Hilo Bay after a rain storm becomes a petri dish of infectious bacteria, University of Hawaii scientists have found.

The warning to swimmers and surfers is clear: If the bay waters are brown or murky, stay out.

Their findings appear in the Oct. 17 issue of the Journal of Environmental Quality.

The scientists tested specifically for Staphylococcus aureus, one of the species that causes staph infections, and fecal indicator bacteria.

Both increased with rainfall and river discharges and were present within Hilo Bay and its rivers, as well as road runoff, sewage and soil, they found.

Of particular concern is methicillin-resistant Staphylococcus aureus or MRSA, which is resistant to common antibiotics like penicillin. MRSA was also found in the bay, but the scientists did not find that its concentrations increased with rainfall.

Tracy Wiegner, a marine science professor at UH-Hilo and one of the study’s authors, said Hawaii has the highest rate of community-acquired staph infections in the country.

“It’s two times the rate of the rest of the U.S.,” Wiegner said in a statement. “That may be because it’s warmer here or because people are in the water more.”

The other authors in the study are Louise Economy, with the state Department of Health; Ayron Strauch, a hydrologist with the Department of Land and Natural Resources; Jonathan Awaya, a biology professor at UH-Hilo; and Tyler Gerken, now a graduate research assistant at the University of Washington.

The group is hoping their research will help further efforts to predict water quality conditions based on rainfall to help advise people of possible health risks when going in the water.

“We are trying to develop real-time models using the water quality buoys, river discharge gauges and rainfall data to be able to make real time predictions,” said Wiegner. “The idea is that you could look at your phone and see what your risk is before going in the water.”

Economy called staph “an opportunistic pathogenic bacterium, meaning that given the right conditions it can cause disease,” adding, “It can invade wounds and cause boils, rashes and even flesh- eating disease. These infections are becoming more and more common in the community and affecting people who were previously healthy.”

Wiegner said, “A good rule of thumb for recreational water users is if the water is brown, turn around. You don’t want to go in with open cuts, and if you do go in, you should always rinse off.”