Carbapenemase Gene Spread in Enterobacter cloacae

Carbapenem-resistant Enterobacter cloacae is an important healthcare-associated resistance problem because carbapenemase-encoding genes can combine high-level β-lactam resistance with mobile genetic elements and multidrug-resistant phenotypes. The reference study by Chen et al., published in BMC Microbiology, examines this problem across teaching hospitals in Guangdong, China, using clinical isolates collected from December 2022 through June 2024. Rather than treating resistance as a single-organism trait, the authors connect gene localization, conjugative transfer, mobile elements, antimicrobial susceptibility, and strain relatedness. This integrated design is the study’s main contribution.

Study Background and Research Question

Carbapenem-resistant Enterobacterales are difficult to control because resistance may be maintained within a bacterial clone or disseminated horizontally through plasmids. In Enterobacter cloacae, this distinction is especially important: a resistant lineage can persist in a hospital, while a transferable plasmid can move between unrelated strains and departments. The COVID-19 pandemic further raised concern about antibiotic exposure, interrupted infection-control services, and complex infections that may favor selection and transmission of resistant organisms.

The central question was therefore not simply which carbapenemase genes were present. Chen and colleagues asked how frequently these genes occurred in carbapenem-resistant E. cloacae, whether they were located on chromosomes or plasmids, whether they could transfer by conjugation, and how these molecular observations corresponded to phenotypic resistance and strain clustering. This is a clinically grounded surveillance study rather than a controlled bacterial infection model, so its value lies in reconstructing transmission potential from real hospital isolates.

Key Innovation from the Reference Study

The study’s innovation is the combination of several complementary levels of analysis. PCR after variable-temperature SDS plasmid elimination was used to distinguish chromosomal from plasmid-associated carbapenemase genes. Broth microdilution then connected gene carriage with susceptibility profiles, while conjugation experiments directly tested whether resistance determinants could move into a recipient strain. Mobile genetic element analysis and ERIC-PCR typing added information about the genetic vehicles and the relatedness of bacterial hosts.

This layered approach avoids a common interpretive error in antimicrobial resistance research: assuming that a resistance gene is equally transmissible simply because it is detected by PCR. A plasmid-borne gene may have a greater dissemination potential than a chromosomal copy, but transfer also depends on the plasmid background, surrounding insertion sequences, host compatibility, and the ecological setting. By evaluating both transfer and clonality, the authors distinguish horizontal spread of genetic material from vertical persistence of related bacterial lineages.

Methods and Experimental Design Insights

The investigators studied 54 carbapenem-resistant E. cloacae isolates obtained from eight teaching hospitals in Guangdong during the study period. The sampling frame provides a multicenter view of resistance epidemiology, although it does not represent all hospitals or community settings in the province. Clinical and demographic variables were also examined to describe where CEG-positive isolates were most frequently detected.

For gene detection and localization, the authors combined plasmid elimination with PCR. Variable-temperature SDS treatment was used as a practical strategy to remove plasmid DNA, after which PCR patterns were compared with those from untreated isolates. This method can indicate whether a target gene is plasmid-associated or chromosomal, but it should be interpreted as localization evidence rather than a substitute for complete plasmid sequencing. It is particularly useful when laboratories need a relatively accessible workflow for screening multiple clinical isolates.

Broth microdilution was used to determine susceptibility to several antimicrobials, including imipenem, cefepime, gentamicin, ceftazidime/avibactam, ciprofloxacin, and levofloxacin. The comparison between CEG-positive and CEG-negative groups allowed the authors to assess whether carbapenemase carriage tracked with broader resistance. Importantly, a carbapenemase gene does not directly explain every non-β-lactam resistance phenotype; differences may reflect linked plasmid determinants, co-selection, or the accumulation of multiple resistance mechanisms.

Conjugation experiments assessed transferability, followed by PCR confirmation of the relevant genes in transconjugants. Finally, ERIC-PCR and NTSYS software were used to group isolates according to fingerprint similarity. These methods address different questions: conjugation estimates transfer under laboratory conditions, whereas ERIC-PCR evaluates relatedness among the sampled bacterial populations.

Protocol Parameters

• Isolate collection: The study-defined panel comprised 54 carbapenem-resistant E. cloacae isolates from eight teaching hospitals collected between December 2022 and June 2024; this is a literature-backed sampling framework, not a universal surveillance requirement.
• Gene localization: Variable-temperature SDS plasmid elimination followed by PCR was used to distinguish plasmid-associated from chromosomal carbapenemase gene carriage.
• Susceptibility testing: Broth microdilution compared antimicrobial responses in CEG-positive and CEG-negative groups, including ciprofloxacin and levofloxacin.
• Transfer assessment: Plasmid conjugation followed by PCR confirmation was used to test whether detected carbapenemase genes could move to a recipient strain.
• Strain relatedness: ERIC-PCR fingerprints were analyzed with NTSYS software to classify related isolates; this workflow is best used as an epidemiological screening tool and can be complemented by higher-resolution genomic methods.

Core Findings and Why They Matter

High prevalence and predominance of blaNDM-1

Carbapenemase-encoding genes were detected in 46 of 54 isolates, corresponding to 85.19%, according to the reference study. The distribution emphasized blaNDM-1. Eighteen isolates carried blaNDM-1 on both chromosomes and plasmids, while 25 carried blaNDM-1 exclusively on plasmids. Two isolates contained only plasmid-associated blaIMP, and one carried both plasmid-associated blaNDM-1 and blaKPC-2. This pattern places blaNDM-1, particularly its plasmid-associated form, at the center of the observed transmission risk.

