Carbapenemase Transmission in Enterobacter cloacae

Carbapenem-resistant Enterobacter cloacae is an important clinical surveillance target because resistance can arise from mobile carbapenemase-encoding genes as well as from clonal expansion. The reference study by Chen et al., published in BMC Microbiology in 2025, examines this problem across eight teaching hospitals in Guangdong Province, China. Rather than treating resistance as a single phenotype, the investigators connected gene carriage, plasmid localization, laboratory transfer, mobile genetic elements, and strain relatedness. The full study is available through the reference publication.

Study Background and Research Question

Carbapenem-resistant Enterobacterales are difficult to control because carbapenemase genes can move between bacterial cells and may be embedded in plasmids, chromosomes, or composite mobile structures. In E. cloacae, this problem is particularly relevant to hospital microbiology: the organism is detected across multiple clinical departments, while its resistance determinants can compromise several antimicrobial classes at once.

Chen and colleagues asked three related questions. First, which carbapenemase-encoding genes were present among carbapenem-resistant E. cloacae isolates collected between December 2022 and June 2024? Second, were these genes located on plasmids, chromosomes, or both, and could they transfer by conjugation? Third, did the isolates form recognizable genetic groups that might indicate clonal dissemination across hospitals or departments? The study also assessed whether gene-positive isolates displayed a broader resistance phenotype than gene-negative isolates.

The pandemic-era context is important but should be interpreted carefully. The authors discuss increased antibiotic exposure, disrupted healthcare services, and complex infections as factors that could favor resistance transmission. However, the collection period alone cannot establish that the COVID-19 pandemic caused the observed gene distribution; a pre-pandemic comparator and longitudinal intervention data would be needed for that conclusion.

Key Innovation from the Reference Study

The principal innovation is the integration of transmission mechanisms at several biological levels. Many surveillance reports identify carbapenemase genes by PCR, while other studies focus on clonal typing or antimicrobial susceptibility. Here, those approaches were combined in one regional collection. This design distinguishes at least three nonexclusive processes: persistence of a resistant clone, movement of a resistance plasmid between strains, and coexistence of resistance genes with mobile genetic elements that may facilitate further rearrangement.

This layered analysis produced a more informative picture than a simple prevalence estimate. The study found that 46 of 54 isolates carried a carbapenemase-encoding gene, but the key observation was the distribution of bla_NDM-1. Some isolates carried the gene on both chromosome and plasmid, whereas another large group carried it exclusively on plasmids. That distinction matters because plasmid localization provides a plausible route for horizontal transfer, while chromosomal integration may support stable inheritance within a lineage.

The work also places mobile genetic elements and strain structure in the same analytical framework. ISEcp1 was the most prevalent of six identified mobile elements, and ERIC-PCR separated the collection into 17 genotypes. The combination helps researchers avoid an overly narrow interpretation in which every resistant isolate is assumed to represent either one outbreak clone or unrestricted plasmid spread.

Methods and Experimental Design Insights

The investigators analyzed 54 carbapenem-resistant E. cloacae isolates from eight teaching hospitals. Variable-temperature sodium dodecyl sulfate plasmid elimination was used to help determine whether carbapenemase genes were plasmid-borne or chromosomal, followed by PCR confirmation. This is a practical genetic workflow when the immediate question is gene localization rather than complete genome architecture.

Phenotypic resistance was measured by broth microdilution. The study compared carbapenemase-encoding-gene-positive and -negative groups across imipenem, cefepime, gentamicin, ceftazidime/avibactam, ciprofloxacin, and levofloxacin. Plasmid conjugation experiments followed by PCR tested whether resistance determinants could transfer to a recipient strain. Finally, ERIC-PCR and NTSYS software were used to estimate genetic relatedness among the 54 isolates.

Protocol Parameters

• Isolate set: Use a clearly defined collection of carbapenem-resistant E. cloacae from multiple clinical sites; the reference cohort covered eight teaching hospitals and the December 2022–June 2024 period.
• Gene localization: Apply variable-temperature SDS plasmid elimination followed by PCR when distinguishing plasmid-associated from chromosomal carbapenemase genes. Interpret this as a localization workflow rather than a substitute for complete plasmid sequencing.
• Susceptibility testing: Use broth microdilution to compare gene-positive and gene-negative isolates across clinically relevant antimicrobial classes. The reference study reports group-level significance, but investigators reproducing the analysis should retain isolate-level MIC distributions and quality-control data.
• Transfer assessment: Pair conjugation experiments with PCR confirmation in transconjugants. A positive laboratory transfer result supports horizontal mobility, but it does not by itself prove patient-to-patient transmission.
• Relatedness analysis: ERIC-PCR can provide a screening view of strain clustering. For outbreak confirmation or plasmid-resolved epidemiology, higher-resolution genome and plasmid analyses would strengthen the design.

