How a Common Skin Germ Travels Through a Newborn Intensive Care Unit, and How Hospitals Try to Stop It

Three studies from Germany and the United States (Knoll et al. 2025, She et al. 2026, Neary et al. 2025) on staph carriage, spread between babies and the role of masks

Many healthy people carry a germ called staph on their skin or in their nose, but in babies born very early it can sometimes cause a serious blood infection. Three recent studies followed this germ from a baby first picking it up, to its spread between babies in a neonatal unit, to what might stop it. They show that carrying staph is common and usually harmless, that some strains spread faster than others, and that masks may help.

Why this germ matters for the smallest babies

Staphylococcus aureus, usually shortened to "staph", is one of the most familiar germs in medicine. Most of the time it simply lives on the skin or inside the nose. Doctors call this "colonisation" or "carriage": the germ is present, but it is not making anyone ill. The trouble comes when it gets past the body's defences, for example through a thin tube placed in a vein (a central line), and enters the bloodstream. Babies born very early have fragile skin and an immune system that is still developing, so they are more vulnerable than older children. There are two broad types of staph. MRSA, short for methicillin-resistant Staphylococcus aureus, resists several common antibiotics. MSSA, methicillin-susceptible Staphylococcus aureus, is the ordinary type that those antibiotics can still treat.

What doctors knew before these studies

For many years, hospitals worried mostly about MRSA, because its resistance to antibiotics made outbreaks frightening. Large studies gradually showed that the ordinary type deserves just as much attention. In 348 neonatal units across the United States, MSSA caused 2,868 of 3,978 serious staph infections in babies, and the researchers concluded that it caused more infections and more deaths than MRSA [1]. A large American research network caring for very small babies, those born weighing less than 1,500 g (about 3.3 pounds), found that about 4 in every 100 of them (3.7%) developed a staph infection of the blood or of the lining around the brain, and that the risk of dying was similar whichever type was involved [2]. A more recent study of 315 American neonatal units found that babies of very low birth weight had more than 20 times the rate of serious staph infection seen in bigger babies [3].

Doctors also learned that carriage usually comes first. A combined analysis of many studies found that babies and children in intensive care who carried MRSA were far more likely to develop an MRSA infection than those who did not [4]. In a German neonatal unit in Heidelberg, babies who carried MSSA in their nose were more likely to become infected, and the germ that caused the infection was genetically the same as the one they had been carrying [5]. This is one reason many units began swabbing babies' noses regularly. Treatments to clear the germ were also tried. An antibiotic ointment called mupirocin removed staph from most babies within days, but many picked it up again after two or three weeks [6]. When parents who carried staph were treated, fewer babies picked up their parents' strain [7]. Still, it was unclear how much of the germ spreads from baby to baby, whether screening actually prevents infection, and which prevention steps work best.

Three studies, three pieces of the puzzle

The first study, led by Knoll and colleagues in Germany, used records from the German Neonatal Network (GNN), which gathers information on very small babies from 68 German hospitals [8]. It included 21,491 babies born weighing under 1,500 g between 2009 and 2023. The team also studied 638 babies from the Priming Immunity at the Beginning of Life (PRIMAL) trial, a study of probiotics (helpful bacteria given by mouth) in babies born at 28 to 32 weeks of pregnancy at 18 German hospitals [9]. They tested these babies' stool for staph in two ways: the usual laboratory method of growing germs on a plate (culture), and a newer method that reads all the genetic material in a sample (sequencing).

The second study, led by She and colleagues, took place in the neonatal intensive care unit of the Children's Hospital of Philadelphia in the United States, a 102-bed unit for the most complex cases plus an 18-bed unit at a second site [10]. Every month from October 2021 to June 2024, staff gently swabbed the noses of all babies in the unit. The researchers read the complete genetic code of 1,446 staph samples from babies, blood tests and hospital surfaces. Because staph strains pick up tiny genetic changes over time, near-identical codes in two babies suggest that the germ passed from one to the other, much as matching fingerprints link people to the same place.

The third study, led by Neary and colleagues, compared 2,728 babies in the neonatal unit at the University of California, Irvine Medical Center before and after COVID-19 rules required every staff member and visitor to wear a mask [11]. Babies were checked for staph by regular swabs, and a positive swab after the third day in hospital was counted as the baby having picked it up in the unit.

What the researchers found

In the German network, about 1.5 in every 100 very small babies developed a staph blood infection after their first days of life [8]. The smallest babies were most affected: almost 3 in 100 of those weighing 500–749 g, compared with fewer than 1 in 100 of those close to 1,500 g. Most of these infections were the ordinary MSSA type. The rate stayed about the same after German hospitals were advised in 2013 to screen babies for staph, although many other things in neonatal care changed over the same years. In the PRIMAL babies, growing germs on a plate found staph in fewer than 8 in 100 stool samples at about one month of age, while genetic sequencing found it in more than 36 in 100. Only 3 of the 638 babies developed a staph blood infection.

