CH. 15 | Microbial Mechanisms of Pathogenicity

15 - 1 Identify the principal portals of entry

Pathogens enter the body through 3 primary avenues:

• Mucous membranes

• Skin

• Parenteral route

Key Concept: Entering the body does not always guarantee disease. Many pathogens have a preferred portal of entry that is required for them to cause disease

Mucous Membranes

• Most common pathway for bacteria and viruses

  • Respiratory Tract: Easiest and most frequent entry point — Microbes are inhaled via moisture drops or dust

    • COVID-19, Influenza, Tuberculosis, Measles

  • Digestive Canal: Entry via contaminated food, water, or fingers — Most microbes are killed by hydrochloric acid, but those that survive can CAUSE disease

    • Typhoid fever, cholera, Hepatitis A, giardiasis

  • Genital System: Sexually contracted — Pathogens may cross unbroken membranes or enter through small abrasions

    • HIV, Chlamydia, Syphilis, Gonorrhea

  • Conjunctiva: Membrane lining the eyelids and eyeballs

    • Pink eye
      
      

Skin

• The largest organ in the body, and a primary defense

  • Unbroken Skin: Mostly impenetrable (impossible to pass through) by microbes

  • Openings: Pathogens can enter through hair follicles or sweat ducts

  • Direct Infection: Some fungi grow on skin keratin, and certain larvae (hookworm) can bore through intact skin
    
    

Parenteral Route

• Occurs when pathogens are deposited directly into tissues beneath the skin or membranes because the barriers have been injured

  • Causes: Punctures, injections, bites, cuts, wounds, surgery, or skin splitting due to swelling or drying

  • Examples: HIV, Hepatitis viruses, Tetanus

15 - 2 Define ID₅₀ and LD₅₀

ID₅₀ (Infectious Dose)

• The number of microbes required to cause an active infection in 50% of a sample population

  • It measures the virulence (degree of pathogenicity) of a microbe

  • Lower ID₅₀ = Higher Efficiency

  • High ID₅₀ = Low Efficiency
    
    

LD₅₀ (Lethal Dose)

• The amount of a toxin or substance required to cause death in 50% of a sample population

  • It measures the potency of a toxin

  • Lower LD₅₀ = More deadly

  • High LD₅₀ = Less deadly

15 - 3 Using examples, explain how microbes adhere to host cells

To understand how microbes cause disease

Adherence (Adhesion)

• Microorganisms attach to surfaces, such as host tissues, using specialized molecules called adhesins

• Attachment phase and is a necessary step for most pathogens to become harmful

• Lock (body cells) & Key (pathogen) system
  
  

Mechanism: Adhesins & Receptors

• Microbes physically bind to nearby cells using specific molecules

  • Adhesins (Ligands): These are surface molecules on the pathogen

    • Usually made of glycoproteins or lipoproteins

  • Receptors: These are complementary surface molecules on the host cell

    • Typically sugars, like mannose
      
      

Examples of Adherence in Action

• Dental Plaque (Streptococcus mutans)

  • This bacterium uses an enzyme to turn glucose into a sticky substance called dextran

  • This forms a “sticky net” — glycocalyx, which allows the bacteria to cling to your teeth

• Respiratory Infections (Influenza & COVID-19)

  • Influenza: Uses a spike protein called HA to bind to sialic acid on your lung cells

  • COVID-19: Uses its famous S (spike) proteins to attach to a specific receptor called ACE2 on human cells
    
    

Overall, this is an important concept because if we can figure out how to alter or block either the adhesin or the receptor, then we can prevent the infection from ever starting

15 - 4 Explain how capsules and cell wall components contribute to pathogenicity

To understand how microbes defend themselves

Capsules

• A capsule is a sticky layer of glycocalyx material that surrounds the bacterial cell wall

  • It works by increasing virulence by impairing phagocytosis. Normally, your immune cells (phagocytes) wrap around a bacterium to destroy it. However, the chemical nature of the capsule makes the bacterium “slippery”, thus preventing the immune cell from sticking to it

  • However, to work around this. If your body produces antibodies against that specific capsule, the immune system can get ahold of the bacterium and destroy it

    • Examples: S. pneumoniae: Strains with capsules cause pneumonia; Strains without them are harmless because your body eats them immediately through phagocytosis
      
