Carbapenemase Gene Transmission in CREC in China

The 2025 study by Chen and colleagues in BMC Microbiology examines how carbapenemase-encoding genes are distributed and transmitted among carbapenem-resistant Enterobacter cloacae (CREC). Using isolates collected from eight teaching hospitals in Guangdong province between December 2022 and June 2024, the authors connect resistance-gene detection with gene localization, antimicrobial susceptibility, conjugative transfer, mobile genetic elements, and strain relatedness. The reference study is particularly relevant to antimicrobial resistance research because it evaluates both the genetic architecture and the dissemination potential of CREC rather than treating resistance as a purely phenotypic property.

Study Background and Research Question

Carbapenem-resistant Enterobacterales are difficult to control because carbapenemase enzymes can hydrolyze carbapenem drugs, while the genes encoding these enzymes may move between bacterial cells. This is a central issue in understanding the beta-lactam antibiotic mechanism: the antibacterial effect depends on intact beta-lactam structures, whereas carbapenemases can destroy those structures before they inhibit cell-wall synthesis. In E. cloacae, resistance may therefore reflect not only the presence of a gene but also its location, genetic context, expression, and ability to disseminate.

The study was motivated by concern that increased antibiotic exposure, disrupted healthcare services, and complex infections associated with the COVID-19 era could alter resistance patterns and transmission networks. Its principal questions were: how frequently do carbapenemase-encoding genes occur in CREC; are they located on plasmids, chromosomes, or both; can they transfer by conjugation; and do gene-positive isolates show distinctive resistance and clonal patterns? Because the sampling period was December 2022 to June 2024, the study provides a contemporary surveillance snapshot rather than a direct before-and-after test of pandemic effects.

Key Innovation from the Reference Study

The main innovation is the integration of several resolution levels in one investigation. Variable-temperature sodium dodecyl sulfate plasmid elimination followed by PCR was used to distinguish plasmid-associated from chromosomal gene carriage. Broth microdilution then linked gene status to antimicrobial susceptibility, while plasmid conjugation experiments tested whether the resistance determinants were functionally transferable. Mobile-element analysis and ERIC-PCR added information about the surrounding genetic environment and the relatedness of the isolates.

This design is more informative than a single PCR prevalence survey. A positive PCR result identifies a resistance determinant, but it does not establish whether that determinant is mobile or whether two isolates acquired it through a shared clone. By combining localization, transfer testing, and genotyping, the authors address both horizontal dissemination through mobile DNA and vertical dissemination through expansion of related bacterial lineages. The approach is therefore useful for bacterial infection model design and hospital epidemiology, although it remains lower resolution than a complete whole-genome analysis.

Methods and Experimental Design Insights

The investigators analyzed 54 CREC isolates from eight teaching hospitals. Carbapenemase-encoding genes were detected by PCR after plasmid elimination experiments designed to help assign their location. Antimicrobial susceptibility was assessed by broth microdilution, and conjugation experiments used PCR to determine whether resistance genes moved into recipient cells. Six classes of mobile genetic elements were evaluated, including ISEcp1. Finally, ERIC-PCR profiles were processed with NTSYS software to group isolates into genotypes and assess possible relatedness across hospitals and clinical departments.

For researchers planning a comparable workflow, the important methodological principle is to treat gene detection, phenotype, mobility, and clonality as separate analytical questions. A plasmid-associated gene that transfers in vitro presents a different epidemiological risk from a chromosomal gene found in a highly related cluster, even when both produce carbapenem resistance. The paper's design makes those distinctions experimentally visible.

Protocol Parameters

• Isolate panel: Analyze clinically recovered CREC isolates from multiple hospitals and record patient sex, age group, clinical department, and specimen type; the reference dataset contained 54 isolates from eight Guangdong teaching hospitals.
• Gene localization: Use variable-temperature SDS plasmid elimination followed by PCR to compare gene detection before and after plasmid curing; interpret the result as evidence for likely plasmid or chromosomal carriage rather than as a complete physical map.
• Susceptibility testing: Apply broth microdilution to compare carbapenemase-encoding-gene-positive and -negative groups across the antimicrobial panel used in the study, including imipenem, cefepime, gentamicin, ceftazidime/avibactam, ciprofloxacin, and levofloxacin.
• Transfer assessment: Perform plasmid conjugation experiments and confirm putative transconjugants by PCR; report transfer rates separately for each gene because a high overall rate can conceal gene-specific differences.
• Relatedness and context: Combine mobile-element detection with ERIC-PCR and NTSYS clustering to distinguish possible clonal spread from independent acquisition of similar resistance determinants.

Core Findings and Why They Matter

High prevalence and dual genetic localization

Carbapenemase-encoding genes were detected in 46 of 54 isolates, corresponding to 85.19%, according to the published results. The dominant determinant was bla_NDM-1. Eighteen isolates, or 33.33% of the total collection, carried bla_NDM-1 on both chromosomes and plasmids, while 25 isolates, or 46.30%, carried it exclusively on plasmids. Two isolates contained only plasmid-borne bla_IMP, and one carried both plasmid-borne bla_NDM-1 and bla_KPC-2.

