Background : Colistin is a last line antibiotic for treating severe infections due to multi-resistant Gram negative bacilli. Disc-diffusion and gradient based methods do not allow reliable detection of colistin resistance for Pseudomonas spp. and Acinetobacter spp.; only liquid methods seem to be satisfactory. The aim of the study is to evaluate the colistin test (COL) that is present in the ATB™ PSE EU strips. This test consists in one single well containing 2 mg/L of colistin sulfate in Mueller Hinton broth.
Material & Methods: The susceptibility of 69 isolates with known reference MICs by broth micro-dilution (BMD), comprising 38 Pseudomonas aeruginosa, 1 Pseudomonas fluorescens (9 S, 30 R) and 31 Acinetobacter spp. (8 S, 23 R), was determined by the COL test of the ATB™ PSE EU strip. Category agreement (CA), Major Errors (ME) and Very Major Errors (VME) were calculated following the EUCAST breakpoints (mg/L): Pseudomonas aeruginosa: ≤2(S) and >2(R) with an area of technical uncertainty (ATU) at 4, Acinetobacter: ≤2 (S) and >2 (R).
Results: Results are summarized in the table below. With this challenging panel (75% R isolates), the colistin resistance detection by ATB™ PSE EU is good for Acinetobacter and promising for Pseudomonas. Among the 6 isolates with a BMD MIC at 4 mg/L (R-ATU), 3 are S and 3 are R by ATB™ PSE EU.
Conclusions: This study shows that the ATB™ method has the potential to detect colistin-resistant Pseudomonas and Acinetobacter well, thus making possible the development of an ATB MIC test.