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BMS 602 Exam 3 With Complete Solution
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BMS 602 Exam 3 With CompleteSolution
Which organism does MacConkey agar select for, and how does it work? ** - Answer
MacConkey is a selective and differential media, that is selective for species of
Enterobacteriaceae.
* This agar will differentiate between Enterobacteriaeceae based on their ability to
ferment lactose.
MAC inhibits growth of : GRAM POSITIVE organisms through the use of bile salts and
crystal violet.
If an organism ferments lactose (E. coli), the colony pH will drop below 6.8, and turn pink
or red. If an organism does not ferment lactose, the colonies will remain white/colorless.
Which organisms are TSI slants differential for? How does it work? - Answer TSI (triple
sugar iron) agar differentiates between species of Enterobacteriaceae on their ability to
reduce sulfur and ferment carbohydrates.
This agar contains:
lactose
sucrose
a small amount of glucose
ferrous sulfate
the pH indicator phenol red
If an organism can ferment any of the sugars present, the medium will turn yellow. Gas
production will collect in the bottom of the tube and push the agar up. (E. coli ferments
all three and produces sulfide gas)
GLUCOSE FERMENTERS: red top, yellow bottom
SULFUR REDUCERS: iron sulfide appears as black precipitate, sometimes obscures
results.
** See slides for specific TSI slant results.
List the results of inocculation on a TSI slant for the following organisms: *** - Answer
Describe the toxins and antigens in the cell wall of common Enterobacteriaceae
, (not-specific). - Answer LPS: found in the outer membrane leaflet of the cell wall. LPS is
a heat stable endotoxin - that is composed of:
- Lipid A: potent endotoxin
- Core Oligosaccharide (five different types)
- O antigen: polysaccharide of repeating units (170 different types)
H antigen (flagella): mobile organisms have flagella
K/Vi Antigen (capsule): heat labile capsules
Pili: play a major role in pathogenesis - allow attachment to host tissues (establish
infection), and sex pili allow for conjugation between organisms - exchange of DNA.
Why is serotyping Enterobacteriaceae important, and when is it used? - Answer
Serotyping is a diagnostic test that tests the organism's reaction with many different
antibodies to determine which antibodies are present. This practice is most common in
the case of outbreaks, and plays a role in determining the source of the
organism/infection.
What are the common virulence factors of Enterobacteriaceae? - Answer Endotoxin:
When the Lipid A component of the cell wall is release, it is recognized by the innate
immune system and stimulates inflammation. A high concentration of Lipid A can lead to
toxic shock and death.
Capsule: prevents opsonization and inhibits complement activation.
Type 3 Secretion System: delivers effector proteins from the bacteria to extracellular
environments or a eukaryotic cell.
** Bacteria scavenge iron with siderophores, which are iron binding compounds. This is
essential because iron is an important growth factor for bacteria.
** Some bacteria produce hemolysins, which lyse red blood cells and release iron,
which siderophores can bind to.
LO (17): Describe the common characteristics of the family Enterobacteriaceae. -
Answer All Enterobacteriaceae share the following characteristics:
- gram NEGATIVE rods
- either non-motile or motile with peritrichous flagella
- do not form spores
- facultative anaerobes
- Simple nutritional requirements (grow rapidly)
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