There are many possible causes for metallic, bitter and/or sour tastes in your mouth including particular medications, allergies, vitamin deficiencies and even diseases like diabetes and hyperparathyroidism. However, it's quite possible that the metallic taste in your mouth is caused by the same bacteria that cause bad breath.
Bitter, metallic and sour tastes tend to be acidic in nature. The bacteria that cause bad breath produce sulfide molecules particularly at the back of your tongue which is the area that is susceptible to sour, bitter and metallic tastes. These bacteria thrive in an acidic environment and the sulfides they produce are weak acids which, of course, help to perpetuate the acidic environment in your mouth. This acidic cycle leads to bad breath and can also lead to a metallic taste in your mouth.
For those who may have an acidic oral environment, here's a helpful hint:
Sprinkle a small amount of baking soda on the bristles of your dry toothbrush. Then, place toothpaste over the baking soda to cover the bristles - then brush. The baking soda neutralizes more of the oral acids and creates a cleaner taste sensation.