Q. Does each member of the family need an individual bar of soap to prevent spreading germs, or do we have to switch to liquid soap?

A. Probably not, especially if the bar gets rinsed off between uses. A study published in 1988 in the journal Epidemiology and Infection concluded that washing even with contaminated bar soap is unlikely to transfer bacteria.

The study, done at the Dial Corporation Technical Center, said that while micro-organisms had recently been isolated from used soap bars, the findings of a low risk of cross-contamination were in agreement with two studies using different methodology.

In the 1988 study, prewashed and softened commercial deodorant soap bars were inoculated with E. coli and P. aeruginosa bacteria at levels 70 times as high as those reported on used soap bars. Then, 16 people were told to wash their hands as usual with the inoculated bars.

“After washing, none of the 16 panelists had detectable levels of either test bacterium on their hands,” the researchers wrote. “These findings, along with other published reports, show that little hazard exists in routine handwashing with previously used soap bars and support the frequent use of soap and water for handwashing.”

Original Source

https://www.nytimes.com/2007/07/10/science/10qna.html