Microbial Inhabitants of Cleaning Appliances and Tools

Nov. 21, 2022

What do sponges, washing machines, dishwashers, showers and toothbrushes have in common? People use them to clean—either their belongings or themselves. However, many of the household spaces, tools and appliances people associate with “clean” are, from a microbial standpoint, far from it. In fact, they are home to diverse microorganisms that are well-adapted to survive harsh conditions, like detergents and high temperatures. Some of these organisms may have pathogenic potential, though the risk of infection is not always clear. Still, there are things one can do to limit microbial growth in and on the appliances and objects associated with housekeeping and personal hygiene.

Kitchen Sponges

A sponge.
Kitchen sponges can contain millions of bacterial cells.
Source: Pille R. Priske/Unsplash
Kitchen sponges are notorious for having high microbial burdens. With their moist and porous surfaces, sponges offer favorable conditions for hordes of bacteria (up to roughly 50 billion bacterial cells per cm3) to grow. Gammaproteobacterial species (e.g., Escherichia coli) are common members of the sponge microbiota. Food-borne pathogens, like Klebsiella pneumonieae, which can infect everything from the lungs to urinary tract, also inhabit sponges, along with various viruses and archaea.

What Can Be Done? 

Avoiding wiping up risky messes (e.g., raw meat) is a good idea. Using alternative scrubbing tools, like dish brushes, which dry faster and harbor less bacteria, is also an option for managing sponge microbes, as is disinfecting the sponge itself. A 2008 study by the United States Department of Agriculture (USDA) showed that microwaving sponges soaked in a ground beef slurry (roughly 31 million bacterial CFU/sponge) for 1 minute reduced bacterial load to 2-3 colony forming units (CFU) per sponge. The efficacy of microwaving sponges, however, may depend on the power of the microwave and how dirty the sponge is (i.e., the microbial load). The USDA also found that running sponges through the dishwasher is a similarly effective strategy, particularly for killing molds and yeasts.

Cleaning Appliances: Dishwashers and Washing Machines

In lieu of (or in addition to) sponges, many people use dishwashers to clean their dishes. However, dishwashers are far from microbe-free. Dishwasher microbial communities are seeded by tap water, which contains bacteria picked up from the plumbing system, and by microbes and food on dirty dishes loaded into the machine. The dishwasher’s frequency of use and its age also shape community composition. For example, researchers found that the make-up of bacterial biofilms on rubber seals of newer dishwashers (0-4 years old) was unique (e.g., had higher abundances of Proteobacteria) from older dishwashers (8 years old), suggesting there are successive shifts in community structure over time. These dynamics are influenced, in part, by interactions with fungi, including pathogen-containing genera like Candida and Cryptococcus, which live on dishwasher rubber seals, water dispensers and drains.

Bacterial and fungal genera detected in 21 dishwashers.
Bacterial and fungal genera detected in 21 dishwasher samples. Numbers represent the number of dishwashers that contained the representative genera.
Source: Raghupath PK et al./Applied and Environmental Microbiology, 2017

Washing machines are another microbially-rich household cleaning appliance, with an estimated average bacterial load of 21,000 colony CFU/cm-2 across various sampling sites (e.g., detergent drawer and rubber door seal). Where do these microbes come from? Clothing traps organisms from human skin, bodily secretions and excretions and the environment, which can transfer between garments during laundering, and to and from the washing machine itself.
washing machine
Washing machine microbial communities are shaped by various factors, including water source and temperature, and how often the machine is used.
Source: Raychan/Unsplash

Like in dishwashers, tap water also contributes to the community. The abundance and types of microbes living in washing machines depends on the machine’s frequency of use, wash cycle (i.e., hot versus cold) and the type of appliance. For instance, front-loader, energy-efficient washing machines found in many homes can harbor residual water in the drum, which creates a moist environment that may promote bacterial growth. Sometimes all it takes is a sniff to detect washing machine microbes—bacteria can degrade detergent and organic material on clothes to generate a funky, unpleasant smell. 

What Can Be Done?

For both dishwashers and washing machines, increasing the temperature of the wash cycle can help control microbial contamination. One can also run a “clean out” cycle (sans clothing or dishes) to wash the appliance. Manually cleaning machine parts (e.g., rubber seals and walls) periodically may also prevent biofilm build-up.

Showers

What about places people go to clean themselves? Showerheads can, literally, shower people with microbes. For instance, non-tuberculosis mycobacterial (NTM) species are notable for their associations with shower aerosols. Ubiquitous in the environment, NTM (e.g., Mycobacterium abscessus and M. avium complex [MAC]) are commonly detected in shower water and showerheads, where they form biofilms. Though generally harmless, these species can cause lung disease in immunocompromised individuals.
showerhead
Showerheads can contain microbial biofilms.
Source: kevin Baquerizo/Unsplash


Shower curtains are also slathered in microbes. Indeed, the “pink mold” that commonly develops along shower walls and curtains is caused by 2 pink-pigmented bacterial species, Serratia marcescens and Aureobasidium pullalans, which feed on soap residues and other organic compounds that spatter around the shower. Oh, and those rubber duckies that make bath time so much fun? Inside, they may contain up to 9.5 million bacterial CFU/cm2.

What Can Be Done?

Disinfecting, or regularly replacing, showerheads, curtains, bath toys and hoses may hinder bacterial growth. This may be more important in homes with high-risk individuals.

