Carbapenemase Gene Transfer in CREC Hospitals

Carbapenem-resistant Enterobacter cloacae complex is an important component of the clinical carbapenem-resistant Enterobacteriaceae burden, yet its resistance architecture is less extensively characterized than that of Klebsiella pneumoniae or Escherichia coli. The reference study by Chen and colleagues addresses this gap by examining carbapenemase-encoding genes, their apparent genomic location, transferability, mobile genetic-element context, and strain relatedness across eight teaching hospitals in Guangdong, China. The work was published as Chen et al., BMC Microbiology 2025, 25:667.

Study Background and Research Question

Carbapenem resistance in E. cloacae can arise through several mechanisms, including carbapenemase production, permeability changes, and combinations of beta-lactamase activity with altered porin expression. Carbapenemase-encoding genes are particularly concerning because they may be carried on plasmids and transferred between bacterial cells. This creates a route by which resistance can spread independently of clonal expansion.

The study was motivated by the possibility that COVID-19-era antibiotic exposure, complex hospital infections, and disrupted healthcare services could alter resistance transmission. Its central questions were practical and epidemiological: how frequently do carbapenem-resistant E. cloacae isolates carry carbapenemase genes; are those genes located on plasmids, chromosomes, or both; can they transfer to a recipient strain; and do resistant isolates represent a few expanding clones or multiple unrelated lineages?

These questions distinguish three processes that are often conflated in antimicrobial resistance research. Horizontal transmission refers to movement of a resistance determinant between cells, vertical dissemination refers to expansion of a successful bacterial lineage, and epidemiological enrichment refers to concentration in particular patient groups, departments, or specimen types. The reference study attempts to assess all three within one regional hospital network.

Key Innovation from the Reference Study

The principal innovation is the combination of complementary assays rather than reliance on PCR positivity alone. PCR established the presence of selected carbapenemase genes, while variable-temperature sodium dodecyl sulfate plasmid elimination was used to infer whether genes were maintained on plasmids or chromosomes. Broth microdilution connected gene status with susceptibility phenotypes. Conjugation experiments then tested whether the resistance determinants could move into a recipient strain.

This design was strengthened by two additional layers. PCR-based mobile-element analysis identified genetic structures associated with mobilization, and ERIC-PCR with NTSYS analysis assessed relatedness among isolates. Consequently, the paper links molecular mechanism, transfer potential, and hospital epidemiology. That integrated view is more informative than reporting the prevalence of bla genes without determining whether they are mobile or distributed among closely related strains.

The study also avoids treating all carbapenemase genes as equivalent. It separately evaluates bla_NDM-1, bla_IMP, and bla_KPC-2, revealing substantial differences in their observed transfer behavior. This gene-specific resolution is important for infection-control interpretation because a prevalent gene may be clinically consequential for different reasons: high abundance, broad host range, efficient transfer, or association with a successful clone.

Methods and Experimental Design Insights

Sampling and molecular characterization

The investigators analyzed 54 carbapenem-resistant E. cloacae isolates collected between December 2022 and June 2024 from eight teaching hospitals in Guangdong. The sample metadata included sex, age group, clinical department, and specimen type. The workflow began with carbapenem-resistant isolates and then divided them according to carbapenemase-encoding gene status.

Variable-temperature SDS plasmid elimination followed by PCR was used to distinguish plasmid-associated from chromosomal gene carriage. This is a useful operational approach when the immediate objective is to compare likely replicon locations, although it should not be interpreted as a substitute for complete plasmid sequencing. Loss of a plasmid can affect bacterial fitness or remove linked genes, and incomplete elimination may complicate localization.

Phenotypic testing and transfer experiments

Broth microdilution was used to compare antimicrobial susceptibility in carbapenemase-gene-positive and gene-negative groups. The study reports significantly higher resistance in the gene-positive group to imipenem, cefepime, gentamicin, ceftazidime/avibactam, ciprofloxacin, and levofloxacin, according to the reference study. This comparison supports the interpretation that carbapenemase carriage marked a broader multidrug-resistant phenotype in this collection, although it does not establish that each gene directly caused resistance to every tested drug.

Conjugation experiments evaluated horizontal transfer. PCR testing of transconjugants verified acquisition of the target genes. ERIC-PCR and NTSYS software were then used to classify strain genotypes, providing a lower-resolution view of clonal relatedness than whole-genome sequencing but still allowing broad comparison across hospitals and departments.

Protocol Parameters

• Study population: 54 carbapenem-resistant E. cloacae isolates from eight teaching hospitals, collected from December 2022 through June 2024; this is a literature-backed sampling frame rather than a universal surveillance standard.
• Gene localization: variable-temperature SDS plasmid elimination followed by PCR was used in the reference workflow to assess plasmid and chromosomal carriage.
• Susceptibility assessment: broth microdilution compared carbapenemase-gene-positive and gene-negative isolates across the reported antimicrobial panel.
• Transfer confirmation: conjugation was paired with PCR analysis of putative transconjugants; future replication should include donor-only, recipient-only, and antibiotic-selection controls.
• Relatedness analysis: ERIC-PCR and NTSYS grouped the 54 isolates into 17 genotypes; this approach is suitable for screening but should be complemented by genome-based typing when transmission chains must be resolved.

