Chapter 14: Infection and Diseases – Microbiology Study Notes

Chapter 14: Infection and Diseases

Chapter Objectives

This chapter explores the fundamental concepts of infection and disease in microbiology, focusing on host-parasite relationships, reservoirs, transmission, and classification of infectious diseases.

• Describe the relationship of host and parasite.

• Describe the relationship between microbes and their hosts.

• Enumerate and describe reservoirs of infectious diseases of humans.

• Describe invasion and establishment of microbes in hosts.

• Describe the nature of infectious disease.

• Describe the movement of pathogens out of hosts.

• Describe the modes of infectious disease transmission.

• Describe the classification of infectious diseases.

• Describe healthcare-associated infections, factors, and control methods.

Host-Parasite Relationship

Balance Between Susceptibility and Resistance

The host-parasite relationship is determined by a balance between host susceptibility and resistance. Factors influencing this balance include:

• Microbial factors: Virulence, portal of entry, dosage, and indigenous microbiota.

• Physiological factors: Injury, stress, malnutrition, and age.

• Cellular factors: Phagocytosis, lymphocyte activity, and interferon production.

• Constitutional factors: Tissue type, organ function, and genetic predisposition.

• Socioeconomic factors: Hygiene, nutrition, and environmental hazards.

Normal Microbiota

Definition and Types

Normal microbiota are microbes that live on and in the human body without causing apparent harm. They are classified as:

• Permanent (Resident) microbiota: Remain part of the normal flora throughout life.

• Transient microbiota: Temporarily present and may be removed by body defenses.

Human Microbiome Composition

The human body contains a vast array of microbial cells, including bacteria, fungi, and viruses. Bacterial cells outnumber human cells, and the microbiome plays a crucial role in health.

Colonization and Establishment

Colonization of normal microbiota begins at birth and is influenced by:

• Birth canal

• First breath

• First food

• Contact with parents, medical staff, linens, and clothes

Colonization is typically established within 24–48 hours post-natal.

Pathogens Crossing the Placenta

Certain pathogens can cross the placenta and affect the embryo or fetus.

Pathogen	Condition in Adult	Effect on Embryo or Fetus
Toxoplasma gondii (Protozoan)	Toxoplasmosis	Abortion, epilepsy, microcephaly, mental retardation, blindness
Treponema pallidum (Bacteria)	Syphilis	Abortion, birth defects, syphilis
Listeria monocytogenes (Bacteria)	Listeriosis	Granulomatosis infantiseptica, death
Cytomegalovirus (DNA Virus)	Usually asymptomatic	Deafness, mental retardation
Rubella virus (RNA Virus)	German measles	Severe birth defects or death

Types of Symbiosis

Symbiotic relationships between microbes and hosts include:

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

Importance of Normal Microbiota

• Prevent attachment of invaders by occupying binding sites.

• Deplete essential nutrients needed by pathogens.

• Produce antimicrobial substances that inhibit pathogen growth.

Normal Microbiota Becoming Opportunists

Normal microbiota can become opportunistic pathogens under certain conditions:

• Immune suppression (e.g., HIV/AIDS, chemotherapy)

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

• Introduction into unusual sites (e.g., wounds, catheters)

Reservoirs of Infectious Diseases

Types of Reservoirs

Reservoirs are sites where pathogens are maintained as sources of infection:

• Animal reservoirs: Zoonoses are diseases that spread from animals to humans (e.g., rabies, influenza).

• Human carriers: Asymptomatic individuals can transmit pathogens.

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

Common Zoonoses

Disease	Causative Agent	Animal Reservoir	Mode of Transmission
Rabies	Lyssavirus	Bats, dogs	Bite of infected animal
Anthrax	Bacillus anthracis	Domestic livestock	Direct contact, inhalation
Ringworm	Microsporum spp.	Domestic animals	Direct contact
Influenza	Influenza virus	Birds, pigs	Contact, inhalation

Invasion and Establishment of Microbes in Hosts

Contamination vs. Infection

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

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

Portals of Entry

Pathogens enter the body through specific portals:

• Skin

• Mucous membranes (respiratory, gastrointestinal, urogenital tracts)

• Placenta

• Parenteral route (circumvents usual portals, e.g., via cuts, bites, injections)

Role of Adhesion in Infection

• Adhesion: Microorganisms attach to host cells using adhesion factors, specialized structures, and attachment molecules.

• Essential for successful colonization and infection.

