Science Buddies: "Ask an Expert"

Hi, I'm doing a science project with my school about the effectiveness of chemical and natural disinfectant against bacteria from a school desk. Our teacher has left and we have a relief (who knows nothing about science ) for the next week or two and we are feeling very lost on what to do next... We have ordered some agar plates from the lab, but we wanted to know how we should test the bacteria. When we swab the desk should we add on the bacteria in a pattern, or apply it evenly to the whole plate? Can we use sterile disks/hole-punches of filter paper soaked in a disinfectant and place it into the middle of the agar plate after the bacteria has been added? How can we calculate how effective each one is... we are thinking of using the area of bacteria killed against the alive bacteria as a percentage and then graphing our results. We ordered enough plates to do a repeat trial for each disinfectant; eucalyptus spray, lemon juice, white vinegar, table salt, and bleach, hydrogen sulfide (lab technician will help us with this) and dettol. Please reply when you can!

Hello BrookeMarshall1210!

Welcome to Science Buddies! Your project seems really interesting, and I'll try to help you with your questions.

First, when you plate your bacteria, you want to create a "lawn" of bacteria. This basically means that there will be an even layer of bacterial colonies on the plate. This can be done by taking a sterile cotton swab and plating it on the agar plate in a zigzag fashion, turning the plate as you go. I would recommend applying it evenly to the entire plate, rather than applying it in a pattern so that your results will be more reliable.

Your proposed project is really similar to one we have on the Science Buddies website
https://www.sciencebuddies.org/science- ... #procedure. This project's procedure can be easily modified to fit your project.

I think soaking the filter paper in disinfectant and measuring the zone of inhibition would be the best way to present your results. Specifically, see step 7 in the attached project. Instead of dipping the filter paper in different dilutions of disinfectants, you would instead dip in in different types of disinfectants (eucalyptus spray, lemon juice, white vinegar, table salt, and bleach, hydrogen sulfide) as you mentioned.

Instead of using the area of bacteria killed against the alive bacteria as a percentage, a better way to quantify the effectiveness of the disinfectants would be to measure the diameter of the zone of inhibition for each disk in millimeters. The larger the diameter, the bigger the zone of inhibition and the more effective to disinfectant will be.

Hopefully, this helped you! Let me know if you have any additional questions.

Here's another project that's even more similar to yours :https://www.sciencebuddies.org/science- ... #materials

This project and the procedure should help clear up any problems you may encounter.

Thanks, Probiotics.

Okay, thank you so much!

Sorry, just another quick question... when I googled some things lots of people used a liquidy substance to inoculate their agar plant. Will we be able to get a even 'lawn' of bacteria from dry swabbing the school desks?

Hi BrookeMarshall1210,

You are correct! To obtain a lawn of bacteria across the area of your agar plate, it would be better to use a liquid. The simplest way to do this, is to soak your swab in sterile water and then spread the water on the surface of a plate. Make sure to divide this water ('bacterial suspension') and spread equal volumes on all plates to obtain bacterial lawns that are more or less similar in density. Also the water that you initially use should not be too much, to prevent excessive dilution of the bacteria!

This is a great project, good luck!
MS