Microbial Interactions, Pathogenicity, and Infectious Disease: Study Notes

Symbiotic Relationships

Definition and Types

Symbiosis refers to the close association between two different organisms, often a microbe and a host. In microbiology, symbiotic relationships are fundamental to understanding how microbes interact with humans and other organisms.

• Symbiosis: 'To live together'; describes the relationship between a microbe and its host.

• Classic definition involves two partners, but in reality, many partners may be involved.

• Types of symbiosis are based on the benefit or harm to the partners:

  • Mutualism: Both partners benefit.

  • Commensalism: One partner benefits, the other is unaffected.

  • Parasitism: One partner benefits at the expense of the other.

Examples

• Mutualism: Bacteria in the human colon provide vitamins and nutrients, while benefiting from a warm, nutrient-rich environment.

• Commensalism: Staphylococcus epidermidis grows on skin, obtaining nutrients without affecting the host.

• Parasitism: Pathogenic protozoa or bacteria harm the host while benefiting themselves.

Normal or Resident Microbiota in Hosts

Definition and Types

Normal microbiota (also called normal flora or indigenous microbiota) are microbes that colonize body surfaces without causing disease. Some body sites are axenic (free of microbes), such as the depths of the lungs.

• Resident Microbiota: Continuously present in specific body sites (skin, digestive tract, vagina, upper respiratory tract, distal urethra).

• Transient Microbiota: Present for a short time; unable to persist due to competition, immune defenses, or physical/chemical changes.

• Acquisition: Microbiota are acquired from first foods, environment, and contact with others.

How Normal Microbiota Become Opportunistic Pathogens

Mechanisms

Normal microbiota can cause disease if they move to unusual sites, if the immune system is suppressed, or if there are changes in the microbiota.

• Movement to Unusual Site: Example: E. coli in the intestine is harmless, but in the urinary tract, it can cause infection.

• Immune Suppression: Diseases, age, cancer treatments, AIDS, or transplants can suppress immunity, allowing opportunistic infections.

• Changes in Resident Microbiota: Broad-spectrum antibiotics can disrupt normal flora, leading to infections (e.g., Candida albicans).

Reservoirs of Infectious Diseases of Humans

Types of Reservoirs

Reservoirs are sources where pathogens persist and from which infection is transmitted.

• Animal Reservoirs: Pathogens in animals (domesticated or wild) can infect humans (zoonoses). Transmission can occur via direct contact, animal feces, eating meat, or arthropod vectors.

• Human Carriers: Asymptomatic individuals can carry and transmit pathogens (e.g., Typhoid Mary, HIV, TB).

• Non-Living Reservoirs: Soil, water, and food can harbor pathogens (e.g., Clostridium spores in soil, water contaminated with feces).

Infection

Movement of Pathogens into Hosts

Infection occurs when a microbe successfully invades and multiplies within the host body.

• Contamination: Microbe is present on the body.

• Infection: Microbe overcomes defenses and invades tissues.

• Possible outcomes: Microbes may be eliminated, become part of the microbiota, or cause infection.

Nature of Infectious Diseases

Definitions and Concepts

• Disease (Morbidity): Injury by pathogen significant enough to interfere with normal function.

• Infection ≠ Disease: Infection is invasion; disease is the result of significant injury.

• Signs: Objective manifestations observed by others (e.g., vomiting).

• Symptoms: Subjective feelings reported by the patient (e.g., nausea).

• Syndrome: Group of signs and symptoms associated with a disease.

• Asymptomatic/Subclinical: Infection present, but no signs.

Causation of Disease (Etiology)

Koch's Postulates

Koch's postulates are criteria to establish a causative relationship between a microbe and a disease.

1. Suspected pathogen must be present in every case of the disease.

2. Pathogen must be isolated and grown in pure culture.

3. Cultured agent must cause disease when inoculated into a healthy, susceptible host.

4. Same pathogen must be re-isolated from the diseased experimental host.

Modern adjustments include recognition that some pathogens cannot be cultured, some diseases are caused by multiple agents, and ethical considerations prevent human experimentation.

• Example of Error: Haemophilus influenzae was once thought to cause influenza, but later found not to be the causative agent.

Virulence Factors of Infectious Agents

Pathogenicity and Virulence

• Pathogenicity: Ability of a microbe to cause disease.

• Virulence: Degree of pathogenicity (how severe or fast the disease is caused).

• Pathogens have virulence factors that nonpathogenic microbes lack.

Major Virulence Factors

• Adhesion factors

• Biofilm formation

• Extracellular enzymes

• Toxins

• Antiphagocytic factors

The Role of Adhesion in Infection

• Adhesion: Process by which microbes attach to host cells.

• Adhesion factors include specialized structures (fimbriae, pili) or proteins.

• Ligands: Surface molecules (e.g., lipoproteins, glycoproteins) that bind to host cell receptors.

• Blocking adhesion can prevent infection.

• Specificity of adhesion determines host and tissue tropism (e.g., Neisseria gonorrhoeae binds only to cells lining the urethra and vagina).

Pathogen	Adhesion Notes
Bordetella (whooping cough)	Has more than 1 adhesion
Plasmodium (malaria)	Can change adhesions over time, evading immune system and attacking more than one cell type

• Avirulent microbes may lack adhesions and thus cannot cause disease.

• Protozoa and helminths have specialized adhesion structures (e.g., suckers, hooks).

Extracellular Enzymes

Types and Functions

• Hyaluronidase: Digests hyaluronic acid (the 'glue' between animal cells), allowing pathogen invasion.

• Collagenase: Breaks down collagen, enabling deeper tissue invasion.

• Coagulase: Causes blood clotting, hiding bacteria from immune system.

• Kinase: Dissolves clots, releasing bacteria to infect new tissues.

• Fungi and protozoa secrete enzymes (e.g., keratinase, mucinase) to invade host tissues.

Examples

• Clostridium perfringens produces hyaluronidase and collagenase.

• Staphylococcus aureus produces coagulase and kinase.

• Streptococcus pyogenes produces streptokinase, dissolving blood clots.

• Fungi causing ringworm secrete keratinase to digest skin, hair, and nails.

• Entamoeba secretes mucinase, digesting the intestinal lining and causing dysentery.

Summary Table: Parasites of Humans

Microscopic	Visible
Bacteria Fungi Protozoa	Parasitic worms Biting arthropods (mites, ticks) Bloodsucking flies, fleas

Additional Info

• Some pathogens can only be identified by epidemiological data, especially when ethical or technical limitations prevent laboratory confirmation.

• Virulence factors are often targets for new antimicrobial therapies and vaccines.