Nature of Infectious Disease

Definition and Manifestations

• Infection: Invasion of host by pathogen.

• Disease (Morbidity): Alteration of normal body functions due to infection.

Manifestations of Disease

• Symptoms: Subjective characteristics (e.g., pain, nausea, fatigue).

• Signs: Objective characteristics (e.g., swelling, fever, rash).

• Syndrome: Group of signs and symptoms.

• Asymptomatic (subclinical): Infections lack symptoms but may have signs.

Symptoms (Sensed by Patient)	Signs (Detected by Observer)
Pain, nausea, headache, chills, sore throat, fatigue, malaise, discomfort, itching, abdominal cramps	Swelling, rash, vomiting, diarrhea, fever, pus, anemia, leukocytosis/leukopenia, bubo, tachycardia/bradycardia

Etiology

• Etiology: Study of the cause of disease. Diseases may have infectious, genetic, or environmental causes.

Koch's Postulates

• Set of criteria used to establish a causative relationship between a microbe and a disease.

• Includes isolation, cultivation, and reproduction of disease in a healthy host.

Virulence Factors of Infectious Agents

• Pathogenicity: Ability to cause disease.

• Virulence: Degree of pathogenicity.

• Virulence factors include adhesion factors, biofilms, extracellular enzymes, toxins, and antiphagocytic factors.

Exotoxins vs. Endotoxins

Feature	Exotoxins	Endotoxins
Source	Mainly Gram-positive and Gram-negative bacteria	Gram-negative bacteria
Chemical Nature	Protein or short peptide	Lipid portion of LPS
Toxicity	High	Low, but may be fatal in high amounts
Heat Stability	Unstable above 60°C	Stable
Effect on Host	Variable, specific	Fever, inflammation, shock
Fever Producing	No	Yes
Antigenicity	Strong	Weak
Representative Diseases	Botulism, tetanus, diphtheria	Typhoid fever, meningitis

Antiphagocytic Factors

• Bacterial capsule: Prevents phagocytosis.

• Antiphagocytic chemicals: Leukocidins, prevent fusion of lysosome and phagocytic vesicle.

Stages of Infectious Disease

• Incubation period: Time between infection and first symptoms.

• Prodromal period: Mild symptoms appear.

• Illness: Most severe signs and symptoms.

• Decline: Signs and symptoms subside.

• Convalescence: Recovery and return to normal health.

Disease	Incubation Period
Staphylococcus foodborne infection	<1 day
Influenza	About 1 day
Cholera	2 to 3 days
Genital herpes	About 5 days
Tetanus	5 to 15 days
Syphilis	10 to 21 days
Hepatitis B	70 to 100 days
AIDS	1 to >8 years
Leprosy	10 to >30 years

Movement of Pathogens Out of Hosts

Portals of Exit

Pathogens exit the host through specific portals, including respiratory tract, gastrointestinal tract, urogenital tract, and skin.

Modes of Infectious Disease Transmission

Three Main Modes

• Contact transmission: Direct, indirect, or droplet.

• Vehicle transmission: Airborne, waterborne, foodborne.

• Vector transmission: Biological (e.g., mosquitoes) or mechanical (e.g., flies).

Classification of Infectious Diseases

Criteria for Classification

• Taxonomic categories

• Body system affected

• Longevity and severity

• Mode of transmission

• Population effects

Longevity and Severity

• Acute: Rapid onset, short duration.

• Chronic: Slow development, long-lasting.

• Subacute: Intermediate between acute and chronic.

• Latent: Pathogen remains inactive before symptoms appear.

Communicability

• Communicable: Spread directly or indirectly.

• Contagious: Easily spread.

• Noncommunicable: Not spread between hosts.

Frequency and Occurrence

• Prevalence: Number of old and new cases in a given time.

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

• Sporadic: Occurs occasionally.

• Endemic: Always present in a population.

• Epidemic: Many people affected in a short period.

• Pandemic: Worldwide epidemic.

Healthcare-Associated (Nosocomial) Infections

Types and Factors

• Exogenous: Pathogen acquired from healthcare environment.

• Endogenous: Pathogen arises from normal microbiota within patient.

• Iatrogenic: Results from modern medical procedures.

• Superinfections: Use of antimicrobial drugs inhibits resident microbiota, allowing other microbes to thrive.

Additional info: These notes provide a comprehensive overview of infection and disease as covered in a college-level microbiology course, suitable for exam preparation and review.