As a microbiology teacher who recently swapped out my vile old automatic dishwasher for a shiny new model, I couldn’t help but wonder, “Does this thing really sanitize the dishes?”
Now unless you work in the fields of medicine or microbiology you probably have not had experience with how very difficult it is to keep anything sterile, completely microbe free. Our bodies, and the world around us, are covered with bacteria and fungal spores. Even when I carefully prepare Petri dishes of bacterial cultures in the laboratory, many of my cultures eventually become contaminated with some uninvited microbe.
So when I began to design the simple experiment that would put my dishwasher to the test, I fully expected to find bacteria on my ‘clean’ dishes. In addition to wondering how clean the dishes were at the end of the cycle, I also wondered whether leaving dishes in the dishwasher for hours after the wash cycle ends allows bacterial populations to grow; whether not immediately putting dishes away results in contamination. Here’s how the test went down.
Dishwasher Testing Experimental Method
Throughout the day, I loaded up my dishwasher with grimy, nasty unrinsed dirty dishes (Photo 1). I then labeled a Petri dish of nutrient agar (a moist nutritious surface for growing bacteria) with three sections:
- Section 1: Sample from Dishwasher containing Dirt Dishes
- Section 2: Sample from Dishwasher Immediately After Wash/Dry Cycle
- Section 3: Sample from Dishwasher 8 hours after End of Wash/Dry Cycle
Before starting the dishwasher, I took a sterile swab and ran it over the inside of the dishwasher door (Photo 2). The sample was then swabbed onto Section 1 of my Petri dish.
Detergent was then added and the dishwasher was run on ‘normal’ with the ‘sanitize’ option selected. As soon as the washer had completed it cycle of washing, rinsing and sterilizing/drying the dishes, I again sampled the inside of the dishwasher door with a sterile swab and ran the swab over Section 2 of the Petri Dish. Eight hours later I took a third sample from the same location.
The Petri dish was then set in a warm location for several days. If any bacteria had been sampled, contaminated sections of the Petri dish would eventually show evidence of bacterial colonies (a dot of millions of bacteria that all arose from one single bacterium).
Dishwasher Experiment Results
After five days, several bacterial colonies were growing in Section 1, the sample taken from the dirty dishwasher. During that time, no bacterial colonies grew in either Section 2 or 3 (Photo 3). Although this is only one experiment, the results do suggest that dishwashers that claim to sanitize actually do remove bacteria from dishes.
There are some bacteria that can produce endospores—tough, dormant structures that are very difficult to destroy. An autoclave, which applies both heat and pressure, is usually required to destroy all endospore-producing bacteria. It is unclear, from this experiment, whether or not endospore producers are destroyed by sanitizing dishwashers.
More Microbiology
For more information on the world of microbes, see the fun educational websites:
