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 have various symbiotic relationships with microorganisms, which can be classified as follows:

• Mutualism: Both organisms benefit. Example: Bacteria in the human colon synthesize vitamins and aid digestion while receiving nutrients and a stable environment.

• Commensalism: One organism benefits, the other is neither helped nor harmed. Example: Mites living in human hair follicles.

• Amensalism: One organism is harmed, the other is unaffected. Example: A fungus secreting antibiotics that inhibit nearby bacteria.

• Parasitism: One organism benefits at the expense of the other. Example: Mycobacterium tuberculosis in human lungs.

Type	Organism 1	Organism 2	Example
Mutualism	Benefits	Benefits	Bacteria in human colon
Commensalism	Benefits	Neither benefits nor is harmed	Mites in human hair follicles
Amensalism	Is harmed	Neither benefits nor is harmed	Fungus secreting antibiotic
Parasitism	Benefits	Is harmed	Tuberculosis bacteria in human lung

Human Microbiome

The microbiome consists of all microorganisms that colonize the body's surfaces without causing disease under normal conditions. These are also called normal microbiota or normal flora. There are two main types:

• Resident microbiota: Remain throughout life, mostly commensal.

• Transient microbiota: Present for short periods; eliminated by competition, body defenses, or changes in the body.

Distribution of Resident Microbiota

Resident microbiota are found in various body regions. The following tables summarize their distribution:

Region	Genera	Notes
Upper Respiratory Tract	Fusobacterium, Haemophilus, Lactobacillus, Moraxella, Staphylococcus, Streptococcus, Veillonella, Candida (fungus)	The nose is cooler than the rest of the respiratory system and has some unique microbiota.

Region	Genera	Notes
Upper Digestive Tract	Actinomyces, Bacteroides, Corynebacterium, Haemophilus, Lactobacillus, Neisseria, Staphylococcus, Treponema, Entamoeba (protozoan), Porphyromonas	Microbes colonize surfaces of teeth, gingiva, lining of cheeks, and pharynx.
Lower Digestive Tract	Bacteroides, Bifidobacterium, Clostridium, Enterococcus, Escherichia, Lactobacillus, Proteus, Shigella, Candida (fungus), Entamoeba (protozoan), Trichomonas (protozoan)	Bacteria are mostly strict anaerobes; some facultative anaerobes are also resident.

Region	Genera	Notes
Female Urinary and Reproductive Systems	Bacteroides, Clostridium, Lactobacillus, Staphylococcus, Streptococcus, Candida (fungus), Trichomonas (protozoan)	Microbiota change as acidity in the vagina changes during menstrual cycles.
Male Urinary and Reproductive Systems	Lactobacillus, Mycobacterium, Peptostreptococcus, Staphylococcus, Streptococcus	Flow of urine prevents extensive colonization.

Region	Genera	Notes
Skin	Corynebacterium, Micrococcus, Propionibacterium, Staphylococcus, Candida (fungus), Malassezia (fungus)	Microbiota live on outer dead layers of skin and in hair follicles and pores of glands.
Eyes	Corynebacterium, Staphylococcus	Tears wash most microbiota from the eyes, so fewer are present compared to the skin.

Acquisition and Changes in the Microbiome

• Humans develop in a sterile environment in the womb.

• The microbiome begins to develop during birth and is established in the first months of life.

• Normal microbiota can become opportunistic pathogens if introduced to unusual sites, during immune suppression, or when the normal microbiota changes.

Reservoirs of Infectious Diseases of Humans

Types of Reservoirs

Reservoirs are sites where pathogens are maintained as a source of infection. There are three main types:

• Animal reservoirs: Zoonoses are diseases that naturally spread from animal hosts to humans (e.g., rabies, malaria).

• Human carriers: Asymptomatic individuals can transmit pathogens to others.

• Nonliving reservoirs: Soil, water, and food can harbor infectious agents, often due to contamination.

The Invasion and Establishment of Microbes in Hosts: Infection

Contamination vs. Infection

• Contamination: Presence of microbes in or on the body.

• Infection: When microbes evade external defenses, multiply, and become established in the body.

Portals of Entry

Pathogens enter the body through specific sites called portals of entry:

• Skin: Pathogens enter through cuts, openings, or by burrowing/digesting outer layers.

