Biologists Find Diverse Bacterial Communities in Microwave Ovens - not really a good thing
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In
a new study, microbiologists from the University of Valencia and Darwin
Bioprospecting Excellence SL investigated the bacterial communities in
microwave ovens and compared the microbial composition of domestic microwaves,
microwaves used in shared large spaces, and lab microwaves.
The
microwave oven bacterial population was dominated by Proteobacteria,
Firmicutes, Actinobacteria, and Bacteroidetes, similar to the bacterial
composition of human skin.
Comparison
with other environments revealed that the bacterial composition of domestic
microwaves was similar to that of kitchen surfaces, whereas lab microwaves had
a higher abundance of species known for their ability to withstand microwave
radiation, high temperatures and desiccation.
Iglesias et
al. showed that microwaves harbor a specialized community of locally
adapted microbial genera, which resembles that reported on kitchen surfaces and
in another extreme, highly irradiated habitat: on solar panels.
Microorganisms
that thrive in ecosystems characterized by extreme environmental conditions
have been well studied to elucidate the evolutionary mechanisms that have
favored their adaptation.
Natural extreme environments represent an exceptional source of novel microbial species, as well as a source of novel secondary metabolites with biotechnological applications. However, one does not need to travel that far in search for extreme environments.
Microwave
irradiation has been used for decades to reduce the presence of microorganisms
in food and extend food shelf life.
The
application of an electromagnetic wave in the range of 300 MHz to 300 GHz to a
dielectric medium such as food, also known as microwave heating, generates heat
to reach lethal temperatures that inactivate most microorganisms, such as Escherichia
coli, Enterococcus faecalis, Clostridium perfringens, Staphylococcus
aureus, Salmonella spp. and Listeria spp.
Recent
work has shown that cell inactivation is associated with deactivation of
oxidation-regulating genes, DNA damage and increased permeability and disrupted
integrity of cell membranes.
Despite
this extensive characterization of the biological effects of microwave
radiation on foodborne bacteria, there are no reports of microwaves as
microbial niches, that is, environments where specific selective pressures (in
this case, thermal shock, microwave radiation, and desiccation) can shape a
specifically adapted microbiome.
“Our
results reveal that domestic microwaves have a more ‘anthropized’ microbiome,
similar to kitchen surfaces, while lab microwaves harbor bacteria that are more
resistant to radiation,” said Dr. Daniel Torrent, a researcher at Darwin
Bioprospecting Excellence SL.
For
the study, Dr. Torrent and his colleagues sampled microbes from inside 30
microwaves: 10 each from single-household kitchens, another 10 from shared
domestic spaces, for example corporate centers, scientific institutes, and
cafeteria, and 10 from molecular biology and microbiology laboratories.
The
aim behind this sampling scheme was to see if these microbial communities are
influenced by food interactions and user habits.
They
used two complementary methods to inventorize the microbial diversity: next
generation sequencing and cultivation of 101 strains on five different media.
In
total, the authors found 747 different genera within 25 bacterial phyla. The
most frequently encountered phyla were Firmicutes, Actinobacteria, and
especially Proteobacteria.
They
found that the composition of the typical microbial community partly overlapped
between shared domestic and single-household domestic microwaves, while lab
microwaves were quite different.
The
diversity was lowest in single-household microwaves, and highest in lab ones.
Members
of genera Acinetobacter, Bhargavaea, Brevibacterium, Brevundimonas, Dermacoccus, Klebsiella, Pantoea, Pseudoxanthomonas and Rhizobium were
found only in domestic microwaves.
Arthrobacter, Enterobacter, Janibacter, Methylobacterium, Neobacillus, Nocardioides, Novosphingobium, Paenibacillus, Peribacillus, Planococcus, Rothia, Sporosarcina,
and Terribacillus were found only in shared-domestic ones.
Nonomuraea bacteria
were isolated exclusively from lab microwaves. There, Delftia, Micrococcus, Deinocococcus and
one unidentified genus of the phylum Cyanobacteria were also common, found in
significantly greater frequencies than in domestic ones.
The
researchers also compared the observed diversity with that in specialized
habitats reported in the literature.
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,” Dr. Torrent said.
“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.”
However,
it was also similar to the microbiome in an industrial habitat: namely, on
solar panels.
The
scientists proposed that the constant thermal shock, electromagnetic radiation,
and desiccation in such highly irradiated environments has repeatedly selected
for highly resistant microbes, in the same manner as in microwaves.
“For
both the general public and lab personnel, we recommend regularly disinfecting
microwaves with a diluted bleach solution or a commercially available
disinfectant spray,” Dr. Torrent said.
“In
addition, it is important to wipe down the interior surfaces with a damp cloth
after each use to remove any residue and to clean up spills immediately to
prevent the growth of bacteria.”
The results were
published in the journal Frontiers in Microbiology.
_____
Alba
Iglesias et al. 2024. The microwave bacteriome: biodiversity of
domestic and laboratory microwave ovens. Front. Microbiol 15;
doi: 10.3389/fmicb.2024.1395751