Lab Exam 1
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What are the two most common pathogenic Gm+ organisms? How does one distinguish between the two in a laboratory?
- The two most common Gm+ organisms are streptococcos and staphylococcus.
- One can distinguish between the two organisms by performing a catalase test
After performing a catalase test, the H2O2 does not bubble. What type qorganism do you have present
Why does staphylococci bacteria cause H2O2 to bubble when inserted?
Staphylococci bacteria, produce an enzyme called catalase that breaks down hydrogen peroxide into water. Oxygen liberated from the peroxide form bubbles
What two organisms does the coagulase test distinguish between?
The coagulase test distinguishes between S. Aureus and S. Epidermidis.
After performing a coagulase test on a catalase positive colony (staphylococcus), you observe clumping in the test tube. What can you conclude?
One can conclude that the organism in question is of S. Aureus, not S. Epidermis
How can one distinguish between different species of steptococcus?
After streaking the culture on a Blood Agar Plate, one can observe the type of hemolysis exhibited on the plate.
A greenish brown coloring around the colony indicates:
A clear zone around the colony indicates
No Zone present around the colony inicates
Alpha Hemolysis is characterisistic of which two streptococcus organisms: they are?
- Alpha streptococci
- S. Pneumoniae
After streaking steptococci organisms on BAP, a greenish brown zone appears around the colonies on your dish. You place optochin on the plate, and later observe a zone of inhibition around the optochin disc. What organism do you have?
Beta hemolytic streptococci are further classified as:
Group A Beta streptococci must be distinguished from other groups of beta hemolytic strep because:
Group A is responsible for erysipelas, scarlet fever and septic sore throat.
On a blood agar plate with a streptococcus culture, sensitivity to bacitracin is observed with a zone of inhibition. This inhibition is characterised by which organisms
- Can either be Group A Beta streptococci or streptococcus pneumonaie
- Alpha Streptococci and other beta groups exhibit no inhibition to bacitracin.
Most gram negative bacteria of medical importance fall into which family?
Where are members of enterobacteriaceae found in the body?
Members of this family are found in urine cultures and may also be isolated from wounds, respiratory sources, blood and some are intestinal pathogens. Some are present normally in certain areas of the body.
Coliform organisms ferment lactose in 48 hours. Name 3 examples of coliform organisms
Which test is the first step in identification of the gram negative rod? What is it used for?
- Triple Sugar Iron Agar (TSI).
- Used for determining carbohydrate fermentation and hyrdogen sulfide production.
After performing a TSI test, what does a acid (yellow) butt indicate?
a yellow butt indicates glucose fermentation
After performing a TSI test, what does an acid (yellow) slant indicate?
lactose or sucrose fermentation
After performing a TSI test, what does a test tube with a yellow butt and yellow slant indicate?
E. Coli or Enterobacter
After performing a TSI test, what does a test tube with a yellow butt and red slant indicate?
Proteus, Salmonella or Shigella
All lactose negative TSI cultures should be further screened in urea broth. This tests for the enzyme urease. If urease is present, what will occur in the urea broth?
If present, this enzyme will break down urea, alter the pH of the medium, and turn it from a yellow to a red color.
If the urea broth is exhibits a pink/red color after 2-4 hrs, what organism is present?
If the urea broth is exhibits a pink/red color after 24 hrs, what organism is present?
- *please note that Klebsiella is lactose positive, however.
What is a citrate agar test used to determine?
Citrate agar is used to determine the utilization of citrate as a sole carbon source.
A change from green to blue color (positive result) in a citrate agar test can mean the presence of which organisms?
What is the lysine iron agar test used to determine?
Determines not only glucose fermentation, but also the presence of the enzymes lysine decarboxylase and lysine deaminase.
Lysine decarboxylase + organisms include
the Salmonella-Arizona group, Escherichia group and the Klebsiella group.
Lysine deaminase + organisms include
the various species of the genus Proteus as well as the Providencia group.
SIM (sulfur, indole, motility) test results
- Bacteria can grow along the stab (non-motile) or throughout the medium (motile)
- After adding 5 drops of Kovac's reagent, if the top of the medium turns red immediately, indole is produced
- If sulfur is produced, the medium will turn black
If the top of the medium of a SIM test turns red after Kovac's reagent is added, what does this indicate?
A positive indole test is characteristic of E. coli and other members of the genus Escherichia.
When doing the sugar tests, organisms that ferment sugar will turn the medium from red to yellow. If a red medium is observed, what organisms does this indicate
A red medium indicated negative results for fermentation and can either be Psuedomonas or Neisseriae
In an Methy Red MR-VP test, positive results are indicated by a red color, while negative results are indicated by a yellow color. What organism is indicated by positive (red) results?
A Vogues-Proskauer MR-VP test for the production of acetylmethylcarbinol. After 48 hours of incubation and addition of indicator, a red color indicates positive results.
What two species is the IMViC test used to distinguish between?
E. Coli and Enterobacter aerogenes
The EMB agar plate allows growth of most Gm- organisms but inhibits most Gm+ organisms. The plate also indicates lactose fermenters. What kind of results do lactose fermenters display
- Lactose fermenters (E. Coli) produce either green sheens or dark centers in their cultures on the EMB plate
- Non lactose fermenters do not show stain
Both Staph Aureus and Staph pyogenes produce Beta Hemolysis on a BAP. What difference in colony growth can indicate one organism over the other?
- S. Aureus grows large colonies
- S. pyogenes grows small colonies
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