Two decades of changing Enterococcus epidemiology and antimicrobial susceptibility in a Brazilian tertiary university hospital, 2002–2023

In the quiet corners of every human body, particularly within the digestive tract, live vast communities of bacteria that usually cause no harm. Among these residents are enterococci, microscopic organisms that are part of our natural flora. However, when these bacteria escape their usual home and enter the sterile environment of a hospital, they can transform from harmless neighbors into dangerous invaders. Hospitals are places where people are already weakened by illness, where their immune systems are compromised, and where they are frequently exposed to powerful medicines designed to kill germs. In this high-stakes environment, bacteria face intense pressure to survive. Those that can withstand the drugs used to treat them are the ones that persist, multiply, and spread. This ability to adapt and resist treatment is a major concern for doctors, because when common medicines stop working, infections become much harder to cure, leading to longer stays in the hospital and a higher risk of death.

For twenty-two years, researchers at a large university hospital in southern Brazil watched this battle unfold. They tracked the behavior of enterococci in their patients, looking for patterns in how these bacteria changed over time. The hospital, located in the city of Londrina, serves as a regional teaching center, meaning it treats a wide variety of complex cases, from routine surgeries to critical care for the most severely ill. The scientists gathered data from thousands of patient samples, separating the bacteria into two main groups: Enterococcus faecalis and Enterococcus faecium. While both are capable of causing infection, they behave differently. The researchers wanted to know which species was more common, where in the hospital they were found, which patients they affected, and most importantly, whether the bacteria were becoming harder to treat with the medicines doctors rely on.

The study began by looking at every positive culture test for these bacteria taken from hospitalized patients between 2002 and 2023. After carefully removing duplicate samples and cases where the bacteria could not be fully identified, the team was left with a clear picture of 6,283 unique infections. The first thing they noticed was that one species, E. faecalis, was far more common than the other, making up nearly four out of every five infections found. The second species, E. faecium, was less frequent but appeared in more dangerous situations. When the researchers looked at where the bacteria were found in the body, urine was the most common source for both types, followed by blood. However, the story changed when they looked at the severity of the illness. While E. faecalis was found in many patients across the general hospital wards, E. faecium was disproportionately found in the intensive care units, where the sickest patients are treated.

The fate of the patients also differed sharply between the two groups. Patients infected with E. faecalis were more likely to be discharged and go home, whereas those infected with E. faecium faced a much higher risk of dying while in the hospital. The data showed that nearly half of the patients with E. faecium infections did not survive, compared to about a quarter of those with E. faecalis. This pattern held true even when accounting for the fact that E. faecium was more likely to be found in older patients and those in critical care. The researchers also observed that the number of infections rose significantly in recent years, peaking in 2021 and 2022, a period that coincided with the global strain on healthcare systems during the pandemic.

Perhaps the most critical finding of the study concerned how the bacteria responded to antibiotics. Doctors use a variety of drugs to fight these infections, but the bacteria have been learning to resist them. For E. faecium, the resistance to powerful drugs like vancomycin had been high for a long time, remaining stubbornly elevated throughout the entire twenty-two-year period. However, the situation with E. faecalis changed dramatically. In the earlier years of the study, only a small fraction of E. faecalis infections were resistant to vancomycin. But in the final years of the study, this resistance surged, rising to a level that was nearly four times higher than before. This means that a drug that was once a reliable backup for treating E. faecalis is becoming much less effective. While resistance to another drug called linezolid remained low for both species, the rise in resistance to other common antibiotics, such as those used for urinary tract infections, was also evident.

The researchers concluded that while E. faecalis remains the most common type of enterococcus found in the hospital, E. faecium is the more dangerous one, causing more deaths and appearing more often in the most critical patients. The study highlights a worrying shift in the hospital's microbiology: the most common bacteria are becoming increasingly resistant to treatment, catching up to the already difficult-to-treat E. faecium. This change suggests that the rules for treating these infections may need to be rewritten. The team emphasized that hospitals must keep a close watch on these bacteria, not just to count them, but to understand which species is causing the trouble and how resistant it has become. Without this constant vigilance, doctors may find themselves fighting infections with fewer and fewer effective weapons, leaving vulnerable patients with fewer options for recovery.