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.