Healthcare Hygiene magazine December 2019 | Page 16

that culture-based surveys of the skin surface, like surveys
conducted in many other microbial habitats, dramatically
underestimate the full extent of bacterial diversity. The
average phylotype richness observed on a single palm surface
was also more than three times higher than the richness
observed in a molecular survey of forearm skin and elbow
skin. Although we would expect the hand surface to have
higher levels of diversity than other skin surfaces because of
the more frequent contact with potential inocula from the
environment, this discrepancy in observed bacterial diversity
is more likely a result of the depth of our sampling, which
allowed us to survey even those rare bacterial taxa present
on the skin surface.”
The researchers add that the total diversity of bacteria
on the hand surface appears to meet or exceed the levels of
bacterial diversity found in other human-associated microbial
habitats, including the mouth and at specific sites within
the lower intestine.
Three phyla (Actinobacteria, Firmicutes, and Proteobacte-
ria) accounted for 94 percent of the sequences, report Fierer,
et al. (2008). The most abundant genera (Proprionibacterium,
31.6 percent of all sequences; Streptococcus, 17.2 percent;
Staphylococcus, 8.3 percent; Corynebacterium, 4.3 percent;
and Lactobacillus, 3.1 percent) were found on nearly all
palm surfaces sampled. According to the researchers, these
genera have previously been found to be abundant in other
molecular surveys of skin bacteria and are considered to be
common skin residents, yet they still represented less than
65 percent of all of the identified sequences. They observe
that the average palm surface has a large number of rare
taxa that may be either transient, short-term colonizers of
skin or more persistent, longer-term residents of the skin
surface that are simply present at relatively low abundances
or whose abundance is determined by specific characteristics
of individual hand surfaces.
The researchers report that qualitatively, the bacterial
communities found on the hand surfaces appear to be more
similar to the communities found on forearm skin than to
the communities found on the forehead or inner elbow,
suggesting that skin bacterial communities are not uniform
across the body and that skin surfaces closer in proximity
may harbor more similar bacterial communities.
What’s fascinating is that Fierer, et al. (2008) found that
handedness significantly influenced bacterial communities,
in that dominant hands have similar overall levels of diversity
as nondominant hands, but the composition of the bacterial
communities on the dominant and nondominant hands
from the same individual was significantly different. Taxa
with relative abundances more than 50 percent greater on
the dominant hand than the nondominant hand included
members of the Enterobacteriales, Lactobacillaceae, Pepto-
streptococcaceae, and Xanthomonadales groups.
“The influence of handedness on palm bacterial commu-
nities is likely due either to differences in skin environmental
conditions (e.g., sebum production, salinity, hydration) or
to the dominant hand coming into contact with different
types of environmental surfaces than the nondominant
hand,” say Fierer, et al. (2008). “Although dominant and
nondominant hands harbor distinct bacterial communities,
the communities on left and right hands from the same
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individual were more similar than we would expect by
chance. However, these communities still shared only 17
percent of their phylotypes on average, indicating that there
is an enormous amount of heterogeneity in skin bacterial
communities within an individual.”
Equally interesting is that men and women harbor
significantly different bacterial communities on their hand
surfaces, the researchers say. Taxa that were shared by
both men and women but were more abundant on the
skin of 1 sex included members of the following groups:
Proprionibacterium (37 percent more abundant on men),
Corynebacterium (80 percent more abundant on men),
Enterobacteriales (400 percent more abundant on women),
Moraxellaceae (180 percent more abundant on women),
Lactobacillaceae (340 percent more abundant on women),
and the Pseudomonadaceae (180 percent more abundant
on women).
The palms of women were also found to harbor signifi-
cantly greater bacterial diversity than those of men, whether
diversity was assessed by examining the overall phylogenetic
structure on each hand or the average number of phylotypes
per hand. The researchers observe, “We do not know what
drives these differences in overall diversity, but differences in
skin pH may be influential. Men generally have more acidic
skin than women, and work from other microbial habitats has
shown that microbial diversity is often lower in more acidic
environments. Other explanations for why men and women
appear to harbor distinct hand bacterial communities may
include differences in sweat or sebum production, frequency
of moisturizer or cosmetics application, skin thickness, or
hormone production.”
Clinicians take note – the time since the last handwashing
also had a significant effect on skin community composition,
Fierer, et al. (2008) emphasize: “Most notably, bacteria be-
longing to the Proprionibacteria, Neisseriales, Burkholderiales,
and Pasteurellaceae taxa were relatively more abundant with
time since last handwashing, whereas other bacteria in the
Staphylococcaceae, Streptococcaceae, and Lactobacillaceae
groups showed the opposite pattern and were relatively
more abundant on hands that had been recently washed.
Although handwashing altered community composition,
overall levels of bacterial diversity were unrelated to time since
last handwashing. Either the bacterial communities rapidly
reestablish after handwashing, or washing (as practiced by
the students included in this study) does not remove the
majority of the bacterial taxa found on the skin surface.”
Eight years after Fierer, et al. (2008) published their
research, Kumar, et al. (2019) assessed the ability of skin
microbes to physically interact (co-aggregate) intergenically.
The bacterial flora from the hands (palm area) of similar age
group students was isolated. The predominant isolates were
selected and identified using 16s rRNA gene sequencing.
A total of 27 bacteria were isolated from the skin (palm
area-fingers) of 10 individuals. The researchers report these
isolates belong to seven bacterial genera and 10 different
species; among 27 isolates, Staphylococcus haemolyticus had
highest co-aggregation partners of 17 followed by Acineto-
bacter spp. and Pseudomonas spp. with 15 partners each.
The study indicates that few microbes have high potential
to influence coaggregation among distinct genera isolated
december 2019 • www.healthcarehygienemagazine.com