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Key for Microbiology Lab 2:
Growth of Bacteria, Part 1

10 points total

1. Describe the growth of M. luteus and E. coli on the agar slants.

M. luteus was bright yellow in color and tended to form individual colonies, rather than a smooth, even growth pattern, so we could call this effuse or beaded growth.

E. coli was off-white in color and showed a spreading growth pattern: usually it appeared as a cloudy film over the whole surface of the slant.


2. Did either or both organisms grow in the area where you stabbed the butt of the slant? What conclusions can you draw about the organisms based on this result?

E. coli should have grown much better in the deep part of the stab than M. luteus. Because there is less oxygen in the butt, we can conclude that M. luteus is probably an obligate aerobe, while E. coli is probably a facultative anaerobe.


3. What would be a good characteristic to use in distinguishing between these two organisms?

The most obvious characteristic is their color: the bright yellow color of M. luteus is certainly a good characteristic to look for.


4. Describe the colony types you observed on your agar plates (the ones you streaked for isolation).

When looking at plates, you're only interested in the individual colonies, not in the areas where the bacteria were close together and didn't form individual colonies.

You should've seen two distinct colony types on this plate:

(1) round, smooth, bright yellow ones with entire (unbroken) margins
(2) flatter, duller, off-white ones with lobate margins
Remember that the arrangement of the colonies is not due to the bacteria themselves, but to how you streaked them on the plate. So, being close together or far apart is not a characteristic of the bacteria!
Also, size is not a reliable characteristic, because colonies will get larger if they're more spread out. Color, texture, height, edges, etc. are your best characteristics.


5. How many different kinds of bacteria do you think were present in the "mixed" culture? What is your evidence for this conclusion?

There are two, because there are two distinct colony types. (They happen to be E. coli and M. luteus again, as some of you guessed, but note that you can't be sure of this identification!)


6. Give two reasons why a streak plate would be a very important first step if you wanted to identify the organisms present in a mixed culture of this kind.

(1) Allows characteristics of individual colonies to be observed
(2) Allows the investigator to determine how many different colony types are present, and therefore gives information about how many types of bacteria may be present (though sometimes you can get very similar colony types from different organisms)
(3) Because one individual, well-isolated colony grows up from one single cell, an individual colony can be used to start a pure culture of that organism.


7. On the back of this sheet, list the eight areas you sampled for section "C." Briefly describe what you observed after incubating the plates. Which areas appeared to be the "cleanest" and "dirtiest?"

You should have at least briefly described what the growth on the plate looked like, as well as discussing the relative amounts.