What the evidence tells us about patient vulnerability, antibiotic exposure and the hospital environment.
Ashleigh Myall
Understanding the spread of multidrug-resistant organisms (MDROs) means looking at the patient and the conditions around them. The organisms already present, the care a patient needs, antibiotic exposure and opportunities for transmission all contribute. Their importance varies between organisms and settings (CDC, 2024).
For Infection Prevention and Control (IPC) teams, the useful question is which of these factors could explain the pattern they are seeing locally. That calls for evidence from microbiology, patient histories and the hospital environment, interpreted together.
This article brings together selected studies on MDRO acquisition and spread, alongside related evidence on carriage and surveillance. The aim is to explain what each type of evidence can tell us, and where uncertainty remains.
Acquisition and Infection Are Different Events
A patient can carry an MDRO without having an infection. A positive result obtained after admission may reveal previously undetected carriage, rather than a new acquisition on the ward. CDC guidance explicitly cautions against using the timing of a clinical culture alone to decide where colonisation occurred (CDC, 2024).
Existing carriage can also become a source of infection within the same patient. In a prospective study of 498 intensive care patients, Klebsiella pneumoniae carriage on admission was associated with subsequent infection, and paired carriage and infection isolates frequently matched. This study included susceptible and resistant strains, so its results should not be treated as an MDRO-specific estimate (Gorrie et al., 2017).
These distinctions change the investigation. We need to consider whether the organism was already present, whether it was newly acquired and what allowed it to cause disease. A rise in positive results does not, on its own, answer those questions.
The Patient’s Condition Shapes Their Risk
Clinical vulnerability and the intensity of care matter. In a study across two New York hospitals, mechanical ventilation, pulmonary disease, days of antibiotic therapy and exposure to other CRE carriers were independently associated with acquisition of carbapenem-resistant Enterobacteriaceae, now generally termed carbapenem-resistant Enterobacterales (Swaminathan et al., 2013).
Among patients already carrying carbapenem-resistant K. pneumoniae, a separate multicentre study found that intensive care admission, abdominal invasive procedures, chemotherapy or radiotherapy, and colonisation at additional body sites were associated with subsequent bloodstream infection (Giannella et al., 2014).
These are different outcomes: acquiring an organism and developing infection after carriage. Devices and procedures can also indicate underlying illness and greater care needs. An association with a catheter, for example, should not automatically be read as evidence that the catheter caused acquisition.
Antibiotics Can Change the Conditions for Spread
Antibiotic exposure can influence the abundance of resistant organisms already carried by a patient. In a prospective study of 51 patients colonised with vancomycin-resistant enterococci (VRE), antibiotics active against anaerobic bacteria sustained high VRE densities in stool. Higher densities were also associated with more frequent contamination of the surrounding environment (Donskey et al., 2000).
This provides a biological reason to consider antibiotic exposure alongside a patient’s potential contribution to spread. Two patients recorded as VRE-positive may not create the same level of environmental contamination. However, this older, small study does not give us a universal shedding rate for every organism, antibiotic or patient.
For an investigation, the timing and type of antibiotic exposure may therefore add useful context. It remains one part of a wider assessment, and prescribing decisions require their own clinical and antimicrobial stewardship review.
The Patients Around You Add Information
Colonisation pressure describes the burden of organism carriage among surrounding patients. In a recent EHR-based study, higher colonisation pressure was associated with acquisition of the same organism across several susceptible and resistant pathogens (Sagers et al., 2026).
This helps explain why patients with similar clinical characteristics can have different exposure histories. The ward, the timing of an admission and the organisms circulating around them can add information that an individual clinical record misses.
Co-location remains evidence of a possible exposure. It does not establish a direct transmission event. Shared staff, equipment or another unobserved source may help explain the association. The value is in identifying a connection that deserves review.
Environmental Reservoirs Can Sustain Transmission
Patients do not need to occupy a room simultaneously for the environment to matter. In the nine-hospital BETR Disinfection trial, adding ultraviolet disinfection to the reference cleaning strategy reduced the combined acquisition or infection outcome among patients entering rooms previously occupied by carriers of target organisms. The relative risk was 0.70; the result applied to that defined population and combined outcome, rather than every organism or cleaning strategy (Anderson et al., 2017).
Reservoirs can also behave differently within the same hospital. A recent South Korean study found persistent sink-associated NDM-producing organisms alongside KPC-producing K. pneumoniae spreading predominantly through person-to-person contact. Environmental and patient-directed strategies were both relevant (Park et al., 2026).
The practical question is whether a recurring pattern fits a persistent location-based source, exposure between patients, or both. Movement records can help organise that investigation, while environmental sampling and microbiological evidence help test the explanation.
Patient Journeys Extend Beyond One Ward
Previous admissions and transfers can connect exposures across organisations. CDC guidance recognises movement between acute care, outpatient and long-term care settings as an opportunity for MDRO dissemination, making prior healthcare contact and communication at transfer relevant to local investigation (CDC, 2024).
Recent work in England illustrates how those connections can inform surveillance. Researchers used patient-sharing networks to select sentinel hospitals for detecting emerging Clostridioides difficile strains. Their simulations supported a network-informed approach to choosing surveillance sites (Pople et al., 2026).
difficile is included here as a related surveillance example, rather than as a synonym for MDROs. The transferable lesson is that the organisation where a result appears may represent only one part of the patient’s relevant journey.
What We Observe Depends on What We Test
Clinical cultures alone can miss asymptomatic carriage, and a positive result needs clinical context to distinguish colonisation from infection (CDC, 2024). This makes screening history part of the evidence, including when samples were taken and which patients were eligible for testing.
When interpreting a change in detected cases, teams should ask whether testing practice also changed. A more complete picture of carriage may initially reveal more cases. Equally, missing results or infrequent screening leave uncertainty about when acquisition occurred. These are reasons to examine the observation process before attributing a trend to a change in transmission.
Bringing the Evidence Into Clinical Review
Taken together, these studies support an assessment that connects patient vulnerability, antibiotics, the surrounding burden of carriage, care contacts and environmental history. No single factor provides a complete explanation. The relevant combination depends on the organism, the setting and the information available.
This is the approach we are taking at NEX. Our platform brings together clinical information, microbiology and patient movements to support infection surveillance, risk assessment and outbreak investigation. NEX is a UKCA-marked Class I medical device in Great Britain, intended to support clinical decision-making (NEX Health Intelligence, 2026a).
We build our risk pipelines to pass supporting evidence alongside the score, so teams can review the recorded factors associated with an elevated estimate. These can include potential exposure to patients carrying an organism and relevant clinical context, depending on the model and available data (NEX Health Intelligence, 2026b).
A statistical explanation describes what contributes to a model’s estimate. Establishing the cause of an individual acquisition requires additional evidence; explanations based on observed associations do not by themselves establish what would happen if a factor were changed (Janzing et al., 2020).
For example, a recorded overlap with a known carrier may justify reviewing the timing, microbiology and wider contact history. It is not confirmation that one patient infected another. NEX does not diagnose infection or confirm transmission, and professional judgement remains with the clinical team.
Our aim is to make possible drivers easier to examine, with the evidence and its limitations visible. That gives IPC teams a clearer basis for deciding what to investigate next and where additional information could change their response.
