Bacteria found in 11 patients at a Maryland hospital were genetically matched to samples from the hospital's own water system, according to an investigation published in the CDC journal Emerging Infectious Diseases.
The organism is Mycobacterium abscessus, a rapidly growing nontuberculous mycobacterium that resists most standard antibiotics and can take months of combination therapy to treat. Hospital infection prevention staff notified the Maryland Department of Health in June 2024 after 10 patients returned positive cultures, and retrospective case finding brought the total to 11.
Whole-genome sequencing did the decisive work. Comparing patient isolates against environmental isolates, investigators found differences of zero to 24 single-nucleotide polymorphisms, close enough to indicate that the infections likely followed exposure to the organism present in the building's water.
The Investigation and What It Turned Up
The inquiry drew in the hospital's infection prevention team, the Maryland Department of Health, the CDC, and the Wadsworth Center laboratory at the New York State Department of Health. Authors were affiliated with those agencies along with University of Maryland Capital Region Health in Largo and the University of Maryland School of Medicine. The published report does not name the facility.
Among the 11 patients, median age was 40, with a range from 24 to 76. Five had surgical procedures across three operating rooms. No two had the same procedure, though the two patients who used the same room had each had cardiac surgery, one involving a heater-cooler machine. Two rooms housed more than one affected patient, but their admissions did not overlap.
Not every positive culture was an active infection. The published patient table classifies some cases as clinical infections and others as contamination or colonization, a distinction that matters for reading the count honestly. Six of the 11 received antimicrobial therapy directed by infectious disease doctors.
Once the source was suspected, the hospital restricted tap water use, installed point-of-use filters, flushed the water system building-wide, and retrained staff on sink safety. No post-mitigation isolates were sequenced, so the published work does not itself confirm the measures cleared the organism.
The full report also records an earlier cluster of three cases at the same facility in 2021, shortly after a newly constructed building was occupied. Water quality testing done before occupancy was within acceptable ranges but did not include specific testing for M. abscessus.
The Broader Problem Hospitals Are Grappling With
This cluster is not an isolated failure at one building. Nontuberculous mycobacteria live naturally in soil, dust and water, and form biofilms inside plumbing that resist standard disinfection. Municipal water arriving at a hospital is treated, but disinfectant levels drop across long pipe runs, low-flow outlets and equipment such as ice machines.
Similar investigations elsewhere have traced clusters to hospital water outlets. A larger cohort study of transplant recipients matched patient isolates to bacteria recovered from tap water, sink basins, drain traps and heater-cooler devices, and concluded that persistent environmental reservoirs, rather than person-to-person spread, were the primary source. Earlier genomic work on a hospital-associated outbreak reached similar conclusions about water outlets.
Infections are hard to treat because M. abscessus resists many antimycobacterial drugs, and courses run for months with multiple agents. That is why 11 cases are a meaningful clinical burden even though the number sounds small.
The risk is concentrated rather than universal. These organisms rarely cause disease in people with healthy immune systems and intact skin. Those infected in health care settings typically have surgical wounds, indwelling lines or catheters, weakened immune systems, or lung disease such as bronchiectasis or cystic fibrosis.
The Realistic Takeaway for Patients and Families
Nothing here suggests hospital tap water is generally unsafe, and no patient should avoid necessary care because of it. The cluster surfaced because a hospital's own infection prevention staff noticed a pattern and reported it, which is surveillance working rather than failing.
Higher-risk patients do have reasonable questions to raise. Someone with a central line, a fresh surgical wound, a transplant or advanced lung disease can ask whether tap water should be avoided for drinking, for filling a nebulizer or humidifier, or for showering during a stay. Many facilities already have water safety plans covering this, and asking prompts the conversation.
Symptoms depend on where the organism lands. Skin and soft tissue infections can appear as slow-healing wounds, drainage, or nodules near a surgical site or line. Pulmonary infections can cause persistent cough, fatigue, fever, and weight loss. These build over weeks, so a wound not healing on schedule after discharge deserves a call to the surgical team.
Families of recently hospitalized patients with an unexplained, persistent infection can ask whether cultures for nontuberculous mycobacteria were ordered, since these organisms do not grow on standard cultures and are easily missed.
Reporting of this kind is voluntary and inconsistent nationally, raising the accountability question underneath the story. No federal rule requires hospitals to publicly disclose nontuberculous mycobacteria clusters the way some other health care-associated infections are reported. Until that changes, the public record depends on facilities and state health departments choosing to publish investigations like this one.
Key Questions Answered
What was found? Eleven patients at a Maryland hospital had cultures positive for Mycobacterium abscessus, and genetic sequencing matched patient isolates to samples from the hospital's water system.
Is the hospital named? The published CDC report does not name the facility. Investigators were affiliated with the Maryland Department of Health, the CDC, two University of Maryland institutions, and the New York State Department of Health.
Were all 11 patients sick? No. Some cases were classified as clinical infections and others as contamination or colonization. Six of the 11 received antimicrobial therapy.
Who is at risk from this bacterium? People with surgical wounds, indwelling lines or catheters, weakened immune systems, or existing lung disease. It rarely causes illness in healthy people.
What did the hospital do about it? It restricted tap water use, installed point-of-use filters, flushed the water system building-wide, and retrained staff on sink safety. No samples were sequenced afterward, so the report cannot confirm the measures cleared the organism.
What should a recently hospitalized patient watch for? Slow-healing wounds, unexpected drainage near a surgical site or line, or a persistent cough with fever and weight loss. Contact the treating team rather than waiting.
