(Reuters) - McDonald's Corp will offer its first-ever 100 percent organic beef hamburger for a limited time in Germany, as a growing number of global diners demand food that is more natural and less processed.

The move from the world's biggest restaurant chain by revenue comes as it is revamping food-sourcing practices as part of new Chief Executive Steve Easterbrook's effort to transform McDonald's into a "modern, progressive burger company."

From Oct. 1 to Nov. 18 McDonald's will offer "McB" burgers, made from organic beef sourced from organic farms in Germany and Austria.

"We have made a great effort to secure sufficient quantities of meat which satisfies the organic requirements and our own quality claims," said Holger Beeck, chief executive of McDonald's Germany.

Sales at McDonald's have slumped, in part due to competition from newer chains including Chipotle Mexican Grill Inc, which for years has offered meat from animals raised without hormones and antibiotics.

McDonald's USA said in March that within two years, it would stop buying meat from chickens raised with antibiotics vital to human health.

That move was cheered by public health and consumer advocates, who are concerned that overuse of antibiotics in meat production is contributing to rising numbers of life-threatening human infections from antibiotic-resistant bacteria dubbed "superbugs."

Still, a shareholder group thinks the company has not gone far enough. It is renewing its call for McDonald's to stop buying any meat from animals raised with antibiotics vital to fighting human infections.

Earlier this month, McDonald's said its 16,000 U.S. and Canadian restaurants would switch cage-free eggs by 2025.