title: Evaluation of susceptibility testing methods for polymyxin.
abstract: BACKGROUND
  The widespread resistance in Gram-negative bacteria has necessitated evaluation of the use of older antimicrobials such as polymyxins. In the present study we evaluated the different susceptibility testing methods for polymyxins B and E against Gram-negative bacteria using the new Clinical and Laboratory Standards Institute (CLSI) guidelines.
  
  
  METHODS
  The susceptibility of 281 multidrug-resistant (MDR) Gram-negative bacteria (GNB) to polymyxin B was evaluated, comparing broth microdilution (BMD; reference method), agar dilution, E-test, and disk diffusion. Disk diffusion testing of polymyxin B was also performed against 723 MDR GNB.
  
  
  RESULTS
  Twenty-four of 281 (8.5%) isolates were found to be resistant to polymyxin B by the reference BMD method. The rates of very major errors for agar dilution and E-test (for polymyxin B) were 0.7% and 1%, respectively, and those for disk diffusion (for polymyxin B and polymyxin E) were 1% and 0.7%, respectively. For the 257 isolates found sensitive by reference BMD, the rates of major errors by agar dilution and E-test (for polymyxin B) were 2.4% and 0%, respectively, and those for disk diffusion (polymyxin B and polymyxin E) were 0% and 0.7%, respectively. Twenty-six (3.6%) of the 723 Gram-negative isolates were resistant to polymyxin B by disk diffusion.
  
  
  CONCLUSIONS
  The E-test and agar dilution methods showed good concordance with BMD. The disk diffusion method can be useful for initial screening in diagnostic laboratories.
authors.affiliation: Department of Laboratory Medicine, Jai Prakash Narain Apex (JPNA) Trauma Centre, All India Institute of Medical Sciences (AIIMS), New Delhi, 110029 India.
authors.forename: Bijayini
authors.initials: B
authors.lastname: Behera
authors.forename: Purva
authors.initials: P
authors.lastname: Mathur
authors.forename: Anupam
authors.initials: A
authors.lastname: Das
authors.forename: Arti
authors.initials: A
authors.lastname: Kapil
authors.forename: Babita
authors.initials: B
authors.lastname: Gupta
authors.forename: Sanjeev
authors.initials: S
authors.lastname: Bhoi
authors.forename: Kamran
authors.initials: K
authors.lastname: Farooque
authors.forename: Vijay
authors.initials: V
authors.lastname: Sharma
authors.forename: M C
authors.initials: MC
authors.lastname: Misra
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D003091:Colistin / Q000494:pharmacology*; D052978:Disk Diffusion Antimicrobial Tests / Q000379:methods / Q000592:standards; D024881:Drug Resistance, Bacterial; D006090:Gram-Negative Bacteria / Q000187:drug effects*; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods / Q000592:standards; D011112:Polymyxin B / Q000494:pharmacology*
chemical_list: D000900:Anti-Bacterial Agents; D011112:Polymyxin B; D003091:Colistin