Carbapenemase Transmission in Resistant Enterobacter cloacae

Carbapenem-resistant Enterobacter cloacae is an important component of the wider carbapenem-resistant Enterobacteriaceae problem. The organism can accumulate β-lactam resistance determinants, persist in healthcare environments, and acquire mobile genetic elements that broaden resistance across bacterial populations. The reference study by Chen and colleagues, published in BMC Microbiology in 2025, addresses this problem through a combined epidemiological, molecular, and transfer-oriented investigation.

Rather than measuring resistance as a purely phenotypic endpoint, the study asks how carbapenemase-encoding genes are positioned within bacterial cells, how readily they move between strains, and whether genetically related isolates are distributed across hospitals and clinical departments. That combination makes the work relevant to septicemia treatment research, surveillance design, and experimental planning for a Gram-negative bacterial infection model.

Study Background and Research Question

The study was motivated by the increasing clinical importance of carbapenem-resistant E. cloacae and by concern that antibiotic use, disrupted healthcare services, and complex infections during the COVID-19 period could influence resistance transmission. The investigators collected 54 carbapenem-resistant E. cloacae isolates from eight teaching hospitals in Guangdong Province between December 2022 and June 2024, as reported in the reference study.

The central research questions were mechanistic and epidemiological: What proportion of these isolates carried carbapenemase-encoding genes? Were the genes located on plasmids, chromosomes, or both? Could the resistance determinants transfer by conjugation? Which mobile genetic elements accompanied them, and did strain-typing data indicate dissemination of related clones between hospitals or departments?

This framing is important because carbapenem resistance is not a single phenotype with a single route of spread. A resistance gene on a conjugative plasmid may move horizontally between unrelated strains, whereas a chromosome-associated determinant may be maintained through clonal expansion. Distinguishing these routes helps determine whether infection-control interventions should focus primarily on patient-to-patient transmission, plasmid dissemination, or both.

Key Innovation from the Reference Study

The main innovation is the integration of four evidence layers in the same isolate collection: carbapenemase-gene detection, intracellular localization, plasmid-transfer testing, and strain relatedness. Many surveillance reports describe the prevalence of genes such as bla_NDM-1, but prevalence alone cannot establish whether a determinant is mobile or whether repeated detection reflects one successful clone. Chen et al. connect those questions experimentally.

The study also separates horizontal dissemination from potential vertical dissemination. Variable-temperature sodium dodecyl sulfate plasmid elimination and PCR were used to investigate whether genes were plasmid-associated or chromosome-associated. Conjugation experiments then tested whether the relevant determinants could move into a recipient strain. ERIC-PCR and NTSYS analysis supplied a complementary view of clonal similarity. This layered design is the paper's most meaningful contribution: it treats resistance transmission as the interaction of genetic location, mobility, and population structure.

The work further places mobile genetic elements around the resistance genes. The frequent detection of ISEcp1 suggests that insertion-sequence context may be useful when comparing isolates and interpreting the potential for gene capture or rearrangement. The data do not by themselves prove the complete evolutionary history of each element, but they identify a practical molecular layer for follow-up sequencing and surveillance.

Methods and Experimental Design Insights

The investigation began with a geographically distributed isolate set rather than a single-ward collection. This increases the opportunity to observe both shared and hospital-specific patterns, although it does not make the sample representative of all clinical settings. The researchers grouped isolates according to carbapenemase-encoding gene status and evaluated resistance phenotypes using broth microdilution.

Protocol Parameters

• Isolate framework: Study carbapenem-resistant E. cloacae collected from eight teaching hospitals in Guangdong between December 2022 and June 2024; use a similarly defined collection when comparing transmission patterns across sites, as described in the reference study.
• Gene detection and localization: Apply variable-temperature SDS plasmid elimination before PCR-based analysis to distinguish likely plasmid-associated from chromosome-associated carbapenemase genes. Treat this as a localization screen rather than a substitute for complete genome sequencing.
• Phenotypic susceptibility: Use broth microdilution to compare carbapenemase-encoding gene-positive and gene-negative groups. Maintain the paper's separation between molecular status and susceptibility phenotype instead of assuming that every gene produces an identical resistance profile.
• Transfer assessment: Pair plasmid conjugation with PCR confirmation in donor, recipient, and transconjugant isolates. This workflow directly tests horizontal transfer and reduces the risk of interpreting gene co-occurrence as mobility.
• Mobile-element analysis: Screen for the selected insertion sequences and other mobile genetic elements in parallel with resistance genes. Co-occurrence can guide mechanistic hypotheses but should not be presented as proof of a functional transposition event without additional experiments.
• Clonal analysis: Use ERIC-PCR and numerical clustering to identify related profiles across departments and hospitals. For high-consequence transmission investigations, consider confirming important clusters with higher-resolution genomic methods.

The experimental sequence is efficient because each method answers a different question. PCR establishes gene presence; plasmid elimination informs location; broth microdilution measures phenotype; conjugation evaluates mobility; mobile-element assays provide genetic context; and ERIC-PCR estimates relatedness. For laboratories developing an antibacterial agent for Gram-negative and Gram-positive bacteria workflow, this separation of questions is more informative than relying on a single susceptibility assay.

