LAS VEGAS -- Officials said a tower of the Tropicana Casino and Resort is expected to reopen Thursday, after it was closed to investigate the discovery of E.coli in a water supply.

The Southern Nevada Health District confirmed Wednesday that the 800-room Island Tower was closed Monday after traces of the bacteria were found during a routine check of a supplemental well used for the tower.

About 490 rooms were affected, and those guests were moved to other rooms at the Las Vegas Strip property, to the company’s Westin Casuarina Las Vegas Hotel Casino and Spa, or to other hotels at Tropicana Entertainment’s expense.

Tropicana spokesman Hud Englehart said there were no reports of illness from any customers or employees using the water in the tower.

Officials said two follow-up tests have come back negative for the bacteria, which can cause severe stomach cramps, diarrhea, vomiting and fever. Most people recover within five to seven days.

The hotel owner sought a third test of the entire property’s water system before the tower reopens Thursday evening.