This is the first study that compared polymyxin B and colistin Etest with broth microdilution test (reference method) to detect susceptibility among carbapenem-resistant P. aeruginosa using the new breakpoints recommended by CLSI [ 10].
When available, the breakpoints recommended by the Clinical Laboratory Standards Institute (CLSI) [ 151] were used to assign the corresponding clinical susceptibility categories: susceptible, intermediate, or resistant.
Results were categorised by using the breakpoints for resistant, intermediate and sensitive recommended by the NCCLS for bacteria isolated from animals (1999).
A non-overlapping 10 kb chromosome-wide window analysis was used to investigate 'hotspots' of CNV using the breakpoints of all CNVs (170 CNVs, 340 unique breakpoints).
Prior to 2002 we were using CLSI breakpoints recommended at the time and were therefore not able to detect any resistance [ 8].
Antimicrobial drug susceptibility was determined by using an agar disk diffusion method, and results were interpreted by using MIC breakpoints recommended for other oxidative gram-negative bacilli by the Comité de l'Antibiogramme de la Société Française de Microbiologie (Paris, France) (6 ).
Breakpoint susceptibility testing was performed using the CLSI recommended broth microdilution MIC method with nine drugs (amikacin, tobramycin, trimethoprim-sulfamethoxazole, cefoxitin, imipenem, linezolid, doxycycline/minocycline, clarithromycin and ciprofloxacin) [Table 1].
Antimicrobial resistance was assessed using breakpoints recommended by the Clinical and Laboratory Standard Institute (CLSI) guidelines 2008 (M100-S18).
Isolates were deemed resistant only when the zone of inhibition was less than or equal to the resistance breakpoint recommended by the NCCLS guidelines [ 44].
Due to the lack of an acknowledged fosfomycin breakpoint for E. faecium, we used the breakpoints of fosfomycin for Enterococcus faecalis proposed by the CLSI.
