McDonald's posted a steeper-than-expected drop in quarterly global sales, hurt by muted demand across key markets, including Europe and the United States where it is expected to face more weakness as the burger giant reels from a deadly E. coli outbreak. Shares of the company were down 2.4 percent before the bell on Tuesday even as it beat profit estimates. Global sales fell 1.5 percent in the third quarter, the biggest decline in four years, compared with analysts' average estimate of