Characterizing Plasmid-Borne Carbapenemase Genes in CREC: Insights from a Multi-Hospital Study in Guangdong, China

Study Background and Research Question

The global rise of carbapenem-resistant Enterobacteriaceae (CRE) poses a critical threat to hospital infection control and public health. Among these, Enterobacter cloacae complex (CREC) is now recognized as a significant contributor to multidrug-resistant infections, following Klebsiella pneumoniae and Escherichia coli in prevalence. The COVID-19 pandemic has intensified these concerns, as disruptions in healthcare services and increased antimicrobial usage have accelerated the spread of resistance. However, details regarding the genetic mechanisms—particularly the carriage and transmissibility of carbapenemase-encoding genes (CEGs) in CREC—remain insufficiently documented at the regional level.

Guanqing Chen and colleagues conducted a comprehensive investigation into CREC isolates from eight teaching hospitals in Guangdong Province between December 2022 and June 2024. Their primary aim was to delineate the distribution, localization, and transfer dynamics of key carbapenemase genes in this important clinical context. Full study details are available in the reference article.

Key Innovation from the Reference Study

The study delivers a nuanced understanding of how carbapenemase resistance determinants—especially the bla_NDM-1 gene—are partitioned between plasmids and chromosomes within clinical CREC isolates. Crucially, the research quantifies the high success rate of horizontal gene transfer (HGT) of these resistance genes through plasmid conjugation, highlighting the potential for rapid dissemination across bacterial populations. The integration of molecular typing, resistance profiling, and epidemiological analysis enables a holistic view of multidrug resistance evolution under pandemic pressures.

Methods and Experimental Design Insights

• Sample Collection: 54 CREC isolates were obtained from eight teaching hospitals, spanning multiple departments and specimen types, from late 2022 to mid-2024.
• Genetic Analysis: The prevalence and localization of carbapenemase genes—including bla_NDM-1, bla_IMP, and bla_KPC-2—were determined using PCR and plasmid elimination via variable temperature SDS treatment.
• Antibiotic Susceptibility: Broth microdilution was used to compare resistance phenotypes between CEG-positive and CEG-negative isolates.
• Plasmid Conjugation: Conjugation experiments assessed the mobility of resistance genes, with transfer events confirmed by PCR.
• Molecular Typing: ERIC-PCR and NTSYS software were employed to genotype strains and analyze epidemiological relatedness.
• Mobile Genetic Elements (MGEs): Six types of MGEs were identified, with ISEcp1 being particularly prevalent.

Protocol Parameters

• Plasmid elimination by variable temperature SDS: Incubate cultures at elevated temperatures in the presence of 0.1–0.5% SDS for multiple passages to cure plasmids prior to PCR analysis of chromosomal versus plasmid gene carriage.
• PCR detection of CEGs: Use gene-specific primers for bla_NDM-1, bla_IMP, and bla_KPC-2. Include controls for both chromosomal and plasmid templates.
• Conjugation assays: Mix donor (CREC) and recipient strains at a 1:1 ratio, incubate on non-selective media, then plate on selective media to identify transconjugants. Confirm gene transfer via PCR.
• Broth microdilution for resistance profiling: Use standard panels for carbapenems, cephalosporins, aminoglycosides, and fluoroquinolones. Compare minimal inhibitory concentrations (MICs) between groups.

Core Findings and Why They Matter

The study documented a strikingly high prevalence of carbapenemase-encoding genes in CREC isolates (85.19%), with bla_NDM-1 being the most frequently detected. Notably, 33.33% of isolates harbored bla_NDM-1 on both plasmids and chromosomes, while 46.30% carried it exclusively on plasmids, underscoring the major role of extrachromosomal gene mobility. The transferability of these genes was demonstrated by a 95.65% success rate in conjugation experiments, indicating robust potential for horizontal dissemination.

Clinically, CEG-positive CREC strains showed significantly higher resistance rates to multiple antibiotics, including imipenem, cefepime, and gentamicin, compared to CEG-negative counterparts. These multidrug-resistant phenotypes pose formidable treatment challenges, particularly in settings with high antibiotic pressure, such as during the COVID-19 pandemic.

Epidemiologically, CEG-positive isolates were most frequently recovered from male patients, elderly individuals, respiratory medicine wards, and sputum samples. Molecular typing revealed 17 genotypes among the isolates, with certain types demonstrating clonal spread across different hospitals and departments, suggesting both local and inter-institutional transmission routes.

Comparison with Existing Internal Articles

The findings of Chen et al. reinforce the central role of plasmid-mediated gene transfer in the spread of antimicrobial resistance—an aspect also highlighted in prior research on the application of Chloramphenicol (2,2-dichloro-N-[(1R,2R)-1,3-dihydroxy-1-(4-nitrophenyl)propan-2-yl]acetamide) as a bacterial protein synthesis inhibitor in molecular biology. For instance, recent articles discuss the utility of chloramphenicol in plasmid selection assays, providing tools for dissecting the stability and mobility of resistance elements. The robust plasmid transfer observed in the reference study aligns with earlier protocol-driven insights into the use of antibiotics for maintaining plasmid-borne genetic elements in laboratory strains.

Moreover, articles such as "Chloramphenicol: Benchmark Antibiotic for Molecular Biology Research" provide detailed rationale for using high-purity chloramphenicol in selection protocols, which is relevant for reproducing experiments that interrogate plasmid dynamics and antibiotic resistance mechanisms in Enterobacteriaceae.

Limitations and Transferability

While the study offers a robust multi-center snapshot of CEG prevalence and mobility in Guangdong, several limitations warrant consideration. The sample size, while substantial, may not capture the full genetic diversity of CREC across China or in other geographic regions. The focus on teaching hospitals may introduce selection bias toward more complex or severe cases. Furthermore, while conjugation experiments confirm high transfer rates for bla_NDM-1 and bla_IMP, the transferability of bla_KPC-2 was not observed in this cohort, suggesting possible context-specific constraints.

Transferability of these findings to community settings or non-Chinese hospitals should be approached cautiously, as local epidemiology and antimicrobial stewardship practices may differ. Nevertheless, the protocols and molecular insights are broadly applicable for laboratories investigating plasmid-mediated resistance in Enterobacteriaceae.

Research Support Resources

To enable similar workflows in molecular biology or clinical microbiology research, investigators frequently employ selection agents that target bacterial protein synthesis. Chloramphenicol (SKU A2512) is a well-established antimicrobial agent that acts by binding the bacterial 50S ribosomal subunit, thereby inhibiting peptidyl transferase activity and blocking translation. This reagent is commonly used in plasmid selection assays to maintain or study plasmid-encoded traits, including resistance determinants. For optimal reproducibility in resistance gene transfer and selection studies, high-purity chloramphenicol—such as that provided by APExBIO—can support stringent experimental protocols.