Microbiology Lab Exam Flashcards
Why does a citrate-positive bacterium produce an alkaline reaction?
The organism also uses ammonium phosphate as a nitrogen source, producing alkaline waste
Why should you use a small amount of inoculum in a citrate test?
A large amount of inactive cells can be mistaken for new growth
Which step of cellular respiration uses citrate?
Krebs Cycle
Can a citrate-positive organism not change the color of a citrate test?
Yes, as long as there is growth
Fluid Thioglycolate Test
A test in which a broth has varying levels of oxygen dispersed throughout the tube, used to determine exact O2 requirements of a sample
Mannitol Salt Agar Test
A test used to determine the salt tolerance of a sample, as well whether the sample ferments mannitol
What happens on a mannitol salt agar plate if the sample can ferment mannitol? Why?
The plate will turn yellow due to acidic byproducts of mannitol fermentation
MacConkey Agar Test
A test used to determine the gram of a sample as well as the sample’s ability to ferment lactose
MacConkey agar test allows for growth of what type of bacteria?
Gram-negative
What color will a MacConkey agar plate be if the sample is positive for lactose fermentation? What pH does this indicate
Red, for acidic pH
MacConkey agar tests are used to isolate and differentiate bacteria in what group?
Enterobacteriaceae
Latex Agglutination Test
A test used to detect S. aureus
The sample is exposed to latex particles covered in fibrinogen and IgG antibodies. If S. aureus is present, latex particles will clump together, creating visible particles
What two virulence factors of S. aureus cause it to react to the latex agglutination test?
Protein A
IgG
The DNase test allows differentiation between what organisms? (3 answers)
S. aureus (+) from other staph species (-)
Serratia (+) from Enterobacter (-)
Moraxella catarrhalis (+) from Neisseria (-)
How does DNase break down DNA?
It hydrolyzes the bonds in the backbone of the DNA molecule
How does a DNase-positive organism present on a DNase test?
Clearing of the agar surrounding the sample
Why does DNase test sometimes include methyl green dye?
Better visualization
Zone of Inhibition
The area in around an antibiotic sample where a bacteria cannot grow
(Kirby-Bauer test)
Minimum inhibitory concentration
The point where the concentration of an antibiotic is insufficient to stop bacterial growth, represented by the margins of the zone of inhibition
(Kirby-Bauer test)
What chemical does the oxidase test look for?
Cytochrome C Oxidase
How is an oxidase test performed? And what change does it produce if positive?
Add a phenylenediamine reagent to colony on solid media
If positive, colony will immediately turn purple/blue
Why must an oxidase test be read within 20 sec of administration?
Reagent can interact w/ o2 in the air to make a false positive
What bacteria are commonly oxidase-positive?
Neisseria, Moraxella, Enterobacteriaceae
What three things does a SIM test test for?
Sulfur Reduction
Indole Production
Motility
What substance is broken down in a SIM test to produce Indole?
Tryptophan
What is the reagent added for the indole result of a SIM test?
Kovak’s reagent
How do Alpha, Beta, and Gamma-hemolytic samples react in a blood agar test? Why?
Alpha - Greenish zone, partial hemolysis
Beta - Clear zone, complete hemolysis
Gamma - No color change, no hemolysis
What specific organism does the CAMP test look for?
Streptococcus agalacticae
What does a CAMP-positive sample look like? Why?
Arrowhead shape near S. aureus, due to CAMP factor that interacts w/ the beta hemolysin in S. aureus
In a TSI test, how can you tell that the sample ferments lactose and/or sucrose?
Yellow butt of slant
