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Micro Lab > Experiments 5,7,11 > Flashcards

Experiments 5,7,11 Flashcards

1
Q

Identify these bacterial shapes and arrangements:

A
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2
Q

Identify these bacterial shapes and arrangements:

A
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3
Q

What bacterial shape is this?

A

Vibrios (bent rods)

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4
Q

What bacterial shape is this?

A

Spiral (Spirilla)

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5
Q

What bacterium is a rod?

A

Bacillus subtilis

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6
Q

Which bacterium is larger, Bacillus subtilis or Staphylococcus epidermidis

A

Bacillus subtilis

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7
Q

Of what value is a simple stain?

A

Determining cell morphology, size, and arrangement.

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8
Q

What is the purpose of heat fixing the smear?

A

It helps the cells adhere to the slide so that they can be stained; it also prevents lysis.

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9
Q

Another method of fixing smears is to use methanol instead of heat. How does alcohol chemically fix the bacteria?

A

Alcohol acts by removing water from the cell. As the bacterial cells are denatured, they retain their structures and adhere to the surface that they are found in.

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10
Q

In heat fixing, what would happen if too much heat were applied?

A

Too much heat would cause the cell’s shape to distort.
Denatures bacterial enzymes that cause the cells to break.

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11
Q

Methylene blue can be prepared as a basic stain or an acidic stain. How would the pH of the stain affect the staining of bacteria?

A

Bacteria is negatively charged, so it can only absorb stain that has an opposite positive charge (aka Basic). Acidic stains carry a negative charge and therefore the bacteria will repel the dye.

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12
Q

Can dyes other than methylene blue be used for direct staining

A

Yes. Crystal violet, basic fuchsin, and safranin are all dyes that can be used in direct staining because they are cationic which means they are positively charged.

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13
Q

Bacteria can be seen without staining. Why then was Koch’s recommendation for fixing and staining important in microbiology?

A

Allows bacteria to be saved and reexamined or shared.

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14
Q

Infection quality control staff in a sterilization unit of a hospital used a simple stain to determine whether bacteria were present in sterilized materials. A simple stain of sterile saline used for respiratory therapy revealed the presence of bacteria. Is the saline contaminated?

A

Yes. It should be free of bacteria, however.

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15
Q

What color will a gram-negative cell stain?

A

Red/pink

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16
Q

What color will a gram-positive cell stain?

A

Purple

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17
Q
  1. Staphylococcus epidermidis
  2. Bacillus subtilis
  3. Escherichia coli

Of these three bacteria, which organism is the largest?

A

Bacillus subtilis

18
Q
  1. Staphylococcus epidermidis
  2. Bacillus subtilis
  3. Escherichia coli

Which bacterial cultures were gram-positive?

A

Staphylococcus epidermidis & Bacillus subtilis

19
Q

Name this bacteria

A

Staphylococcus epidermidis

20
Q

Name this bacteria

A

Bacillus subtilis

21
Q

Name this bacteria

A

Escherichia coli

22
Q

Why will gram-positive cells more than 24 hours old stain gram-negative?

A

When bacteria die their cell walls degrade and may not retain the primary stain.

23
Q

Can iodine be added before the primary stain in a Gram stain?

A

No

24
Q

List the steps of the Gram-staining process, and identify the color of G+ and G- cells in each step.

A
25
Q

Which step can you omit without affecting determination of the Gram reaction?

A

Safranin

26
Q

Suppose you performed a Gram stain on a sample from a pure culture of bacteria and observed a fiels of red and purple cocci. Adjacent cells were not always the same color. What do you conclude?

A

Bacteria is G+, but because it could have possibly been an old sample, the cells could not retain the dye.

27
Q

Suppose you are viewing a Gram-stained field of red rods and purple cocci though the microscope. What do you conclude?

A

It is a mixed culture

28
Q

Human cells can be stained with crystal ciolet and safranin, so why can’t human cells be Gram stained?

A

Human cells do not have cell walls so the primary stain will be removed easily.

29
Q

Considering you can’t identify from a Gram stain, why might a physician perform a Gram stain on a sample before prescribing an antibiotic?

A

G+ and G- cells are different in the compositions of their cell walls. By determining its’ properties the physician can pick the correct antibiotic that goes with that wall type.

30
Q

What will a streak plate with two species of bacteria look like?

A

You can identify two distinguishable species because they will appear different on the streak plate.

31
Q

Where will the colonies be located in the pour plate? Where will they be the largest?

A

Colonies will be embedded in the agar. Pour plates are prefferable for anaerobic bacteria so they will be largest deeper within the medium.

32
Q

How can you tell whether a mixed culture has different bacterias?

A

The appearance of microbes will allow you to distinguish whether or not a mixed culture has multiple bacterias.

33
Q

How do the colonies on the surface of the pour plate differ from those suspended in the agar?

A

Anaerobes proliferate suspended in pour plate agars, colonies on the surface tolerate oxygen

34
Q

How could your streak plate technique be improved?

A

Keep in mind a small amount of concentrated bacteria will go a long way, therefore by streaking the plate at a better angle you can isolate better

35
Q

How do you know if you got a pure culture in your subculture?

A

If the colonies appear the same.

36
Q

What is a contaminant?

A

Presence of unwanted organisms

37
Q

How would you determine whether a colony was a contaminant on a streak plate? On a pour plate?

A

Streak plate: The contaminant will be located in an area where you did not inoculate (outside of the streak line). And will also appear different

Pour plate: The appearance of the contaminant will be different from the rest.

38
Q

What would happen if the plates were incubated a week longer? A month?

A

A week longer the organisms on the plate will grow more. The plate will eventually become too full, and you will be unable to isolate. A month longer the bacteria will have used up all the nutrients in the agar and die.

39
Q

Could some bacteria grow on the streak plate and not be seen if the pour plate technique is used?

A

Yes, some aerobic organims will not grow on the pour plate but will grow on the streak plate

40
Q

What is a disadvantage of the streak plate technique? Of the pour plate technique?

A

Streak Plate: Colony isolation problems

Pour plate: Preparation is time consuming, reduced growth rate of aerobes, and the danger of killing heat-sensitive organisms.

41
Q

Will the isolated colonies always be in the fourth sector on the streak plate?

A

No. Depending on how well you streak you may be able to find colonies within the 3rd sector.

42
Q

Colony-forming units per ml =

A

of Colonies

—————————————

Dilution x Amount Plated

Micro Lab (4 decks)

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