Ampicillin is a broad spectrum bactericidal antibiotic belonging to beta lactam group .It produces it’s antimicrobial action by prevents cell wall synthesis by inhibiting bacterial trans peptidase & carboxypeptidase enzymes , causing an osmotic imbalance which destroys the bacteria in it’s growth phase .

Colistin is a polypeptide antibiotic with a bactericidal activity against G –ve bacteria. It acts like a cationic surfactant alternating the permeability of the cell membrane by combining with lipoprotein which makes the bacteria losses the nutrients.

The association is a synergistic and produces alternation is the bacterial metabolism and so induces death. Also it acts in reducing the bacterial endotoxins activity in the tissue fluids. The association is active against Gram + ve bacteria such as : Actinomyces spp , Bacillus anthracis , Clostridium spp ., Corynebacterium spp ., Erisipelothrix rhusiopathiae, listeria monocytogene , Staphylococcus spp . & Streptococcus spp. And Gram – ve bacteria such as: Actinobaccilus spp, Bordetella bronchiseptica, E. coli, Proteus mirabilis, Pastuerella spp, Klebsiella pneumonia, Pseudomonas aeruginosa, Salmonella spp. Shigella spp., Homophiles spp. indication: it is used for treatment of diarrhea caused by susceptible organisms such as salmonellosis caused by Salmonella spp . And Colibacillosis caused by e – coli.