A deadly, drug-resistant superbug outbreak that began last summer at the National Institutes of Health Clinical Center claimed its seventh victim Sept. 7, when a seriously ill boy from Minnesota succumbed to a bloodstream infection, officials said Friday.

The boy was the 19th patient at the research hospital to contract an antibiotic-resistant strain of the bacterium Klebsiella pneumoniae that arrived in August 2011 with a New York woman who needed a lung transplant. But his case marked the first new infection of this superbug at NIH since January — a worrisome signal that the bug persists inside the huge brick-and-glass federal facility in Bethesda.

“It’s heartbreaking,” said John Gallin, the physician-researcher who directs the clinical center. “What happened this summer was a very unfortunate case. All of these cases are hugely sad cases.”

The boy arrived in Bethesda in April after complications arose from a bone marrow transplant he received last year. His underlying condition — a severe genetic defect that crippled his immune system — increased his risk of acquiring the superbug, as did the steroids and other drugs the boy was given to combat complications from the transplant.

“We worried he was set up for a bad infection,” said Gallin.

On July 25, routine rectal swabs of patients for hospital-borne infections — a measure put in place during the worst of the outbreak last fall — detected the superbug in the boy.

Genetic analysis showed the boy’s strain matched that of the superbug that arrived last year. It eventually spread to 17 additional patients, of whom 11 died. Six of those deaths were directly attributed to the superbug by NIH staff. The NIH did not make the outbreak public until describing it in a scientific publication last month.

As the superbug spread last fall, NIH staff members built a wall to isolate infected patients, ripped out plumbing that harbored the bacteria, hired monitors to ensure doctors and nurses were properly scrubbing their hands and even blasted patients’ rooms with vaporized disinfectant.

By January, those measures had apparently halted the spread. For six months, no new patients became infected.

But in July, the boy tested positive for the superbug. Clinic staffers isolated him in the intensive-care unit and raced to treat the infection.

The boy’s superbug originally appeared vulnerable to one antibiotic, but after a week of therapy, the infection grew impervious to that drug, too, Gallin said. The NIH obtained an experimental antibiotic, but it also failed.

“This kid probably got this infection because a patient who was a carrier [of the superbug] was on the same unit,” said Gallin. “There was undoubtedly some intrahospital transmission despite our best efforts.”

Swabs picked up the superbug on a railing outside the boy’s room, but Gallin said it’s impossible to know whether the boy or someone else deposited it there.

Gallin said that earlier this year, two other patients arrived at the clinical center carrying different strains of potentially deadly drug-resistant Klebsiella. Neither of those strains has spread to other patients, Gallin said. One of those two patients was treated at two hospitals in Maryland before transferring to NIH.

Gallin declined to name those hospitals and, citing medical confidentiality, also declined to provide details about the boy, such as his age.

The NIH clinical center is a federal facility that is not required to report hospital-borne infections to the state. Nor does it have to report this type of infection to the federal Centers for Disease Control and Prevention, although Gallin said the CDC has been working closely with NIH to help control the outbreak.

Nonetheless, NIH officials informed Maryland health department officials about the Klebsiella case last week, state health department spokeswoman Dori Henry said Friday.

Montgomery County health officials were not aware of the death, a county health department spokeswoman said, although Gallin said the NIH had also informed the county of the death.

Klebsiella infections are a major problem for severely ill hospitalized people whose immune systems are weakened. Experts, including Gallin, are quick to reassure the public that such hospital-borne infections pose no risk to healthy people outside hospitals.

In 2011, about 80 percent of Maryland’s acute-care hospitals had at least one patient with carbapenem-resistant bacterial infection — the larger class of infections to which resistant Klebsiella belongs — Henry said in an e-mail.

Nationwide, about 6 percent of hospitals are battling outbreaks of this class of superbugs, according to the CDC, which has stepped up nationwide surveillance. Strains similar to those seen at NIH have spread across the world since first appearing in North Carolina in 2001, Gallin said.

Maryland said it is working with hospitals, nursing homes and other organizations to control and prevent infections through good hand hygiene, screening of patients for bacteria, room cleaning, and the judicious use of antibiotics.

At Suburban Hospital in Bethesda, which is near the NIH clinical center, hospital officials said they have had no outbreaks of drug-resistant Klebsiella. Only once or twice in the past year has there been evidence of such an infection, said Rita Smith, manager of the hospital’s infection control efforts.

The most common drug-resistant infections at Suburban, part of the Johns Hopkins health system, include methicillin-resistant Staphylococcus aureus, or MRSA, E. coli, and Clostridium difficile.