Bronchitis can be caused by bacteria and in many cases where bacteria are present, is caused by both a virus and bacteria especially following an upper respiratory tract infection. 
The Odds Are… It’s Viral
Bronchitis is predominantly a viral infection.
Unless your physician has diagnosed your bronchitis as bacterial, the overwhelming odds are that your bronchitis is caused by a virus. Viral bronchitis accounts for up to 95% of all bronchitis cases. The remaining 5% or so of acute bronchitis cases that are not associated with viruses are divided among a variety of factors such as bacterial infections, toxic chemical exposure, and gastric reflux. 
Viral or Bacterial Bronchitis?
Most incidence of bronchitis are caused by the same viruses that cause a cold or the flu. Bronchitis caused by a virus will generally follow a cold or flu-like illness. The color of sputum is not always an indicator of the type of bronchitis but often in the case of viral bronchitis, the sputum is clear or white and may accompany a mild fever. In the case of bacterial bronchitis, the sputum may appear yellowish or green and may come on abruptly with a fever and chest pain. 
Antibiotics and Bacterial Bronchitis
Antibiotics are ineffective against viral infections such as those that cause most bronchitis conditions.** Because the body builds up a resistance to antibiotics, your physician will not prescribe them unless a bacterial infection is the suspected cause of your bronchitis.
If bacterial bronchitis is suspected, your physician can run blood or sputum tests to diagnose the condition.
 “Bronchitis – Cedars-Sinai.” Cedars-Sinai – A Non-Profit Hospital in Los Angeles . N.p., n.d. Web. 1 Mar. 2012. <http://www.cedars-sinai.edu/Patients/Health-Conditions/Bronchitis.aspx>.
 “Bronchitis .” WESTERN NEW MEXICO UNIVERSITY. N.p., n.d. Web. 1 Mar. 2012. http://www.wnmu.edu/admin/studentaff/Health/aliments/Bronchitis.htm
 HUESTON, M.D, WILLIAM J., and ARCH G. MAINOUS III, PH.D. “Acute Bronchitis.” American Academy of Family Physicians 57.6 (1998): 1270-1276. Print.