UAMS MICRO LAB EXAM 1 QUESTIONS
AND ANSWERS WITH COMPLETE
SOLUTIONS 100% CORRECT!!!
Microbiology: Culture Techniques & Laboratory Practices
Why are agar plates incubated upside-down (inverted)?
✔✔ To prevent condensation from forming on the lid and dripping onto the
agar surface.
(Dripping moisture can cause bacterial colonies to run together, distorting colony
morphology and preventing isolated colony quantification).
Why must the inoculating loop be flame-sterilized (or disposed of) between
streaking each quadrant during a four-quadrant streak for isolation?
✔✔ To progressively dilute the bacterial load and facilitate isolation of pure
individual colonies (CFUs).
(This process ensures well-isolated colonies for accurate enumeration and
downstream identification).
Why can a single isolated colony on an agar plate be used to subculture and
start a pure culture?
✔✔ Because an isolated colony originates from a single viable bacterial cell
(or single CFU).
(Under proper incubation and nutrient conditions, that single cell multiplies
exponentially into millions of genetically identical daughter cells).
What is meant by the term "colonial morphology" in clinical microbiology?
✔✔ The visual phenotypic characteristics of a bacterial colony on solid media.
(Includes size, shape/elevation, margin/border, color, opacity, texture/viscosity,
and hemolytic reactions).
What are the primary visual signs of bacterial growth in a liquid medium
(broth)?
✔✔ Cloudiness or turbidity throughout the liquid.
, (May also present as a pellicle on the surface, sediment/button at the bottom, or
floccullent clumping).
Why is the bottom of an agar plate labeled and not the lid? -ANSWER✔✔Because
lids are not attached to the agar plate and can be dropped/fall off. If the lid falls off,
the plate should be labeled with the patient information.
Which media used in this exercise are selective for gram positive organisms? Gram
negative organisms? -ANSWER✔✔Gram positive media: CNA, PEA
Gram negative media: EMB, MAC
In a gram stained smear what color are the gram positive organisms? Gram
negative organisms? -ANSWER✔✔Gram positive MOs: purple
Gram negative MOs: pink/red
Why must young or actively growing cultures be used for the gram stain? -
ANSWER✔✔Older organisms may have atypical gram stains because they may
not take up the stain very well. Gram positive MOs can appear gram negative after
24-48 hours.
What is the color of an overdecolorized organism? An underdecolorized organism?
-ANSWER✔✔Overdecolorized MO: Pink/red (gram positives look gram negative)
Underdecolorized MO: Purple (gram negatives look gram positive)
What is the function of the iodine solution in the Gram stain? If it were omitted
how would staining results be affected? -ANSWER✔✔Iodine functions as a
mordant/fixative to keep the crystal violet inside the bacterial cell wall. It forms an
crystal violet-iodine complex. If omitted, gram positive organisms appear gram
negative.
AND ANSWERS WITH COMPLETE
SOLUTIONS 100% CORRECT!!!
Microbiology: Culture Techniques & Laboratory Practices
Why are agar plates incubated upside-down (inverted)?
✔✔ To prevent condensation from forming on the lid and dripping onto the
agar surface.
(Dripping moisture can cause bacterial colonies to run together, distorting colony
morphology and preventing isolated colony quantification).
Why must the inoculating loop be flame-sterilized (or disposed of) between
streaking each quadrant during a four-quadrant streak for isolation?
✔✔ To progressively dilute the bacterial load and facilitate isolation of pure
individual colonies (CFUs).
(This process ensures well-isolated colonies for accurate enumeration and
downstream identification).
Why can a single isolated colony on an agar plate be used to subculture and
start a pure culture?
✔✔ Because an isolated colony originates from a single viable bacterial cell
(or single CFU).
(Under proper incubation and nutrient conditions, that single cell multiplies
exponentially into millions of genetically identical daughter cells).
What is meant by the term "colonial morphology" in clinical microbiology?
✔✔ The visual phenotypic characteristics of a bacterial colony on solid media.
(Includes size, shape/elevation, margin/border, color, opacity, texture/viscosity,
and hemolytic reactions).
What are the primary visual signs of bacterial growth in a liquid medium
(broth)?
✔✔ Cloudiness or turbidity throughout the liquid.
, (May also present as a pellicle on the surface, sediment/button at the bottom, or
floccullent clumping).
Why is the bottom of an agar plate labeled and not the lid? -ANSWER✔✔Because
lids are not attached to the agar plate and can be dropped/fall off. If the lid falls off,
the plate should be labeled with the patient information.
Which media used in this exercise are selective for gram positive organisms? Gram
negative organisms? -ANSWER✔✔Gram positive media: CNA, PEA
Gram negative media: EMB, MAC
In a gram stained smear what color are the gram positive organisms? Gram
negative organisms? -ANSWER✔✔Gram positive MOs: purple
Gram negative MOs: pink/red
Why must young or actively growing cultures be used for the gram stain? -
ANSWER✔✔Older organisms may have atypical gram stains because they may
not take up the stain very well. Gram positive MOs can appear gram negative after
24-48 hours.
What is the color of an overdecolorized organism? An underdecolorized organism?
-ANSWER✔✔Overdecolorized MO: Pink/red (gram positives look gram negative)
Underdecolorized MO: Purple (gram negatives look gram positive)
What is the function of the iodine solution in the Gram stain? If it were omitted
how would staining results be affected? -ANSWER✔✔Iodine functions as a
mordant/fixative to keep the crystal violet inside the bacterial cell wall. It forms an
crystal violet-iodine complex. If omitted, gram positive organisms appear gram
negative.
