Summary
Summary Chapter 15 MBY 161
- Course
- (MBY161)
- Institution
- University Of Pretoria (UP)
This is a well-written summary of chapter 15 of MBY 161. The microbial mechanisms of pathogenicity.
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Chapter 15: Microbial mechanisms ofpathogenicity
Properties of microbes that contribute to pathogenicity:
o ability to cause disease bu overcoming the defenses of a host
o virulence --- the degree or extent of pathogenicity
How microbes enter a host
pathogenicity—the ability of a microbe to cause disease by overcoming the defences of a
host
virulence--- the degree of pathogenicity of a microbe
disease is due to the accumulation of microbial waste products
portals of entry--- the avenue by which a pathogen gains access to the body
o different portals of entry
mucous membranes
mucous membranes lining
o respiratory tract, gastrointestinal tract, genitourinary tract,
and conjunctiva
o Most pathogens enter through the mucous membranes of
the gastrointestinal and respiratory tracts.
o The respiratory tract is the easiest and most frequently
traveled portal of entry for infectious microorganisms.
common cold, pneumonia and TB
o Microorganisms can gain access to the gastrointestinal tract
in food and water and via contaminated fingers.
thyphoid fever and cholera
o genitourinary tract --- portal of entry for pathogens that are
contracted sexually.
o Examples of STIs are HIV infection, genital warts,
chlamydia, human herpesvirus-2, syphilis, and gonorrhea.
skin
Some microbes gain access to the body through openings in the
skin, such as hair follicles and sweat gland ducts.
direct deposition beneath the skin and membranes (parenteral route)
parenteral route--- a portal of entry for pathogens by deposition
directly into tissues beneath the skin and mucous membranes
o Other microorganisms gain access to the body when they
are deposited directly into the tissues beneath the skin or
into mucous membranes when these barriers are penetrated
or injured
o HIV, hepatitis viruses and gangrene
, Preferred Portal of entry
o Many pathogens have a preferred portal of entry that is a prerequisite to their
being able to cause disease. If they gain access to the body by another portal,
disease might not occur
Numbers of invading microbes
o If only a few microbes enter the body, they will probably be overcome by the
host’s defenses. However, if large numbers of microbes gain entry, the stage is
probably set for disease. The likelihood of disease increases as the number of
pathogens increases
o ID50 is the virulence of a microbe. ID50 --- the number of microbes required to
produce a demonstrable infection in 50% of the test host population. infectious
dose 50% of a sample population.
o potency of a toxin is often expressed as the LB50 --- lethal dose for 50% of the
inoculated hosts within a given period
Adherence--- attachment of a microbe or phagocyte to another’s plasma membrane or
other surface
o The attachment between pathogen and host is accomplished by means of surface
molecules on the pathogen called adhesins or ligands that bind specifically to
complementary surface receptors on the cells of certain host tissues
o adhesins--- a carbohydrate-specific binding protein that projects from prokaryotic
cells; used for adherence, also called a ligand
Adhesins may be located on a microbe’s glycocalyx or on other microbial
surface structures, such as pili, fimbriae, and flagella
majority is glycoproteins or lipoproteins
o receptors--- an attachment for a pathogen on a host cell.
typically sugars like mannose
o If adhesins, receptors, or both can be altered to interfere with adherence, infection
can often be prevented (or at least controlled).
o Biofilms--- a microbial community that usually forms as a slimy layer on a
surface
Microbes have the ability to come together in masses, cling to surfaces,
and take in and share available nutrients in communities called biofilms
Examples
dental plaque on teeth, the algae on the walls of swimming pools,
and the scum that accumulates on shower doors.
Forms when
a surface is moist and contains organic matter
The first microbes to attach are usually bacteria. Once they adhere to the
surface, they multiply and secrete a glycocalyx that further attaches the
bacteria to each other and to the surface
Biofilms resist disinfectants and antibiotics
This characteristic is significant, especially when biofilms colonize
structures such as teeth, medical catheters, stents, heart valves, hip
replacement components, and contact lenses.
How bacterial pathogens penetrate host defenses
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