Minimum inhibitory concentration or MIC, is the lowest concentration of an antibiotic needed to inhibit the growth of a microbe after its incubation overnight. [1] Colistin is an antibiotic belonging to the polymyxin group of antibiotics, and is used to treat gram negative bacterial infections. [2] In this experiment we tested the antibiotic Colistin at several concentrations against Escherichia coli. This was done to find out the minimum inhibitory concentration (MIC) needed for Colistin to work effectively as a bactericidal. The postulated MIC for Colistin was 2.0 micrograms per milliliter. To test this postulated MIC, several serial dilutions were used to plate specific concentrations of both the antibiotic and the bacteria. The data collected …show more content…
[2, 3] Chemically, colistin is a complex polypeptide antibiotic that targets the bacterial cell membrane as the site for its antimicrobial actions. [3, 4] Specifically the mode of action for colistin is to bind itself to the lipopolysaccharides (LPS) that are present in gram negative bacterium. [5, 6] Colistin associates itself with the cell membrane of its target bacteria through electrostatic interactions; the positively charged polypeptide of colistin binds to the negatively charged LPS molecules found on gram-negative bacteria. Colistin then causes the displacement of cations in the phosphate groups of the membrane lipids. [2, 5, 6] The displacement of those specific cations destabilizes the outer cell membrane and leads to the leakage of the intracellular contents of the bacteria, thus bacterial death results. [5, 6] Of course the bactericidal effects of an antibiotic depends on its dosage for a given bacteria. For the antibiotic colistin research points to 2.0 μg/mL as the effective dosage for Escherichia coli, though the exact dosages for Colistin are not agreed upon …show more content…
[6] This nonpathogenic strain of E. coli was developed in lab for laboratory uses. [6] Although some strains of E. coli have antibiotic resistance to Colistin, the strain we used was not of concern. Acquired resistance to polymyxin has been observed in E. coli via the substitution of phosphate groups in LPS. [7, 8] For our purposes of finding a MIC experimentally, using the DH5Alpha was suitable as it is a very well developed strain that is efficient because of its mutations for general laboratory procedures. [6] Our experiment will look for the MIC of Colistin to understand whether or not it is actually 2.0 μg/mL when it works against the DH5Alpha strain of E coli at the lowest possible