Carbapenemase Gene Transmission in Carbapenem-Resistant Enterobacter cloacae

Study Background and Research Question

Carbapenem-resistant Enterobacter cloacae (CREC) is an important component of the broader carbapenem-resistant Enterobacteriaceae problem. Resistance in this species is particularly concerning when carbapenemase-encoding genes (CEGs) are carried on mobile plasmids, because the resistance determinant can move between bacterial cells rather than remaining confined to a single lineage. The study by Chen and colleagues, published in BMC Microbiology in 2025, addresses this issue using isolates collected from teaching hospitals in Guangdong, China. The full report is available in the reference study.

The research was motivated by concern that antibiotic exposure, disrupted healthcare services, and complex infections during and after the COVID-19 pandemic could alter the distribution and transmission of resistance determinants. Rather than asking only how frequently CREC was detected, the investigators asked several linked questions: Which CEGs were present? Were they located on chromosomes, plasmids, or both? Could they transfer by conjugation? Which mobile genetic elements accompanied them? Finally, did the isolates represent expansion of related clones, independent acquisition events, or both?

Key Innovation from the Reference Study

The study’s innovation is its integrated view of resistance transmission. Many surveillance reports provide gene prevalence or antimicrobial susceptibility data, whereas this work connected gene identity with genetic location, experimentally measured transferability, mobile-element context, and strain-level relatedness. That combination is important because a high prevalence of blaNDM-1 has a different epidemiological meaning when the gene is chromosomal, plasmid-borne, or present in both compartments.

The investigators also combined two transmission perspectives. Horizontal dissemination was evaluated through plasmid elimination, conjugation experiments, and PCR confirmation of transferred genes. Vertical dissemination was explored through ERIC-PCR fingerprinting and clustering with NTSYS software. This design does not replace whole-genome sequencing, but it provides a practical framework for distinguishing transferable resistance from the spread of closely related bacterial backgrounds in a multicentre collection.

Methods and Experimental Design Insights

The sample set comprised 54 CREC isolates obtained between December 2022 and June 2024 from eight teaching hospitals in Guangdong, according to the published methods and results. The study included clinical and epidemiological descriptors such as sex, age group, hospital department, and specimen type. These variables allowed the molecular findings to be interpreted alongside the clinical distribution of the isolates.

For genetic characterization, the authors used a variable-temperature sodium dodecyl sulfate plasmid elimination approach followed by PCR. This strategy was used to infer whether resistance genes were associated with plasmids or chromosomes. It is a useful screening-level method because loss of a plasmid-associated determinant after treatment supports plasmid localization, although interpretation depends on experimental controls and does not provide the structural resolution of complete plasmid sequencing.

Antimicrobial susceptibility was assessed by broth microdilution. The investigators compared CEG-positive and CEG-negative groups across several agents, including imipenem, cefepime, gentamicin, ceftazidime/avibactam, ciprofloxacin, and levofloxacin. Broth microdilution provides a quantitative phenotype that can be directly compared between molecular subgroups, but resistance patterns should still be interpreted gene by gene rather than attributed to all carbapenemases as a single class.

Plasmid conjugation experiments were used to test whether CEGs could move into a recipient strain. PCR analysis of transconjugants then confirmed the identity of transferred genes. The study also examined six mobile genetic elements, with particular attention to ISEcp1. Finally, ERIC-PCR generated strain fingerprints, and NTSYS software was used to group the 54 isolates into related genotypes.

Protocol Parameters

• Isolate collection: Use a clearly defined CREC panel with hospital, department, specimen, and patient metadata; the reference study analyzed 54 isolates from eight teaching hospitals.
• Plasmid localization: Apply variable-temperature SDS plasmid elimination followed by targeted PCR when a screening-level distinction between plasmid-associated and chromosomal CEGs is required.
• Phenotypic testing: Use broth microdilution to compare susceptibility profiles between gene-defined groups rather than relying only on categorical screening results.
• Transferability testing: Pair conjugation with PCR confirmation of recipient or transconjugant colonies; growth under selective conditions alone is insufficient evidence of CEG transfer.
• Strain relatedness: ERIC-PCR can provide an accessible fingerprinting layer, while whole-genome sequencing should be considered when precise clone, plasmid, or outbreak reconstruction is required.
• Quality controls: Include donor, recipient, and negative-control conditions, and interpret mobile-element PCR results alongside gene-location and transfer data.

Core Findings and Why They Matter

CEGs were detected in 46 of 54 isolates, or 85.19%, in the reference collection. The dominant determinant was blaNDM-1. A substantial subgroup carried blaNDM-1 on both chromosomes and plasmids, while another major subgroup carried it exclusively on plasmids. Smaller groups contained plasmid-associated blaIMP or a combination of blaNDM-1 and blaKPC-2. This distribution indicates that the same resistance gene may participate in different evolutionary and transmission pathways within CREC.

