4.3 Host-Microbe Interactions (Part 3)

What are the three things a pathogen must do to successfully cause disease?

Adhere to the host's tissues, invade and acquire nutrients, and evade the immune system long enough to replicate or reproduce

Generally, how do pathogens adhere to the host's tissues?

Pathogens make a variety of adherence structures

Generally, how do pathogens invade tissues and acquire nutrients?

Pathogens make a variety of enzymes and binding proteins.

Generally, how do pathogens evade the immune system long enough to replicate or reproduce?

Pathogens use a variety of methods to confuse or suppress the immune system.

Fimbriae

Adhesion structures that are thin and hair-like, usually dozens or 100s per cell

Pili

Adhesion structures that are longer and thicker than fimbriae, usually only a few on each cell

Fimbriae and pili

Adhesion structures made of protein fibers that have protein on the very end called adhesins

Adhesins

Allow bacteria to attach to specific proteins on specific kinds of cells.

Capsules and Slime Layers

Adhesion structures made of sugars with chemical stickiness

Capsules

Made of densely packed and organized sugars; organization usually makes it appear sharp on microscope images.

Slime Layers

Made of loosely packed and disorganized sugars; lack of organization usually makes it appear fuzzy on microscope images.

What is a biofilm?

A community of microbes that stick to surfaces and secrete a sticky matrix called extracellular polymeric substances (EPS), which provides protection and enhanced adhesion

What is EPS (extracellular polymeric substance)?

A gel-like mixture secreted by biofilm cells that traps bacteria and protects them from immune defenses

Biofilm on teeth

Streptococcus mutans forms a biofilm on tooth enamel that causes dental caries (tooth decay)

How do viruses attach to host cells?

Most viruses use spike proteins to bind to specific host-cell receptors

How can some viruses attach without spikes?

They bind directly to host receptors using their capsomeres

What are invasins?

Microbial proteins that promote tissue invasion or bind nutrients needed for growth

What is the cytopathic effect?

Cell injury or death caused by toxins or infections that releases nutrients into surrounding tissues

What does coagulase do?

An enzyme made by Staphylococcus aureus that enhances coagulation, forming a protective clot to hide from phagocytes

What does streptokinase do?

An enzyme made by Streptococcus that breaks down clots, allowing bacteria to escape and spread

How does hyaluronidase aid tissue invasion?

It destroys hyaluronic acid (the sugar "glue" holding epithelial cells together) so bacteria can spread

How does collagenase aid tissue invasion?

It breaks collagen fibers in connective tissue, damaging skin and mucosal barriers

How do toxins help pathogens invade?

By killing host cells (cytopathic effect) and releasing nutrients such as sugars and amino acids

What are A-B toxins?

Two-part bacterial toxins:A (active) and B (binding) that enter cells and destroy them; example: C. difficile binary toxin

What are the two main strategies pathogens use to evade the immune system?

(1) Hiding from host defenses and (2) Undermining host defenses

What is antigenic masking?

When pathogens coat themselves with host self-antigens, making immune cells unable to recognize them as foreign

Antigenic masking in S. aureus

Staphylococcus aureus binds extracellular-matrix proteins like collagen and fibronectin to avoid recognition by T cells and antibodies

What is antigen mimicry?

Pathogens display surface antigens that mimic host molecules, tricking the immune system into ignoring them

How do capsules and slime layers contribute to antigen mimicry?

They contain sugars similar to those found on human cells, making pathogens harder to distinguish from self

Neisseria meningitidis immune evasion

Builds its capsule from sialic acid (the same sugar found on human cell membranes) helping it blend in

What is antigenic variation?

Rapid mutation or switching between different antigen sets so immune cells can't mount a lasting response (e.g., influenza, SARS-CoV-2)

Trypanosoma brucei antigenic variation

The protozoan that causes sleeping sickness changes its surface antigens repeatedly, forcing the host to restart immune responses

How do some pathogens escape phagocytosis?

They prevent destruction inside macrophages by killing the cell or living safely inside it

Anthrax immune evasion

Bacillus anthracis produces a lethal A-B toxin that kills macrophages after phagocytosis, allowing escape

Q fever immune evasion

Coxiella burnetii survives and replicates inside lysosomes of phagocytes, making it mostly immune to destruction