# An Investigation of Colistin Heteroresistance in Klebsiella pneumoniae Isolates in Iran

## TL;DR

This study reports the first case of colistin heteroresistance in Klebsiella pneumoniae isolates in Iran, highlighting challenges in antibiotic susceptibility testing.

## Contribution

First report of colistin heteroresistance in K. pneumoniae using PAPs in Iran.

## Key findings

- 2 out of 79 colistin-susceptible isolates were heteroresistant with a MIC of 0.5 μg/ml.
- Heteroresistance was unstable, as no MIC increase occurred after serial passaging.
- Heteroresistant isolates belonged to ST377 and ST15 sequence types.

## Abstract

According to the World Health Organization 2024 bacterial priority pathogens list, carbapenem‐resistant Enterobacterales (CRE) were listed among the critical priority pathogens. Heteroresistance (i.e., a bacterial isolate that appears susceptible but harbors resistant subpopulations) represents a challenge in traditional laboratory testing, which may lead to treatment failure with colistin. This phenomenon has been studied in many bacteria, including K. pneumoniae. To our knowledge, this is the first report of colistin heteroresistance in K. pneumoniae isolates using population analysis profiles (PAPs) in Iran.

Between 2019 and 2020, 100 K. pneumoniae isolates were collected from various samples of hospitalized patients in Iran. This study primarily determined antibiotic resistance by antimicrobial susceptibility testing. Thereafter, the prevalence of colistin heteroresistance in K. pneumoniae isolates was evaluated by the PAP test. Heteroresistant isolates were typed by multilocus sequence typing (MLST).

The MIC test showed that 79 (79%) of the 100 K. pneumoniae isolates were susceptible to colistin. Overall, 2/79 colistin‐susceptible isolates were classified as heteroresistant isolates by the PAP method, with a colistin MIC of 0.5 μg/ml. Importantly, after 5 serial passaging on colistin‐free plates, there was no increase in the MIC of the colistin‐resistant subpopulations, showing that heteroresistance cases were unstable. MLST revealed that heteroresistant isolates belong to ST377 and ST15.

In conclusion, the current study contributes to our understanding of the challenges posed by heteroresistant isolates in clinical laboratories. Since heteroresistant isolates may be misidentified as susceptible by standard tests, these findings raise concerns regarding the interpretation of colistin susceptibility results.