Stress may make it easier for germs to infect skin wounds as researchers below proved.

Investigators created skin wounds in mice that were exposed to stressful living conditions. The researchers then applied Streptococcus bacteria to the wounds, and compared the healing rates of the stressed mice with those of mice with skin wounds that were also exposed to the bacteria but did not undergo the same levels of stress.

Mice that had been stressed out prior to wounding and infection showed a 30% delay in wound healing at 3 and 5 days compared with the mice that were not stressed, the report indicates.

In addition, the investigators found that after 5 days, the stressed mice had 100,000 times more opportunistic bacteria in their wounds than the non-stressed mice.

Seven days after the bacteria exposure, about 85% of the wounds in the stressed mice were infected, versus about 27% of the wounds in the non-stressed mice.

In this study, stress increased the rate of wound infection by threefold. Stress disrupts the body's equilibrium, in turn significantly impairing its ability to control and eradicate bacterial infection during wound healing.

The bottom line is that stress shuts down either the recruitment or the function of those immune cells needed to fight infection.

Research in humans has found that psychological stress can take a toll on the immune system by reducing the concentration of cytokines, proteins that help to ward off infections. In one study, skin wounds on the arms of women who had higher levels of the stress hormone cortisol had lower levels of key compounds released by the body to mediate healing.

Brain, Behavior, and Immunity online 2001;10.1006