MM - LU7

Normal Microbial Flora of Humans

  • Normal microbial flora: Microorganisms found on/in the body that do not cause disease in normal hosts but can cause disease when the balance is disrupted.

  • Synergistic relationships: Organisms work together to produce a stronger host response than individual effects.

  • Location: Found on the skin, in the respiratory tract, mouth, stomach, intestines, and genitourinary tract.

Key Sites and Microbial Examples

  • Skin:

    • Bacteria: Staphylococcus, Micrococcus, Corynebacterium, Propionibacterium, Streptococcus

    • Fungi: Candida, Malassezia

  • Gastrointestinal Tract:

    • Bacteria: Streptococcus, Neisseria, Lactobacillus, Bacteroides

    • Protozoa: Entamoeba gingivalis

  • Oral Cavity:

    • Bacteria: Bacteroides, Clostridium, Lactobacillus

  • Upper Respiratory Tract:

    • Bacteria: Lactobacillus, Streptococcus

  • Genital Tract:

    • Bacteria: Gardnerella, Lactobacillus

  • Ear & Eye:

    • Bacteria: Staphylococcus, Streptococcus

Transmission of Microorganisms

  • Most microbial diseases are communicable (transmissible between hosts).

  • Sources of microbes can include:

    • Human hosts (sick/asymptomatic carriers)

    • Environmental reservoirs (animals, water)

  • Pathogen requirements for successful infection include:

    • Surviving host transfer

    • Attachment/penetration of host tissues

    • Withstanding host defenses

    • Inducing damage to host tissues.

Exit of Microbes from the Host

  • Possible exit routes:

    • Aerosolized through speaking, sneezing, coughing.

    • Skin flakes, oral/respiratory secretions (kissing, drooling).

    • Insect bites, blood removal.

    • Fecal matter and urine.

Transmission to Humans

  • Routes of transmission include:

    • Droplets, Dust, Sewage, and Contaminated Water

    • Airborne pathogens

    • Contaminated Foods and Medications

    • Animal Bites or Excretions

Routes of Entry

  • Entry points for microbes:

    • Respiratory tract (airborne particles)

    • Mouth (food, water, kissing)

    • Blood/Blood transfusions

    • Sexual contact

    • Wounds/Injections

Endogenous Spread Routes

  • Possible routes:

    • From mouth/upper respiratory tract to lower respiratory tract

    • From skin bacteria to local lesions/blood

    • From anus to urinary tract/eyes

    • From open wounds to other body areas.

Microbial Pathogenicity and Virulence

  • Microbes use various mechanisms to produce disease.

Attachment Mechanisms

  • Microbes must attach to host tissues for infection.

  • Attachment specificity is due to receptor sites.

  • Mechanisms include: pili and specialized proteins.

Circumvention of Defense Mechanisms

  • Some bacteria decrease phagocytosis and can evade host defenses.

    • E.g., capsular substances block phagocyte attachment, while some bacteria destroy phagocytes.

    • Intracellular pathogens can survive inside phagocytes, evading the immune response.

Tissue Damage Mechanisms

  • Stimulating Inflammatory Responses: Some microbes induce strong immune responses causing tissue alteration (e.g., Streptococcus pneumonia).

  • Enzyme Secretion: Bacteria can secrete enzymes that destroy tissues (e.g., collagenase, hemolysins).

  • Toxin Production:

    • Exotoxins: Soluble toxins secreted into the environment.

    • Endotoxins: Toxins from gram-negative bacteria, causing signs like fever and shock.

Study of Infectious Disease

  • Includes:

    • Causative Microbe: Description and classification.

    • Pathogenesis and Clinical Disease: Mechanisms causing illness.

    • Transmission and Epidemiology: Modes and frequency of disease distribution.

    • Diagnosis: Identifying diseases.

    • Treatment: Responses to medications.

    • Prevention and Control: Reducing infectious agents, vaccines.

Immunity and Immunology

  • Immunity: Host ability to resist infection.

  • Immunology: Science of immune responses against foreign challenges, including self vs. non-self recognition.

  • Types of immune responses:

    • Nonspecific resistance (innate immunity)

    • Specific immunity (acquired immunity).

Natural Immunity

  • Offers resistance through inherited characteristics, representing the first line of defense, lacking immunological memory.

Specific Immunity

  • Responds to specific foreign agents with improvement on repeated exposure. Antigens provoke antibody production which helps eliminate pathogens.