In Philadelphia, about one in four babies carried MSSA in their nose in any given month, compared with about one in twenty for MRSA [10]. The team found 69 groups of near-identical staph that had passed between babies within the unit. About one in three of the times a baby picked up staph could be traced to another baby in the unit, and so could about one in three of the blood infections linked to these strains. Babies who shared staph had usually been cared for in the same area of the unit or by the same care team. Hospital surfaces carried strains matching three of these groups. Most striking, the groups that caused at least one blood infection stayed in the unit much longer, a typical 421 days compared with 128 days, and spread to new babies faster than those that did not.

In Irvine, babies were less likely to pick up MRSA during the masking period: the statistical estimate suggested less than half the rate, after allowing for differences between the babies [11]. Masks made no clear difference to the ordinary MSSA type. Babies whose mothers had carried staph in the year before birth were more likely to pick up MSSA, which fits earlier research showing that mothers and babies often share the same strain [12].

What these findings mean, and what they do not

These are all observational studies: they describe what happened and what went together, but they cannot prove that one thing caused another. From July 2022 the Philadelphia unit also treated every baby found to carry staph with mupirocin ointment, alongside a hand-washing campaign and extra cleaning, so much of the spread happened despite those efforts. The masking study counted only 32 babies who picked up MRSA, and its result only just reached statistical significance, so it is promising but not certain. Visitor limits also changed at the same time as masking. Masks have been shown to reduce staff carriage of a resistant staph strain on hospital units caring for patients with that strain [13], which makes the finding believable.

The researchers do not fully agree on what to do next. The German team argues that because many babies carry staph but very few become ill, trying to clear the germ from every baby would mean treating a great many babies to help a few, and might upset the helpful bacteria that are settling into a baby's gut [8]. The Philadelphia team suggests that quickly spotting and stopping the fastest-spreading strains could prevent infections [10]. The Irvine team suggests that treating parents who carry staph might protect babies [11]. None of these approaches has yet been tested head-to-head, so it remains an open question.

What this means for families

If your baby's swab shows staph, try not to be alarmed. Carriage is common, and most babies who carry staph never become ill. Swabbing helps the team know which germs are present and protect other babies. Things families can do are simple and powerful: clean your hands carefully every time you enter the unit and before touching your baby, follow any mask or gown guidance, and ask the team whether testing or treatment for parents makes sense. More about how neonatal units prevent infection in very premature babies is available on In[Neo]Sight (preventing infection in extremely preterm babies).

What researchers are working on next

Researchers want to test whether targeting only the fastest-spreading strains works better than treating every baby who carries staph, to check staff and family members as well as babies, and to learn whether nurturing healthy gut bacteria can help keep staph in check.

Conclusion

Staph carriage is common in newborn intensive care and usually harmless, but in the smallest babies it can lead to serious infection. These three studies show that the germ often spreads between babies within a unit, that the strains causing infection tend to linger and spread faster, and that masks may reduce spread of the resistant type. How best to clear the germ, and from whom, is still being worked out.

References

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  2. Shane AL, Hansen NI, Stoll BJ, et al. Methicillin-resistant and susceptible Staphylococcus aureus bacteremia and meningitis in preterm infants. Pediatrics. 2012;129(4):e914-922. doi:10.1542/peds.2011-0966 ↩
  3. Jennings MR, Elhaissouni N, Colantuoni E, et al. Epidemiology and Mortality of Invasive Staphylococcus aureus Infections in Hospitalized Infants. JAMA Pediatr. 2025;179(7):747-755. doi:10.1001/jamapediatrics.2025.0429 ↩
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  8. Knoll RL, Podlesny D, Fortmann I, et al. Staphylococcus aureus colonization and bloodstream infection in very preterm infants. Gut Microbes. 2025;17(1):2592423. doi:10.1080/19490976.2025.2592423 ↩
  9. Van Rossum T, Haiß A, Knoll RL, et al. Bifidobacterium and Lactobacillus Probiotics and Gut Dysbiosis in Preterm Infants: The PRIMAL Randomized Clinical Trial. JAMA Pediatr. 2024;178(10):985-995. doi:10.1001/jamapediatrics.2024.2626 ↩
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  13. Lacey S, Flaxman D, Scales J, Wilson A. The usefulness of masks in preventing transient carriage of epidemic methicillin-resistant Staphylococcus aureus by healthcare workers. J Hosp Infect. 2001;48(4):308-311. doi:10.1053/jhin.2001.1024 ↩