Ch 14 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, which can impact health and disease.

• Mutualism: Both organisms benefit. Example: Bacteria in the human colon synthesize vitamins and aid digestion.

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

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

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

Parasite, Host, and Pathogen

• Parasite: An organism that lives on or in a host and gets its food at the expense of its host.

• Host: The organism that harbors the parasite.

• Pathogen: A parasite that causes disease in its host.

Microbiome of Humans

Resident and Transient Microbiota

The human microbiome consists of all the microorganisms that colonize the body’s surfaces without normally causing disease. These are also called normal microbiota, normal flora, or indigenous microbiota.

• Resident microbiota: Remain throughout life, mostly commensal.

• Transient microbiota: Present for short periods; eliminated by competition, body defenses, or chemical/physical changes.

Acquisition of the Microbiome

• Humans are free of microorganisms in the womb.

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

Some Resident Microbiota by Body Site

Body Site	Genera	Notes
Skin & Eyes	Corynebacterium, Micrococcus, Propionibacterium, Staphylococcus, Candida (fungus), Malassezia (fungus)	Microbes are on the outer, dead layers of skin and in hair follicles and pores of glands.
Upper Digestive Tract	Actinomyces, Bacteroides, Corynebacterium, Haemophilus, Neisseria, Streptococcus, Treponema	Microbes colonize surfaces of teeth, gums, cheeks, and pharynx.
Lower Digestive Tract	Bacteroides, Clostridium, Escherichia, Lactobacillus, Proteus, Shigella	Mostly strict anaerobes; some facultative anaerobes are also present.
Upper Respiratory Tract	Streptococcus, Haemophilus, Neisseria, Staphylococcus	Nose is cooler than rest of respiratory system; lungs are axenic (no natural microbiota).
Female Urinary & Reproductive	Lactobacillus, Streptococcus, Candida (fungus)	Microbiota change as acidity in the vagina changes during menstrual cycle.
Male Urinary & Reproductive	Lactobacillus, Mycobacterium, Staphylococcus	Flow of urine prevents extensive colonization.

How Normal Microbiota Become Opportunistic Pathogens

Normal microbiota can cause disease under certain conditions and are then called opportunistic pathogens.

• Introduction into unusual site in the body

• Immune suppression

• Changes in the normal microbiota (e.g., antibiotic use)

• Stressful conditions

Reservoirs of Infectious Diseases of Humans

Types of Reservoirs

Reservoirs are sites where pathogens are maintained as a source of infection. Most pathogens cannot survive for long outside their host.

• Animal reservoirs: Diseases naturally spread from animal hosts to humans (zoonoses).

• Human carriers: Asymptomatic infected individuals can be infective to others.

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

Common Zoonoses

Disease	Causative Agent	Animal Reservoir	Mode of Transmission
Tapeworm infestation	Dipylidium caninum	Dogs	Ingestion of larvae transmitted in dog saliva
Malaria	Plasmodium spp.	Monkeys	Bite of Anopheles mosquito
Anthrax	Bacillus anthracis	Domestic livestock	Direct contact with infected animals
Rabies	Lyssavirus sp.	Bats, skunks, foxes, dogs	Bite of infected animal

The Invasion and Establishment of Microbes in Hosts: Infection

Contamination vs. Infection

• Contamination: The mere presence of microbes in or on the body.

• Infection: When an organism evades the body’s external defenses, multiplies, and becomes established in the body.

Portals of Entry

Pathogens enter the body through specific sites called portals of entry. The three major pathways are:

• Skin: Entry 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: Not a true portal; pathogens are deposited directly into tissues beneath the skin or mucous membranes (e.g., via puncture wounds).

The Role of Adhesion in Infection

Adhesion is the process by which microorganisms attach themselves to host cells, a critical step for establishing infection.

• Adhesion factors include specialized structures and attachment molecules (adhesins or ligands).

• Attachment proteins on microbes bind to specific receptors on host cells, determining host specificity.

• Blocking or altering these interactions can prevent infection.

• Some bacteria form biofilms, enhancing their ability to colonize and resist host defenses.

The Nature of Infectious Disease

Infection, Disease, Morbidity, Pathogenicity, and Virulence

• Infection: Invasion of the host by a pathogen.

