Metronidazole is used to combat anaerobic bacteria and protozoa.  Once anaerobes take in this drug it is reduced and releases toxins which will kill the bacteria or protozoa.  This is useful in treating some bacterial overgrowths in the intestines.  Some believe that the root cause of Crohn's Disease is a bacteria, but there is no proof of this to date; antibiotics are useful in treating symptoms of Crohn's that include infection.

Ampicillin

Ampicillin is an antibiotic that works by preventing bacteria from dividing.  Ampicillin may be helpful in fighting bacteria that other drugs cannot and this is why it is useful to have a range of different types of antibiotics to use in the treatment of Crohn's symptoms.

Ciprofloxacin

Ciprofloxacin is an antibiotic that kills bacteria by interfering with enzymes necessary to cell division.  It is sometimes used in combination with Metronidazole to fight complex infections.