Carbapenemase-producing Enterobacterales (CPE) are multidrug-resistant bacteria that can colonize the gastrointestinal tract of hospitalized patients and spread within intensive care units (ICUs). Some colonized individuals may carry a particularly high bacterial burden and contribute disproportionately to environmental contamination and transmission to other patients. Identifying these high-risk individuals could improve infection prevention and control strategies.

This prospective observational study will be conducted in the Adult Intensive Care Unit of Hospital Italiano de Buenos Aires. Patients identified as colonized with CPE through routine surveillance will undergo quantitative culture and real-time polymerase chain reaction (PCR) testing of rectal swabs to measure bacterial load. Environmental samples will also be collected from high-touch surfaces surrounding colonized patients. In selected cases, molecular typing methods will be used to evaluate genetic relatedness between patient and environmental isolates and to investigate possible transmission events.

The primary objective is to determine the correlation between bacterial load measured by culture and the PCR cycle threshold (Ct) value. Secondary objectives include evaluating the association between bacterial load and environmental contamination, and assessing whether patients with higher bacterial loads are more likely to contribute to transmission within the ICU. Results may help identify patients with increased dissemination potential and support targeted infection prevention interventions.

Carbapenemase-producing Enterobacterales (CPE) represent a major global public health threat because of their extensive antimicrobial resistance and their ability to spread within healthcare facilities. Gastrointestinal colonization is recognized as the main reservoir for transmission. Previous studies have suggested that a subset of colonized patients, often referred to as "super-spreaders," carry a substantially higher bacterial burden and may account for a disproportionate share of environmental contamination and transmission events.

Although bacterial load can be quantified using culture-based and molecular methods, there is currently no validated cycle threshold (Ct) value from routinely used real-time PCR assays that reliably identifies patients with high dissemination potential. Establishing a correlation between Ct values and bacterial burden could provide a practical and accessible tool for infection prevention programs.

This prospective observational cohort study will be conducted in the Adult Intensive Care Unit of Hospital Italiano de Buenos Aires. All patients identified through the institutional surveillance program as colonized with CPE will be eligible for inclusion. For each CPE-positive patient, rectal swabs obtained as part of routine surveillance will undergo quantitative culture on selective chromogenic media and molecular testing using real-time PCR targeting carbapenemase genes. Bacterial load will be estimated using colony-forming unit counts and PCR Ct values.

Environmental contamination will be assessed through systematic sampling of high-touch surfaces in the patient's surroundings, including bed linen, bedside furniture, and medical equipment. Environmental isolates will undergo microbiological and molecular characterization. In selected situations, whole genome sequencing or pulsed-field gel electrophoresis will be performed to evaluate genetic relatedness among isolates recovered from patients, environmental samples, and secondary cases.

The primary outcome is the correlation between quantitative bacterial load and PCR Ct values. Secondary outcomes include the extent of environmental contamination associated with different bacterial loads and the occurrence of transmission events involving genetically related strains. Multivariable analyses will evaluate the influence of relevant clinical and epidemiological factors, including length of stay, fecal incontinence, prior antimicrobial exposure, immunosuppression, and other potential confounders.

The study involves minimal risk because all patient samples are obtained within the framework of routine infection control surveillance. Findings from this study may improve the identification of patients with increased transmission potential and support more targeted and efficient infection prevention strategies in critical care settings.

Inclusion Criteria

• Age 18 years or older.
• Hospitalized in the Adult Intensive Care Unit of Hospital Italiano de Buenos Aires during the study period.
• Positive surveillance rectal swab for carbapenemase-producing Enterobacterales.

Exclusion Criteria

-Patients colonized or infected with carbapenemase-producing Enterobacterales who remained in the same room for less than 48 hours.

Adult patients hospitalized in the Adult Intensive Care Unit of Hospital Italiano de Buenos Aires during the study period, with a positive surveillance rectal swab for carbapenemase-producing Enterobacterales detected through the institutional infection control surveillance program.

Individual participant data underlying the results reported in publications will be available after de-identification. Data may be shared with qualified researchers upon reasonable request to the principal investigator and after approval by the Hospital Italiano de Buenos Aires Ethics Committee, in accordance with institutional policies and applicable regulations regarding data protection and patient confidentiality.

What IPD Will Be Shared? De-identified demographic data Clinical characteristics Rectal swab quantitative culture results PCR cycle threshold (Ct) values Environmental sampling results Molecular characterization and genomic data used in analyses Derived study variables

Beginning 6 months after publication and ending 5 years after publication.

Researchers who provide a methodologically sound proposal may access de-identified participant data for scientific purposes. Requests will be reviewed by the principal investigator and the institutional ethics committee. Data sharing agreements may be required before data release.