Despite being perceived as sterile, hospital sinks can harbor dangerous bacteria. (Monkey Business Images/Shutterstock)
In a nutshell
- A year-long study of hospital sink drains found 67 different bacterial species living there, many resistant to multiple antibiotics. One particularly concerning strain, Pseudomonas aeruginosa, showed 21% resistance to at least one type of antibiotic.
- Despite rigorous cleaning protocols including bleach and pressurized steam, bacteria thrive in hospital drains. A new intensive care unit’s drains developed diverse bacterial populations within months of opening, matching levels found in established wards.
- Hospital sink drains may serve as reservoirs for dangerous bacteria that can potentially spread to vulnerable patients. The presence of the blaVIM gene in some bacteria indicates resistance even to last-resort antibiotics, raising concerns about future treatment options.
PALMA DE MALLORCA, Spain — Behind the sterile appearance of hospital rooms lurks an unexpected threat: the humble sink drain. While they may seem innocuous, these common fixtures harbor complex communities of bacteria that could pose serious risks to patient health. A new year-long study from Spain has pulled back the curtain on these microscopic tenants, revealing both familiar and emerging bacterial threats, including several that resist our most powerful antibiotics.
At a modern university hospital in Majorca, built in 2001, researchers tracked bacterial populations in sink drains across five different wards from February 2022 to February 2023. Despite implementing state-of-the-art cleaning protocols, including routine bleach cleaning and biweekly disinfection with chemicals and pressurized steam, the findings, published in Frontiers in Microbiology, paint a concerning picture of these oft-overlooked reservoirs of potentially dangerous microorganisms.
“Here we show that hospital sink drains host bacterial populations that change over time, despite impeccable cleaning protocols in the particular hospital we looked at,” says senior study author Margarita Gomila, a professor at the University of the Balearic Islands in Spain.
Hospital-acquired infections (HAIs) have become one of healthcare’s most pressing challenges. These infections, which patients contract while receiving treatment for other conditions, affect millions globally each year. In the European Union and European Economic Area alone, more than 3.5 million patients are affected annually, leading to over 90,000 deaths and costs of up to €24 billion (approximately $25.171 billion). These infections strain healthcare systems significantly, consuming up to 6% of public hospital budgets.
The rise of antibiotic-resistant bacteria has complicated this challenge considerably. When antibiotics are widely used in hospitals, they create an environment where resistant strains of bacteria are more likely to survive and multiply. These hardy survivors can then share their resistance genes with other bacteria, similar to passing along survival instructions. This process can eventually lead to the emergence of new, more dangerous bacterial strains.
Using cotton swabs, researchers sampled six drains in each of five wards: two intensive care units (including a brand-new one), hematology, short stays, general medicine, and a microbiology laboratory. They collected samples four times throughout the year, culturing the bacteria under various conditions to ensure they could identify as many species as possible. Through DNA barcoding and mass spectrometry, they identified 1,058 bacterial specimens.
The results revealed a complex ecosystem living within hospital drains. Most concerning was the prevalence of antibiotic-resistant bacteria. Nearly all tested bacteria showed resistance to multiple types of antibiotics, including both common and last-resort medications. Among Pseudomonas aeruginosa isolates, 21% proved resistant to at least one class of antibiotics. Multiple Klebsiella and Enterobacter strains showed resistance to third-generation cephalosporins, though they remained susceptible to carbapenem antibiotics.
The research team identified 67 different bacterial species across the drains. This diversity surprised even the researchers, given the hospital’s rigorous cleaning protocols. Among the most common were Pseudomonas and Stenotrophomonas species. P. aeruginosa appeared frequently and raised particular concerns. The World Health Organization considers this bacterium one of the greatest threats to humans regarding antibiotic resistance, as it can cause severe infections like ventilator-associated pneumonia and sepsis.
Different wards housed their own distinct bacterial communities. In general medicine, Klebsiella pneumoniae dominated the bacterial population. Both general medicine and intensive care units showed significant populations of Acinetobacter johnsonii and A. ursingii. The short-stay ward harbored specific strains of Enterobacter, while intensive care and hematology units showed a notable presence of Staphylococcus aureus.
“The bacteria we found may originate from many sources, from patients, medical personnel, and even the environment surrounding the hospital. Once established in sink drains, they can spread outwards, posing significant risks to immunocompromised patients above all,” says Gomila.
The study revealed that bacterial diversity fluctuated over time with no clear seasonal pattern. The ICU and general medicine wards consistently showed the highest diversity, likely due to the variety of patients they treat and the range of medical procedures performed there. In contrast, the microbiology laboratory, with its stringent cleaning protocols, yielded the fewest bacterial isolates.