• Mucous membranes: Line body cavities open to the environment; respiratory tract is the most common site.

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

• Parenteral route: Circumvents usual portals; pathogens deposited directly into tissues (e.g., via punctures).

The Role of Adhesion in Infection

Adhesion is the process by which microorganisms attach to host cells, a critical step for colonization and infection. Microbes use specialized structures or attachment molecules (adhesins/ligands) to bind to host cell receptors. Some bacteria form biofilms for collective attachment.

The Nature of Infectious Disease

Manifestations of Disease

• Symptoms: Subjective characteristics felt only by the patient (e.g., pain, nausea).

• Signs: Objective manifestations observed or measured by others (e.g., fever, rash).

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

• Asymptomatic (subclinical) infections: Lack symptoms but may have signs.

Etiology: Causation of Disease

Etiology is the study of the cause of disease. Robert Koch developed postulates to prove that a specific pathogen causes a specific disease:

1. The suspected agent must be present in every case of the disease.

2. The agent must be isolated and grown in pure culture.

3. The cultured agent must cause the disease when inoculated into a healthy host.

4. The same agent must be reisolated from the diseased experimental host.

Virulence Factors of Infectious Agents

Virulence is the degree of pathogenicity. Virulence factors include:

• Adhesion factors

• Biofilms

• Extracellular enzymes

• Toxins (exotoxins and endotoxins)

• Antiphagocytic factors

The Stages of Infectious Disease

Infectious diseases typically progress through five stages:

1. Incubation period: No signs or symptoms.

2. Prodromal period: Vague, general symptoms.

3. Illness: Most severe signs and symptoms.

4. Decline: Declining signs and symptoms.

5. Convalescence: No signs or symptoms.

The Movement of Pathogens Out of Hosts: Portals of Exit

Pathogens leave the host through portals of exit, often the same as portals of entry, via materials the body secretes or excretes (e.g., saliva, feces, urine, blood).

Modes of Infectious Disease Transmission

Contact Transmission

• Direct contact: Physical interaction (e.g., touching, kissing).

• Indirect contact: Via fomites (inanimate objects).

• Droplet transmission: Pathogens spread in droplets of mucus (e.g., coughing, sneezing).

Vehicle Transmission

• Airborne: Pathogens travel more than 1 meter via aerosols.

• Waterborne: Spread through contaminated water (fecal-oral route).

• Foodborne: Spread via contaminated, undercooked, or improperly stored food.

• Bodily fluid transmission: Blood, urine, saliva, etc., can carry pathogens.

Vector Transmission

• Biological vectors: Arthropods that transmit pathogens and serve as hosts for part of the pathogen's life cycle (e.g., mosquitoes, ticks).

• Mechanical vectors: Passively transmit pathogens on their bodies (e.g., houseflies, cockroaches).

Classification of Infectious Diseases

Diseases can be classified by taxonomic categories, affected body systems, longevity/severity, transmission mode, and population effects. Key terms include:

• Acute disease: Rapid onset, short duration.

• Chronic disease: Slow development, long-lasting.

• Latent disease: Appears long after infection.

• Communicable disease: Transmitted from host to host.

• Noncommunicable disease: Not transmitted between hosts.

Epidemiology of Infectious Diseases

Frequency of Disease

• Incidence: Number of new cases in a given area during a given period.

• Prevalence: Total number of cases in a given area during a given period.

Epidemiological Studies

• Descriptive epidemiology: Tabulation of data concerning a disease (location, time, patient info).

• Analytical epidemiology: Determines probable cause, mode of transmission, and prevention methods.

• Experimental epidemiology: Tests hypotheses about disease causation (e.g., Koch's postulates).

Healthcare-Associated (Nosocomial) Infections

• Exogenous: Acquired from the healthcare environment.

• Endogenous: Arise from normal microbiota within the patient.

• Iatrogenic: Result from medical procedures.

• Superinfections: Result from antimicrobial use disrupting normal microbiota.

Prevention: Handwashing is the most effective method to reduce healthcare-associated infections.

Public Health and Disease Control

• Public health agencies (e.g., CDC, WHO) monitor, report, and control infectious diseases.

• Efforts include enforcing cleanliness, immunization, vector control, and public education.