الفهرس 
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Abstract
The aim of the present work was to study the antimicrobial activity of colistin, a member of the polymyxins family which was internationally revisited in spite of being an old antibiotic, factors likely to affect, its mechanism otact,on and possible mechanisms of resistance.
Sixty three bacterial strains and isolates obtained from difiereni sources were used in this study. They were identified by classical microscopical and biochemical methods as follows: tO A. baumannil. 26 P. aerugmosa. I 5 E. coil and 8 Kiebsielia species. Out of the 59 collected isolates. 26 were PDR, being resistant to anti-pseudomonal penicillins. cephalosporins. arninoglycosides. carbapenems. monobactams and quinolones but sensitive to colistin and polymyxin B. These PDR isolates were classified as: tO A. haumainjil. 10 P. aeruginosa, 3 E. coil and 3 Klebsiella species upon testing their susceptibility by the disc diffusion technique. These PDR isolates showed resistance to most antibiotic classes except polymyxins.
The bacteriostatic activity of colistin and polymyxin 13 was investigated through MIC determination against the PDR clinical isolates. The MIC range ofcolistin and polymyxin B against the PDR clinical isolates lied between 0.625-5 p.g/rnl. All the PDR isolates under investigation were considered sensitive to polyrnyxins as their MIC did not exceed the resistance breakpoint.
The bactericidal activity of colistin was investigated by the viable count technique against 10 PDR clinical isolates using two concentrations of colistin which are V2 MIC and MIC. Colistin was rapid bactericidal in a concentration dependent manner. Rapid and significant declines in bacterial bioburden were observed after 3 hr in most of the tested PDR clinical isolates. 1-lowever, re-growth was observed as early as 6 hr and substantial regrowth occurred at 24 hr in all the tested PDR clinical isolates at ‘/2 MIC and most of them atMIC.
1-leteroresistance phenomenon; which may be defined as a phenotypic manifestation of resistance within an apparently genetically homogeneous strain; was studied by selecting different colonies belonging to the same organism and inoculating them on agar plates containing different concentrations of colistin. Those which grow on higher colistin concentrations are the heteroresistant populations. Some of the selected colonies were able to grow on agar plates containing up to 10 MIC corresponding to each tested isolate. It was found that when the isolates were passed first in V2 MIC of colistin (induced resistance); the number of colonies which were able to grow in the presence of higher colistin concentrations was higher.