Signs of Winter 3: What’s in the Kitchen?

Human Microbiome. From Hou et al. Sig. Transduct. Target. Ther. 7 2022.

(Click on the following link to listen to an audio version of this blog … What’s in the kitchen

The numbers used to describe the bacteria communities in, on, and around human beings are staggering!  There are between three and ten times more bacterial cells in and on “you’ (and every other person, too) than there are cells of all of your tissues and organs. These bacteria help the body to resist infections by pathogens from the environment, assist in the digestion and absorption of food, synthesize a number of vitamins, and produce a number of hormones that modulate and coordinate the organ systems of the body (including the immune system!). Many aspects of our individual microbiomes are established at birth (especially our gut microbiomes), but the growing science of probiotics indicates that vital microbial systems can be modified by diet.

Our surrounding environment is even more rich in bacteria than our bodies are. In a study published in BMC Research Notes (October 28, 2019) a research team sampling kitchen surfaces and utensils found an average of 1.54 million bacteria per square centimeter in the kitchen they examined!  Further, other studies have shown that a gram of soil can contain between 100 million and 1 billion bacteria!

Back in the 1990’s Deborah and I taught a summer set of courses for elementary-aged kids at Penn State. We called the summer program “Kids in College,” and it was a very successful, very diverse with dozens of courses and sports camps and hundreds of students. One of the classes that we taught was called “Hands-On Science.” In this class the students went out and observed, and sampled and explored and then we pulled their observations together in a scientific way.

Phot by NOAA. Public Domain

One of our projects we did was to look for bacteria around the Penn State New Kensington campus. One afternoon, 20 eight and nine years olds fanned out across the campus armed with sampling swabs and Petre plates. They stuck the swabs everywhere they could think and then wiped them across the Petre plate agar. In a few days we had a picture of the microbial community of the campus!

The students had been most excited to take samples from the bathrooms thinking that they would the places for the highest levels of bacteria. They were very disappointed to find that the bathrooms were remarkably free of bacterial contaminants! Cleaning works!

The places that they found the most bacteria were on the bottoms of their shoes and anywhere in the hallways that had foot traffic but lacked regular mopping and cleaning. Those spots covered the Petre plates to overflowing!

There are a lot of bacteria out there!

Sponge, Photo by pxfuel.

Back in December 2018 I wrote about the bacteria that grow on kitchen sponges. Some of these bacteria are harmless but some of them are powerful human pathogens! Sponges are ideal bacterial habitats, and, as they are used to wipe up messes and clean surfaces, they get infused with a rich diversity and abundance of bacteria. In a paper published in Scientific Reports seven years ago (July 19, 2017) a research team at the University of Furtwangen in Germany identified 362 species of bacteria living in a kitchen sponge at densities of 82 billion bacteria per cubic inch of sponge! Typical methods of disinfecting a sponge (microwaving it, washing it in the dishwasher or laundry, soaking it in vinegar or other cleaning solutions, or cooking it in hot water) do initially reduce the overall numbers of bacteria, but some of the most pathogenically dangerous species in the sponge are the most likely to survive the treatment and proliferate.

“When people at home try to clean their sponges,” stated the lead researcher on the project,” they make it worse.”

What should you do with your kitchen sponge? It quickly becomes a dispersal agent for bacteria and the more you try to clean it, the more pathogenic it becomes! The researchers recommend that you either throw the sponge away and replace it with a new one every week or so, or disinfect it and then use in some other house location where food is not handled (like a bathroom or a basement).

Cellulose dish cloths

Deborah and I stopped using sponges in the kitchen altogether. We have been using reuseable, cellulose, kitchen cloths which we regularly throw into the laundry and wash with the towels in hot water. They are supposed to stand up to 200 washings and when they begin to fray or wear out, they are recyclable.

There are, also, two recent studies that expose new areas of bacterial contamination in our kitchens.

A paper recently published in Frontiers of Microbiology (August 7, 2024) described the microbial communities that live inside of microwave ovens! Microwaves have been used extensively over the years to sterilize materials and also to cook food. One side benefit of cooking food is generating appropriately high temperatures to kill potentially dangerous bacteria in the food materials.

Photo by Mrbeastmodalday, Wikimedia Commons

The environment of the microwave oven is, though, continuously enriched with bacteria (from the air, from spilled and splattered food, from surface contamination of the dishes and plates being put in the microwave and also from the hands of the people loading and unloading the dishes and plates from the microwave. This community of microorganisms is never completely killed by the microwaving process and is, in fact, under a steady selection process that favors bacterial strains that are resistant to microwaves and the heat they generate.

The researchers in this study sampled 30 microwave ovens. Ten ovens were from household kitchens, ten were from shared kitchens at offices and other workplaces, and ten were from molecular biology and microbial biology laboratories. The research team found 747 genera of bacteria thriving inside of these microwave ovens!

Many of these bacteria were relatively harmless and corresponded to the typical bacteria found on kitchen and laboratory surfaces. Many of the bacteria were also typical of species found on the skin microbiomes of humans. These microwave bacterial communities, though also included a number of significant human pathogens (like Klebsiella, Enterococcus and Aeromonas).

The researchers concluded that microwave ovens were not more bacterially toxic than the rest of the surfaces of a kitchen. They stressed, however, that the inside surfaces of a microwave oven need to be as thoroughly cleaned as the rest of the surfaces in a kitchen.

Photo by Lymantria, Wikimedia Commons

Several recent articles published in popular and professional cooking magazines have referred to a paper published a number of years ago in Food Service Technology  (August, 2002). This study looked at a small sample of oven mitts and pot holders from a variety of types of kitchens and sampled them for bacteria.

Oven mitts and pot holders are essential pieces of personal protective equipment (PPE) for kitchen workers and they are frequently contaminated with cooked and raw food and also with microbial species that are resident on or transiently present on the skin of the PPE user. Oven mitts and pads are also frequently used and then set aside or hung up to await their next use with little or no thought to cleaning.

The study examine 10 oven mitts and 3 potholders from kitchens that included kitchens in hospitals. The PPE from hospitals were particularly enriched in bacteria but all of the PPE were found to have rich microbial communities. The hospital PPE’s bacteria included some very significant human pathogens including Bacillus cereus, a variety of coliforms, Pseudomonas spp. and Clostridium perfringens.

The external surfaces of the mitts had higher levels of bacteria that the internal surfaces, but significant numbers of bacteria were found inside of the oven mitts. This suggests that the oven mitts may serve as contamination foci for the hands of the kitchen workers using them.

It was noted that no E. Coli, Listeria spp. or Salmonella spp. were found in or on the mitts and pot holders tested. The researchers, though, strongly recommended that all oven mitts and hot pads used in a kitchen be regularly, and thoroughly cleaned and sanitized.