Infection, Infectious Diseases, and Epidemiology: Study Notes

Symbiotic Relationships Between Microbes and Their Hosts

Types of Symbiosis

Symbiosis refers to the close association between two different biological species. In microbiology, humans and microbes engage in various types of symbiotic relationships:

• Mutualism: Both organisms benefit from the relationship.

• Commensalism: One organism benefits, while the other is neither helped nor harmed.

• Amensalism: One organism is harmed, and the other is unaffected.

• Parasitism: One organism (the parasite) benefits at the expense of the other (the host), which is harmed.

Example: Termites and their gut protozoa exhibit mutualism, as protozoa help digest cellulose, benefiting both organisms.

Normal Microbiota in Hosts

Definition and Types

Normal microbiota (also called normal flora or indigenous microbiota) are microorganisms that colonize the body's surfaces without causing disease under normal conditions. There are two main types:

• Resident microbiota: Remain a part of the normal microbiota throughout life and are mostly commensal.

• Transient microbiota: Present temporarily; cannot persist due to competition, immune responses, or physical/chemical changes in the body.

Distribution of Resident Microbiota

Resident microbiota are found in various body regions, each with characteristic genera and ecological notes.

Body Site	Genera	Notes
Upper Digestive Tract	Actinomyces, Lactobacillus, Corynebacterium, Haemophilus, Bacteroides, Streptococcus, etc.	Microbes colonize surfaces of teeth, gums, lining of cheeks, and throat; many species in saliva.
Lower Digestive Tract	Bacteroides, Bifidobacterium, Clostridium, Enterococcus, Escherichia, etc.	Mostly strict anaerobes; some are facultative anaerobes.

Body Site	Genera	Notes
Upper Respiratory Tract	Fusobacterium, Haemophilus, Lactobacillus, Moraxella, Staphylococcus, etc.	Microbiota found in nose and throat; lower respiratory tract is typically sterile.

Body Site	Genera	Notes
Female Urinary and Reproductive Systems	Bacteroides, Clostridium, Lactobacillus, Staphylococcus, Streptococcus, Candida (fungus), etc.	Microbiota change with menstrual cycle; acidity varies.
Male Urinary and Reproductive Systems	Bacteroides, Fusobacterium, Lactobacillus, Mycobacterium, Staphylococcus, Streptococcus	Flow of urine prevents colonization of urinary tract.

Body Site	Genera	Notes
Eyes and Skin	Corynebacterium, Micrococcus, Propionibacterium, Staphylococcus, Candida (fungus), Malassezia (fungus)	Microbiota live on outer dead layers of skin and in hair follicles; tears wash most microbes from eyes.

Acquisition of Normal Microbiota

Humans develop microbiota during and after birth. The womb is generally considered sterile, and colonization begins during the birthing process and continues in the first months of life.

How Normal Microbiota Become Opportunistic Pathogens

Normal microbiota can become opportunistic pathogens under certain conditions, such as:

• Introduction into unusual body sites

• Immune suppression

• Changes in the abundance of normal microbiota

• Stressful conditions

Reservoirs of Infectious Diseases of Humans

Types of Reservoirs

Reservoirs are sites where pathogens are maintained as sources of infection. Types include:

• Animal reservoirs: Zoonoses are diseases naturally spread from animal hosts to humans.

• Human carriers: Asymptomatic individuals can transmit pathogens.

• Nonliving reservoirs: Soil, water, and food contaminated by feces or urine.

Animal Reservoirs and Zoonoses

Zoonoses can be acquired by direct contact, eating animals, or via arthropod vectors. Humans are often dead-end hosts, making eradication difficult.

Human and Nonliving Reservoirs

Human carriers may be asymptomatic or develop illness later. Nonliving reservoirs include contaminated soil, water, and food.

The Invasion and Establishment of Microbes in Hosts: Infection

Exposure to Microbes: Contamination and Infection

• Contamination: Presence of microbes on body surfaces.

• Infection: Microbes evade defenses, multiply, and establish in the body.

Portals of Entry

Pathogens enter the body through specific portals:

• Skin: Entry via cuts, openings, or by burrowing/digesting outer layers.

• Mucous membranes: Most common; includes respiratory and gastrointestinal tracts.

• Placenta: Usually a barrier, but some pathogens can cross and infect the fetus.

• Parenteral route: Circumvents usual portals; pathogens deposited directly into tissues.

The Role of Adhesion in Infection

Adhesion is the process by which microbes attach to host cells, a critical step for colonization. Adhesion factors include specialized structures and attachment molecules (ligands/adhesins). Blocking these can prevent infection. Some bacteria form biofilms for enhanced attachment.

The Nature of Infectious Disease

Infection vs. Disease

• Infection: Invasion by a pathogen.

• Disease (Morbidity): When infection leads to altered normal body functions.

Manifestations of Disease

• Symptoms: Subjective effects felt by the patient (e.g., pain, fatigue).

• Signs: Objective effects observed by others (e.g., fever, rash).

• Syndrome: Group of symptoms and signs that characterize a disease.

• Asymptomatic (subclinical): Infections without symptoms but with detectable signs.

Terminology of Disease

Etiology

Etiology is the study of the cause of disease. The germ theory of disease states that infections by pathogenic microorganisms cause disease. Robert Koch developed postulates to link specific pathogens to specific diseases.

Exceptions to Koch’s Postulates

• Some pathogens cannot be cultured.

• Some diseases are caused by multiple pathogens or cofactors.

• Ethical issues prevent testing in humans for some pathogens.

Virulence Factors of Infectious Agents

Pathogenicity is the ability to cause disease; virulence is the degree of pathogenicity. Virulence factors include:

• Adhesion factors

• Biofilms

• Extracellular enzymes

• Toxins

• Antiphagocytic factors

Note: Virulence does not indicate disease severity.

Extracellular Enzymes

Secreted by pathogens to dissolve host chemicals, aiding invasion and evasion of defenses. Mutants lacking these enzymes are often avirulent.

Toxins

Toxins are chemicals that damage host tissues or trigger harmful immune responses. Toxemia is the presence of toxins in the bloodstream. Two main types:

• Exotoxins: Secreted proteins, often highly toxic.

• Endotoxins: Lipopolysaccharide (LPS) components of Gram-negative bacteria, released upon cell death.

Antiphagocytic Factors

These factors help pathogens evade phagocytosis by host immune cells, allowing prolonged infection. Examples include bacterial capsules and chemicals that inhibit phagocyte function.

The Stages of Infectious Disease

Infectious diseases typically progress through five stages:

1. Incubation period

2. Prodromal period

3. Illness

4. Decline

5. Convalescence

The Movement of Pathogens Out of Hosts: Portals of Exit

Pathogens exit the host via portals often shared with entry sites, typically in secretions or excretions.

Transmission of Infectious Diseases

Transmission occurs from a reservoir or portal of exit to a new host's portal of entry. Main modes include:

• Contact transmission: Direct, indirect, or droplet spread.

• Vehicle transmission: Via air, water, or food.

• Vector transmission: Via arthropods (e.g., mosquitoes, ticks).

Classification of Infectious Diseases

Diseases can be classified by affected body system, taxonomic group, longevity/severity, transmission mode, or population effects.

Epidemiology of Infectious Diseases

Definitions and Measures

Epidemiology is the study of disease occurrence and transmission. Key measures:

• Incidence: Number of new cases in a given area and time.

• Prevalence: Total number of cases in a given area and time.

Occurrence is also described by frequency and geographic distribution.

Patterns of Disease Occurrence

• Endemic: Disease normally present in a population.

• Sporadic: Occasional cases.

• Epidemic: Sudden increase above normal levels.

• Pandemic: Epidemic spread over multiple continents.

Nationally Notifiable Infectious Diseases

Certain diseases must be reported to public health authorities for monitoring and control.

Epidemiological Studies

• Descriptive epidemiology: Tabulates data on disease cases, location, and time; identifies index case.

• Analytical epidemiology: Determines probable cause, transmission, and prevention; often retrospective.

Hospital Epidemiology: Healthcare-Associated (Nosocomial) Infections

Types of healthcare-associated infections:

• Exogenous: Acquired from the healthcare environment.

• Endogenous: Arise from patient's own microbiota.

• Iatrogenic: Result from medical procedures.

• Superinfections: Result from antimicrobial use disrupting normal microbiota.

Control: Aggressive measures, especially handwashing, are essential to reduce infection rates.

Epidemiology and Public Health

Public health agencies at local, national, and global levels (e.g., CDC, WHO) coordinate efforts to monitor, report, and control infectious diseases.