Twenty seconds of scrubbing with soap is one of the best ways to protect yourself and the people and things you touch from disease-causing germs. But how exactly does soapy suds kill pathogenic bacteria and viruses that infect us?
Soap’s germ-zapping superpowers are built into its molecular structure: a “head” attached to a long “tail,” according to Dr. Lee Riley, physician, professor and chairman of the Division of Infectious Diseases and Vaccinology at the University of California. (UC Berkeley. The head is hydrophilic, or water loving, while the tail is hydrophobic – water fearing or water repelling. This hydrophobic tail has an affinity for fat, and all bacteria and some viruses – including SARS-CoV-2, coronavirus which causes the disease COVID-19 – have a lipid membrane, which makes it vulnerable to the fat puncture tail of a soap molecule.
“The tail fits into the [bacteria’s] lipid membrane, and that’s how it ends up being killed,” Riley told Live Science.
Some types of pathogens have very robust cell walls, so they can survive even after the hydrophobic tail of soap has penetrated their membrane. But even in these cases, soap molecules can defeat bacteria and virus by surrounding and isolating them.
When soap attacks these pathogens, the tails of the soap molecules attach to the lipid membrane of the cell, with the hydrophilic heads facing outward. This forms a tiny ball of soap molecules, known as a micelle, around the pathogen, Dr. John Swartzberg, physician, clinical professor emeritus and infectious disease expert at UC Berkeley, told Live Science. Bacteria or viruses are easily captured by the micelles because the exterior of the micelle is hydrophilic, so it is easily swept off your hands and down the drain – along with its pathogenic prisoners – when you rinse the soap off with water.
Related: Is it possible for something to be “germ-free”?
In 2010, researchers evaluated the soap’s effectiveness by having 20 volunteers contaminate their hands a total of 480 times with diarrhea-causing bacteria. Subjects were then randomly assigned to perform one of three actions: washing hands with soap, washing hands with water only, or not washing hands at all. Their hands were then tested for diarrhea-causing insects, the scientists reported in The International Journal of Environmental Research and Public Health (opens in a new tab).
In the group of subjects not washing their hands, the bacteria was present in 44% of the participants. Among those who washed their hands with water alone, bacteria were present in 23% of the study subjects. And in the group that washed their hands with soap and water, bacteria were detected in only 8% of people, according to the study.
Soap molecules are so effective in eliminating and annihilating germs on our hands that antibacterial soap is completely unnecessary and can be harmful; by driving the evolution of antibiotic-resistant strains of bacteria, Swartzberg said. And when we wash antibacterial soap down the drain, it can promote the rise of antibiotic-resistant bacteria in the local water supply.
On top of that, antibacterial soap kills all bacteria, even the good ones we depend on to stay healthy, Riley added.
However, one key element necessary for soap to do its job is often overlooked. And now is the time. It takes at least 20 seconds for the tail of the soap molecule to sufficiently bind to pathogens on your hands or another surface. Reducing that timeframe could mean missing out on the full protective effect of soap use, Swartzberg said.
Originally posted on Live Science.