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Salty Pits

Science Snack
Salty Pits
How hospitable is your armpit?
Salty Pits
How hospitable is your armpit?

Explore how different deodorants work and what that means for the bacteria in your armpit.

COVID-19 Learning Note: You can use the same technique in this Snack to see how well you wash your hands. Bacteria on your hands will grow on the agar plates, but viruses will not. Place a thumb or palm on two different plates before and after you wash your hands, and see if there’s a difference in the amount of bacteria that grows. How well did you wash?

Tools and Materials
  • Deodorants (look for non-antiperspirant varieties such as Crystal Body Deodorant Spray, ideally in liquid form—spray or roll-on) 
  • Cotton swabs 
  • Nutrient agar or bacto agar 
  • Distilled water (not shown)
  • Petri dishes with lids (6-cm diameter ones work well)
  • Bacterial sample such as E. coli (available at a biological supply store), to be used as a control
  • Marker that will write on plastic
  • Optional: A sweaty friend

Prepare agar plates for bacterial culture by melting agar in distilled water:

  1. Measure  the recommended agar-to-water ratio for your product (31 grams per liter for nutrient agar is common). Make enough to cover the bottom of the Petri dishes you will use; you will need approximately 5 ml of agar solution for a 6-cm Petri dish.  
  2. Heat the solution in a microwave until all of the agar is dissolved, stirring from time to time. 
  3. Let the agar solution cool to the touch and then pour enough to cover the bottom of the Petri dishes. Swirl the dish after you pour to help the agar spread completely. This is your agar plate. 
  4. Make at least four plates total (two for each test sample or control).
To Do and Notice

Pre-treat two of the agar plates with the deodorant: Wet a cotton swab with the liquid from inside the spray or roll-on, then dab the plates gently so that they are covered with the wet solution. Let the deodorant solution soak into the agar completely. 

Dip a fresh cotton swab into the bacterial control sample and streak a line onto one deodorant-treated and one untreated plate. (This will show whether a known sample of bacteria will grow in these environments.)

Take a sample from your armpit by wiping it with another cotton swab. Streak this sample onto one deodorant-treated plate. Take a second sample from the same source and streak it onto an untreated plate. 

Optional: Make two more agar plates, pre-treat one with deodorant, and apply a sample from your friend’s armpit to both the deodorant-treated and the untreated plates.

Place the lids on your plates and label them so that you know which have deodorant applied and where the samples came from. 

Store your plates upside down (with the agar on top) in a warm place overnight. 

The next day, check out your plates for bacterial cultures (they'll look like wet, white spots). Where did bacteria grow?

What’s Going On?

Did bacteria grow on your untreated plates? If the control sample didn’t grow, try remaking the plates. If the control grew, but the armpit sample did not, it means your plates are okay, but there may not have been enough bacteria on your armpit swab.

Was there bacteria on the plate that you treated with deodorant?

Pure deodorants reduce body odor by masking the smell or inhibiting bacterial growth. The main type of bacteria on your skin is Staphylococcus epidermidis, which grows in small, white colonies. These and other bacteria create waste products that contribute to our body odor.

If bacteria grew on the untreated plate, but not the treated plate, the deodorant you used probably contains something that prevents bacteria from growing, such as alcohol or high salt concentrations.

Was there a difference between your bacteria control and the sample you took from your body?