Question

When performing the streak for isolation technique, your result is unexpected: There is only growth obtained in quadrant 1. The growth observed in quadrant 1 is as expected, heavy with no isolated colonies. However, no growth is visible in any other quadrants.

Which of the following errors could explain this unexpected result?
A) The loop did not cross into quadrant 1 when streaking quadrant 2
B) You forgot to flame the loop in between quadrants
C) All of the errors listed here could be responsible for this unexpected result
D) Not enough bacteria was obtained from the starting culture when streaking quadrant 1

Answer 1

A) The loop did not cross into quadrant 1 when streaking quadrant 2

Step-by-step explanation:

There are many different techniques to isolate bacteria populations. Using solid media such as agar, you can perform the streak for isolation technique.

What you want to do by using this technique is to get an isolated colony of the bacteria of interest. And to get it, you need to dilute a sample or inoculum of bacteria as much as possible by streaking it. Every time you do it, you take fewer amounts of cells from the original sample, separating them each time more, to the point of having only one single cell apart by a few millimeters from another cell. The isolated cell divides and forms a new pure colony.

So basically, you need to imaginary divide the agar plate into four quadrants.

1. You sterilize the loop carefully and take a sample of bacteria, trying to get the smallest amount of inoculum. You will apply the loop with the sample in quadrant one and spread it. You close the plate and turn it, placing the second quadrant in front of you.
2. You sterilize the loop again on the flame and let it cool. You perform the same motion in quadrant two but carefully touching the edge of quadrant one, so you can take some of the cells that you have already spread on it. Once you spread quadrant two, you close the plate and turn it again, so quadrant three is in front of you.
3. You sterilize the loop again on the flame and let it cool. You perform the same motion in quadrant three but carefully touching the edge of quadrant two. Close and rotate the plate.
4. You sterilize the loop and repeat the same motion in the last quadrant. Close the plate and take it to incubation. Finally, you put the loop on the flame for the last time.

• The importance of extending the streaks into the anterior quadrant is to take a few cells from there and to spread them in the following quadrant. In this way, you are diluting the original inoculum, and separating the cells each time more. The fourth quadrant should have a few isolated cells that will reproduce and form a pure colony. If you do not touch the anterior quadrant while streaking, you will not have any cell in the quadrant, and you will not observe any colony after the incubation.
• The importance of sterilizing the loop every time you change the quadrant is to avoid contamination with other species. If you do not flame the loop, the result will be too many colonies of different species, probably touching themselves, in each quadrant.

Answer 2

A) The loop did not cross into quadrant 1 when streaking quadrant 2

Step-by-step explanation:

The streak for isolation technique is a method used to get isolated bacterial colonies. This technique consists of, in a petri dish with culture medium, where, with the help of an inoculation loop, the bacteria are scratched on one side of the dish. This side is called the first quadrant. This inoculation loop must then cross with the first quadrant and be dragged to the second quadrant, where new lines must be made. This allows a part of the bacteria from the first quadrant to be carried over to the second. The inoculation loop must cross the second quadrant again to inoculate the third quadrant, and so on.

In this case, we can see that for bacterial colonies to grow in all quadrants, it is essential that the loop crosses in the first quadrant to cross out the second quadrant. If not, only the first quadrant will have bacterial colonies.