      

Cell Wall Components

• Certain chemicals in the cell wall help bacteria stick to you and resist being digested

  • M Protein: Found on the surface and fimbriae of Streptococcus pyogenes

    • Function: It is heat-resistant and acid-resistant. It helps the bacteria attach to your cells and helps them resist phagocytosis by white blood cells

  • Opa Protein: An outer membrane protein used by N. gonorrhoeae

    • Function: It works with fimbriae to attach the bacteria firmly to host cells. Once attached, the host cell actually pulls the bacteria inside

  • Mycolic Acid: A waxy lipid found in the cell wall of Mycobacterium species, etc

    • Function: Even if a white blood cell manages to swallow the bacteria, the waxy mycolic acid prevents the bacteria from being digested

    • The bacteria can actually multiply inside the immune cell

15 - 5 Compare the effects of coagulases, kinases, hyaluronidase, and collagenase

To understand how bacteria use enzymes to spread or hide

Coagulases

• An enzyme that turns fibrinogen (a blood protein) into fibrin (clotting threads)

  • They create a blood clot around the bacteria, and from here the clot acts like a fortress that protects the bacteria from phagocytosis and isolates them from other immune defenses

  • Produced by some species of Staphylococcus
    
    

Kinases

• An enzyme that break down fibrin and digest clots

  • They dissolve the clots the body creates to “wall off” an infection, and the benefit of this is that it allows the bacteria to spread through the body instead of staying trapped in one spot

  • Fibrinolysis (Streptokinase)
    
    

Hyaluronidase

• An enzyme that digests hyaluronic acid, a sugar that holds connective tissue cells together

  • They cause tissue blackening and allow microbes to spread from the initial site of infection to other places

  • Produced by Streptococci, etc
    
    

Collagenase

• An enzyme that breaks down collagen, the main protein in the connective tissue of muscles and organs

  • They destroy the structural framework of tissues, facilitating the spread of gas gangrene

  • Produced by Clostridium species

15 - 6 Define and give an example of antigenic variation

Antigenic Variation

• The process by which some pathogens alter their surface proteins (antigens) so that the body’s antibodies can no longer recognize or bind to them
  
  

Examples

• Neisseria gonorrhoeae: This bacterium has several different copies of the Opa-encoding gene. It can express different antigens over time, making it very difficult for the immune system to clear the infection

• Influenzavirus (The Flu): This virus constantly undergoes antigenic changes, which is why you need a flu shot every year, because the antibodies from last year wouldn’t be able to recognize this year’s version

15 - 7 Describe how bacteria use the host cell’s cytoskeleton to enter the cell

To understand how bacteria enter a cell

Mechanism (Hijacking Actin)

• While some bacteria enter through simple attachment, others take a more active approach by manipulating the host’s internal “scaffolding.”

  • Invasins: These are surface proteins produced by certain bacteria (like Salmonella and E. coli) upon contact with the host cell’s plasma membrane that can rearrange nearby actin filaments of the cytoskeleton, allowing the bacteria to be engulfed by the host cell
    
    

Entry via “Membrane Ruffling”

• When Salmonella makes contact, its invasins cause the host cell’s plasma membrane to look like the splash of a liquid hitting a solid surface

  • The “ruffle” is this disruption of the cytoskeleton that creates “ruffles” in the membrane

  • Macropinocytosis: The bacterium sinks into these ruffles and is engulfed by the host cell. This is essentially the cell “drinking” the bacterium by mistake

15 - 9 Describe the function of siderophores

Bacteria are living organisms that need nutrients to grow, and for most pathogenic bacteria, iron is the most critical resource.

