Clinical characteristics, specific resistance patterns, and molecular mechanisms of carbapenem-resistant Morganella morganii isolates

Xiangkuo Zheng^1,2Weiliang Zeng²Yan Liu²Luozhu Feng³Jiayin Zheng²Tieli Zhou²Changrui Qian^2*Cui Zhou^2*

• ¹School of Medicine, Huzhou University; Key Laboratory of Vector Biology and Pathogen Control of Zhejiang Province, Huzhou, China
• ²Department of Clinical Laboratory, the First Affiliated Hospital of Wenzhou Medical University; Key Laboratory of Clinical Laboratory Diagnosis and Translational Research of Zhejiang Province, Wenzhou, China
• ³Department of Clinical Laboratory, the First Affiliated Hospital of Ningbo University, Ningbo, China

Objectives: The emergence and spread of carbapenem-resistant Morganella morganii (M. morganii) pose a serious challenge globally. This study aimed to reveal the clinical characteristics, specific patterns, and molecular mechanisms of carbapenem-resistant M. morganii. Methods: 170 M. Morganella clinical isolates were recovered from the First Affiliated Hospital of Wenzhou Medical University (Wenzhou, China) from January 2016 to December 2017. Carbapenem MICs of these isolates were determined by antimicrobial susceptibility testing. Carbapenem determinants, including carbapenem resistance genes (blaKPC-2, blaVIM, blaIMP, blaNDM, and blaOXA-48) and extended-spectrum β-lactamase (ESBLs) genes (blaTEM, blaCTX-M-1, and blaSHV) were analyzed by Polymerase chain reaction (PCR). PCR and sequencing assays were also performed to detect the penicillin-binding protein (PBP) mutations. In addition, efflux pump activity on carbapenem-resistant M. Morganii isolates was tested. Quantitative real-time PCR (qRT-PCR) was used to determine the mRNA relative expression levels of outer membrane porin-encoding genes ompC, PBP activator-encoding genes lpoA and lpoB. Results: 26 imipenem-resistant and 108 imipenem-intermediate M. morganii isolates were identified, which accounted for high rates of 15.29% and 63.53%, respectively. No isolate was resistant to meropenem and ertapenem. The prevalence rates of ESBLs-encoding genes blaTEM and blaCTX-M-1 in 26 carbapenem-resistant M. morganii isolates were 30.77% and 11.54%, while carbapenemases-encoding genes were not detected. Additionally, carbapenem-resistant M. Morganella isolates had more specific PBP mutations than tested carbapenem-susceptible and carbapenem-intermediate isolates. Of note, there was an association between efflux pump phenotypes and reduced imipenem susceptibility in 13 carbapenem-resistant M. morganii isolates. qRT-PCR revealed that transcript levels of ompC showed no significant difference among carbapenem-resistant, carbapenem-intermediate, and carbapenem-susceptible groups, while a significant difference of lpoA and lpoB gene expression was observed among three groups above. The imipenem-resistance group isolates had more PBP mutations. Conclusion: M. morganii were commonly non-susceptible to carbapenem antimicrobials imipenem while susceptible to meropenem and ertapenem. We found that low expression of PBP activators-encoding genes lpoA and lpoB were primary underlying mechanism, as well as carrying more specific PBP mutations. Meanwhile, the overexpression of efflux pump might the resistance of imipenem against M. Morganella.