We hope to be cured when we stay in hospital. But too often, we acquire new infections there. Such 'healthcare-associated infections' (HAI) are a growing problem worldwide, taking up an estimated 6% of global hospital budgets. In the EU alone, HAIs add up to more than 3.5 million cases per year, resulting in 2.5 million disability-adjusted life years, a cost of up to €24 billion, and 90,000 deaths. They are likewise the sixth leading cause of death in the US.
Patients with lowered immune defenses, and in some hospitals, poor adherence to hygiene protocols, allow HAIs to thrive. Furthermore, antibiotics are widely used in hospitals, which tends to select for hardy, resistant strains of bacteria. When such resistance genes lie on mobile genetic elements, they can even jump between bacterial species, potentially leading to novel diseases.
"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," said Dr Margarita Gomila, a professor at the University of the Balearic Islands in Spain, and the senior author of a study in Frontiers in Microbiology .
"These results highlight that controlling bacterial growth in drains, and preventing colonization by new strains of such hard-to-disinfect niches, is likely a global problem."
Rigorous cleaning protocols
Gomila and her coworkers focused on sink drains in a single modern university hospital on the island of Majorca, built in 2001 and managed by the health service of the Balearic Islands. Cleaning protocols there are state-of-the-art: sinks and their drains are routinely cleaned with bleach, as well as disinfected with chemicals and pressurized steam every fortnight, or every month in non-patient areas. Once a year, drainpipes are hyperchlorinated at low temperature.
Four times between February 2022 and February 2023, they used cotton swabs to sample six drains in each of five wards: two for intensive care, including a brand-new one; one ward each for hematology, short stays, and general medicine; as well as a microbiology laboratory. They cultured the sampled bacteria on five different media and at two different temperatures, and identified the resulting 1,058 isolates with DNA barcoding and mass spectrometry. They then used an automated platform to test whether each of 219 isolates were resistant to a range of antibiotics.
The authors identified a total of 67 different species from the drains. The diversity in most drains went up and down over time with no clear pattern – seasonal or otherwise. The greatest diversity occurred in general medicine and intensive care, while the fewest isolates were found in the microbiology laboratory. Strikingly, the new intensive care unit, opened in July 2022, already showed a high level of bacterial diversity from the opening, on a par with its longer-established twin.
Dominant across wards were six Stenotrophomonas species as well as Pseudomonas aeruginosa, a pathogen known to cause ventilator-associated pneumonia and sepsis, and characterized by the WHO as one of the greatest threats to humans in terms of antibiotic resistance. At least 16 other Pseudomonas species were also found at various times and in various wards, but especially in the short-stay ward.
Other notorious hospital-associated pathogens found repeatedly were Klebsiella pneumoniae in the general medicine ward, Acinetobacter johnsonii and Acinetobacter ursingii in general medicine and intensive care, Enterobacter mori and Enterobacter quasiroggenkampii in the short-stay ward, and Staphylococcus aureus in intensive care and hematology.
"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," said Gomila.
Antibiotic resistance
Of the species found here, Klebsiella, Enterobacter, and P. aeruginosa feature among the so-called ESKAPE group of bacteria, known to thrive in hospital settings and to show frequent multi-resistance and a high potential for causing illness.
In the present study, 21% of P. aeruginosa isolates were found to be resistant to at least one class of antibiotics. Multiple Klebsiella and Enterobacter strains detected proved resistant to the third-generation antibiotic cephalosporin, but not to the carbapenems commonly used today against multidrug-resistant infections.
Worryingly, the blaVIM gene, which makes its carriers resistant even to carbapenems, was detected sporadically in a minority of P. aeruginosa strains from the two intensive care wards, the general medicine ward, and the short-stay ward.
The authors concluded that hospital drains can serve as reservoirs for both known and emerging pathogens, some of which exhibit strong antibiotic resistance.
"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," reminded first author José Laço, a PhD student in Gomila's laboratory.