Siderophores

• Bacterial iron-binding proteins

  • How it works

    • They work by taking iron away from the host’s iron-transport proteins by binding the iron even more tightly

    • Once the siderophore has formed into an iron-siderophore complex, it binds to specific siderophore receptors on the surface of the bacterium

    • Then, the bacterium pulls the whole complex inside, releasing the iron into its own cytoplasm to power its growth and reproduction
      
      

Alternative Methods of Stealing Iron

• Direct Binding: Some pathogens have receptors that bind directly to our iron-transport proteins or hemoglobin, taking the whole unit into the bacterial cell

• Cell Murder (Toxins): When iron levels are low, some bacteria release toxins that kill host cells, thereby making their iron more accessible and available for the bacteria

15 - 10 Provide an example of direct damage, and compare this to toxic production

Direct Damage

• This occurs when a pathogen physically destroys the host cell it is currently inhabiting or attached to

  • Mechanism: As pathogens multiply inside a host cell, the cell becomes crowded with metabolic waste and new microbes. Eventually the pressure causes the cell to rupture (lyse)

  • Once the cell bursts, the newly formed pathogens are released and can immediately infect neighboring cells

  • Examples include viruses, as well as intracellular bacteria and protozoa
    
    

Toxin Production

• Toxins are poisonous substances produced by microorganisms that contribute significantly to their pathogenicity

15 - 11 Contrast the nature and effects of exotoxins and endotoxins

Exotoxins

• A protein toxin released from living, mostly gram-positive bacterial cells

• Highly specific — targets particular cells or metabolic functions

• Extremely potent even in small amounts

• Soluble in body fluids, easily spread through blood

• The body produces antitoxins against them

• Can be inactivated into toxoids for use as vaccines
  
  

Endotoxins

• Part of the outer portion of the cell wall (lipid A) of most gram-negative bacteria: released on destruction of the cell

• Lipid portions of lipopolysaccharides (LPS) — part of the cell wall

• Produced only by gram-negative bacteria

• Released when the bacteria die and the cell wall lyses

• Less specific in their effects

• Produce the same signs and symptoms, regardless of the species of the microorganism

15 - 12 Outline the mechanisms of action of A-B toxins, membrane-disrupting toxins, superantigens, and genotoxins

A-B Toxins

• Bacterial exotoxins consisting of two polypeptides

  • Two components: A (active/enzyme) and B (binding)

  • B component binds to the host cell receptor

  • Toxin enters via receptor-mediated endocytosis

  • A and B are separated inside the cell

  • A component disrupts cell function, often by inhibiting protein synthesis

  • B component is recycled back to the cell membrane
    
    

Membrane-Disrupting Toxins

• Cause lysis of host cells by disrupting plasma membranes, either by:

  • Forming protein channels in the membrane

  • Disrupting the phospholipid portion

    • Key Types:

      • Leukocidins — Kill pathogenic WBCs (neutrophils and macrophages) by forming protein channels; produced mainly by staphylococci and streptococci

      • Hemolysins — Destroy red blood cells via protein channels; streptococcal hemolysins are called streptolysins
        
        

Superantigens

• An antigen that activates many different T cells, thereby eliciting a large immune response

  • Binds to proteins on macrophages, nonspecifically stimulating T cell proliferation

  • T cells release massive amounts of cytokines

  • Excessive cytokines cause fever, nausea, vomiting, diarrhea, shock, and even death
    
    

Genotoxins

• A-B toxins that target DNA or RNA

• Cause mutations, disrupt cell division, and may lead to cancer

• Produced by gram-negative bacteria; Salmonella, and some E. coli

15 - 13 Identify the importance of the LAL assay

Limulus Amebocyte Lysate (LAL) Assay

• A test to detect the presence of bacterial endotoxins

• Detects endotoxins in drugs, medical devices, and body fluids (even minute amounts)

• Critical because sterilized materials can still contain endotoxins when when no live bacteria are present

15 - 14 Using examples, describe the roles of plasmids and lysogeny in pathogenicity

Plasmids and Pathogenicity

• Plasmids are small, circular DNA molecules that replicate independently from the main chromosome

• Two Key Groups:

  • R (resistance) factors — carry antibiotic resistance genes

  • Virulence factors — carry genes that determine a microbe's pathogenicity

• Examples of virulence factors encoded by plasmids:

  • Tetanus neurotoxin

  • Heat-labile enterotoxin (E. coli)

  • Staphylococcal enterotoxin D

  • Adhesins and coagulase (S. aureus)

  • Fimbria specific to enteropathogenic E. coli

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Lysogeny and Pathogenicity

• Bacteriophage — A virus that specifically infects and destroys bacteria

• Prophage — A bacteriophage (virus) genome that has inserted itself into a host bacterium's DNA
  
  

• Bacteriophages can incorporate their DNA into the bacterial chromosome, becoming a prophage — this state is called lysogeny

