Standard & Poor’s Ratings Service said Thursday it put Whole Foods Market Inc., which runs natural foods grocery stores, on credit watch with negative implications.

Specifically, Standard and Poor’s may adjust the company’s current “BBB-” corporate credit rating. The credit agency made the decision after Whole Foods announced plans to buy Wild Oats Market for about $565 million and take on $106 million in debt. “BBB-” is the agency’s lowest investment-grade rating.

In a statement, Standard & Poor’s said the acquisition “will be financed with additional debt, which may result in a material change in credit metrics.”

Shares of Whole Foods Market Inc. rose $6.04, or 13.2 percent, Thursday to close at $51.74 on the Nasdaq Stock Market.

Copyright 2006 Associated Press.