Chapter 15: Microbial Mechanisms of Pathogenicity (Exam 3)

Opportunistic Pathogens

A pathogen that does not normally cause infection but might if host immune system is compromised or when normal microbiota change their location in the host

Primary pathogen

Cause infection/disease in host regardless of immune system or normal microbiota

Pathogenicity

Ability of a host to cause disease

Virulence

Degree of pathogenicity
-High virulence=More likely to cause infection

Virulence Factors

Traits of a pathogen that aid pathogen in escaping immune responses and successfully establishing infection
-Structures (Capsule, pili)
-Enzymes

Portals of Entry

Route a pathogen uses to enter the host cell
-Preferred
-Mucous membranes
-Skin
-Parenteral

Preferred Portal of Entry

Many pathogen cause infections only if they enter the host through their preferred portal of entry

Skin Portal of Entry

Very few pathogens capable of using this portal

Parenteral Portal of Entry

Microbes are deposited directly into tissues when barriers are penetrated
-cut, wound, burn, bite

ID50

Infectious does needed to cause in 50% of a sample population
-Measures virulence of a microbe; lower ID50=higher virulence

LD50

Lethal dose that death of 50% of a sample population
-Measures potency of a toxin

Adherence

Attachment to a host cell

Adhesins/ligands

On pathogen and bind to RECEPTORS on host cells

Adhesins/ligands examples

-Glycocalyx (capsule, slime layer)
-Fimbriae
-Pili

How do capsules assist pathogens in evading host defense mechanisms?

Impair phagocytosis; avoid attachment step

Streptococcus Pneumoniae (Pneumonia) mechanism to avoid host defense mechanisms

Capsules: Impair phagocytosis; avoid attachment step

Haemophilus Influenza (Pneumonia and meningitis) mechanism to avoid host defense mechanisms

Capsules: Impair phagocytosis; avoid attachment step

Bacillus anthracis (Anthrax) mechanism to avoid host defense mechanisms

Capsules: Impair phagocytosis; avoid attachment step

Yersina pestis (Plague) mechanism to avoid host defense mechanisms

Capsules: Impair phagocytosis; avoid attachment step

How do Cell Wall components assist pathogens in avoiding host defense mechanisms?

-M protein
-Opa Protein
-Waxy lipid

M Protein

On cell surface mediates attachment of bacterium to host and helps resist phagocytosis
-Streptococcus pyogenes

Opa Protein

Allows attachment to host cells
-Neisseria gonorrhea

Waxy Lipid

Resits digestion by phagocytic cells and bacteria can even multiply inside phagocytes
-Mycobacterium tuberculosis

What are the different mechanisms that pathogens can use to evade host defense mechanisms?

-Capsules
-Cell Wall Components
-Enzymes
-Antigenic Variation
-Penetration into the host

What enzymes can be used to evade host defense mechanisms?

-Coagulases
-Kinases
-Hyaluronidase
-Collagenase
-IgA proteases

How do coagulases evade host defense mechanisms?

Coagulate fibrinogen in to a blood clot. Protects from phagocytosis and isolate it from other host defenses
-Species of staphylococcus

How do kinases evade host defense mechanisms?

Digest fibrin clots; aid pathogen in its spread in the host
-Streptococcus pyogenes

How does hyaluronidase evade host defense mechanisms?

Digests polysaccharide hyaluronic acid that hold cells together; helps the spread

How does collagenase evade host defense mechanisms?

Breaks down collagen

How do IgA Proteases evade host defense mechanisms?

Destroy IgA antibodies to escape antimicrobial effect of antibodies

How does antigenic variation assist a pathogen in evading host defense mechanisms?

-Altering the surface antigen

What species use antigenic variation?

Neisseria gonorrhea: Has several copies of genes for Opa proteins so if host starts producing antibodies for one type, it can change the type of protein being expressed

Influenza virus: Causes flu and can change surface antigen very frequently

How can a pathogen penetrate the host in order to evade host defense mechanisms?

-Invasions
-Using actin to move
-Survival inside phagocytosis

Invasions

Use invasins to rearrange actin filaments and cause membrane ruffling of host cells which aids in pathogen entry
-Salmonella

Using Actin to Move

Use host actin to propel themselves through host cytoplasm and move from one host cell to the next; allows the spread of pathogen while remaining in the host cell
-Shigella and listeria

Survival inside Phagocytosis

1. Bacteria requiring a low pH to replicate (Coxiella burnetiid)
2. Escape from phagosome before it fuses with lysosome; remains in cytoplasm (Listeria and Shigella)
3. Prevents fusion of lysosome (TB)

Exotoxins

Proteins produced and secreted by bacteria

Exotoxins Bacterial Source

G+

Exotoxins Relation to Microorganism

Metabolic product of growing cell

Exotoxins Chemistry

Proteins

Exotoxins Pharmacology

Specific

Exotoxins Heat Stability

Unstable

Exotoxins Toxicity

High; cause disease in low #s

Exotoxins Fever-producing

No

Exotoxins Immunology

Vaccine

Exotoxins Lethal Dose

Small

Exotoxins Representative Diseases

Tetanus, botulism, scarlet fever

Endotoxins Bacterial Source

G-

Endotoxins Relation to Microorganism

Present in LPS of outer membrane of cell wall

Endotoxins Chemistry

Lipid Portion (lipid A)

Endotoxins Pharmacology

General

Endotoxins Heat stability

Stable

Endotoxins Toxicity

Low; need large number to cause disease

Endotoxins Fever-producing

Yes

Endotoxins Immunology

No vaccine

Endotoxins Lethal Dose

Large

Endotoxins Representative Disease

Typhoid fever, UTI, Meningococcal meningitis