The Hand Microbiome, Bacterial Diversity, and Defining ‘Clean’ in Hand Hygiene
By Kelly M. Pyrek
The average adult has about 22 square feet of skin. As the human body’s largest organ, skin’s expansive real estate allows for numerous microorganisms to reside and multiply on the epidermis. Researchers have established that microorganisms can survive for a range of hours to months on surfaces without any kind of intervention, and 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 range from more than 1 × 106 colony forming units (CFU)/cm2 on the scalp, 5 × 105 CFUs/cm2 in the axilla, and 4 × 104 CFU/cm2 on the abdomen to 1 × 104 CFU/cm2 on the forearm. Total bacterial counts on the hands of HCWs 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
Through the wonders of microbial sequencing and typing, there may come a time when it will be possible to tell from the bacteria on an object which individuals have touched it. For example, Seong, et al. 2017 emphasized the significance of bacterial flora found on the surface of the skin and its application in personal identification. In particular, hand surface can be an environment of primary interest, as it harbors higher level of microbial diversity than other parts of the skin. In this study, diversity of microbial communities inhabiting the palms of different individuals was explored using culture-based methods A total of 686 bacterial strains were obtained and identified based on 16S rRNA gene sequence analysis. Among the isolates, the genera Staphylococcus, Micrococcus and Enhydrobacter were recovered as major taxa. Twenty strains could be recognized as candidates for novel species, as the 16S rRNA gene sequence similarities with known species were around 97 percent. Variations in the bacterial composition among individuals could be observed, the researchers found, and say their data confirmed a high potential of palm microbial flora in the forensic application for personal identification.