Hi there! I'm Jayme Dyer! This video is about streaking for
singles. Not that kind of streaking... The "singles"
in "streaking for singles" are single colonies of microbes like
bacteria, yeast, or even archaea. These tiny single-celled organisms are too
small to see by eye but a single cell can divide
so many times over a few hours or days that it makes a pile of cells big enough
that we can see it. That's called a colony and because all
the cells in the colony originated from that first starting cell they are all
pretty close to genetically identical.
Scientists use single colonies of microbes for lots of different reasons.
Here's one example,
in this cool paper scientists were studying how wild yeasts have sex.
Yeasts are a favorite food of fruit flies and the scientists suspected that
the yeasts might be doing it in the fly's digestive system.
To test that, they took two different yeasts that had different mutations.
If they made it the offspring would have both mutations.
They fed the yeast to flies and collected the fly poop on petri plates.
But the scientists couldn't just look at the cells in the fly poop and tell which
ones had which mutations. They needed to separate the cells and
because it's hard to do genetics on individual cells,
They needed to grow each one into a colony then they could test whether each
colony had each mutation. So, how do you go from a pile of mixed up
cells to single colonies? Well, you streak for singles! To get
single colonies, you need to put individual cells far enough away from
each other on a petri plate that when they grow and make a colony, the colonies
are not touching. But you can't pick up an individual cell
and put it on a petri plate because the cell's too small to see. So what you
have to do is dilute the cells by streaking them onto a petri plate.
It works like this: when i dip my paintbrush in paint
it starts with a lot of paint on it. When I rub it on the paper,
in the beginning lots of paint molecules come off the brush but
eventually, the paint is so diluted that I can't even see it anymore.
When you streak for singles, you do the same thing but instead of using paint,
you use microbial cells. To pick up the cells you don't use a
paint brush, you use a sterile inoculating loop or –
my favorite – a toothpick. Make sure it's sterile! Once the cells
are on your toothpick, you streak them onto a petri plate.
If you streak enough, then the cells on the toothpick will eventually become
diluted enough that you'll be laying down
individual cells. Which, if they're far enough apart, they'll grow into single
colonies. Remember, they're too small to see. You'll
only see the colonies after they've had time to grow.
When i was diluting the paint with my paintbrush, I had a big piece of paper
to dilute my paint with. But petrie plates are small, so we've got
to be efficient with the space.
You can streak for singles with one toothpick on one plate but you have to
streak a lot to get the cells diluted enough.
That means you can't be afraid to let your lines touch each other.
If you streak like this, you won't get single colonies.
So, let your lines touch.
You can be more efficient with the space on your petri plate if you make
multiple streaks. Look how quickly I can get the paint diluted if I use multiple
fresh paintbrushes. You can do the same when streaking for
singles. Streak your first toothpick onto just a
portion of the plate. Discard that toothpick and get a new
sterile one now. Streak from the region you just covered
into a new portion of the plate. Make sure you streak over the previous
streaks and into a fresh part of the plate.
Repeat once more.
By using multiple toothpicks, you can even streak multiple strains for singles
on one petrie plate. Making multiple overlapping streaks is
almost guaranteed to give you single colonies.
One last thing, auger is basically like jello. So, it's super easy to accidentally
stab your toothpick into it. So be sure to streak gently.
Don't forget that when working with microbes, you should use protective
equipment that is appropriate for the organisms you're working with and
always use sterile technique.
Remember the mating yeast? The scientists
found that the yeast waited until after they had been pooped out before they had
sex. Possibly as a way to avoid mating with
their siblings. What interesting questions will you answer by streaking
for singles?