Exercise 3: Examine your plates from the last lab

Use the following pictures to answer the questions on pages 18 to 21 of your lab manual, and to fill in table 3.1. After carefully examining the pictures, make sure that you are able to:

Recognize different colony types on a Petri plate.

Tell the difference between the colonies of bacteria and the colonies of fungi.

Describe the colonies of bacteria and fungi using the terms given on pages 18 and 19 of the lab manual. Note that you should be able to describe the size, form, elevation, margin, color and surface qualities from the pictures below. (It’s not really possible to describe the texture of a colony from a picture, because you would need to stab the colony with a needle to figure this out. But you should still be familiar with the terms for texture given in your lab manual.)

Define the term ‘contamination’ as it applies in a microbiology lab.

PLATES LEFT OPEN TO THE AIR

The following picture shows a T-soy plate that was left open to the air in the lab room for 30 minutes. After exposing the plate to the air, it was covered with a lid and incubated upside down in a 28°C incubator for 7 days, for colonies to form. Most of the colonies on this plate appear to be bacterial, although two of them are probably fungi. How can you tell the difference between the colonies of bacteria and fungi?

It’s fairly easy to tell the difference between the colonies of bacteria and filamentous fungi, because the growing hyphae give the fungal colonies a ‘fuzzy’ appearance. The colonies of most fungi also grow quite a bit bigger than the colonies of bacteria do. (Although watch out, there are some bacteria that form huge colonies!)

What is a lot more difficult is telling the difference between the colonies of bacteria and the colonies of yeast. (Remind me, what is a yeast again?) Because they generally look the same on a Petri plate. In fact, most of the small compact glossy colonies on this plate are probably bacterial. But the only way to know for sure would be to pick a small amount of the colony and view it with a microscope.

PLATES TOUCHED WITH FINGERS

The following pictures show a plate touched with the lab instructor’s fingers. First I touched the left side of the plate with my unwashed fingers. Then I carefully washed my hands and touched the other side of the plate with my washed fingers. I tried to keep my method of touching the plate the same both times so that the results would be comparable. (Each time, I touched the plate with my index finger, and rubbed back and forth a few times to spread around my bacteria.) The pictures show a top view and a side view of the same plate. The side view should make it easier to describe the ‘elevation’ of the colonies. (Although most of these colonies appear to be pretty flat.)

Now, from looking at this plate, you might be tempted to think that I had a ton of bacteria on my fingers before I washed them, and then after washing, most of the bacteria had been removed. However, I do this experiment at the beginning of every term… so I know better! I actually repeated the experiment later the same day. Again, I touched one side of the plate with my unwashed fingers, and the other side after washing. The only difference is the way I touched the plate. (This time, I touched with three fingers and just pressed them down gently on the surface, without moving my fingers around.)

SECOND WASHED AND UNWASHED FINGERS PLATE:

You can see on this second plate that my results seem to be reversed. This time I collected relatively few microbes from my unwashed fingers, and a lot more (including fungi) after washing my hands. Did I make a mistake somewhere? Probably not. This just shows why it is so important to include replicates when you perform experiments in the lab. If you only carry out an experiment once, how can you be sure that the results you observe are reproducible? In this case, you can see that the results of my finger touching experiment were not reproducible. Sometimes I get more stuff growing from my unwashed fingers, and sometimes I get more after washing my hands!

So what’s the moral of the story here?

No! it is not that you shouldn’t wash your hands! After all, public health officials tell us that proper hand-washing is one of the most important things we can do to protect ourselves and the others around us from transmission of disease opens in new window. (And yes, this is supported by scientific experimentation!)

But the moral of the story is that when you’re working in a microbiology lab, it doesn’t matter how well or how often you wash your hands. Your hands are always going to be covered in microbes. And if you’re going to learn how to work with pure cultures of bacteria, you’ll need to consider the microbes living on your skin and how to keep them out of your experiments. (More on pure cultures below.)

PLATES INOCULATED WITH A SWAB COLLECTED FROM A SURFACE

The last plate(s) were inoculated with a specimen collected from a surface in the lab using a sterile swab. The first surface I chose was the window sill from beside one of the work stations in our lab. I hypothesized that this would be a good place to find fungi, as the window sill collects a lot of dust. And I knew that you would need to see some fungal colonies so that you could practice describing their characteristics. As I expected, the dust on the window sill harbored quite a lot of fungal spores, which have now grown to give large fuzzy colonies. Notice that even fungi show a great diversity of colony types. There are also several bacterial colonies on this plate.

Because I wanted to test more than one surface, I cheated and tried this experiment twice. In my second try, I inoculated my plate with a swab taken from the inside of my shoe. You’ll remember that the inner soles of my shoes are supposed to be treated with an antimicrobial compound to reduce foot odor. I hypothesized that because of this antimicrobial treatment, I would fail to obtain any colonies on my Petri plate.

Well, score one for the bacteria! So much for my antimicrobial shoes! (Admittedly these shoes are pretty old. And it looks like they’ve been worn a lot. Maybe the antimicrobial treatment has broken down.)

One of the things you’ll notice from looking at this plate, is that using a swab to inoculate a plate is not a very good way to get isolated colonies. In lab period 2 you’ll learn two strategies that microbiologists use to get well isolated colonies that can be used for future experiments.

SO WHAT IS ALL THIS STUFF WE’RE SEEING ON THE PETRI PLATES?

I have just one word to describe all of the stuff we’re seeing on all of the plates shown above. And unfortunately, it’s my least favorite word in microbiology! That word is contamination. Contamination is what happens when the microbes from our environment get onto our Petri plates, or into our test tubes and ruin our experiments. You can see from the pictures above that microbes are everywhere. Just about every surface in our lab is covered in microbes. Your skin and fingers are covered in microbes. There are even microbes floating around in the air we breathe. And if just one of these microbes gets onto your Petri plate, it will grow, it will multiply, and your experiment is contaminated.

That’s because microbiologists work with pure cultures. What is a pure culture? A pure culture means a single strain of a single species of microbe, growing on a Petri plate, or in a tube of liquid medium, where every colony in the culture is the same. Every cell is genetically identical, because they all arose from a single parental cell. If just one contaminant falls into your culture, your culture is no longer a pure culture. It’s contaminated!

How can you avoid this? A microbiologist needs to learn how to work aseptically. Aseptic technique is the set of techniques that a microbiologist uses to work with pure cultures and make sure those pure cultures stay free from contamination. You will learn all about aseptic technique in your next lab period.