Chapter 14 Pathogenicity Microbial Mechanisms of Pathogenicity - - PowerPoint PPT Presentation

Chapter 14

ROBERT W. BAUMAN

MICROBIOLOGY

Pathogenicity

Microbial Mechanisms of Pathogenicity

• Pathogenicity -The ability to cause disease
• Virulence - The extent of pathogenicity

– Virulence Factors

• Adhesion factors
• Extracellular enzymes
• Toxins
• Antiphagocytic factors

• ID50: Infectious dose for 50% of the test

population

• LD50: Lethal dose (of a toxin) for 50% of

the test population

Numbers of Invading Microbes

Bacillus anthracis

Portal of entry ID50 Skin 10-50 endospores Inhalation 10,000-20,000 endospores Ingestion 250,000-1,000,000 endospores

• Adhesions/ligands bind to receptors on host

cells

– Glycocalyx Streptococcus mutans – Fimbriae Escherichia coli – M protein Streptococcus pyogenes – Opa protein Neisseria gonorrhoeae – Tapered end Treponema pallidum

Adherence

• Surface lipoproteins or glycoproteins,

–called ligands, –bind host cell receptors

• Ability to change or block the ligand or its

receptor can prevent infection

• Inability to make attachment proteins or

adhesins renders the microorganisms avirulent

Adhesins Proteins

– Coagulase Coagulate blood – Kinases Digest fibrin clots – Hyaluronidase Hydrolyses hyaluronic acid – Collagenase Hydrolyzes collagen – IgA proteases Destroy IgA antibodies – Siderophores Take iron from host iron-binding proteins – Antigenic variation Alter surface proteins

Exoenzymes

Penetration into the Host Cell

Figure 15.2

Toxins

• Toxin

Substances that contribute to pathogenicity

• Toxigenicity Ability to produce a toxin
• Toxemia

Presence of toxin the host's blood

• Toxoid

Inactivated toxin used in a vaccine

• Antitoxin

Antibodies against a specific toxin

Exotoxin

Source *Mostly Gram + Metabolic product By-products of growing cell Chemistry Protein Fever? No Neutralized by antitoxin Yes LD50 Small

Exotoxins

Figure 15.4a

• A-B toxins
• r type III

toxins

Exotoxins

Figure 15.5

• Superantigens or type I toxins

– Cause an intense immune response due to release of cytokines from host cells – Fever, nausea, vomiting, diarrhea, shock, death

Exotoxins

• Membrane-disrupting toxins or type II

toxins

– Lyse host’s cells by:

• Making protein channels in the plasma membrane

(e.g., leukocidins, hemolysins)

• Disrupting phospholipid bilayer

Exotoxins

Exotoxins

Exotoxin Lysogenic conversion

• Corynebacterium

diphtheriae A-B toxin. Inhibits protein synthesis. +

• Streptococcus pyogenes

Membrane-disrupting. Erythrogenic. +

• Clostridium botulinum

A-B toxin. Neurotoxin +

• C. tetani

A-B toxin. Neurotoxin

• Vibrio cholerae

A-B toxin. Enterotoxin +

• Staphylococcus aureus

Superantigen. Enterotoxin.

Endotoxin

Figure 15.4b

Endotoxins

Source Gram– Metabolic product Present in LPS of outer membrane Chemistry Lipid Fever? Yes Neutralized by antitoxin No LD50 Relatively large

Endotoxins

Figure 15.6

• Certain factors prevent phagocytosis by the

host’s phagocytic cells

–Bacterial capsule

• Can be slippery making it difficult for phagocytes to

engulf the bacteria

• Often composed of chemicals found in the body and

not recognized as foreign

–Antiphagocytic chemicals

• Some prevent fusion of lysosome and phagocytic

vesicles

• Leukocidins directly destroy phagocytic white blood

cells

Antiphagocytic Factors

Cytopathic Effects of Viruses

Table 15.4

• Fungal waste products may cause

symptoms

• Chronic infections provoke an allergic

response

• Mycotoxins

– Neurotoxins: Phalloidin, α-amanitin

• Amanita

– Aflatoxin

• Aspergillus

Pathogenic Properties of Fungi

– Ergot toxin: alkaloid

• Claviceps

– Tichothecene toxins inhibit protein synthesis

• Fusarium

– Proteases

• Candida, Trichophyton
• Capsule prevents phagocytosis

– Cryptococcus

Pathogenic Properties of Fungi

• Presence of protozoa
• Protozoan waste products may cause

symptoms

• Avoid host defenses by

– Growing in phagocytes – Antigenic variation

Pathogenic Properties of Protozoa

• Use host tissue
• Presence of parasite interferes with host

function

• Parasite's metabolic waste can cause

symptoms

Pathogenic Properties of Helminths

• Neurotoxins produced by dinoflagellates

– Saxitoxin

• Paralytic shellfish poisoning

Pathogenic Properties of Algae

Mechanisms of Pathogenicity

Figure 15.9