The plasmid-associated component was especially important because conjugation experiments showed transfer of CEGs from 44 of 46 CEG-positive isolates, corresponding to a 95.65% success rate in the study design. Transfer was observed for blaNDM-1 in 42 of 44 tested cases and for blaIMP in both tested cases, whereas the single blaKPC-2-containing isolate did not produce a successful transfer event. These results support a high laboratory-measured transfer potential for the dominant genes, while also showing that transferability cannot be inferred from gene presence alone.

ISEcp1 was the most frequently identified mobile genetic element, detected in 47 of the 54 isolates. Isolates carrying four mobile-element types simultaneously formed the largest subgroup. The biological implication is not that every element directly mobilizes every CEG, but that resistance determinants were embedded in genetically dynamic backgrounds with multiple possible routes for rearrangement or movement.

CEG-positive isolates displayed significantly higher resistance rates than CEG-negative isolates to the antimicrobial agents examined, including imipenem, cefepime, gentamicin, ceftazidime/avibactam, ciprofloxacin, and levofloxacin, as reported in the study’s susceptibility analysis. This association reinforces the clinical significance of CEG surveillance, but it does not establish that one carbapenemase gene caused every resistance phenotype. Porin changes, efflux, additional beta-lactamases, and unrelated resistance determinants may also contribute.

ERIC-PCR classified the isolates into 17 genotypes. Types E and G were the most common, each accounting for 11 isolates, and were recovered from different departments in five hospitals. Two type E isolates showed a Dice coefficient of 100%, suggesting very close fingerprint similarity. The simultaneous evidence for related strains and transferable plasmids is the study’s central epidemiological message: CREC dissemination may involve both clonal expansion and gene movement across bacterial backgrounds.

The epidemiological profile also showed the highest detection frequencies among male patients, older adults, respiratory medicine departments, and sputum specimens. These patterns identify where enhanced screening or infection-control attention might be prioritized, but they should not be interpreted as evidence that sex, age, department, or specimen type independently causes CEG carriage.

Comparison with Existing Internal Articles

The internal article on immunomodulation and Gram-negative infection research emphasizes host-cell responses, including dendritic-cell activation and intracellular signaling. That perspective is complementary but distinct from the Guangdong study: the reference paper characterizes pathogen-side resistance transmission, whereas the internal article discusses how an antibacterial perturbation may be evaluated in immune assays.

A second internal overview on mechanistic and translational workflows is broader and more application-oriented. In comparison, Chen and colleagues provide a narrower, evidence-led epidemiological contribution based on clinical isolates, molecular localization, transfer experiments, and fingerprinting. The reference study should therefore be used as the foundation for resistance surveillance and transmission hypotheses, not as evidence for host-directed efficacy or treatment outcomes.

Limitations and Transferability

The study has several limitations that affect how broadly its conclusions can be transferred. The isolates came from eight teaching hospitals in one province, so the observed gene distribution may not represent other regions, hospital types, or community settings. The collection was also observational and hospital-based; it cannot determine whether specific antibiotic exposures, patient transfers, or infection-control events caused the detected patterns.

Plasmid elimination and conjugation provide valuable functional evidence, but laboratory transfer does not prove that a gene moved between those same strains in patients or in the hospital environment. ERIC-PCR is useful for screening relatedness but has lower resolution than whole-genome sequencing and detailed plasmid reconstruction. In addition, the study did not establish patient outcomes, treatment responses, or the clinical effectiveness of any antimicrobial intervention.

Why this cross-domain matters, maturity, and limitations

Linking hospital epidemiology with experimental infection research is useful because the molecular context of an isolate should guide downstream model selection. A plasmid-borne, conjugation-capable CEG raises different questions from a stable chromosomal determinant: the former is relevant to donor-recipient transfer assays, environmental persistence studies, and mixed-population experiments, whereas the latter may be more informative for lineage-focused surveillance. This bridge is scientifically mature at the level of hypothesis generation, but translation to patient treatment, immune modulation, or sepsis outcomes remains limited because those endpoints were not measured in the reference study.

Research Support Resources

For related laboratory work, researchers can use Polymyxin B (sulfate) (SKU C3090) as a research reagent in carefully controlled Gram-negative bacterial infection research, including isolate-specific antibacterial comparisons, a dendritic cell maturation assay, or selected sepsis and bacteremia models. Its membrane-active activity should not be assumed to overcome carbapenemase-associated resistance, so susceptibility and cytotoxicity controls remain essential. Although it is described as an antibiotic for bloodstream and urinary tract infections, the product is intended for scientific research use only; the product information lists storage at -20 °C and advises prompt use of prepared solutions.