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Background  We evaluated the distribution of carbapenem and colistin resistance mechanisms of clinical  E. coli  and  K. pneumoniae  isolates from Iran.    Methods  165 non-duplicate non-consecutive isolates of  K. pneumoniae  and  E. coli  were collected from hospitalized patients admitted to Iran's tertiary care hospitals from September 2016 to August 2018. The isolates were cultured from different clinical specimens, including wound, urine, blood, and tracheal aspirates. Antibiotic susceptibility testing was performed by disc diffusion and microdilution method according to the Clinical and Laboratory Standards Institute (CLSI) guideline. The presence of extended spectrum β-lactamases (ESBLs) genes, carbapenemase genes, as well as fosfomycin resistance genes, and colistin resistance genes was also examined by PCR-sequencing. The ability of biofilm formation was assessed with crystal violet staining method. The expression of colistin resistance genes were measured by quantitative reverse transcription-PCR (RT-qPCR) analysis to evaluate the association between gene upregulation and colistin resistance. Genotyping was performed using the multi-locus sequencing typing (MLST).    Results  Colistin and tigecycline were the most effective antimicrobial agents with 90.3% and 82.4% susceptibility. Notably, 16 (9.7%) isolates showed resistance to colistin. Overall, 33 (20%), 31 (18.8%), and 95 (57.6%) isolates were categorized as strong, moderate, and weak biofilm-producer, respectively. Additionally,  bla  TEM ,  bla  SHV ,  bla  CTX-M ,  bla  NDM-1 ,  bla  OXA-48-like  and  bla  NDM-6  resistance genes were detected in 98 (59.4%), 54 (32.7%), 77 (46.7%), 3 (1.8%), 17 (10.30%) and 3 (1.8%) isolates, respectively. Inactivation of  mgrB  gene due to nonsense mutations and insertion of IS elements was observed in 6 colistin resistant isolates. Colistin resistance was found to be linked to upregulation of  pmrA - C, pmrK, phoP , and  phoQ  genes. Three of  bla  NDM-1  and 3 of  bla  NDM-6  variants were found to be carried by IncL/M and IncF plasmid, respectively. MLST revealed that  bla  NDM  positive isolates were clonally related and belonged to three distinct clonal complexes, including ST147, ST15 and ST3299.    Conclusions  The large-scale surveillance and effective infection control measures are also urgently needed to prevent the outbreak of diverse carbapenem- and colistin-resistant isolates in the future.

annals of clinical microbiology and antimicrobials

Molecular characteristics of antibiotic-resistant Escherichia coli and Klebsiella pneumoniae strains isolated from hospitalized patients in Tehran, Iran