Chapter 15

Microbial Mechanism of Pathogenicity

• Pathogens have to enter the system to cause disease

• Regions/areas of the body used by microbes to enter the system are portals of entry

• Respiratory tract : the easiest and most frequently used – pneumonia, tuberculosis, common cold

• Gastrointestinal tract: contaminated food or water. Dysentery, cholera, typhoid fever.

• Genitourinary tract – syphilis, gonorrhea,

lymphogranuloma venereum.

• Skin: Intact skin protects us from microbes. Necator americanus - larvae

• Parenteral route: established by cuts, insect bite – malaria, tetanus

• Preferred portal of entry: Salmonella typhi – rubbed on skin – no disease – ingested – typhoid fever.

• Infective dosage – A few enter the body – immune system gets rid of the microbes.

• Numerous bacteria – cause disease

• Virulence factors

• Capsules – capsulated bacteria escape from phagocytes.

• M – protein : found on the surface of Streptococcus pyogenes .

• It helps the bacteria to attach itself to the epithelial cells of the host.

• Enzymes

• Leukocidins – Streptococci – kills WBCs.

• Hemolysin – Streptococci – kills RBCs.

• Hyaluronidase – hyaluronic acid – polysaccharide in the connective tissue.

• Produced by Clostridium – spread from the initial site of infection.

• Collagenase – Clostridium – breaks down collagen (protein) in the connective tissue.

• Endotoxin – gram- bacteria – lipidA – septic shock

• Exotoxin is usually associated with gram+ bacteria – protein – diphtheria toxin.

• Plasmids – R plasmids make the bacteria resistant to antibiotics.

• Lysogeny – lysogenized bacteria are more virulent – phage DNA codes for a toxin – lysogenized Corynebacterium diphtheriae.

• Fungus – Amanita phalloides – neurotoxin –

damage to the liver – death within a week.

Algae – Alexandrium – paralytic shellfish poisoning.

Protozoa – Plasmodium causes damage to RBCs.

• Helminths – Necator americanus – drinks blood.

• Viruses – obligate intracellular parasites.

Destroy host cells when viruses reproduce themselves.