Microbiology Chapter 15: Microbial Mechanisms of Pathogenicity

Infectious disease

• A disease caused by a pathogenic microbe

Communicable disease

• An infectious disease that can be passed from human to human

Iatrogenic disease

• A disease contracted from a medical procedure

Nosocomial disease

• A disease contracted in a hospital setting

Signs

• Objective or measurable indications of disease

- Ex: Vomiting

• “If a vet can see it in their patient, it’s a sign”

Symptoms

• Subjective feelings of disease

- Ex: Nausea

• “If it takes words from a patient to describe it, it’s a symptom”

Zoonosis

• A disease transmitted from animal to human

Incubation Phase

• First phase of disease

• The time from infection to first symptoms

• Microbe multiplies during this time

- Microbial burden normally not enough to transmit disease, yet

Prodromal

• Second phase of disease

• First signs or symptoms of disease

• Microbe is still increasing

Illness

• Third phase of disease

• Person starts to fell ill

• Microbe volume at a peak

- Decline to start soon

Decline

• Fourth phase of disease

• Signs and symptoms begin to go away

• Microbe is being removed by the immune system

- Person is susceptible to a secondary infection

Convalescence

• Fifth phase of disease

• Patients start to recover

• Time patient spends in this phase is variable

Acute

• Timing of disease

• Infected, diseased, recovers

Chronic

• Timing of disease

• Infected, slow progression of disease, pathology occurs over long time

Laten

• Timing of disease

• Infected, disease may be delayed, microbe goes dormant

Koch’s Postulates

• Multi-step process for determining if a microbe causes disease

• Invented by Robert Koch’s

• Step 1: Microbe must be present in all diseased organisms and absent from healthy ones

• Step 2: Microbe must be separated from organism and grown on it’s own

• Step 3: Microbe must cause disease when inoculated into a susceptible, healthy organism

• Step 4: The same microbe must be separated from second diseased organism and grown on it’s own

Pathogenicity

• The ability for a microbe to cause disease

Virulence

• How well the virus causes disease

ID50

• Measurement

• The number of microbes needed to cause infection in 50% of inoculated animals

LD50

• Measurement

• The number of microbes needed to cause lethal infection in 50% of inoculated animals

Primary pathogens

• Classification of microbe

• Causes disease regardless of the host’s microbiota or immune system status

Opportunistic pathogens

• Classification of microbe

• Only causes disease if the host’s immune system is compromised

Exposure

• Stage of pathogenesis

• The act of communing into contact with a specific pathogen

• Microbe must get inside the body

- Typically via mucus membrane

- Possible through broken barriers

Mucus membrane

• Eyes

• Mouth

• Nose

• Anus

• Vagina

• Urethra

Adhesion

• Stage of pathogenesis

• The act of a pathogen staying in the body once it has made entry

Adhesins

• Factors that help a pathogen adhere to the body

• Necessary to keep the microbe in the body

• Includes

- Fimbriae

- Glycocalyces

Invasion

• Stage of pathogenesis

• The pathogen disseminates through the body to it’s preferred tissue or cells

Virulence factors

• Things that helps pathogens disseminate through the body

• Includes

- Toxins

- Exoenzymes

Infection

• Successful multiplication of the pathogen in the host

Local infection

• An infection that is isolated to one part of the body

- Ex: Pimple

Focal infection

• A local infect that spread to a second location

Systemic infection

• An infection that has spread throughout the body

Adhesins

• A protein or glycoprotein that is on the surface of the pathogen

• Binds to a receptor on the surface of a host cell

Protein F

• Adhesins

• Located on Streptococcus pyogenes

• Binds to respiratory epithelia cells

Type 1 fimbriae

• Adhesins

• Located on ETEC

• Binds to intestinal epithelial cells

Exoenzymes

• Enzymes secreted outside the pathogen

• Helps for deeper invasion

Hyaluronidases

• Exoenzyme

• Degrades hyaluronic acid

- Allows spread through tissues

Phospholipases

• Exoenzyme

• Degrades phospholipid bilayers

Nucleases

• Exoenzyme

• Chews up DNA from dead cells to prevent trapping

Proteases

• Exoenzyme

• Degrades proteins

- Allows spread through tissue

Toxins

• Biological poisons that damage tissue

LPS

• Toxin

• Lipid A portion = toxin that stimulates the inflammatory response

- Cases shock

• Located on the outsides of gram negative

Cholera toxin

• Toxin

• Causes secretion of fluids and electrolytes from intestinal cells

Botulinum toxin

• Toxin

• Inhibits release of acetylcholine

- Muscles can’t contract

Tetanus toxin

• Toxin

• Inhibits release of GABA

- Muscles can’t relax

White blood cells of various types

Proinflammatory molecules

Capsules

• Virulence factors

• Slippery coating that makes it hard for antibody or phagocytes to bind

• Purpose: Immune evasion

Proteases

• Virulence factors

• Functions to degrade antibody that might be attached

• Purpose: Immune evasion

Antigenic variation

• Virulence factors

• Genetic recombination in the gene for a surface protein

- Antibody can only match one protein

• Purpose: Immune evasion

Antigenetic drift

• Mechanism in viruses

• Small mutations in the spike (usually) that slightly alter binding of antibodies

• Purpose: Immune evasion

Antigenetic shift

• Mechanism in viruses

• Reassortment of viral genome

- Each viral genome is in multiple distinct sections

• Purpose: Immune evasion

Gliotoxin

• Mycotoxins of the fungus Aspergillus

• Inhibits inflammation and phagocytic cells

• Kills cells in host

Aflatoxin

• Mycotoxins of the fungus Aspergillus

• Can cross the placenta barrier

Flora

• The microorganisms that are naturally in the human body

• Mainly located in the mouth and colon

• When normal, not dangerous to humans

- If disturbed, can become dangerous

Hyaluronic Acid

• Substance that holds epithelial cells together

• When destroyed pathogens are able to gain entry through tissue

Helminth

• Worms

Glycan Gimmickry

• Immune evasion technique of fungus

• Alters outer sugars on cell wall to look more like human cells