Microbiology: Bacteria and Diseases Study Guide

Basic Terminologies in Microbiology and Disease

• Disease: Defined as the result of an undesirable relationship between the host and the pathogen. It is clinically marked by an interruption in the normal functioning of body parts.

• Infection: This refers to the invasion of the body by pathogenic microorganisms. It is critical to note that the term "infection" is not synonymous with "disease."

• Symbiosis: A prolonged and close interaction between organisms of different species. It is an umbrella term for various types of biological relationships.

• Mutualism: A specific form of symbiosis in which both organisms involved benefit from the relationship.

• Commensalism: A form of symbiosis where one organism benefits from another organism without causing it any harm.

• Parasitism: A form of symbiosis where one organism benefits from another organism and simultaneously causes harm to that host organism.

• Pathogenicity: The fundamental ability of an organism to produce disease. An organism capable of producing disease in humans is classified as pathogenic.

• Virulence: This describes the specific degree of pathogenicity of an organism, or the quantitative measure of how effectively an organism can produce disease.

• Contamination: Defined as the presence of unwanted materials (which may be chemical, biological, or radiological) in locations where they should not be or at concentrations that exceed the normal range. The presence of these substances does not necessarily lead to harm.

• Pollution: This refers to the presence of contaminants that can cause adverse biological effects to humans and their communities. A key distinction is that while all pollutants are contaminants, not all contaminants are considered pollutants.

• Bacteremia: The simple presence of bacteria in the blood.

• Septicemia: The presence of actively multiplying bacteria in the blood, usually originating from a specific source of infection. This condition is clinically referred to as Sepsis.

• Pyemia: The presence of pus-producing bacteria in the bloodstream. This is a severe form of septicemia or blood poisoning that leads to the formation of multiple abscesses in various internal organs.

• Viremia: The presence of viruses in the blood.

• Toxemia: The presence of toxins in the blood.

The Chain of Infection: Factors Influencing Infection Occurrence

• The development of an infectious disease is the consequence of an interaction among three components:

  • The etiologic agent.

  • The host.

  • The environment.

• The Chain Process: Transmission begins when a pathogenic organism leaves its host or a reservoir through a portal of exit. A susceptible organism then acquires the infection through a specific mode of transmission, entering the body of the susceptible host via a portal of entry. Once inside, the organism multiplies and produces disease.

The Six Components of the Chain of Infection

1. Pathogen / Infectious Agent

   • Includes bacteria, viruses, fungi, and parasites.

   • Virulence factors: Depends on the number of organisms, their potency, their ability to enter and survive in the body, and the susceptibility of the host.

   • Examples:

     • Smallpox virus: Highly virulent; infects almost everyone exposed.

     • Tuberculosis bacillus: Infects a smaller number of people, usually those with weakened immune systems, those who are undernourished, or those in crowded conditions.

     • HIV and Hepatitis B/C: These viruses can enter and survive in the body for years before disease symptoms manifest.

2. Reservoir

   • The site where an infectious agent normally resides, multiplies, and serves as a continual source of disease-producing microorganisms.

   • It provides necessary conditions for survival and opportunities for transmission.

   • Animal Reservoirs: Some diseases are transmitted from animals to humans, known as zoonotic infections. Humans are often incidental or "dead-end" hosts. Examples: anthrax, plague, and rabies.

   • Human Reservoirs: Directly transmitted between individuals (e.g., respiratory pathogens, STIs).

   • Types of Carriers (Infected humans who may not manifest disease or who continue to harbor organisms after recovery):

     • Asymptomatic / Healthy / Passive Carriers: Infected but manifest no symptoms.

     • Incubatory Carriers: Transmit the agent during the incubation period of the illness.

     • Chronic Carriers: Harbor the organism for months or longer after initial infection.

     • Convalescent Carriers: Individuals who recovered from the disease but remain capable of transmitting it.

   • Environmental Reservoirs:

     • Soil: Associated with the fungus Histoplasma capsulatum.

     • Water: Reservoir for Entamoeba histolytica (protozoan causing amoebiasis).

     • Aquatic Vegetation: Plants like watercress and "kangkong" harbor Fasciola hepatica larvae, which cause liver damage.

   • Fomites: Inanimate objects that serve as vectors, including:

     • Medical instruments (stethoscopes, surgical equipment, syringes, needles).

     • Common items (cups, eating utensils, bedding, wound dressings).

3. Portal of Exit

   • The route by which an agent leaves its host, usually where the agent is localized.

   • Examples:

     • Schistosoma haematobium: Invades the urinary bladder; exits via urine.

     • Respiratory Infections: Exit via droplets or aerosols.

     • Sexually Transmitted Infections: Exit via vaginal or urethral secretions.

     • Blood-borne Pathogens: Exit via blood-sucking arthropods (e.g., Plasmodium spp., the agent of malaria).

4. Mode of Transmission

   • Direct Contact:

     • Environmental contact: e.g., Schistosoma spp. larvae entering skin when wading in fresh water containing snails.

