# The Use of CHROMID® Colistin R for the Detection of Colistin-Resistant Gram-Negative Bacteria in Positive Blood Cultures

## TL;DR

This study shows that CHROMID® Colistin R can quickly detect colistin-resistant Gram-negative bacteria in blood cultures, helping guide antibiotic treatment.

## Contribution

The study demonstrates the effectiveness of CHROMID® Colistin R for rapid detection of colistin resistance in Gram-negative bacteria from blood cultures.

## Key findings

- CHROMID® Colistin R detected 93% of colistin-resistant isolates within 18 hours.
- Only 9 colistin-susceptible isolates grew on CHROMID® Colistin R, including Pseudomonas aeruginosa.
- The method provides a rapid diagnostic tool for colistin resistance in Gram-negative bacteria from blood cultures.

## Abstract

The aim of this study was to assess the utility of CHROMID® Colistin R for direct detection of colistin-resistant Gram-negative bacteria from positive blood cultures. A total of 390 blood cultures from hospitalised patients containing Gram-negative bacteria were included in this study. These blood cultures were referred to clinical laboratories in the United Kingdom and Türkiye. A further 16 simulated positive blood culture bottles were included that contained a range of colistin-resistant strains as well as susceptible control strains. Fluid from each positive blood culture was diluted 1/200 in saline and 10 µL aliquots cultured onto cystine-lactose-electrolyte-deficient agar and CHROMID® Colistin R. All recovered bacteria were identified, and for Gram-negative bacteria, their minimum inhibitory concentration of colistin was measured using the broth microdilution method. From a total of 443 Gram-negative isolates, 57 colistin-resistant isolates were recovered, of which 53 (93%) grew on CHROMID® Colistin R within 18 h. Of the 377 isolates determined to be colistin-susceptible, only 9 isolates were able to grow, including 6 isolates of Pseudomonas aeruginosa. For positive blood cultures that are shown to contain Gram-negative bacteria, culture on CHROMID® Colistin R is a useful diagnostic tool to detect susceptibility or resistance to colistin within 18 h.