• A change in microbial characteristics due to a prophage is called lysogenic conversion

• Lysogenic cells are medically important because some bacterial pathogenesis is caused by the prophages they carry

• Examples of toxins/virulence factors encoded by phage genes:

  • Diphtheria toxin

  • Botulinum neurotoxin

  • Erythrogenic toxins

  • Shiga toxin in E. coli O157

  • Cholera toxin — lysogenic phages in V. cholerae can even transmit the toxin gene to nonpathogenic strains, increasing the number of pathogenic bacteria

15 - 15 List 11 cytopathic effects of viral infections

Cytopathic Effects (CPE)

• A visible effect on a host cell, caused by a virus, that may result in host cell damage or death

• Cell junction disruption — junctions between cells are broken down (e.g., SARS-CoV-2 and Alphainfluenzavirus disrupt pulmonary alveoli junctions; cilia also stop moving)

• Cytokine storm — excessive cytokine production increases inflammation, damaging tissues and organs, sometimes fatally (e.g., SARS-CoV-2, Alphainfluenzavirus)

• Macromolecular synthesis stops — some viruses irreversibly halt mitosis within the host cell (e.g., Simplexvirus)

• Lysosome release — host cell lysosomes release their enzymes, destroying intracellular contents and causing cell death

• Inclusion bodies — viral nucleic acids or proteins accumulate in the cytoplasm or nucleus (e.g., Negri bodies in rabies); useful diagnostically

• Syncytium formation — adjacent infected cells fuse into a large multinucleate giant cell (e.g., measles, mumps, common cold)

• Altered cell function with no visible changes — host cell function is changed without visible morphological effects (e.g., measles virus reduces IL-12 production via CD46 receptor)

• Antigenic changes on cell surface — viral proteins alter the cell surface, triggering host antibody response that kills the infected cell even if the virus is noncytocidal

• Chromosomal changes — chromosomal breakage occurs; oncogenes may be activated or contributed by the virus, potentially leading to cancer

• Cell transformation — cancer-causing viruses transform host cells into abnormally shaped cells that lose contact inhibition, leading to unregulated cell growth

• Interferon production — infected cells produce alpha and beta interferons (coded by host DNA) which protect neighboring cells by inhibiting viral protein synthesis and triggering apoptosis of infected cells; however, most viruses can partially evade interferons by blocking their synthesis

15 - 16 Discuss the causes of symptoms in fungal, protozoan, helminthic, and algal diseases

Fungi

• Generally lack well-defined virulence factors; toxic effects are indirect (fungus already growing in/on host)

• Chronic infections (e.g., athlete's foot) can provoke allergic responses

• Disease caused by toxin production

• Can be caused by capsules, toxins, and allergic responses
  
  

Protozoa

• Symptoms triggered by the presence of protozoa and their waste products

• Some protozoa change their surface antigens while growing in a host, thus avoiding destruction by the host’s antibodies

• Mechanisms vary by organism:

  • Plasmodium — invades and reproduces within host cells, causing rupture (malaria)

  • Toxoplasma — enters macrophages via phagocytosis, prevents acidification/digestion, survives and grows inside

  • Giardia — attaches via a sucking disc, digests host cells and tissue fluids

• Some evade the immune system through antigenic variation:

  • Trypanosoma continuously switches surface antigens, staying ahead of antibody responses; can produce up to 1,000 different antigens, allowing infection to last decades
    
    

Helminths

• Symptoms caused by the physical presence of the parasite

  • Use host tissues for growth or form large parasitic masses, causing cellular damage

  • Example: Wuchereria bancrofti blocks lymphatic circulation → accumulation of lymph plasma → grotesque swelling of limbs (lymphatic filariasis)

• Metabolic waste products of the parasites also contribute to disease symptoms
  
  

Algae

• Disease caused by neurotoxin production during harmful algal blooms that can cause paralysis when ingested by humans

15 - 17 Differentiate portal of entry and portal of exit

Portal of Entry

• Routes through which microbes enter the body

• Microbes tend to use a preferred route of entry specific to the pathogen
  
  

Portals of Exit

• Routes through which microbes leave the body

• Found in secretions, excretions, discharges, or shed tissue

  • The respiratory tract & digestive canal are the most common

  • Skin or wound infections