity lowered, bacteria released into bloodstream).
• Environment:
  • Waterborne diseases: Cholera, Typhoid, Legionella.
  • Foodborne diseases: Salmonella, Campylobacter, E. coli.
  • Surfaces and soil: Clostridium botulinum and C. tetani.
• Animals (Zoonoses): A significant source of novel human diseases with many having animal origins.
  • Examples: Anthrax, E. coli, Plague.
  • Parasites: Toxoplasmosis.
  • Viruses: Avian influenza, Middle East Respiratory Syndrome coronavirus (MERS-CoV), Ebola, Rift Valley fever.
  • Other: BSE (Bovine Spongiform Encephalopathy).

Role of Animals in Infectious Disease

• Zoonoses: diseases naturally transmitted between animals and humans.
• Animals can serve as reservoirs of pathogens, contributing to the emergence of pandemics.
• Exposure to infected animals or contaminated environments is a significant risk factor.

Role of Occupation in Infectious Disease Exposure

• Traditionally, occupations involving close contact with animals or animal products (e.g., farmers, tanners, slaughterhouse workers) have faced increased risks.
• Modern occupations (e.g., laboratory workers, healthcare professionals) can also be at risk of exposure to infectious agents.

Scale of Disease

• Additional examples include chickenpox, Ebola, and 2009 HIN1 influenza

The Role of Animals in Infectious Disease

• Humans can contract animal viruses (e.g., bird flu - H5N1; swine flu - H1N1).
• Viral recombination can increase pathogenicity.
• Antigenic shift: Combination of two or more different strains, resulting in a new form of virus.
• Antigenic drift: Accumulation of mutations within regions, reducing effectiveness of immune system.

Koch's Postulates (and Updated versions)

• Set of conditions required to determine the specific cause of disease.
• Modern molecular methods exist to study pathogens and virulence.

The Disease Process

• Transmission and exposure: The stage when a pathogen is transferred to a host.
• Attachment and Invasion: The process of a pathogen attaching to a host cell and/or entering the host.
• Colonization and Damage: The pathogen multiplies and infects the host tissue.

Transmission and Exposure

• Transmission is the movement of disease from one host to another.
• Types include vertical (mother to child), horizontal (person-to-person), vehicle-borne (through an object), and vector-borne (through an organism).
• Routes of disease include inhalation, ingestion, and contact.

Attachment and Invasion

• Pathogens bind to host cell receptors to avoid host defenses.
• Virulence factors (pili, adhesins, capsules, enzymes) play a role in attachment, invasion, and evading immune defenses.

Biofilms

• Biofilms are groups of microorganisms that attach to each other and/or a surface.
• They are frequently a cause for chronic infections and are often resistant to treatment.
• Biofilms are a major concern for individuals with cystic fibrosis.

Colonization and Damage: Consequences

• Microorganisms need suitable nutrients to grow in host tissues.
• Growth requires resistance to host defense mechanisms.
• Damage can be caused through toxin accumulation, enzymatic digestion, and host cell death.

Microbial Toxins

• Microbial toxins damage tissues and promote infections.
• Endotoxins (LPS): Part of the cell wall, released when the bacterial cell dies. Can cause shock.
• Exotoxins: Secreted, heat-sensitive toxins from bacterial cells. Various mechanisms of damage (cell death, disrupting nerves).
• Examples of microbial toxins like Botulinum Toxin.

Microbial Exotoxins

• Types (based on effect): Cytotoxins, neurotoxins, enterotoxins.
• Further divided into types I, II, and III based on their mechanistic targets.
• Example toxins and their targets like Cholera toxin, Diphtheria toxin, and Shiga toxin.

Microbial Endotoxins

• Large molecule that is a component of gram-negative bacterial cell walls.
• They are released into the tissue when the bacteria dies/is destroyed
• Responsible for many serious clinical effects such as fever, septic shock

Haemolysins

• Proteins that break down red blood cells.
• Classified based on their effects: Alpha, Beta, and Gamma hemolysis.

Non-Bacterial Diseases (Mycoses)

• Diseases caused by fungi, which can be persistent (chronic) and affect various parts of the body (often common skin diseases).
• They are common for individuals with lowered immunity.

Non-Bacterial Diseases (Viral)

• Viruses can lead to both communicable (spreadable) and non-communicable diseases.
• Examples include herpes, HIV, influenza, and virus-related cancers (e.g., HPV and cervical cancer), and some illnesses caused by animal-derived viruses (e.g., influenza, swine flu, and others).

Extra Reading

• Prescott's Microbiology, Brock Microbiology: Recommended texts.
• Percival et al. (2015): Referred article for the specific subject.

Explore the critical aspects of disease sources and pathogenesis in the MPharm programme. This quiz covers pathogenic virulence factors, the spread of diseases, and public health challenges posed by microbial infections. Understand the impact of antimicrobial resistance and the factors influencing individual susceptibility to infections.