Toothbrushes

Besides showering, toothbrushing is a key part of personal hygiene regimens. Toothbrushes are one of the most densely colonized objects in the home. Microbes on toothbrushes include human oral taxa (e.g., Streptococcus species, which are abundant members of the oral microbiota), as well as those associated with the skin microbiota and the home environment (e.g., the air). Pathogens like Acinetobacter baumanii, Staphyloccocus aureus and Candida albicans, have also been detected.
Toothbrushes.
Toothbrushes are colonized by microbes from the mouth, skin and environment.
Source: Towfiqu barbhuiya/Unsplash
The toothbrush microbiome depends on toothbrush age, toothpaste and period of use, among other factors. 

What Can Be Done?

There are ways to disinfect toothbrushes: microwaving a toothbrush for 1 minute can reduce bacterial growth, as can soaking it in 3% hydrogen peroxide or Listerine® mouthwash. One’s best bet is to store toothbrushes in an upright position to allow them to dry (a closed container promotes more microbial growth than leaving the brush exposed to the air). Moreover, it is recommended that toothbrushes be replaced every 3-4 months, or more often if the bristles look worn out.

How do Microbes Survive in Harsh Household Environments?

In many of the above spaces and tools, microbes are exposed to harsh chemicals and detergents, temperatures and mechanical forces—or a combination of all these stressors. How do they survive?

The types and severity of stressors microbes face largely depend on location. For example, microbes in the detergent drawer of washing machines must tolerate compounds like bleach, surfactants or perfumes while those in the door seal must be able to survive fluctuating periods of dry and wet conditions and varying pH values. Human behavior matters, too. Given 90% of energy used by washing machines goes toward heating the water, more people opt for cold water cycles to increase efficiency. In the U.S., the median cold-water wash temperature is 14.4°C (57.9°F)—a steep drop from the recommended 40-60°C (104-140°F) needed to kill most bacteria.

In general, biofilm formation is a key requisite for microbial survival in hostile household environments. The sticky biofilm matrix helps bacterial cells adhere to surfaces, while also protecting them from environmental assaults. For example, showerhead conditions likely select for biofilm-forming species that can withstand high water pressure. Certain materials, like rubber, a common component of appliances, offer favorable surface for biofilm growth. In addition, some microbes can metabolize plastics, surfactants and detergents found within these locations and use them as a nutrient source.

Often, microbes that thrive throughout the home are adept at dealing with many stressors simultaneously. For instance, polyextremotolerant fungal species (e.g., black yeast species like Exophiala dermititidis, a rare cause of fungal infections) can withstand everything from extreme temperatures to radiation. Despite being present in low abundance in the environment, they are commonly isolated from dishwashers, underscoring how relatively inhospitable conditions select for hearty microbes. To that end, there is a purported link between polyextremotolerance and opportunistic pathogenicity, suggesting that some of the same traits that facilitate microbial survival also allow for host colonization and infection. Relatedly, bacterial antibiotic resistance genes, which have been detected in shower drains, dishwasher seals, toothbrush heads and other locations, are linked to stress responses. Thus, hostile conditions in and on household objects may promote growth of antibiotic-resistant bacteria.

What Are the Risks?

With that in mind, can people get sick from the tools they use to clean? Direct evidence of infections caused by cleaning appliances, spaces and objects, specifically in household environments, is sparse. One study showed that a domestic style washing machine was responsible for transmitting Klebsiella oxytoca, a bacterium that can cause diverse infections (e.g., pneumonia, wound infections and more), to newborns in a neonatal intensive care unit. When the machine was removed, transmission stopped, thus highlighting washing machines as a potential transmission route for pathogens.

Similarly, an outbreak of infections caused by Saprochaete cllavata (a yeast that can sicken immunocompromised people) at a cancer center in Marseilles, France was traced to a dishwasher with a faulty heater, suggesting it served as a “vector of contamination.” Sponges may also spread microbes around—contamination of kitchen sponges with fecal coliforms (e.g., E. coli) or Staphylococcal aureus was predictive of other kitchen surfaces having the same contamination.
Relative abundances of mycobacterial clades in showerheads samples across 21 regions in Europe and the U.S
Relative abundances of mycobacterial clades in showerheads samples across 21 regions in Europe and the U.S. Researchers found a significant association between abundances of mycobacteria in showerheads and NTM lung disease prevalence in states throughout the U.S.
Source: Gebert MJ et al./mBio, 2018

In the realm of personal hygiene, an analysis of over 650 households across the U.S. and Europe demonstrated that geographic regions with high levels of pathogenic mycobacteria in residential showerheads generally overlapped with regions where NTM lung disease was most prevalent, suggesting a possible association between shower head aerosols and disease manifestation. On a related note, researchers showed that the toothbrushes of children with cystic fibrosis who were treated with antibiotics for lung infections contained viable pathogens (i.e., Pseudomonas aeruginosa and S. aureus). They concluded the toothbrushes may spread bacteria that could trigger new infections in the lower airways.

Nevertheless, it's worth acknowledging that the above examples are largely associative. The presence of microbes is not inherently harmful—they are, after all, everywhere. Even if viable opportunistic pathogens are detected, this is not enough to determine their disease-causing potential. Ultimately, more research is needed to uncover direct links between microbes in and on home cleaning places and tools and human health.

Want to learn more about myths, policies and best practices for disinfecting the home? Check out this next article. 


Author: Madeline Barron, Ph.D.

Madeline Barron, Ph.D.
Madeline Barron, Ph.D. is the Science Communications Specialist at ASM. She obtained her Ph.D. from the University of Michigan in the Department of Microbiology and Immunology.