Teeth staining and bacteria
Stains on teeth can be divided into two groups: External (extrinsic) stains – seen on the surface of the teeth and are caused by an accumulation of dark-colored foods and drinks (coffee, red wine…), tobacco and by chromogenic bacteria found in dental plaque
Intrinsic stains (internal) – can be caused by many factors such as the use of tetracycline, vitamins with iron, high doses of fluoride in the natural water supply etc. Discolored or dark tooth may be a sign of previous trauma to your child's tooth
Chromogenic bacteria are believed to be responsible for brown, black, green, and orange stains
Brown and black stains are typically seen along the gum line of the teeth, either as a thin line or as a broad band. The bacteria, residing on the teeth as part of dental plaque, produce hydrogen sulfide which reacts with iron in the saliva to form a bacterial plaque that is usually black or has black discoloration to it
Unfortunately reducing iron intake will have virtually no effect on the formation of the stains. The amount of iron in human saliva is very low to start with and one can't reduce .it low enough without causing iron deficiency. One good thing about the phenomenon is that these patients seem to develop few cavities
The green stain can be challenging to remove and is generally found on upper front teeth
The orange or orange-yellow stains are usually close to the gum tissue. These stains are easily removed
How can one treat it
Because the stain is most likely due to the bacteria present in plaque, good home care of brushing, flossing, use of mouth wash in addition to a professional cleaning should remove the stains ,but they do tend to return
http://www.pediatricdentistsf.com/faqs_firstvisit_stains.php
Yours with a smile
Dr. Varda 
ד"ר ורדה צ'יוקליה-שור 