One particularly striking finding emerged from the new intensive care unit, which opened in July 2022. Within months, this new facility developed bacterial diversity comparable to its established counterpart. This rapid colonization suggests that standard prevention measures might not be sufficient to prevent bacterial establishment in hospital drains.
The research also uncovered a worrying development regarding antibiotic resistance. In several wards, including both intensive care units, general medicine, and the short-stay ward, researchers detected the blaVIM gene in some P. aeruginosa strains. This gene makes bacteria resistant to carbapenem antibiotics, medications that doctors often rely on as a last resort against multidrug-resistant infections.
Beyond identifying present bacterial species, the study highlights the dynamic nature of these drain ecosystems. Despite regular cleaning and disinfection, bacterial populations persist and evolve. This resilience poses a significant challenge for infection control in healthcare settings.
“Cleaning protocols are important and should be frequently applied, especially in wards that are kept separate precisely to slow the spread of potentially harmful bacteria. But to get to the bottom of the problem, it’s essential to study the source of these bacteria and their routes of transmission,” says study author José Laço, a Ph.D. student in Gomila’s laboratory.
As healthcare facilities worldwide grapple with growing cases of antibiotic-resistant infections, understanding these bacterial reservoirs becomes increasingly crucial. Hospital drains, often overlooked in infection control strategies, may play a more significant role in spreading dangerous bacteria than previously recognized. The study reinforces that no detail is too small when it comes to infection control.
Paper Summary
Methodology
Researchers conducted a systematic survey of bacterial life in hospital sink drains over a full year. They selected five different hospital wards in a modern Majorcan hospital: two intensive care units (including one that opened during the study), hematology, short stays, general medicine, and a microbiology laboratory. Using cotton swabs, they collected samples from six drains in each ward four times between February 2022 and February 2023. The samples were processed within two hours in laboratory conditions, where researchers used five different types of growth media (nutrient mixtures) to culture the bacteria, some containing antibiotics to specifically identify resistant strains. They grew these cultures at both room temperature and body temperature (37°C) for varying periods, then used sophisticated identification techniques, including mass spectrometry and DNA analysis, to determine exactly which bacterial species were present.
Results
From 1,058 bacterial samples, researchers identified 67 different species. Pseudomonas aeruginosa appeared most frequently across all wards, with 21% of these bacteria showing resistance to at least one type of antibiotic. The ICU and general medicine wards displayed the highest bacterial diversity. A particularly concerning finding was the presence of the blaVIM gene in some bacteria, which confers resistance to last-resort carbapenem antibiotics. The newly opened ICU quickly developed bacterial populations similar to the existing one, demonstrating rapid colonization despite modern cleaning protocols. The microbiology laboratory, with its stringent cleaning procedures, showed the lowest bacterial counts.
Limitations
This research focused on a single hospital, which may limit how broadly the findings can be applied to other healthcare facilities worldwide. The study only examined bacteria that could be grown in laboratory conditions, potentially missing species that don’t thrive in artificial environments. While the research identified numerous antibiotic-resistant bacteria, it couldn’t determine whether these bacteria actually caused infections in patients. Additionally, the study period of one year, while substantial, may not capture longer-term patterns in bacterial population changes.
Discussion and Takeaways
This study reveals hospital sink drains as significant reservoirs for potentially dangerous bacteria, particularly antibiotic-resistant strains. The findings suggest that even modern cleaning protocols may not fully address the challenge of bacterial colonization in drains. The research highlights the need for improved infection control strategies, especially in high-risk areas like intensive care units. The rapid colonization of the new ICU’s drains indicates that preventing bacterial establishment may be more challenging than previously thought. The study also emphasizes the importance of monitoring bacterial populations in healthcare settings and potentially developing new approaches to drain maintenance and cleaning.
Funding and Disclosures
This research received funding through the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement. The views and opinions expressed reflect those of the authors only and do not necessarily reflect those of the European Union or the granting authority. The authors declared no conflicts of interest that could influence their research.
Publication Information
The study, “Yearlong analysis of bacterial diversity in hospital sink drains: culturomics, antibiotic resistance and implications for infection control,” was published in Frontiers in Microbiology on February 14, 2025. The research team included José Laço, Sergi Martorell, Maria del Carmen Gallegos, and Margarita Gomila from the University of the Balearic Islands and Hospital Son Llàtzer in Palma, Spain.