Core Findings and Why They Matter

Carbapenemase-encoding genes were detected in 85.19% of the collection, or 46 of 54 isolates, according to the reference study. Among all isolates, 18 carried bla_NDM-1 on both chromosomes and plasmids, while 25 carried it only on plasmids. Two isolates carried only bla_IMP on plasmids, and one carried both plasmid-associated bla_NDM-1 and bla_KPC-2. The predominance of NDM-1, particularly in plasmids, identifies a plausible high-priority route for regional dissemination.

Gene-positive isolates had significantly higher resistance rates to imipenem, cefepime, gentamicin, ceftazidime/avibactam, ciprofloxacin, and levofloxacin than the gene-negative group. The result is consequential for antibacterial research because it links a molecular marker with a multidrug-resistant phenotype rather than with carbapenem resistance alone. At the same time, the finding should not be interpreted as universal susceptibility prediction: resistance can also reflect porin changes, efflux, altered target expression, or additional beta-lactamases.

Conjugation showed a high transfer capacity in the tested system. The overall reported success rate was 95.65%, or 44 of 46 tested isolates. Transfer was observed for bla_NDM-1 in 42 of 44 tests and for bla_IMP in both of two tests, whereas the single tested bla_KPC-2 case did not transfer. These results support the conclusion that horizontal dissemination is not merely theoretical in this collection. They do not, however, quantify transfer frequency under hospital environmental conditions.

Six mobile genetic elements were identified, with ISEcp1 present in 87.04% of all isolates, or 47 of 54. Isolates carrying four mobile-element types simultaneously were the largest subgroup, representing 40.74% of the collection. This pattern suggests a genomic environment capable of repeated recombination or mobilization, although the study does not establish the direction or timing of each genetic event.

ERIC-PCR divided the isolates into 17 genotypes. Types E and G were each found in 11 isolates, representing 20.37% apiece, and these types originated from different departments in five hospitals. Two type E isolates shared a Dice coefficient of 100%, a finding compatible with close genetic relatedness. Epidemiologically, the highest detection frequencies occurred among male patients, older patients, respiratory medicine cases, and sputum specimens. These distributions identify useful surveillance strata, not independent risk factors, because the study was not designed as a controlled risk-factor analysis.

Comparison with Existing Internal Articles

The reference study complements the internal review Ceftolozane/Tazobactam: Mechanism and Evidence. That article focuses on PBP binding, exposure relative to MIC, antipseudomonal activity, and resistance considerations for a cephalosporin–beta-lactamase inhibitor combination. In contrast, Chen et al. focus on the epidemiological and genetic architecture of carbapenem resistance in E. cloacae. Reading them together separates two questions that are often conflated: how an antibacterial agent acts, and how resistance determinants circulate among clinical isolates.

A second related resource, Imipenem: Bridging Mechanism and Strategy in Translational Research, discusses connections between molecular action and experimental strategy. Its mechanistic perspective can help frame susceptibility assays, but the Guangdong study supplies the stronger evidence for plasmid localization, conjugative transfer, and hospital-associated strain clustering. Neither resource should be used to infer that a mechanistic profile overcomes carbapenemase-mediated resistance.

Limitations and Transferability

The sample size was modest and geographically concentrated in eight teaching hospitals in one province. The results therefore provide a detailed regional snapshot rather than a national prevalence estimate. Selection of isolates from clinical specimens may also enrich for severe or treatment-exposed infections and may not represent colonizing populations in patients, healthcare workers, or the hospital environment.

Several conclusions are mechanistically suggestive rather than definitive. Conjugation demonstrates transfer under laboratory conditions, but direct transmission pathways require contact tracing, environmental sampling, time-resolved isolate collection, and genomic comparison. ERIC-PCR is useful for screening relatedness but generally has lower discriminatory and structural resolution than whole-genome sequencing. Similarly, SDS-based plasmid elimination and PCR can support localization, yet they do not fully resolve plasmid backbones, insertion sites, copy number, or co-located resistance genes.

The study also lacks a pre-pandemic comparison in the presented cohort, so changes attributable specifically to COVID-19-associated prescribing or healthcare disruption remain uncertain. Transferability to other regions will depend on local antibiotic consumption, plasmid populations, infection-control practices, and the prevalence of NDM, IMP, KPC, and other carbapenemases. These limitations do not weaken the central observation; they define the next level of evidence needed for intervention planning.

Why this cross-domain matters, maturity, and limitations

The cross-domain value lies in connecting hospital molecular epidemiology with antibacterial research and assay design. The study is mature enough to support surveillance priorities, gene-localization experiments, and transfer-risk hypotheses, but it does not establish clinical treatment effectiveness, immune response modulation, or performance in a sepsis animal model. Those questions require separate pharmacology, host-response, and in vivo studies and should not be inferred from the present isolate-based evidence.

Research Support Resources

For comparable in vitro susceptibility and resistance-characterization workflows, researchers can use Imipenem (SKU P10075), a semisynthetic thienamycin antibiotic and beta-lactam antibiotic targeting PBPs. Its described broad-spectrum activity includes gram-negative and gram-positive bacteria, but the product is intended for scientific research use only. In this study context, it is best treated as a standardized test compound for phenotype comparison, not as evidence that plasmid-mediated carbapenemase resistance can be therapeutically bypassed.