Carbapenemase Gene Dynamics in Enterobacter cloacae During COVID-19

Study Background and Research Question

Carbapenem-resistant Enterobacteriaceae (CRE) remain a global health threat, with Enterobacter cloacae among the top contributors to clinical resistance burdens in China. The COVID-19 pandemic, marked by increased antibiotic consumption and disrupted healthcare workflows, raised concerns about the selection and dissemination of multidrug-resistant bacteria. However, the detailed molecular epidemiology and transmission mechanisms of carbapenemase-encoding genes (CEGs) within carbapenem-resistant Enterobacter cloacae (CREC) during this period had not been rigorously characterized. The reference study (Chen et al., BMC Microbiology 2025) addresses this knowledge gap by analyzing CREC isolates from eight teaching hospitals in Guangdong Province, China, collected between December 2022 and June 2024.

Key Innovation from the Reference Study

The central innovation of this research lies in its comprehensive mapping of the distribution, genetic context, and transfer dynamics of CEGs in a representative clinical population during an unprecedented public health crisis. By distinguishing between chromosomal and plasmid locations of resistance genes and quantifying their horizontal transfer efficiency, the authors provide actionable insights into the mechanisms underpinning persistent and emerging carbapenem resistance. The study also integrates epidemiological data, linking gene prevalence with patient demographics, clinical departments, and sample types—offering a nuanced understanding of transmission risk factors.

Methods and Experimental Design Insights

• Sample Collection and Identification: Fifty-four non-redundant CREC isolates were obtained from eight tertiary hospitals, representing a cross-section of departments and patient demographics during the ongoing COVID-19 pandemic.
• Genetic Characterization: The team employed variable temperature Sodium Dodecyl Sulfate (SDS) plasmid elimination and polymerase chain reaction (PCR) to detect CEGs, focusing on bla_NDM−1, bla_IMP, and bla_KPC−2 loci.
• Antibiotic Susceptibility Testing: Broth microdilution assays quantified resistance profiles, comparing CEG-positive and -negative strains across multiple antibiotic classes, including imipenem, cefepime, gentamicin, ceftazidime/avibactam, ciprofloxacin, and levofloxacin.
• Plasmid Conjugation and Transfer Dynamics: Plasmid conjugation experiments assessed the horizontal transferability of CEGs, while PCR and mobile genetic element (MGE) analysis determined the frequency and diversity of transfer vectors.
• Genotypic and Epidemiological Analysis: Enterobacterial repetitive intergenic consensus (ERIC)-PCR and NTSYS software enabled the grouping of isolates into genotypes, correlating these clusters with clinical and demographic variables.

Protocol Parameters

• Plasmid elimination: Variable temperature SDS treatment, followed by PCR for confirmation, is recommended for distinguishing chromosomal versus plasmid CEG localization.
• Antimicrobial susceptibility: Broth microdilution protocols should be standardized (e.g., 0.5 McFarland inoculum, 18–24h incubation at 35°C) when comparing CEG-positive and -negative isolates.
• Plasmid conjugation: Use filter mating with a suitable recipient (e.g., E. coli J53), and confirm gene transfer via PCR; record transfer frequency for each CEG type.
• Genotyping: ERIC-PCR with appropriate primer sets and cluster analysis using software such as NTSYS yields robust epidemiological clustering for outbreak tracing.

Core Findings and Why They Matter

The study uncovered several key findings that illuminate the evolving landscape of carbapenem resistance:

• High CEG Carriage and Multidrug Resistance: 85.2% of CREC isolates harbored carbapenemase-encoding genes. The bla_NDM−1 gene, associated with broad-spectrum β-lactam resistance, was most frequently detected—present on both chromosomes and plasmids in 33.3% of isolates, and exclusively on plasmids in 46.3%. This dual localization enhances both vertical and horizontal gene dissemination.
• Efficient Plasmid-Mediated Transfer: Plasmid conjugation experiments showed a 95.7% overall transfer success for CEGs, with bla_NDM−1 at 95.5% and bla_IMP at 100%. This underscores the high risk of rapid resistance spread within and across clinical settings.
• Mobile Genetic Elements and Diversity: Six distinct MGEs were identified, with ISEcp1 being the most prevalent (87%), and 41% of isolates carrying four MGE types simultaneously. The diversity and abundance of MGEs potentiate the mobilization and persistence of resistance determinants.
• Epidemiological Correlates: Higher CEG detection rates were observed in male and elderly patients, respiratory medicine departments, and sputum specimens. Genotyping revealed 17 distinct clusters, with two major types accounting for over 40% of the population, spanning multiple hospitals and departments—a signature of both clonal expansion and interfacility transmission.
• Antibiotic Resistance Phenotypes: CEG-positive isolates displayed significantly higher resistance to multiple antibiotics compared to CEG-negative counterparts, highlighting the clinical challenge posed by these strains (see study data).

Together, these findings emphasize the urgent need for molecular surveillance and targeted infection control interventions, especially during periods of healthcare system stress.

Comparison with Existing Internal Articles

Several internal resources dissect the mechanisms and translational implications of monocyclic β-lactam antibiotics like Aztreonam in the context of Gram-negative resistance. For example, Aztreonam’s Mechanistic Impact: Strategic Leverage in Translational Research discusses the compound’s specificity and utility in dissecting β-lactam resistance, while Aztreonam: Synthetic β-Lactam Antibiotic for Gram-Negativ... highlights its research applications in both antimicrobial and pharmacological assays. Unlike these reviews, the present reference study delivers direct epidemiological and molecular evidence of gene transfer networks and resistance phenotypes in clinical Enterobacter cloacae—data critical for contextualizing the role of synthetic β-lactam antibiotics in resistance modeling and experimental design.

Limitations and Transferability

While the study provides high-resolution insights into CEG dynamics in Guangdong hospitals, several limitations affect generalizability. The isolates represent a single region and a pandemic-impacted interval, which may not capture nationwide or inter-pandemic trends. Whole-genome sequencing could further clarify the genetic contexts and evolutionary trajectories of key resistance genes. Nevertheless, the combined use of plasmid elimination, conjugation assays, and genotyping offers a transferable framework for similar epidemiological investigations in other settings.

Research Support Resources

To facilitate translational research on antibiotic resistance and Gram-negative pathogens, researchers may employ Aztreonam (SKU A5931), a monocyclic β-lactam antibiotic with validated activity against Gram-negative aerobic bacteria. Aztreonam's robust inhibition of bacterial cell wall synthesis and its well-characterized effects on bone marrow progenitor cells and hepatic cytochrome P450 enzymes make it suitable for modeling multidrug resistance and pharmacological interactions, as reported in the product dossier. For optimized experimental protocols and advanced assay strategies, APExBIO provides technical documentation and workflow recommendations tailored to resistance and cell-based research. This approach supports reproducibility and data comparability in laboratory studies aligned with current resistance epidemiology.