Science Buddies: "Ask an Expert"

What are some alternatives to use to speed up the growth of the bacteria? Could I use an oven?

Hi,

What type of bacteria are you growing and what temperature do you want to achieve? Bacteria are very versatile and there are extreme psycryophiles that will grow at minus 15 degrees C and extreme thermophiles that grow at 80 degrees C. Most likely you are working with mesophilic bacteria that grow well between ambient 20 and 37 degrees C. If so, you can speed up their growth by incubating your samples in an enclosed container with a light bulb turned on, like your closed oven. However, I don't recommend using the oven because a family member will invariably come in and turn the oven on without looking and destroy your project. As an alternative, you could put a light bulb in a cardboard box or check the temperature on top of your water heater. Make sure the temperature is not above 42 degrees Centigrade, as this could also kill the bacteria. If you are working with Petri dishes, avoid an area with too much air circulation, as you want to avoid drying out the plates before the colonies can grow.

If you don't find a warmer spot, then you will be using ambient temperature and it will just take a little longer for the bacteria to grow. Whatever temperature you use, this will be one of the controlled parameters in your experiment.

Donna Hardy

I am working with E. Coli, so the ideal temperature is about 37 degrees C.

Also, how long does it take for zones of inhibition to develop? I know it's usually around two to four days at room temperature, but it's been a day and nothing seems to have happened at all. Is this normal? Wouldn't there at least be something to signify the growth of the colonies? The disinfectant being used is a diluted soap solution originally containing .15% triclosan. I started doing the experiment two weeks after the bacteria was shipped, so is it possible that they died? Thank you!!

Hi,

What temperature are you using? E. coli will grow very slowly at room temperature of 20-22 degrees C.

What type of agar are you using? Is it possible that the surface of the agar has dried out?

How did you prepare the lawn of bacteria for the sensitivity test?

Normally, E. coli would grow up overnight and the zones of inhibition would be visible within 18-24 hours.

If it has been 2 weeks since you received your bacteria, and you just transferred them to the agar plate, then they could still be recovering and in lag phase, and getting ready to grow. Did you store your culture in the refrigerator? Did the culture dry out? Here is a website that describes the bacterial growth curve. Your older culture was probably in late stationary or early death phase, and if there are any survivors, it will take them time to start growing. Here is a website that describes a typical bacterial growth curve; this is very important background information for your project.

http://en.wikipedia.org/wiki/Bacterial_growth

So continue to incubate your plates for a few more days.

The guidelines from the Science Buddies website contain helpful information for culturing bacteria and also techniques for troubleshooting:

https://www.sciencebuddies.org/science- ... ains.shtml

https://www.sciencebuddies.org/science- ... ques.shtml

Ideally, you want to make a “lawn” of bacteria from a freshly grown culture. If possible, I recommend growing up a broth culture overnight, making a 1:100 dilution in sterile culture medium, and transferring the diluted culture to a sterile Petri dish with agar to make the lawn and then place the antibiotic disks on the agar surface. Using a culture that has grown up to late stationary phase will give consistent, reproducible results.

If you cannot duplicate your experiment, and nothing grows, let me know and I will give you some advice for writing up your results. You have a complete project, but if you don’t obtain results, some creativity will be needed.

Donna Hardy

Actually, it seems like putting the agar plates in the oven sped up the bacteria's growth greatly. I can see streaks running across the plate. However, it doesn't seem like any zones of inhibition developed, so I'm thinking I should use a stronger disinfectant. I'm going to redo the experiment with Lysol this time.

EDIT: Also, I have a tube with Staphylococcus epidermidis broth, but somehow, the broth solidified. I'm not sure what to do...

EDIT 2: Ok, I tried using Lysol this time, and I let the bacteria grow in the oven with the light on. I can see the colonies, but only on one plate did zones of inhibition develop. And for some reason, there was a lot of liquid in the plates.