Exercise 4: Observe your plates

When you look at the pictures below, the first thing you should look at is the streaking pattern. This is the exact streaking pattern I used in the streak plate video I showed on the first day of this lab. (And you’ll notice it’s just a little bit different from the streaking pattern shown on pages 26 and 27 of the lab manual.) There are many streaking patterns that work, and every microbiologist has their own opinion on how to streak a plate properly. When I was a student, I was told that the pattern shown in the lab manual is the best way to get isolated colonies. After years of streaking plates myself, and now years of watching students streak, I can tell you that is not true. The best pattern is whichever one works for you!  

Notice that their are a few things all microbiologists will agree on. A excellent streak plate should achieve well isolated colonies, and it should be free of contamination.

Of course, it takes a while to learn how to do ‘excellent’ streak plates. Most students find that their first few tries fail to produce colonies, and worse, are rife with contamination. (Remember to always keep your Petri plates covered! When you’re streaking, you lift the lid only as much as you need to reach under it with your loop. And then you get it re-covered as quickly as you can after streaking.)

You’re mostly going to see excellent streak plates in the pictures below. However, after this lab has finished, I’ll post a Lab 2 review in the same folder on UM Learn. This review has pictures of streak plates gone wrong, and a discussion of how to fix them. Because you will only be streaking plates in one lab period this year (the first of your two in-class sessions), it’s especially important that you examine these problem plates so that you are ready to avoid making the same mistakes.

Micrococcus luteus streak plate

Begin by looking at the streak plate made from the pure culture of Micrococcus luteus. You should be able to recognize isolated colonies.