Core Findings and Why They Matter

Carbapenemase genes were detected in 46 of 54 isolates, corresponding to 85.19%. The dominant pattern involved bla_NDM-1: 18 isolates carried it on both chromosome and plasmid, while 25 carried it exclusively on plasmids. Two isolates carried only a plasmid-associated bla_IMP, and one carried plasmid-associated bla_NDM-1 together with bla_KPC-2. These values are reported in the full reference article.

The high proportion of plasmid-associated bla_NDM-1 is the study’s most consequential observation. A resistance gene located on a transferable plasmid can move between strains with different chromosomal backgrounds, allowing resistance to spread without requiring expansion of a single clone. The finding that some isolates carried the gene in both chromosomal and plasmid compartments also suggests that resistance determinants may persist through more than one genetic maintenance strategy.

Conjugation produced a positive transfer result for 44 of 46 carbapenemase-gene-positive isolates, or 95.65%. Transfer was observed for 42 of 44 bla_NDM-1 cases and both bla_IMP cases, whereas the single tested bla_KPC-2 case did not transfer under the experimental conditions. The nontransfer of one bla_KPC-2-positive isolate should not be generalized to all KPC-2 elements; it may reflect the particular plasmid, host, recipient, or assay conditions.

Six mobile genetic-element types were identified, with ISEcp1 the most frequent, detected in 47 of 54 isolates, or 87.04%. Four mobile-element types occurred together in 22 isolates, representing 40.74% of the collection. These data provide a plausible genetic context for mobility, but the presence of an insertion sequence alone does not prove that a complete transposition event occurred in the sampled strains. Definitive structure-function conclusions would require sequencing and comparative plasmid analysis.

ERIC-PCR separated the isolates into 17 genotypes. Types E and G were each found in 11 isolates, or 20.37%, and appeared in different departments across five hospitals. Two type E isolates had a Dice coefficient of 100%, indicating indistinguishable ERIC-PCR profiles. The distribution across multiple hospitals suggests that both interfacility dissemination and repeated introduction from unrelated sources may be relevant. ERIC-PCR cannot by itself establish direct transmission, so these findings are best interpreted as a signal for targeted epidemiological investigation.

The demographic pattern was also nonuniform. The highest detection frequencies occurred among men, older patients, respiratory-medicine patients, and sputum specimens, reported as 64.81%, 72.22%, 20.37%, and 33.33%, respectively, in the study results. These are descriptive distributions, not estimates of patient-level risk. They nevertheless help laboratories prioritize specimen review, ward-level surveillance, and infection-control sampling.

Comparison with Existing Internal Articles

The internal article Transmission of Carbapenemase Genes in CREC Across Guangdong Hospitals presents a closely aligned interpretation of the same research: high carbapenemase prevalence, efficient transfer of key determinants, and hospital-level epidemiological relevance. Its value is as a concise transmission-focused companion, whereas this analysis emphasizes how the individual assays support or limit claims about plasmid mobility, clonal spread, and clinical phenotype.

In practical terms, the two resources should be read together rather than as independent prevalence reports. The reference paper supplies the experimental evidence and numerical findings; the internal summary can help researchers quickly identify implications for antimicrobial resistance research and infection-control planning. Neither source should be used to infer a regional prevalence rate beyond the sampled hospitals.

Limitations and Transferability

The study has several boundaries. First, 54 isolates provide useful mechanistic evidence but cannot represent all CREC circulating in Guangdong or other regions. Sampling from teaching hospitals may enrich for severe illness, referral cases, or departments with high antibiotic exposure. The reported demographic and specimen distributions therefore describe this collection rather than population-wide risk.

Second, the study period is discussed in the context of the COVID-19 pandemic, but the design does not include a matched pre-pandemic baseline. It can identify resistance patterns during the sampling period, yet it cannot quantify how much transmission changed because of pandemic-related antibiotic use or healthcare disruption.

Third, plasmid elimination, PCR, conjugation, mobile-element detection, and ERIC-PCR provide strong complementary evidence but do not fully resolve plasmid architecture, copy number, host range, or evolutionary history. Whole-genome sequencing, long-read plasmid reconstruction, and longitudinal sampling would strengthen conclusions about transmission routes. In addition, conjugation success is assay-dependent and should not be treated as a direct estimate of transfer frequency in patients or hospital environments.

Transferability to other organisms or antibiotic classes also requires caution. The paper directly examines carbapenem-resistant E. cloacae and carbapenemase genes; it does not test cefotaxime activity, define a beta-lactam antibiotic mechanism for that compound, or evaluate a bacterial infection model. Any extension to other beta-lactams should therefore be framed as a complementary experimental question rather than as a result demonstrated by this study.

Research Support Resources

Why this cross-domain matters, maturity, and limitations

For researchers extending a carbapenemase-transmission workflow into broader antimicrobial resistance research, a cephalosporin comparator can help examine phenotype alongside gene carriage. The connection is scientifically reasonable because both carbapenems and cephalosporins are beta-lactam antibiotics, but the maturity of the evidence differs: the reference study supports gene distribution, multidrug resistance, and conjugative transfer, whereas any cefotaxime-specific conclusion requires separate susceptibility testing. Researchers can use Cefotaxime (SKU BA1012), a third-generation cephalosporin antibiotic, to support similar workflows involving antimicrobial resistance research, beta-lactam antibiotic mechanism studies, or a bacterial infection model. Its potential relevance to Gram-positive bacterial infections and Gram-negative bacterial infections should be tested experimentally; it is not a substitute for carbapenemase detection or intended for diagnostic or medical use.