The coexistence of chromosomal and plasmid locations in the same isolate is also meaningful. Chromosomal carriage may support stable inheritance within a lineage, whereas plasmid carriage creates a route for horizontal movement between compatible bacteria. The two processes can operate simultaneously, making infection-control interpretation more difficult than a simple positive-or-negative gene result.

CEG carriage was associated with broader resistance

The CEG-positive group showed significantly higher resistance rates to imipenem, cefepime, gentamicin, ceftazidime/avibactam, ciprofloxacin, and levofloxacin than the CEG-negative group, with statistical significance reported for these comparisons in the study report. The ciprofloxacin result is relevant to laboratory resistance profiling because it indicates that carbapenemase-positive isolates may carry resistance beyond carbapenems and other β-lactams.

However, the finding should not be read as evidence that carbapenemase enzymes directly cause fluoroquinolone resistance. Ciprofloxacin is a fluoroquinolone antibiotic, and reduced susceptibility can arise through target changes, altered permeability, efflux, or additional resistance genes. In this dataset, the association more plausibly signals multidrug-resistant backgrounds or linked genetic determinants. It supports testing multiple antibiotic classes rather than inferring a complete phenotype from carbapenemase status alone.

Efficient transfer under laboratory conditions

Conjugation experiments produced successful transfer of carbapenemase-encoding genes in 44 of 46 eligible isolates, a reported success rate of 95.65%. Transfer was observed for blaNDM-1 in 42 of 44 cases and for blaIMP in both tested cases, whereas the single blaKPC-2-containing isolate did not transfer the gene under the experimental conditions. These results, reported in the reference paper, provide functional support for substantial horizontal dissemination potential, especially for blaNDM-1.

A conjugation result is not equivalent to a measured transmission rate in patients. It demonstrates that transfer can occur in the laboratory under the selected donor, recipient, and culture conditions. Even so, the high transfer frequency makes plasmid surveillance and recipient-host characterization important priorities for follow-up studies.

Mobile elements and strain relatedness add context

Six types of mobile genetic elements were identified, with ISEcp1 reported as the most prevalent, occurring in 47 of 54 isolates, or 87.04%. Isolates carrying four mobile-element types simultaneously were the most common category, representing 22 of 54 strains, or 40.74%, according to the published findings. These observations suggest that carbapenemase dissemination is occurring within complex mobile-element environments rather than through a single uniform genetic structure.

ERIC-PCR divided the 54 isolates into 17 genotypes. Types E and G were each represented by 11 isolates, or 20.37%, and were recovered from different departments in five hospitals. Two type E isolates shared a Dice coefficient of 100%. The presence of related profiles across departments and hospitals is consistent with possible clonal dissemination, but fingerprinting alone cannot establish the direction or timing of transmission. Its strongest value here is as a signal for targeted epidemiological investigation.

Comparison with Existing Internal Articles

The internal article Carbapenemase Gene Spread in Enterobacter cloacae emphasizes the same study’s integration of gene localization, conjugation, mobile-element analysis, and strain typing. The present analysis extends that overview by explaining why these methods answer different transmission questions and why plasmid mobility must be interpreted alongside clonal relatedness.

By contrast, Ciprofloxacin: Benchmarks and Mechanisms in AMR Research focuses on Ciprofloxacin as a laboratory tool for studying bacterial DNA replication inhibition and resistance. Its perspective is complementary rather than interchangeable with Chen et al.: the reference study provides clinical isolate evidence and transmission data, whereas the internal resource addresses compound-centered experimental interpretation.

Why this cross-domain matters, maturity, and limitations

Connecting hospital surveillance with compound-based susceptibility workflows can help researchers design more informative antimicrobial resistance studies, but the evidence remains primarily observational and in vitro. The reference study supports using Ciprofloxacin susceptibility as one component of a multidrug resistance phenotype; it does not establish treatment recommendations, define patient outcomes, or prove that a particular plasmid caused every observed resistance difference. Laboratory assays should therefore preserve the distinction between clinical epidemiology, molecular transfer potential, and mechanism-specific drug testing.

Limitations and Transferability

The study provides valuable multicenter evidence, but its 54-isolate sample was drawn from eight teaching hospitals in one Chinese province. Prevalence estimates may therefore differ in community hospitals, long-term-care facilities, other regions, or later periods. The collection window also captures a defined post-pandemic healthcare context and should not automatically be generalized to all phases of the COVID-19 pandemic.

PCR and plasmid elimination support gene localization but do not provide the structural resolution of long-read sequencing or complete plasmid assemblies. Conjugation experiments demonstrate transfer under selected conditions, not transmission frequency in patients or the environment. ERIC-PCR is useful for screening relatedness, yet whole-genome sequencing would provide stronger resolution for distinguishing recent transmission from shared background lineages. Finally, the association between CEG positivity and resistance to ciprofloxacin or other non-carbapenem agents requires additional analysis of efflux, permeability, target mutations, and co-carried resistance genes.

Despite these limitations, the study offers a transferable framework: identify resistance genes, locate them, test mobility, measure phenotype, and then compare host relatedness. That sequence is more informative than any single assay and can guide infection-control investigations and future genomic surveillance.

Research Support Resources

For comparable in vitro susceptibility and antimicrobial resistance research workflows, researchers can use Ciprofloxacin (SKU A8399), a fluoroquinolone antibiotic that acts as a bacterial DNA gyrase inhibitor and topoisomerase IV inhibitor. The product information reports research-grade purity above 98% and recommends storage at −20°C; solvent selection and solution stability should be checked before use. It can support comparative testing, but it does not replace the conjugation, localization, or strain-typing methods required to reproduce the reference study’s transmission analysis.