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Flashcards in Laboratory Information Deck (22)
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1
Q

Interpretation of Test Results

A

Disease + Disease -
Test + A (TP) B(FP) A+B
Test - C(FN) D(TN) C+D
A +C B + D

Sensitivity: true positive rate
the number of people that test positive and have the disease out of all the people that have the disease
You know patient has the disease. What is the chance that the patient will test positive?
A/(A+C) -----------------------------------------------------
specificity: true negative rate
the number of people that test negative and do not have the disease out of all the people that do not have the disease
You know the patient does not have the disease. What is the chance that the patient will test negative
D/(D+B) -----------------------------------------------------
positive predictive value (PPV)
The number of people that actually have the disease among the people that test positive for the disease
You know the patient has tested positive. What is the chance that the patient has the disease?
A/(A+B) -----------------------------------------------------
negative predictive value (NPV)
the number of people that don’t have the disease among the people that test negative for the disease
You know the patient has tested negative. What is the chance that the patient does not have the disease?
D/(C+D)
2
Q

Complex Media

A

Undefined composition of nutrients –>include a carbon source, agar, degraded source of cellular material

3
Q

Selective Media

A

-used to isolate or identify particular
organisms–>through use of some inhibitory agent that does not allow the growth of “irrelevant” bacteria from mixed cultures

eg. Making a particular sugar the only carbon source screens out bacteria that can’t metabolize that sugar
eg2. adding a dye, antibiotic, salt or specific inhibitors which affect the metabolism or
enzyme systems of the organisms

Types of selective media: MacConkey media, CNA blood agar

4
Q

Differential Media

A

-used to differentiate closely related organisms or
groups of organisms
-Owing to the presence of certain dyes or chemicals in the media, the organisms will produce characteristic changes or growth patterns that are used for identification or differentiation

eg. MacConkey agar: distinguishes lactose fermentors from non-fermentors

–>blood agar is also a differential media

5
Q

Non-selective Media

A
  • support the growth of many different bacteria

eg. Blood agar, chocolate agar, and TSAYE

6
Q

Types of chemicals in selective media/function 4

A
  • sodium azide: selects for gram positive bacteria
  • bile salts (sodium deoxycholate): selects for Gram negative enteric bacteria only
  • colistin and nalidixic acid: inhibits growth of many Gram negative bacteria (selects for Gram positive)
  • crystal violet: hydrophobic molecules that disrupt bacterial membranes
7
Q

MacConkey media

A

A SELECTIVE and DIFFERENTIAL media used for Enterobacteriaciae organisms
–>also used to differential Salmonella and Shigella from Enterobacteriaciae

  • contains bile and crystal violet
  • selects for gram negative enteric bacteria: these can grow in this media because of their long hydrophilic repeated sugars on the LPC hat repels the hydrophobic growth inhibitors of the media
  • distinguishes lactose fermentors from non-fermentors: the acid from the fermentation of lactose causes the colonies to appear pink/red and leads to a precipitation of the bile salts which causes a zone of cloudiness around the colonies
  • –>non-lactose fermentors make transparent non colored colonies
8
Q

CNA blood agar

A

A SELECTIVE media

  • contains colistin and nalidixic acid
  • inhibits growth of gram negatives
9
Q

TSAYE medium

–>Tryptic Soy Agar-Yeast Extract

A

-non selective media: used to isolate and cultivate a wide variety of bacteria

10
Q

Blood Agar Plates (BAP)

A

non-selective and differential media
–>for fastidious human pathogenic organisms

  • used to differentiate bacteria based on their ability to lyse RBC by secretion of hemolysins (hemolytic enzymes)
  • contains RBCs
11
Q

Types of Hemolysis

A

1) Alpha: partial hemolysis as indicated by a greenish pigmented zone of clearing around the bacterial colony on the BAP; green pigment within agar itself
2) Beta: complete hydrolysis as indicated by total zone of clearing around bacterial colony on the BAP; can even read through a good zone of clearing
3) Gamma: no hemolysis production as observed by lack of change in RBCs in the BAP around the bacterial colony; however, must still observe growth on the plate

12
Q

Chocolate Agar

A

non-selective media
–>for fastidious human pathogenic organisms eg Neisseria and Hemophilus ssp.

-contains pancreatic casein digest, peptone, hemoglobin, cornstarch, minerals and lysed RBCs

13
Q

Sorbitol MacConkey (SMAC) Agar

A

A SELECTIVE and DIFFERENTIAL media

A variation of the MacConkey agar that substitutes sorbitol for lactose as the main carbon sorce so that bacteria that are able to ferment sorbitol appear pink o red

–>selects and differentiates E. coli O157 from other E.coli spp

14
Q

Eosin methylene blue (EMB) agar

A

A SELECTIVE and DIFFERENTIAL media

  • contains eosin and methylene blue
  • selects for Gram negative bacteria
  • differentiates b/n lactose and non-lactose fermentors on basis of color change
  • –>strong acid by products produce a metallic green sheen
  • –>weak acid by products produce a pinkish purple color
  • –>no acid by products make colorless colonies
15
Q

Oxidase test

A

qualitative procedure for determining the presence or absence of cytochrome c oxidase activity in bacteria

16
Q

indole test

A

biochemical test used to test an organisms ability to break down the amino acid tryptophan into indole
–>indole pos bacteria contain typtophanase

17
Q

Analytical Profile Index (API)

A

used to identify the enteric Gram negative rods

  • ->20 separate test compartments are on the strip
  • ->water dilution of specific bacterial concentration added to each well
  • ->some color changes due to pH change, some have end products that are ID’d with additional reagents
18
Q

Thayer-Martin Media

A

Selective–against gram positive and inhibits growth of many vaginal normal flora microorganisms —>ultimately selecting from Neisseria gonnorohea

19
Q

Columbia CNA Agar

A

A selective medium for Staphylococci and Streptococci, this agar contains nalidixic acid, colistin sulfate and 5% sheep’s blood in a Columbia agar base. The medium suppresses growth of Gram negative rods while permitting unrestricted growth of Stephylococci, hemolytic Streptococci, and Enterococci. CNA is selective for many Gram positive organisms.

20
Q

Catalase test

A

Catalase is an enzyme that rapidly decomposes two molecules of hydrogen peroxide to water an oxygen.

21
Q

Coagulate test

A

Coagulase plays a role in blood clotting. It is thought to act by bonding with Coagulase Reacting Factor that is derived from prothrombrin. This complex then reacts w/ fibrinogen to form fibrin. Coagulase is an extracellular enzyme that is a VIRULENCE factor of some Staphylococci (S. aureus): survival mechanism to restrict phagocytosis.

22
Q

Rapid strep test

A

uses color immunochromatographic dipstick technology w/ rabbit antibodies coated on the nitrocellulose membrane. The sample is subjected to a chemical extraction of a carb antigen unique to Group B Strep. The test stick is then placed in the extraction mixture and the mixture migrates along the membrane. If group B step is present in the sample, it will form a complex w/ the anti group B step antibody conjugated color particles. the complex will then be bound by the anti-group b strep capture antibody and a visible blue test line will appear to indicate a positive result.