     • Person-to-person contact: Skin-to-skin, kissing, or sexual transmission. Warts transmit via skin lesion contact; Infectious Mononucleosis (Epstein Barr Virus) via saliva ("Kissing Disease"); Syphilis and Gonorrhea via secretions.

     • Droplet Spread: Occurs during coughing or sneezing. Droplets are defined as being larger than $\text{5 microns}$. Proximity is required as they travel only a few feet before falling.

   • Indirect Contact:

     • Airborne Transmission: Via dust or aerosols. Aerosols are droplets with nuclei smaller than $\text{5 microns}$. They remain suspended longer and travel farther. Example: Cryptococcus neoformans via aerosolized bird droppings; Measles (Paramyxovirus genus Morbilivirus) via aerosols.

     • Vehicle Transmission: Transmission via media such as food, water, milk, or biologic substances (blood/secretions). Fomites (bedding/clothing) are also vehicles.

     • Vector Transmission: Usually insects.

       • Mechanical: Passive transport on insect feet/body (e.g., cockroaches/flies carrying feces to food).

       • Biological: Active transport. The organism multiplies inside the vector. Examples: Malaria via female Anopheles mosquito; Dengue, Chikungunya, and Zika via mosquito bites; Plague (Yersinia pestis) via rat flea bites.

5. Portal of Entry

   • How the agent enters the host to access tissues for multiplication. Often the same as the portal of exit (e.g., respiratory in/out).

   • Specific Entry Routes:

     • Mouth: Food and water-borne organisms.

     • Skin Penetration: Infective larvae like Schistosoma haematobium.

     • Blood/Blood Products: Hepatitis B and HIV.

6. Susceptible Host

   • Factors affecting susceptibility: Immune status (most important), nutritional status, chronic alcohol intake, and the presence of invasive devices or natural barrier breakdowns.

Classification of Infectious Diseases

Based on Behavior in Host/Population

• Communicable Disease: Spread from one host to another directly or indirectly.

• Contagious Disease: Easily and rapidly spread (e.g., measles, chickenpox).

• Fulminant Infection: Results in patient death over a very short period. Example: Meningococcemia (death can occur hours after hospital confinement).

• Non-communicable Disease: Not spread between persons. Usually caused by normal flora or organisms introduced via skin breaks (e.g., Clostridium tetani causing tetanus).

Based on Source of Microorganism

• Exogenous: Source is outside the body (e.g., Cholera, nosocomial infections).

• Endogenous: Source is inside the body. Example: Escherichia coli moving from the colon to the urinary tract (common in women due to proximity of the anal orifice to the urethra).

Based on Occurrence

• Sporadic: Occurs occasionally.

• Endemic: Constantly present at low levels in a population. Example: Malaria in Palawan; Schistosomiasis in Leyte.

• Epidemic: A great number of people in a locality develop the disease in a short period.

• Pandemic: Worldwide occurrence or involving at least three regions globally.

Based on Severity or Duration

• Acute: Develops rapidly; lasts a short time.

• Chronic: Develops slowly; lasts long periods. Example: Tuberculosis; Hepatitis B (can be acute or chronic).

• Latent: Agent remains inactive for a time but can reactivate to produce symptoms.

Based on Extent of Host Involvement

• Localized: Invading organisms limited to a small area (e.g., boils, abscesses).

• Systemic (Generalized): Organisms or products spread throughout the body via blood or lymph.

• Focal: Localized infection spreads to specific areas (e.g., arising from teeth, tonsils, or sinuses).

• Primary Infection: Acute infection causing the initial illness.

• Secondary Infection: Caused by opportunistic pathogens after the primary infection weakens defenses. Common/devastating in AIDS patients.

• Subclinical (Inapparent) Infection: Does not cause noticeable illness. Example: Hepatitis A in some individuals.

Stages of an Infectious Disease

1. Incubation Period: The interval between the entry of the agent and the first appearance of signs/symptoms. Length varies based on organism virulence, number of microorganisms, and host resistance.

2. Prodromal Period: A short period of early, mild, and non-specific symptoms (e.g., fever, cough, malaise in measles).

3. Period of Illness: The most severe stage / maximal invasion. Signs and symptoms are distinctive of the disease.

   • Clinical Indicators: Bacterial infections typically show increased neutrophil counts; viral infections typically show high lymphocyte counts.

   • Outcomes: Resolution/recovery, fulminant infection (death), progression to chronic form, or progression to a carrier state.

4. Period of Decline (Defervescence): Signs and symptoms subside. Temperature returns to normal. Patient is highly vulnerable to secondary infections during this stage.

5. Period of Convalescence: The recovery period where the patient regains strength and the body returns to its pre-diseased state.

Reference

• Bartolome, Fe. A. & Quiles, Elizabeth P. Microbiology and Parasitology: A Textbook and Laboratory Manual for Health Sciences. Second Edition. Quezon City: C & E Publishing, Inc. p. 133-145.