الفهرس 
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Abstract
T
he carbapenemase-producing Enterobacteriaceae are among the most clinically significant multidrug-resistant bacteria. Its rise has led to an increase in the use of the polymixin B and colistin because of limited effective alternative antibiotic treatment.
Because these bacteria usually remain susceptible to polymyxins, an old class of antimicrobial drugs almost abandoned in the 1970s because of their potential toxicity, interest in polymyxins (colistin and polymyxin B) has been renewed worldwide.
The increasing use of colistin explains why acquired colistin resistance may now be added to the multi resistant traits in Enterobacteriaceae.
Acquired resistance to colistin in Enterobacteriaceae results mostly from modification of lipopolysaccharide. Addition of phospho-ethanolamine, 4-amino-l-arabinose cationic groups, or both to lipopolysaccharide decreases polymyxin binding to the bacterial outer membrane. Addition of these groups may be associated with chromosome encoded mechanisms (mutations in Pmr AB or PhoPQ two-component systems or alterations of the mgr B gene).
Rapid identification of polymyxins resistant Enterobacteriaceae is extremely important for timely detection, treatment and implementation of infection control measures to prevent their spread. Many phenotypic and molecular commercially available and in-house laboratory tests have been described for detection of polymyxins resistant Enterobacteriaceae.
Polymyxin NP test is based on carbohydrate metabolism. Acid formation associated with carbohy¬drate metabolism in Enterobacteriaceae can be observed through the color change of a pH indicator from red to yellow.
In this study we aimed to Evaluate the diagnostic efficacy of the rapid polymyxin NP test for the diagnosis of colistin-resistant Enterobacteriacae and Determine the prevalence of mcr-1 and mcr- 2 genes among the colistin-resistant Enterobacteriacae.
During this study, forty two Enterobacteriaceae isolates collected from different clinical samples submitted for routine culture and sensitivity in Central Microbiology Laboratory Ain Shams University Hospitals were included.
Among the forty two tested isolates by polymyxin NP test, 26 isolates were sensitive (26/42, 61.9%), including 22 Klebsiella isolates (22/26, 84.6%), and four E.coli isolates (4/26, 15.3%).While among the sixteen resistant isolates, fourteen Klebsiella isolates (14/16, 87.5%) and two E.coli isolates (2/16, 12.5%) were resistant as shown in table (7).
Sixteen 16/42 (38.1%) isolates were positive for polymyxin NP test. 18 isolates out of 42 were resistant to colistin (42.9%) by colistin MIC broth microdilution method and 15 isolates out of 42 were resistant to polymyxin (35.7%) by polymyxin MIC broth microdilution. Among the 42 tested isolates for MCR 1 gene by PCR, 17 isolates were positive seventeen isolates were positive for MCR-1 gene (17/42, 40.5%), while 5 isolates were positive for MCR 2 gene by PCR (5/42, 11.9%).
Polymyxin NP test showed an overall sensitivity, specificity, PPV and NPV compared to MIC brothmicrodilution for colistin of 77.8%, 91.7%, 87.5% and 84.6 % respectively.
Polymyxin NP test showed an overall sensitivity, specificity, PPV and NPV compared to MIC brothmicrodilution for polymyxin B of 93.3%, 92.6%, 87.5% and 96.2 % respectively.
In conclusion the rapid polymyxin NP test is easy-to-perform, rapid, sen¬sitive, and specific. It detects polymyxin-resistant and -sus¬ceptible isolates from any Enterobacteriacea species, regard¬less of the molecular mechanism of polymyxin resistance. This test offers the possibility of detecting polymyxin resis¬tance from bacterial cultures from infected samples before any antimicrobial drug susceptibility testing results are obtained. Results are obtained at least 16 h sooner with this test than with the reference broth micro¬dilution method. This test is as reliable as the reference dilu¬tion technique but much less complex.