الفهرس 
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Abstract
This study was done during the period between January 2018 and June 2020, a totally 1118 different samples were obtained from different clinical sources included urine, blood, sputum, and surgical wound of patients admitted at Minia University, Azhar Assuit University and Al-Hussien hospitals, Egypt. Samples were examined by Gram stain, cultivated on Blood and MacConkey agar. Isolates were identified by standard microbiological techniques in the Medical Microbiology and Immunology Department, Faculty of Medicine, Minia University.
Antimicrobial susceptibility testing was performed for all Enterobacteriaceae isolates by disc diffusion against seven different antibiotics and colistin susceptibility was further detected by BMD and AD methods.
Molecular detection of colistin resistance mechanisms either chromosomal or plasmid mediated was done through conventional PCR for plasmid mcr genes, amplification then sequencing analysis of chromosomal mgrB, pmrB and phoQ genes, and comparison between gene expression of resistant and sensitive isolates.
Out of 1118 samples, 509 Enterobacteriaceae isolates represented 45.5% were detected. Colistin resistance was detected in 38 isolates (8.4) distributed among E. coli and K. pneumoniae isolates only. MDR were detected in 50.4% and 44.1% of total E. coli and K. pneumoniae isolates, respectively, while detected in 90% of ColREC and 100% ColRKP.
Mcr genes couldn’t be detected in any colistin resistant isolates while mutations in chromosomal genes were detected.
PmrB gene mutations were detected in colistin resistant K. pneumoniae isolates and phoQ mutation was associated with high level colistin resistance. In E. coli, mgrB mutation were detected in resistant as well as sensitive isolates but phoQ mutations were detected in some colistin resistant isolates and associated with high level colistin resistance.
Overexpression of phoQ gene in E. coli and pmrB gene in K. pneumoniae was detected in colistin resistant isolates in comparison to sensitive isolates, with positive correlations between mutations in these genes and MIC of colistin. Also, there were positive correlation between expression levels of phoP and phoQ, phoP and, and pmrB and phoQ that pointed to incidence of cross-regulatory interaction between PhoP/PhoQ and PmrA/PmrB systems.