medical laboratory scientist-microbiology summary
DESCRIPTION
ASCP certified exam review. Microbiology basics for students taking Medical Laboratory Scientist exam. This is not comprehensive review. Some points to rememberTRANSCRIPT
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ASCP – Medical Laboratory Scientist
Test Prep Review – Brief Summary
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Microbiology
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Morphology – Cocci
Cocci Spherical, long chain or
pairs
short chain – S. pneumonia
irregular clusters -Staphylococci, Micrococcus
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Morphology - Bacilli
Bacilli Rod-shaped Ends are rounded or
tapered Can be in chains, but
not this is not an identifying characteristic
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Morphology - Spiral
Spiral form Vibrios: curved rods
(resembling commas)
Spirilla: S-shaped, but not extended
Spirochetes: helical and flexible spiral shaped
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Stains: Gram stain
Basic dyes – cation is colored Acidic dyes – anion is colored Bacteria have high affinity for basic
dyes
Gram stain:
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Acid-Fast Stain
Mycobacterium leprae bacteria that have infected this tissue have been stained red with an acid-fast stain
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Gram Positive Bacteria
AerobicCocci: Streptococcus, Staphylococcus Enterococcus, Gemelia,Leuconostoc, Micrococcus
Anaerobic
Bifidobacterium, Clostridium, Eubacterium, Actinomyces,Peptostreptococcus, Propionibacterium
Aerobic or Facultative anaerobicBacilli:Bacillus, Corynebacterium, Erysipelothrix, Gardnerella, Lactobacillus, Listeria, Norcardia, Streptomyces
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Gram Negative Bacteria
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Aerobic Cocci:
Neisseria, Moraxella
Anaerobes
Bacteroides, Fusobacterium, Porphyromonas, Prevotella, Veillonella
Aerobic or Facultative anaerobicBacilliEnterobacteriaceae: Citrobacter, Escherichia, Enterobacter, Klebsiella, Morganella, Proteus, Providencia, Salmonella, Serratia, Shigella, Yersinia
Other:Acinetobacter, Aeromonas, Bordetella, Brucella, Campylobacter, Eikenella, Francisella, Haemophilus, Helicobacter, Legionella, Pasteurella, Plesiomonas, Pseudomonas, Burkholderia, Stenotrophomonas, Chryseobacterium, Vibrio, HACEK, Bartonella, Capnocytophagia
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Triple Sugar Iron Agar Test (TSI)TSI is a differential medium that contains lactose, sucrose, a small amount
of glucose (dextrose), ferrous sulfate, and the pH indicator phenol red
It is used to differentiate enterics based on the ability to reduce sulfur and ferment carbohydrates.
If an organism can ferment any of the three sugars present in the medium, the medium will turn yellow.
If an organism can only ferment dextrose, the small amount of dextrose in the medium is used by the organism within the first ten hours of incubation. After 10 hours, the reaction that produced acid reverts in the aerobic areas of the slant, and the medium in those areas turns red, indicating alkaline conditions.
The anaerobic areas of the slant, such as the butt, will not revert to an alkaline state, and they will remain yellow. (example: Salmonella and Shigella.
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Triple Sugar Iron Agar Test (TSI)If an organism can reduce sulfur, then hydrogen sulfide (H2S) gas will
be produced
H2S will react with the iron to form iron sulfide, which appears as a black
precipitate.
If the black precipitate is formed, it can mask any acid/alkaline results. Sulfur reduction requires an acidic environment, so if the black precipitate is present, some fermentation took place.
If the organism could ferment only dextrose, red color will appear; if it could ferment either lactose and/or sucrose, yellow color will appear.
If the fermentation produced gas, fissures in the medium could be observed, or the entire slant may be raised above the bottom of the test tube
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Examples of the TSI test
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Micrococcus luteus - uses the amino acids and does not grow in the butt of the slant
This would be read K/NC
Bacillus megaterium - ferment sugars but doesn’t grow in the anaerobic area of the butt.
This would be read A/NC
Enterobacter aerogenes - ferment the sugars but turn to the amino acids
This would be read as K/A.
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Examples of TSI test
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Enterobacter cloacae - exhibits fermentation of glucose and gas production but no sulfur reduction
This would be read K/A,G
Staphylococcus aureus - exhibits acidic fermentation
This would be read A/A.
Salmonella typhimurium - ferments glucose & reduces sulfur
This would be read K/A, H2S
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TSI TestResults (slant/butt) Symbol Interpretation
Red/yellow K/AGlucose fermentation only; Peptone catabolized
Yellow/yellow A/AGlucose and lactose and/or sucrose fermentation
Red/red K/K No fermentation; Peptone catabolized
Red/no color change K/NCNo fermentation; Peptone used aerobically
Yellow/yellow with bubbles A/A,GGlucose and lactose and/or sucrose fermentation; Gas produced
Red/yellow with bubbles K/A,GGlucose fermentation only; Gas produced
Red/yellow with bubbles and black precipitate
K/A,G, H2SGlucose fermentation only; Gas produced; H2S produced
Red/yellow with black precipitate
K/A, H2SGlucose fermentation only; H2S produced
Yellow/yellow with black precipitate
A/A, H2SGlucose and lactose and/or sucrose fermentation; H2S produced
No change/no change NC/NC No fermentation
A=acid production; K=alkaline reaction; G=gas production; H2S=sulfur reduction
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Identification of Enteric Bacteria
Can they ferment lactose
Can they use citric acid as their sole carbon source?
ShigellaProduces lysine decarboxylase
Can they use citric acid as their sole carbon source?
SalmonellaGenerally
produces H2SEscherichia
Do they produce acetoin?
Citrobacter Enterobacter
Yes
YesYes
Yes
No
NoNo
No
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Reference
http://www.austincc.edu/microbugz/triple_sugar_iron_agar.php
Microbiology, text book
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