Healthcare Hygiene magazine December 2019 | Page 14

cover story
The Hand Microbiome:
Bacterial Diversity, and
Defining ‘Clean’ in
Hand Hygiene
By Kelly M. Pyrek
T
he average adult has about 22
square feet of skin. As the human
This is the third
body’s largest organ, skin’s expansive
in a series
real estate allows for numerous micro-
of articles
organisms to reside and multiply on the
examining how
epidermis. Researchers have established
“clean” is being
that microorganisms can survive for a
defined in the
range of hours to months on surfaces
healthcare
without any kind of intervention, and
environment.
the skin is no different. Studies indicate
that microbes can flourish on hands for hours, and they can
number as low as 2 million to as high as 10 million or more
on fingertips, hands, forearms and elbows.
Specifically, total aerobic bacterial counts can reach 1 ×
104 CFU/cm2 on the forearm, while total bacterial counts
on the hands of healthcare workers have ranged from 3.9
× 104 to 4.6 × 106 CFU/cm2. Fingertip contamination
ranged from 0 to 300 CFU when sampled by agar contact
methods. Investigators documented that although the
count of transient and resident flora varies considerably
among individuals, it is often relatively constant for any
given individual.
Skin can be understood as its own “ecosystem” according
to Fredricks (2001), who explains, “There are multiple
niches within the ecosystem of the skin. The axilla may be
as different from the trunk as a tropical rain forest is from
a desert. The various regions of the skin are noted to have
different populations of microbial inhabitants, reflecting
their different niches. Colony counts of aerobic bacteria
from moist areas such as the axilla or toe web spaces can
reach 107 bacteria per cm2, whereas dry areas such as the
forearm or trunk may harbor 102 or fewer bacteria per
cm2. Anaerobic bacteria are also present on human skin,
with colony counts up to 106 bacteria per cm2. In addition,
skin structures within a specific skin zone may harbor unique
microbes. The stratum corneum, cellular layer, hair shaft and
follicle, eccrine, apocrine, and sebaceous glands may each
have associated microflora.”
The hands carry a mix of good bacteria along with
potentially pathogenic microorganisms, so the importance
of washing one’s hands in the clinical setting cannot be
overstated, but let’s examine the issue from a microbiological
perspective, to gain a better understanding of exactly what
is on our hands in the first place.
The Hand Microbiome
Before the microbiome became the center of the health
NOTE
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data universe, Fierer, et al. (2008) studied the influence of sex,
handedness, and washing on the diversity of hand-surface
bacteria. The researchers examined the palm surfaces of the
dominant and nondominant hands of 51 healthy young-adult
volunteers to characterize bacterial diversity on hands and
to assess its variability within and between individuals.
By using a novel pyrosequencing-based method, they
found that the diversity of skin-associated bacterial commu-
nities was surprisingly high; a typical hand surface harbored
more than 150 unique species-level bacterial phylotypes, and
the researchers identified a 4,742 unique phylotypes across
all of the hands examined. “Although there was a core set
of bacterial taxa commonly found on the palm surface, we
observed pronounced intra- and interpersonal variation in
bacterial community composition; hands from the same
individual shared only 17 percent of their phylotypes, with
different individuals sharing only 13 percent. Women had
significantly higher diversity than men, and community
composition was significantly affected by handedness, time
since last handwashing, and an individual’s sex. The variation
within and between individuals in microbial ecology illustrated
by this study emphasizes the challenges inherent in defining
what constitutes a ‘healthy’ bacterial community.”
Fierer, et al. (2008) explain that the skin is “a body habitat
with complex regional variations in cellular architecture and
environmental exposures, where bacterial density may be
as high as 107 cells per square centimeter. Many of these
bacteria are not simply passive or transient colonizers of
the skin surface, but rather appear to be adapted to the
specific rigors associated with living in different regions of
the skin including frequent skin shedding, antimicrobial host
defenses, exposure to soaps and detergents during washing,
exposure to UV radiation, and low moisture availability.”
Researchers have found, through culture-based and
molecular approaches, that there may be a core set of
bacterial taxa commonly found on skin surfaces, but there is a
significant amount of variability in the composition of skin-as-
sociated bacterial communities that is not well understood.
Fierer, et al. (2008) selected to study the palms because
they characterized them as “likely one of the more dynamic
skin microbial habitats, given the nearly constant and varied
exposure to environmental surfaces and the frequency
of perturbations caused by handwashing. In addition,
pathogens may inhabit the palmar surface, and efforts to
reduce disease transmission by handwashing are a key public
health concern.”
The researchers note, “The average palm surface
harbors >150 distinct species-level bacterial phylotypes.
Not surprisingly, this number of unique phylotypes exceeds
the number of bacterial types typically cultivated from the
skin surface by at least an order of magnitude 8, confirming
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