BOSTON – Researchers observed a direct relationship between bacterial load on the hands and the risk of cross-transmission following hand-to-hand contact.

The researchers noted that at least 1 log10 of Esherichia coli must be present for transmission to occur.

A research team lead by Didier Pittet, MD, MS, director of the Infection Control Programme and World Health Organization (WHO) Collaborating Centre on Patient Safety at the University of Geneva Hospitals, asked six healthcare workers with different hand sizes to serve as either “contaminator” or “host” in a series of experiments.

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“One important issue derived from our study is that it sets a target that may be useful for the norms evaluating hand hygiene products, and also for future studies comparing different hand hygiene products,” said Fernando Bellissimo-Rodrigues, MD, PhD, a research fellow at the University of Geneva Hospitals and one of the study’s co-authors. Dr Bellissimo-Rodrigues presented the results during ASM 2016.

Contaminators’ hands were contaminated in suspensions of inoculum of E coli ATCC 10536 ranging from 103 cfu/mL to 106 cfu/mL. One hand of the contaminator was then cultured using the “fingertips” method while the other held the host’s hand for 1 minute with fingers interlocked.

The researchers then took cultures from the hosts’ fingertips to assess the degree of cross-transmission.

Cross-contamination risk was greatest at the highest inoculum levels (P<.001). After adjusting for gender and hand size, risk for cross-transmission increased and by 46.8-fold when inoculum was <4 log10 compared with inoculum <3 log10, Researchers said the risk increased 4.4-fold when the inoculum ranged 3 log10 to 3.5 log10 and 16.4-fold when it ranged 3.5 log10 to 4 log10,


1. Bellissimo-Rodrigues F, Soule H, Pires D, Gayet-Ageron A, Pittet D. Assessing the Risk of Hand-to-hand Transmission of Bacteria. Presented at: ASM Microbe 2016. June 16-20, 2016; Boston, MA.