INTRODUCTION
With approximately 1014 bacteria in the gut of every
adult, bacteria outnumber human cells 10 to
1 in the body. These diverse bacteria play important
roles in digestion and the maintenance of a healthy
immune system. However, if the number or composition of
bacteria is altered, certain opportunistic species may become
pathogenic and cause disease, including cancer, usually via
inflammatory mechanisms. These mechanisms have only recently
become a major topic of research. New studies have
examined how the microbiome contributes to inflammatory
bowel diseases and their association with colon cancer. The
gut microbiome may also be linked to disease beyond this
organ system,1,2 including in the skin.3
The skin is another major component of the human microbiome.4 Each person has around 1012 bacteria on the surface of
the skin, in the pilosebaceous units, and elsewhere in the dermis.5 Though fewer in number than bacteria in the colon, they
may nonetheless have similar functions in immune regulation,
as well as in prevention or promotion of neoplastic processes.
Here, we review the role of the microbiome in cancer with a
focus on how the skin microbiome may be involved in its pathogenesis
and potentially prevention and treatment.
Gut Microbiome-Induced Gastrointestinal Cancer
The gut microbiome is well-characterized, with approximately
500 to 1,000 different species present in any individual human.6
Several of these species have been implicated in carcinogenesis. Most prominent among these is Helicobacter pylori, a
common bacterial inhabitant of the human stomach and an opportunistic
pathogen. It is widely believed to play a critical role
in the development of gastric adenocarcinoma and gastric mucosa-
associated lymphoid tissue (MALT) lymphoma.7 In these
cases, chronic inflammation from H. pylori can induce malignant
transformation of B cells.7 For MALT lymphoma, treatment
of H. pylori infection with antibiotics is often sufficient to resolve
the lymphoma.8
Many other associations of human cancer with the gastrointestinal
microbiome exist, but are not as well-characterized.
The bacterium Campylobacter jejuni is often associated with
immunoproliferative small intestinal disease lymphoma,9 and
antibiotics are frequently successful as a treatment. However,
C. jejuni’s mechanism of carcinogenesis is unknown. Streptococcus
bovis is associated with colonic neoplasia10 and may
trigger this disease via chronic inflammation through innate
immune pathways.11 Salmonella typhi infection has been linked
to gallbladder cancer,12 probably through inflammation as well.
Indeed, the mechanisms by which specific bacteria induce cancer
vary, but usually involve chronic inflammation at the site of
the tumor. T-helper (Th)17 inflammation, in particular, has been
associated with these malignancies, and can be promoted by
microflora such as Helicobacter.13 In mouse models, several
additional species of bacteria were associated with cancer.14
Interestingly, the risk of different types of cancer could be increased
or decreased in germ-free or antibiotic-treated mice.14
