LAS VEGAS, Jan. 29, 2013 /PRNewswire/ -- Fifteen resorts operated by MGM Resorts International (NYSE: MGM) have achieved the prestigious Green Key rating for environmental conservation from the largest sustainable operations certification body in the world. The accomplishment peaks MGM Resorts' previous achievement of 12 properties designated with "Green Key" honors. Since a previous assessment in 2010, Bellagio has risen in rank from "4 Green Keys" to "5 Green Keys" by updating energy efficient equipment and initiating new maintenance programs. Bellagio has also implemented new operating policies and procedures, and enhanced its recycling efforts and sustainable food and beverage practices. Circus Circus Las Vegas rose from "3 Green Keys" to "4 Green Keys" in the most recent assessment. Three other properties - ARIA Resort & Casino, Vdara Hotel & Spa and Mandalay Bay Resort have maintained their "5 Green Keys" designation since 2010. Less than two percent of all current hotels that are rated have reached the elite "5 Keys" category. MGM Resorts properties that achieved a "4 Green Keys" rating during the recent assessment include: Luxor, Excalibur, New York-New York, Monte Carlo, The Mirage, Circus Circus Las Vegas, MGM Grand Las Vegas, The Signature at MGM Grand, MGM Grand Detroit, Beau Rivage in Biloxi, Miss. and Gold Strike in Tunica, Miss. This is the first-ever Green Key assessment conducted at The Signature at MGM Grand, Beau Rivage in Biloxi, Miss., and Gold Strike in Tunica, Miss. Green Key Global designates its ratings to hotels that exemplify the highest standards of environmental and social responsibility throughout their properties. Green Key has more than 2,900 member hotels and is the foremost "green" ranking, certification and audit program in North America. Green Key evaluates hotels on a scale of one to five Keys, with five Keys being the highest ranking. Results are based on a comprehensive evaluation of the hotel's sustainability efforts.