This distribution matters because plasmid carriage creates a plausible route for horizontal spread, while dual chromosomal and plasmid localization may provide multiple genetic reservoirs within the same bacterial population. The findings also caution against assuming that a single isolate has only one relevant resistance compartment.

Gene-positive isolates showed broader resistance

The gene-positive group had significantly higher resistance rates than the gene-negative group for imipenem, cefepime, gentamicin, ceftazidime/avibactam, ciprofloxacin, and levofloxacin, with statistical significance reported for these comparisons in the reference paper. The result supports a multidrug-resistance phenotype associated with carbapenemase-encoding genes, but it should not be interpreted as proof that every detected gene directly causes resistance to every tested drug. Co-resident determinants, permeability changes, efflux, and other resistance mechanisms may contribute.

Very high in vitro transfer potential

Conjugation experiments produced successful transfer of carbapenemase-encoding genes in 44 of 46 gene-positive isolates, or 95.65%. Transfer was reported for 42 of 44 bla_NDM-1-positive cases, for both of the two bla_IMP cases, and not for the single bla_KPC-2 case. These gene-specific results are more informative than the aggregate rate: they identify plasmid-borne bla_NDM-1 as the most consequential transfer concern in this collection.

ISEcp1 was the most prevalent of the six detected mobile genetic elements, occurring in 47 of 54 isolates, or 87.04%. Isolates carrying four mobile-element types simultaneously were the largest category, comprising 22 of 54 isolates, or 40.74%. Together, these observations suggest a complex genetic environment in which resistance determinants may be mobilized or maintained by more than one element.

Clonal structure and clinical distribution

ERIC-PCR and NTSYS analysis divided the 54 isolates into 17 genotypes. Types E and G were each represented by 11 isolates, or 20.37%, and isolates belonging to these groups came from different departments in five hospitals. Two type E isolates shared a Dice coefficient of 100%. This pattern is compatible with both multicenter dissemination of related strains and repeated acquisition of mobile genes, but ERIC-PCR alone cannot establish a transmission chain.

The highest detection frequencies occurred among men, older patients, respiratory-medicine cases, and sputum specimens: 35 of 54 isolates, 39 of 54, 11 of 54, and 18 of 54, respectively, as reported by the study authors. These are characteristics of the sampled isolates, not population-level risk estimates. They can nevertheless help prioritize screening and sampling strategies in respiratory wards and other settings with substantial antibiotic exposure.

Comparison with Existing Internal Articles

The internal article Transmission Dynamics of Carbapenemase Genes in CREC in China provides a concise molecular-epidemiology overview of the same study and emphasizes plasmid-borne bla_NDM-1, multidrug resistance, and infection-control implications. The reference paper supports that overview with the underlying experimental detail: it shows how plasmid elimination, conjugation, mobile-element profiling, and ERIC-PCR were combined to reach those conclusions. For literature review purposes, the internal summary is useful for rapid orientation, whereas the original article should remain the source for numerical interpretation and methodological appraisal.

Limitations and Transferability

The study has several limits that affect how broadly its findings can be transferred. The sample consisted of 54 isolates from eight teaching hospitals in one Chinese province, so the reported percentages should not be treated as national prevalence estimates or as representative of community isolates, non-teaching hospitals, or other geographic regions. The sampling window also does not provide a matched pre-pandemic control period. Consequently, the data cannot by themselves demonstrate that COVID-19-related antibiotic use caused the observed resistance profile.

Plasmid elimination and PCR provide practical evidence of gene location, but they may not resolve complete plasmid structures, insertion sites, copy numbers, or all co-carried resistance genes. Similarly, ERIC-PCR can identify broad relatedness but does not provide the single-nucleotide resolution needed to confirm patient-to-patient transmission. Whole-genome sequencing, plasmid reconstruction, and longitudinal sampling would strengthen conclusions about transmission pathways.

Conjugation success is also an experimental measure of transferability under laboratory conditions, not a direct estimate of transfer frequency in patients or hospital environments. The finding that most tested determinants transferred should therefore guide surveillance and containment hypotheses rather than be translated into a clinical risk percentage. Finally, the association between carbapenemase genes and resistance to several non-carbapenem agents does not replace isolate-specific susceptibility testing.

Research Support Resources

For antimicrobial resistance research, Cefotaxime (SKU BA1012) can support comparator assays involving a third-generation cephalosporin antibiotic, beta-lactam antibiotic mechanism studies, or a bacterial infection model. It may also be relevant when researchers examine resistance across Gram-positive and Gram-negative organisms, but it was not the test agent in this CREC study and should not be used to infer carbapenemase status without validated susceptibility methods. The supplied solid is reported for storage at -20 °C; freshly prepared solutions are recommended for short-term experimental use rather than prolonged solution storage.