Medical Laboratory Scientist-microbiology summary

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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


Morphology - Bacilli

Bacilli Rod-shaped Ends are rounded or

tapered Can be in chains, but

not this is not an identifying characteristic


Morphology - Spiral

Spiral form Vibrios: curved rods

(resembling commas)

Spirilla: S-shaped, but not extended

Spirochetes: helical and flexible spiral shaped


Stains: Gram stain

Basic dyes – cation is colored Acidic dyes – anion is colored Bacteria have high affinity for basic

dyes

Gram stain:


Acid-Fast Stain

Mycobacterium leprae bacteria that have infected this tissue have been stained red with an acid-fast stain


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


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


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.


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


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.


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


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


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


Reference

http://www.austincc.edu/microbugz/triple_sugar_iron_agar.php

Microbiology, text book



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