Core Findings and Why They Matter

Carbapenemase-encoding genes were detected in 46 of the 54 isolates, corresponding to 85.19% of the collection, according to the reference study. The dominant determinant was bla_NDM-1. Notably, 18 isolates carried bla_NDM-1 on both chromosomes and plasmids, while 25 carried the gene exclusively on plasmids. Two isolates contained only a plasmid-associated bla_IMP gene, and one carried plasmid-associated bla_NDM-1 together with bla_KPC-2. These results show that the same broad resistance phenotype can arise from different genetic architectures.

The location data are particularly significant. A plasmid-only pattern creates a plausible route for horizontal transmission between compatible bacterial hosts, whereas dual chromosomal and plasmid localization may support both persistence within a lineage and onward mobility. The study therefore supports a model in which resistance can be maintained vertically while also being redistributed horizontally.

Phenotypically, the carbapenemase-encoding gene-positive group had significantly higher resistance rates to imipenem, cefepime, gentamicin, ceftazidime/avibactam, ciprofloxacin, and levofloxacin than the gene-negative group, with the comparisons reported as statistically significant in the paper. This pattern indicates that carbapenemase carriage was associated with broader multidrug resistance rather than an isolated change in carbapenem susceptibility. It also cautions against selecting treatment or experimental conditions from a single antimicrobial readout.

Conjugation results supplied the strongest evidence for mobility. Carbapenemase-encoding genes transferred successfully in 44 of 46 relevant experiments, a 95.65% success rate. Transfer was observed for bla_NDM-1 in 42 of 44 experiments and for both tested bla_IMP genes, whereas the single bla_KPC-2 result did not transfer. These gene-specific differences matter: a resistance determinant can be epidemiologically important even when its transfer behavior differs from that of another carbapenemase gene.

Six mobile genetic element types were identified. ISEcp1 was the most prevalent, detected in 47 of 54 isolates, or 87.04%, and isolates carrying four mobile-element types simultaneously formed the largest category at 22 of 54, or 40.74%, as reported in the study. ERIC-PCR divided the collection into 17 genotypes. Types E and G were each found in 11 of 54 isolates, or 20.37%, and occurred across different departments in five hospitals. Two type E isolates shared a Dice coefficient of 100%. Together, these observations support concurrent plasmid-mediated spread and dissemination of related strains.

The clinical distribution also provides surveillance clues. The highest detection frequencies occurred among male patients, older individuals, respiratory-medicine cases, and sputum specimens, accounting for 35 of 54, 39 of 54, 11 of 54, and 18 of 54 isolates, respectively, according to the reference article. These are descriptive distributions, not evidence that sex, age, department, or specimen type independently causes gene carriage. Their practical value is in identifying settings where targeted sampling and infection-control review may be most informative.

Comparison with Existing Internal Articles

The internal article Meropenem: β-Lactam Antibiotic Carbapenem for Reliable Assays is oriented toward assay optimization and experimental workflow planning. It can complement the present paper when researchers need to translate resistance-surveillance concepts into controlled bacterial assays, but it does not replace the Guangdong isolate data or the conjugation evidence.

A second resource, Meropenem and Carbapenem Resistance: Mechanisms, Models, and Translational Frontiers, provides broader discussion of resistance mechanisms and infection-model applications. In contrast, the reference study offers primary multicenter evidence focused specifically on E. cloacae, gene localization, mobile elements, and transmission. The most defensible use of these resources is therefore complementary: the BMC Microbiology article supplies the evidence base, while the internal guides can help organize laboratory workflows and model selection.

Limitations and Transferability

The study has several limits that affect interpretation. First, the sample contains 54 isolates from one province and eight teaching hospitals, so prevalence estimates should not be generalized to other regions, community settings, or non-teaching institutions without validation. The collection period also describes a defined 2022–2024 surveillance window; it cannot, by itself, quantify how transmission changed relative to a pre-pandemic baseline.

Second, PCR and variable-temperature SDS plasmid elimination provide useful evidence about gene location but may not resolve complete plasmid structures, copy numbers, integration sites, or the precise genetic arrangements surrounding each determinant. Conjugation demonstrates transfer under the tested laboratory conditions, not necessarily the frequency of transfer in patients, wastewater, or hospital surfaces. Similarly, ERIC-PCR is useful for screening relatedness but has lower resolution than whole-genome sequencing for distinguishing recent transmission from shared ancestry.

Third, the clinical distributions are descriptive and may reflect sampling intensity, referral patterns, antimicrobial exposure, or specimen availability. The association between carbapenemase genes and multidrug resistance is important, but it does not establish that every co-resistant phenotype is caused directly by the detected carbapenemase. Future work would benefit from longitudinal sampling, complete plasmid and chromosome sequencing, quantitative transfer measurements, and integration with patient-contact or environmental data.

Despite these constraints, the framework is transferable. A laboratory studying carbapenem-resistant bacterial infections can reproduce the conceptual sequence of gene detection, localization, phenotype testing, transfer assessment, and strain comparison while adapting the assays to its own organism and biosafety requirements. For septicemia treatment research, this approach helps distinguish an antimicrobial's apparent activity against a strain from the strain's capacity to disseminate resistance.

Research Support Resources

Researchers can use Meropenem (SKU A5124) as a defined carbapenem exposure in related susceptibility assays, resistance-selection studies, or a Gram-negative bacterial infection model, provided concentrations, controls, and endpoints are established in the approved protocol. It is a β-lactam antibiotic carbapenem and an antibacterial agent for Gram-negative and Gram-positive bacteria. The product is supplied for scientific research use only and is not intended for diagnostic or medical purposes.