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. 2008 Nov 18;105(46):17994-9.
doi: 10.1073/pnas.0807920105. Epub 2008 Nov 12.

The influence of sex, handedness, and washing on the diversity of hand surface bacteria

Noah Fierer  1 , Micah Hamady, Christian L Lauber, Rob Knight
Affiliations

The influence of sex, handedness, and washing on the diversity of hand surface bacteria

Noah Fierer et al. Proc Natl Acad Sci U S A. .

Abstract

Bacteria thrive on and within the human body. One of the largest human-associated microbial habitats is the skin surface, which harbors large numbers of bacteria that can have important effects on health. We examined the palmar 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. We used a novel pyrosequencing-based method that allowed us to survey hand surface bacterial communities at an unprecedented level of detail. The diversity of skin-associated bacterial communities was surprisingly high; a typical hand surface harbored >150 unique species-level bacterial phylotypes, and we identified a total of 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% of their phylotypes, with different individuals sharing only 13%. Women had significantly higher diversity than men, and community composition was significantly affected by handedness, time since last hand washing, 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; addressing these challenges will be critical for the International Human Microbiome Project.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Relative abundances of the most abundant bacterial groups on the hand surfaces, with the hand samples divided into categories of sex (A), time since last hand washing (B), and the dominant versus the nondominant hand (C). Error bars are 1 standard error of the mean. For the number of sequences and number of samples included in each category and the full taxonomic description of the hand surface bacterial communities see Table S1. Superscripts on the taxon name indicate the phylum or subphylum: 1, Actinobacteria; 2, Firmicutes; 3, Betaproteobacteria; 4, Gammaproteobacteria; 5, Alphaproteobacteria; 6, Bacteroidetes.
Fig. 2.
Fig. 2.
Average pairwise bacterial community similarity between left and right hands from the same individual (circles) and between hands from different individuals (squares) as measured by using the unweighted UniFrac similarity index (bottom axis, open symbols) or the percentage of phylotypes that are shared between pairs (top axis, filled symbols). Average pairwise values and 95% confidence intervals are shown. For these analyses, 2,500 sequences were randomly selected per sample, and only those samples represented by >2,500 sequences were included (n = 51 and 5,100 pairwise comparisons for intraindividual comparison and interindividual comparisons, respectively).
Fig. 3.
Fig. 3.
Differentiation in hand-surface communities between sexes (A), dominant versus the nondominant hands (B), time since last hand washing (C), and time since last hand washing for each sex (D) determined by using the unweighted UniFrac algorithm. The length of the branches corresponds to the degree of differentiation between bacterial communities in each category. All of the branch nodes shown here were found to be significant (P < 0.001), indicating that each of these categories harbored distinct bacterial communities.
Fig. 4.
Fig. 4.
Rarefaction curves showing differences in bacterial diversity on palm surfaces from men and women. (A) Phylogenetic diversity estimated by measuring the average total branch length per sample after a specified number of individual sequences have been observed (36). (B) Diversity estimated by determining the average number of unique phylotypes per hand. For these analyses, we randomly selected 2,400 sequences per hand sample, and thus the average number of phylotypes per hand is lower than for the full dataset (Table 1). Bars indicate 95% confidence intervals.
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