Infectious Disease

Pathogens are microorganisms that cause disease. A host is any organism that is capable of supporting the nutritional and physical growth requirements of another organism. Infection is the presence and multiplication of a parasitic organism in the host.

Pathology depends on: –Organism –Site of infection Treatment depends on the microorganism –Prokaryote vs. eukaryote vs. virus –Resistance to antibiotics

Not all interactions between bacteria and humans are harmful – normal microbiota prevents the growth of pathogens. Most microorganisms can be opportunistic pathogens which can cause disease if the host is immunocompromised or if they can enter the body in a place where they are not normally found. ( E. coli urinary tract infections)

Prions Infectious proteins –Improperly folded proteins that cause other proteins to take on that shape –Extremely difficult to destroy –Discussed later with transmissible spongiform encephalopathies (TSE’s)

Bacteria Prokaryotes –Lack a nucleus and membrane-bound organelles –Structurally simple but metabolically complex –Contain both DNA and RNA –Single circular chromosome –Plasmids : extrachromosomal DNA that provides information (antibiotic resistance) that can be shared with other bacteria.

Bacteria Cytoplasmic membrane Cell wall – peptidoglycan May also produce a capsule of protein or carbohydrate – protects from the host immune system.

Virulence factors Exotoxins – proteins released from a bacterial cell – enzymes that lead to cell death or dysfunction Endotoxins- complex molecules made of lipid and polysaccharides found in the cell wall of gram-negative bacteria. Usually released when the cell is killed, can cause clotting, bleeding, inflammation, hypotension and fever.

Virulence factors Adhesion factors – allow bacteria and viruses to attach to the host. –Receptor –Ligand or adhesin

Virulence factors Evasive factors – – capsules, and slime layers evade phagocytes –Leukocidin C –Some survive and reproduce inside phagocytes –Coagulase –Enzymes that break up antibodies –Alter surface antigens during the disease course –Urea splitters raise environmental pH

Virulence factors Invasive Factors- products that facilitate the penetration of anatomic barriers and host tissue. These can destroy cell membranes, connective tissue, intercellular matrices, and structural protein complexes.

Viral Infection Viruses extremely small – can infect bacteria Usually just composed of DNA (or RNA) + protein “coat” or capsid Can’t reproduce on their own – need to use a host cell

Infection Adsorbed to host cell receptor Penetration Coat removal Uses host enzymes to replicate nucleic acid and proteins New viruses are assembled Virus is released –Lytic cycle

Cellular effects Decreased synthesis of host proteins Disruption of lysosomal membranes Changes in host cell membrane proteins Transform into cancer cell Tissue damage may promote bacterial infection