Zaifeng Si,1,2,* Shengmei Zhao,3,* Yingying Hao,3 Qian Wang,3 Yanfa Zhong,2 Yue Liu,3 Ran Chen,3 Yan Jin,3 Zhiming Lu1,3 

1Department of Clinical Laboratory, Shandong Provincial Hospital, Cheeloo College of Medicine, Shandong University, Jinan, Shandong, People’s Republic of China; 2Department of Clinical Laboratory, Dezhou Municipal Hospital of Traditional Chinese Medicine, Dezhou, Shandong, People’s Republic of China; 3Department of Clinical Laboratory, Shandong Provincial Hospital Affiliated to Shandong First Medical University, Jinan, Shandong, People’s Republic of China

*These authors contributed equally to this work

Correspondence: Yan Jin; Zhiming Lu, Email sdjinyan@163.com; luzhiming@sdu.edu.cn

Background: Enterobacter cloacae complex (ECC), which includes major nosocomial pathogens, causes urinary, respiratory, and bloodstream infections in humans, for which colistin is one of the last-line drugs.
Objective: This study aimed to analyse the epidemiology and resistance mechanisms of colistin-resistant Enterobacter cloacae complex (ECC) strains isolated from Shandong, China.
Methods: Two hundred non-repetitive ECC strains were collected from a tertiary hospital in Shandong Province, China, from June 2020 to June 2022. Whole-genome sequencing and bioinformatics analyses were performed to understand the molecular epidemiology of the colistin-resistant ECC strains. The nucleotide sequences of heat shock protein (hsp60) were analyzed by using BLAST search to classify ECC. The gene expression levels of ramA, soxS, acrA, acrB, phoP, and phoQ were assessed using RT-qPCR. MALDI-TOF MS was used to analyse the modification of lipid A.
Results: Twenty-three colistin-resistant strains were detected among the 200 ECC clinical strains (11.5%). The hsp60 cluster analysis revealed that 20 of the 23 ECC strains belonged to heterogeneous resistance clusters. Variants of mgrB, phoPQ, and pmrAB, particularly phoQ and pmrB, were detected in the 23 ECC strains. The soxS and acrA genes were significantly overexpressed in all 23 colistin-resistant ECC strains (P < 0.05). Additionally, all 23 ECC strains contained modified lipid A related to colistin resistance, which showed five ion peaks at m/z 1876, 1920, 1955, 2114, and 2158. Among the 23 ECC strains, 6 strains possessed a phosphoethanolamine (pETN) moiety, 16 strains possessed a 4-amino-4-deoxy-L-arabinose (−L-Ara4N) moiety, and one strain had both pETN and −L-Ara4N moieties.
Conclusion: This study suggests that diverse colistin resistance existed in ECC, including unknown resistance mechanisms, exist in ECC. Mechanistic investigations of colistin resistance are warranted to optimise colistin use in clinical settings and minimise the emergence of resistance.

Keywords: Enterobacter cloacae complex, ECC, epidemiology, colistin, resistance mechanism