SUN-TIMES MEDIA July 25, 2012 11:40AM

Updated: July 25, 2012 8:16PM

Evanston health officials have closed all five city public beaches to swimming for Wednesday as a result of high bacterial counts, the city said.

The city tests on the water from the five beaches, one dog beach and Northwestern Universitys beach on a daily basis. The samples take 18 hours in an incubator to test. When E. coli counts exceed 235 chloroform forming units, an indicator of bacterial colonies in the water, the Evanston Health Department closes the beach.

Sample results for Wednesday indicated a count of 238 to about 800 CFU at Evanston beaches, the city said.

Factors such as natural die-off, wind and wave action, and ultraviolet light from the sun will help to reduce the level of bacteria. The length of time this takes is unpredictable, but it is usually less than 24 hours, the city said.

The water needs to be resampled before the beaches will be allowed to reopen.

Residents who want to check beach status for Evanston beaches in advance have several options. They can call the lakefront operations recorded message at (847) 859-7822 or check the citys beach status page at http://www.cityofevanston.org. Starting this season, beach patrons also are able to sign up for text or email alerts from that page.

Escherichia coli (E. coli) bacteria live in the digestive systems of humans and other warm-blooded animals, so they are found in sewage and other wastewater. Most strains are not harmful, but some are, and they can indicate the presence of other disease-causing bacteria, the city said.

E. coli can cause gastroenteritis-type illnesses that come with symptoms such as diarrhea, nausea, vomiting, abdominal pain, headache and low-grade fever. Skin rashes and earaches also may be experienced.

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On sweltering day, high bacteria levels close all Evanston beaches