With the advancement of technology, the microwave became a staple in human life due to its two main uses: cooking and heating food, and the belief that it kills bacteria on food during heating.
The common belief is that the heat generated by microwaves destroys bacteria and other pathogens. Microwaves heat food by causing water molecules within the food to vibrate, producing heat. If the food is heated evenly and reaches a high enough temperature (typically above 165°F or 74°C), it can effectively kill bacteria. However, a new study has found that microwave ovens can harbour a variety of microbial communities within itself.
Recent research shows that certain bacteria, including potentially harmful strains such as Klebsiella, Enterococcus, and Aeromonas, can survive and even thrive in household microwaves. This challenges the long-held notion that microwaves are as sterile as previously assumed.
Since the Industrial Revolution, microbes have successfully colonised novel habitats such as marine oil spills, plastic floating in the oceans, industrial brownfields, and even the interior of the International Space Station. Surprisingly, one extreme environment harbouring a specialised community of highly adapted microbes is much closer to home: inside microwaves.
This discovery, reported for the first time in a study published in Frontiers in Microbiology by researchers, is significant not only from a hygiene perspective but also for its potential biotechnological applications. If the strains found inside microwaves can be harnessed, they could prove useful in industrial processes that require particularly resilient bacteria.
“Our results reveal that domestic microwaves have a more ‘anthropized’ microbiome, similar to kitchen surfaces, while laboratory microwaves harbour bacteria that are more resistant to radiation,” said Daniel Torrent, one of the authors in a statement.
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What the study revealed
Torrent and colleagues sampled microbes from 30 microwaves: 10 from single-household kitchens, 10 from shared domestic spaces (such as corporate centers, scientific institutes, and cafeterias), and 10 from molecular biology and microbiology laboratories. The goal of this sampling was to determine whether microbial communities are influenced by food interactions and user habits.
They used two complementary methods to assess microbial diversity: next-generation sequencing and the cultivation of 101 strains on five different media. In total, the researchers identified 747 different genera within 25 bacterial phyla. The most frequently encountered phyla were Firmicutes, Actinobacteria, and especially Proteobacteria.
The study found that the composition of the typical microbial community partially overlapped between shared domestic and single-household microwaves, while laboratory microwaves had a distinct microbial profile. Microbial diversity was lowest in single-household microwaves and highest in laboratory ones.
Specific genera, including Acinetobacter, Bhargavaea, Brevibacterium, Brevundimonas, Dermacoccus, Klebsiella, Pantoea, Pseudoxanthomonas, and Rhizobium, were found exclusively in domestic microwaves. In contrast, Arthrobacter, Enterobacter, Janibacter, Methylobacterium, Neobacillus, Nocardioides, Novosphingobium, Paenibacillus, Peribacillus, Planococcus, Rothia, Sporosarcina, and Terribacillus were unique to shared domestic microwaves.
Nonomuraea bacteria were isolated exclusively from laboratory microwaves. Additionally, Delftia, Micrococcus, Deinococcus, and one unidentified genus of the phylum Cyanobacteria were found in significantly greater frequencies in laboratory microwaves compared to domestic ones.
The authors compared the observed microbial diversity with that in specialised habitats reported in several other studies. As expected, the microbiome in microwaves resembled that found on typical kitchen surfaces.
“Some species of genera found in domestic microwaves, such as Klebsiella, Enterococcus, and Aeromonas, may pose a risk to human health. However, it is important to note that the microbial population found in microwaves does not present a unique or increased risk compared to other common kitchen surfaces,” said Torrent.
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Impact on general public health
Klebsiella, Enterococcus, and Aeromonas are bacteria that can have significant impacts on human health, particularly in vulnerable populations such as those with weakened immune systems or underlying health conditions.
Klebsiella
Impact on Health:
Klebsiella pneumoniae is the most common species associated with human infections. It can cause a range of serious infections, including:
- Pneumonia: Especially in hospitalised patients or those with chronic lung disease.
- Urinary Tract Infections (UTIs): Common in hospital settings, especially in catheterized patients.
- Sepsis: A life-threatening response to infection that can lead to organ failure and death.
- Antibiotic Resistance: Klebsiella is known for developing resistance to multiple antibiotics, including carbapenems, leading to difficult-to-treat infections.
Enterococcus
Impact on Health:
Enterococcus faecalis and Enterococcus faecium are the most clinically relevant species.
- Urinary Tract Infections (UTIs): Enterococci are a common cause of UTIs, particularly in hospital environments.
- Bacteremia: Infection of the bloodstream that can lead to sepsis.
- Endocarditis: Infection of the inner lining of the heart chambers and valves, which can be life-threatening.
- Antibiotic Resistance: Enterococcus, particularly Enterococcus faecium, is often resistant to vancomycin (known as VRE – Vancomycin-Resistant Enterococcus), making infections difficult to treat.
Aeromonas
Impact on Health:
Aeromonas hydrophila is the most common species associated with human infections.
- Gastrointestinal Infections: Aeromonas can cause diarrhea, especially in children and immunocompromised individuals.
- Wound Infections: Particularly in wounds exposed to contaminated water, leading to cellulitis or more severe soft tissue infections.
- Bacteremia: Though less common, it can occur, especially in immunocompromised individuals or those with underlying health conditions.
- Resistance Concerns: Some strains of Aeromonas show resistance to certain antibiotics, complicating treatment.
General impact
These bacteria pose a significant health risk, particularly in hospital settings where they can cause healthcare-associated infections (HAIs). Their ability to resist multiple antibiotics makes them challenging to treat, leading to prolonged hospital stays, increased healthcare costs, and higher mortality rates in severe cases.
Parallel evolution
The authors also noted that the microwave microbiome was similar to that found in an industrial habitat, specifically on solar panels. They suggested that the constant thermal shock, electromagnetic radiation, and desiccation in these highly irradiated environments have repeatedly selected for highly resistant microbes, much like in microwaves.
“For both the general public and laboratory personnel, we recommend regularly disinfecting microwaves with a diluted bleach solution or a commercially available disinfectant spray. Additionally, it is important to wipe down the interior surfaces with a damp cloth after each use to remove any residue and clean up spills immediately to prevent bacterial growth,” advised Torrent.
(Edited by Neena)
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Shambhu Kumar is a science communicator, making complex scientific topics accessible to all. His articles explore breakthroughs in various scientific disciplines, from space exploration to cutting-edge research.