• Disease (Morbidity): Results if the invading pathogen alters normal body functions.

• Pathogenicity: Ability of a microorganism to cause disease.

• Virulence: Degree of pathogenicity; determined by virulence factors.

Manifestations of Disease: Symptoms, Signs, and Syndromes

• 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 still have signs.

Causation of Disease: Etiology

• Etiology: The study of the cause of disease.

• Diseases can have various causes, including infectious agents, genetics, and environment.

Koch’s Postulates

Robert Koch developed a set of criteria 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.

Limitations: Some pathogens cannot be cultured, some diseases are caused by multiple agents, and ethical issues may prevent testing in humans.

Virulence Factors of Infectious Agents

• Adhesion factors (e.g., fimbriae, glycocalyces)

• Biofilms

• Extracellular enzymes (e.g., hyaluronidase, collagenase, coagulase, kinase)

• Toxins (exotoxins and endotoxins)

• Antiphagocytic factors (e.g., capsules, leukocidins)

Toxins: Exotoxins and Endotoxins

• Exotoxins: Secreted proteins, usually by Gram-positive bacteria; highly toxic and heat-labile.

• Endotoxins: Lipid A component of Gram-negative bacterial LPS; released upon cell death, heat-stable, less potent but can cause fever, shock, and inflammation.

Feature	Exotoxins	Endotoxins
Source	Mainly Gram-positive and some Gram-negative bacteria	Gram-negative bacteria
Chemical Nature	Protein	Lipid A of LPS
Toxicity	High	Low
Heat Stability	Unstable	Stable
Fever Producing	No	Yes
Antigenicity	Strong	Weak

Antiphagocytic Factors

• Bacterial capsules: Composed of chemicals not recognized as foreign; slippery and difficult for phagocytes to engulf.

• Antiphagocytic chemicals: Prevent fusion of lysosome and phagocytic vesicles; leukocidins destroy phagocytic white blood cells.

The Stages of Infectious Disease

Infectious diseases typically progress through five stages:

1. Incubation period: Time between infection and appearance of symptoms.

2. Prodromal period: Short period of mild symptoms.

3. Illness: Most severe stage; characteristic signs and symptoms.

4. Decline: Symptoms subside as immune response/treatment reduces pathogen numbers.

5. Convalescence: Recovery and return to normal function.

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. They are commonly found in materials the body secretes or excretes (e.g., respiratory droplets, feces, urine, semen).

Modes of Infectious Disease Transmission

Contact Transmission

• Direct contact: Physical interaction between hosts.

• Indirect contact: Via fomites (inanimate objects).

• Droplet transmission: Pathogens spread in droplets of mucus by exhaling, coughing, or sneezing.

Vehicle Transmission

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

• Waterborne: Spread via contaminated water (often 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).

• Mechanical vectors: Arthropods that passively carry pathogens on their bodies (e.g., flies).

Classification of Infectious Diseases

Classification Schemes

• By taxonomic category

• By body system affected

• By longevity and severity (acute, subacute, chronic, latent)

• By how they are spread (communicable, contagious, noncommunicable)

• By effects on populations

Epidemiology of Infectious Diseases

Definition and Frequency of Disease

• Epidemiology: The study of where and when diseases occur and how they are transmitted in populations.

• 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.

Occurrence Terms

• Endemic: Disease is always present at a stable frequency.

• Sporadic: Disease occurs occasionally.

• Epidemic: Disease occurs at a greater frequency than usual.

• Pandemic: Epidemic occurs on more than one continent simultaneously.

Epidemiological Studies

• Descriptive epidemiology: Tabulation of data concerning a disease, including location, time, and patient information.

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

• Experimental epidemiology: Tests hypotheses about the cause of disease, often using Koch’s postulates.

Hospital Epidemiology: Healthcare-Associated (Nosocomial) Infections

• Exogenous: Pathogen acquired from the healthcare environment.

• Endogenous: Pathogen arises from normal microbiota within the patient.

• Iatrogenic: Results from modern medical procedures.

• Superinfections: Use of antimicrobial drugs inhibits some resident microbiota, allowing others to thrive.

Prevention: Aggressive control measures, especially handwashing, are the most effective way to reduce healthcare-associated infections.