Induced colistin resistance as an identifying marker for Aeromonas phenospecies groups.

AIMS

To investigate the taxonomic interest of colistin resistance as an identifying marker for Aeromonas phenospecies groups.

METHODS AND RESULTS

Colistin resistance was investigated in 387 Aeromonas isolates identified at species level using a 14-test format protocol with miniaturized tests combined with determination of urocanic acid utilization whenever necessary. Colistin resistance, determined by the disc diffusion method, was unreliable when compared with minimum inhibitory concentration (MIC) determination. In some strains, the MIC values and resistance rates of colistin could be increased after overnight induction with a 50- microg colistin disc in 20 ml of Mueller-Hinton broth (2.5 mg l(-1)). Colistin-induced resistance level was raised to 85.8% in the Aeromonas hydrophila complex, 2.1% in the A. caviae complex and 2.5% in the A. veronii complex except for A. jandaei (100% colistin resistant). This new marker allowed the identification of 96.2 and 93.6% of Aeromonas isolates to phenospecies and species level, respectively.

CONCLUSIONS

Colistin-induced colistin resistance is a new phenotypic marker for Aeromonas isolates.

SIGNIFICANCE AND IMPACT OF THE STUDY

With the present protocol, colistin resistance determination may improve the identification of Aeromonas isolates to phenogroup level, when results obtained by conventional biochemical methods are ambiguous.