ex 8 & 9 Flashcards
(19 cards)
importance of pure cultures
isolate a species from contaminants
purpose of streak plating
obtain isolated colonies of bacteria each of which represents a pure culture that multiplied from a single cell
steps of streak plating
1) label bottom of agar plate with name, lab section, date, source of inoculum
2) flame loop until it turns red allow it to cool and aseptically obtain a loopful of broth culture labeled unknown mixture 8
3) lift one edge of the petri plate cover and streak 1st sector by making as many streak without overlapping previous streaks.
4) flame loop and let it cool, turn plate, streak 2nd quadrant
5) flame loop and let it cool, turn plate, streak 3rd quadrant
6) flame loop, let it cool, turn plate, streak 4th quadrant
7) incubate the plated in an inverted position at 37 degrees Celsius, isolated colonies develop (24-48 hrs)
MSA
selective:
inhibits the growth of gram neg and ALLOWS growth of gram pos
differential:
bacteria that can ferment mannitol runs the agar from red to yellow
EMB
selective:
inhibits the growth of gram pos and ALLOWS growth of gram neg
differential:
bacteria can ferment lactose colonies changes color to dark purple, metallic green, or dark pink
E coli description on EMB
metallic green
S. aureus vs S. epidermis on MSA
S. aureus:
yellow on MSA and ferments mannitol
S. epidermis:
grew but did not turn agar yellow b/c it does not ferment mannitol
Which plates do Staphylococcus epidermidis from the lab grown on?
Staphylococcus epidermis grew on nutrient agar and mannitol sugar (MSA) agar.
Explain why EMB medium is both selective and differential (which bacteria does it allow to grow and what characteristic does it differentiate).
Selective?
Differential?
EMB medium is selective because it inhibits the growth of gram positive bacteria and allows the growth of gram negative bacteria. EMB medium is differential because the bacteria that can ferment lactose colonies changes color to dark purple, metallic green, or dark pink.
Which plates do Escherichia coli from the lab grow on?
Escherichia coli grew on nutrient agar and eosin methylene blue (EMB) agar.
Looking at the results of your pure cultures (E. coli, S. aureus, etc) and your Unknown 9 culture, can you tell which species of bacteria are in Unknown 9 (and name both species)? How did you come to these conclusions?
Unknown 9 culture was a mixture of Escherichia coli and Staphylococcus aureus. I came to these conclusions because the unknown 9 culture grew on EMB agar and on MSA agar. The colony color was metallic green on the EMB agar. The agar color turned from red to yellow on the MSA agar. Both of these observations occured on the plates of the bacteria by itself.
You have an unknown, pure culture that has been streaked on MSA and EMB. You see no growth on MSA. You observe significant growth on EMB, but colonies appear to be cream colored. What can you tell me about your unknown, pure culture?
The unknown pure culture is gram negative since it grows on EMB, because the colonies appear cream colored it means the bacteria cannot ferment lactose. If the bacteria could ferment lactose the colonies would appear metallic green, dark purple, or dark pink.
Which plates do Pseudomonas aeruginosa from the lab grow on?
Pseudomonas aeruginosa grew on the nutrient agar and eosin methylene blue (EMB) agar.
For differential analysis of ________, you look at the color of the colonies and for _________you look at the color of the agar.
EMB; MSA
On a streak plate, how would you determine whether or not you got any contamination?
You would be able to determine if there was contamination if there is a colony growing that you did not streak.
or differences in colony color and/or shape
Does Unknown 9 from the lab represent a pure culture or a mixed culture? How can you tell?
Unknown 9 was a mixed culture because it grew on both on agar plates. If it was a pure culture it would have only grown on either EMB agar or MSA agar, because it grew on both agar plates we can conclude it was mixed culture.
Explain why MSA medium is both selective and differential (which bacteria does it allow to grow and what characteristic does it differentiate).
selective?
differential?
MSA medium is selective because it inhibits the growth of gram negative bacteria and allows the growth of gram positive bacteria. MSA is differential because the bacteria that can ferment mannitol turns the agar from red to yellow.
Which plates do Staphylococcus aureus from the lab grow on?
Staphylococcus aureus grew on the nutrient agar and mannitol salt agar (MSA).
What is the main purpose of the streak plate technique?
The main purpose of the streak plate technique is to obtain isolated colonies of bacteria, each of which represents a pure culture that multiplied from a single cell.
