Ampicillin is one of the most valuable semisynthetic penicillins because whilst it retains the typical Gram – positive activity of benzyl -penicillins it possesses greatly enhanced activity against Gram – negative bacteria , so it can be described as a true bactericidal broad spectrum antibiotic.
Ampicillin is absorbed in the upper part of the intestine and excreted in a high concentrations in the bile, this means that ampicillin may be returned to the intestinal lumen through an enterohepatic circulation which makes it valuable in the treatment of enteric infections .
Colistin is a polypeptide antibiotic which is the least toxic of the five antibiotics known as Polymyxins .
they are narrow spectrum antibiotics and bactericidal in action against Gram – negative organisms
Colistin is poorly absorbed from the GI tract after oral administration, thus offering specific advantage in the treatment and control of GIT infections .
