Brass and copper doorknobs spread far fewer germs than steel or aluminum ones because they naturally disinfect themselves!
Several different metals have been shown to display an antimicrobial capacity called the oligodynamic effect. These include ions of mercury, silver, copper, lead, zinc, bismuth, gold, aluminum, and alloys such as bronze and brass. Objects made of silver and copper are most toxic to bacteria, which is why they are commonly included in mineral sanitizers for swimming pools.
All you germaphobes out there should be encouraged by the fact that you don’t need to avoid knobs containing copper - pure copper handles have been shown to kill off some organisms as quickly as just 15 minutes, and brass (a copper alloy) will be completely germ-free in seven hours or less!