Chapter 25 microbial infection and pathogenesis

infection

a microbe is established and growing in a host, can be harmful or not to host. (infection to microbiome)

pathogens

Organisms that cause disease, or tissue damage in a host

Pathogenicity

the ability of an organism to cause disease

microbial adherence

adherence of pathogens to tissue via receptor molecules on the cell surface

bacterial capsules

forms a thick coating outside the plasma membrane and cell wall and serves two important functions in bacterial pathogenicity.

adherence structures:

Fimbriae, Pili, and Flagella

Fimbriae, Pili, and Flagella

bacterial cell surface protein structures that function in attachment.

Growth of the Microbial Community

example: Human Dental Caries (tooth decay)

Dental caries, or cavities, are an oral microbial disease.

Streptococcus sobrinus and Streptococcus mutans and other bacteria attach and reproduce and form a biofilm (plaque).

Invasiveness

ability of a pathogen to grow in host tissue at densities that inhibit host function

Bacteremia

the presence of bacteria in the bloodstream

Septicemia

bloodborne systemic infection, may lead to massive inflammation, septic shock and death (lethal in a quarter to half of all cases).

Infection

any situation in which a microorganism (not a member of the local flora) is established and growing in a host

Virulence

the relative ability of a pathogen to cause disease

Attenuation

the decrease or loss of virulence

Attenuated strains

of various pathogens are valuable to clinical medicine because they are often used for the production of viral vaccines, e.g. measles, mumps, polio, rubella.

pathogenicity islands

Several genes that direct invasion are clustered together on the chromosome

SPI1 and SPI2

•Salmonella pathogenicity islands contains genes that promote a more systemic disease. Salmonella also contains antibiotic antibiotic resistance plasmids (R plasmids).

Compromised Host

•(susceptible to infection)

•The pathogen-host interaction is dependent upon both the host and the pathogen.

•Certain medical procedures (e.g., surgery) or underlying conditions predispose individuals to  develop diseases.

Nosocomial infections (Hospital-acquired)

•affect nearly 2 million people each year.

•Infections with viruses, such as HIV, weaken the
immune system.

Opportunistic infections

are those caused by organisms that do not cause disease in healthy hosts

Invasiveness

•requires a pathogen break down host tissues. This is often done with enzymes that attack host cells.

Hyaluronidase

breaks down host tissues

Coagulase and streptokinase

manipulate clotting.  Coagulase forms clots, while streptokinase breaks them down.

Exotoxins

•proteins released from the pathogen cell as it grows and can cause damage at distant sites

•three categories

•AB toxins

•cytolytic toxins

•Superantigen toxins

Diphtheria Exotoxin

•Blockage of Protein Synthesis

•Can cause a thick covering in the back of the throat. It can lead to difficulty breathing, heart failure, paralysis, and even death

AB toxin

•that is made up of an Active (A) domain and a binding (B) domain

•The A domain adds an ADP-ribosyl group to elongation factor thermos unstable (EF-2), which prevents its function in translation.

Enterotoxins

•exotoxins whose activity affects the small intestine

•typically cause massive secretion of fluid into the intestinal lumen, resulting in vomiting and diarrhea

•example: cholera toxin

•Cytolytic Exotoxins (Cytotoxins)

•Soluble proteins that work by degrading cytoplasmic membrane integrity, causing cell lysis and death

hemolysins

Toxins that lyse red blood cells are called

Staphylococcal a-toxin

kills nucleated cells and lyses erythrocytes.

•Superantigens (SAG’s)

•cause an overstimulation of the immune system (usually of T cells leading to a “cytokine storm”)

•can lead to shock and death

•generally due to a localized infection, but with systemic effects

Mainly Gram +ve bacteria example: . S aureus and S pyogenes

lipopolysaccharide (LPS)

•portion of the cell envelope of certain gram-negative Bacteria, which is a toxin when solubilized

•LPS is a stimulator of the immune system

•Generally, less toxic than exotoxins and released when bacterial cell dies

•Limulus amoebocyte lysate (LAL)

•Presence of endotoxin can be detected by the Limulus amoebocyte lysate (LAL) assay.