Microbial Pathogenesis I

• pathogenicity of the microbe
• resistance/susceptibility of the host

organisms that live on or in a host organism causing damage to the host

• quantitative measure of pathogenicity
• cell number that will elicit a pathogen response in a host within a given time period

microorganism is established and growing in host

damage or injury to the host that impairs host function

• lethal dose₅₀
• dose of an agent that kills 50% of animals in a test group

• reduced virulence
• often happens with lab strains where virulence not continually selected for

• exposed surfaces
• breaks
• wounds
• mucous

• selective, often multiple mechanisms
• capsule/slime layer - promotes adherence and binding to surfaces
• adherence proteins - protein on the cell can bind to receptors from the host
• lipoteichoic acid - facilitate binding to receptor
• fimbrae (pili) - Type I pili, adhere to mannose residues on host cell surfaces, uniformly distributed and produce a static attachment

• availability of nutrients and minerals
• localization

• expression of receptors for host iron carriers
• synthesis, secretion and uptake of siderophores (high affinity iron-binding molecules), often in pathogenicity or "fitness" islands - can remove iron from animal iron-binding proteins

ability of an organism to cause disease based on action of toxin

ability of the organism to grow in host tissues in such large numbers that the pathogen inhibits host function

• virulence factors
• very potent and have specific functions
• released extracellularly
• *proteins - denatured by heat

• cytolytic toxins
• AB toxins
• superantigen toxins

act on the cytoplasmic membrane, causing cell lysis and cell death

blood agar plate - can detect hemolysins (toxins that rupture red blood cell resulting in hemolysis)

• secreted by bacteria
• fragment B promotes specific binding to host cell receptor
• proteolytic cleavage occurs
• A into host cytoplasm where it disrupts host cell function

• large genomic regions
• present in genomes of pathogenic strains and not in nonpathogenic strains- occurrence is widespread

• *often linked to a tRNA gene
• often different GC content
• often unusual codon usage
• acquired through horizontal gene transfer

encode a variety of virulence factors

• tRNA gene flanked by direct repeats
• mobility factors: integrase, transposases, insertion sequence elements (remnants of phage or plasmids)
• gene clusters with specific functions - enhance virulence/fitness