Patterns of pathogenic microbial contamination of handwashing sinks change over time following new installations

Summary

Background

Environmental contamination of handwashing sinks and drains can negatively impact hand hygiene practices and facilitate pathogen transmission. This study determined what contaminations occurred after sink and drain installation, when and for how long, as well as differences in contamination patterns depending on the purpose of sink use in a non-outbreak setting.

Methods

The study was conducted from November 2022 to October 2023. Two new sinks and drainpipes replaced the existing sinks in a general internal medicine hospital ward staff station for use before and after patient care. A third single sink and drainpipe was installed in the staff break room for non-medical use. Bacterial samples were collected from the water tap, sink basin, drain and drainpipe by swab or sponge on days 1 (installation day) and 3 in week 1, in week 2 and monthly thereafter. The cleaning staff members were responsible for cleaning the sink basins and drains daily with detergent.

Results

Within the first week after installation, pathogenic microbial contamination (Pseudomonas aeruginosa, Citrobacter freundii and Stenotrophomonas maltophilia) of drainpipes was observed in both the staff station and the break room. Taps were contaminated 2 weeks after installation at the staff station. The contamination cleared but recurred repeatedly until the 12-month point. Pathogens in drainpipes persisted for longer in the staff station than in the break room.

Conclusion

The intermittent microbial contamination patterns of the handwashing sinks changed over time following their installation. Pathogenic microbes persisted, especially in drainpipes and in sinks used for clinical purposes, as opposed to non-clinically used sinks.

Introduction

Pathogenic microorganisms that colonise hospital sinks can cause healthcare-associated infections by contaminating patients, medical staff and medical equipment through water splashback from the sink base during use [1]. Previous studies have reported that such contamination of the sink base and drain occurs due to microorganisms growing up from the drainpipe [2]. Disinfectants, such as hypochlorous acid, acetic acid and quaternary ammonium, as well as heating devices for drainpipes, have been evaluated for infection control. However, many studies have highlighted persistent contamination by multi-drug-resistant organisms (MDROs) once they have colonised sink drains and drainpipes [[3], [4], [5]]. Additionally, because sink contamination and infection prevention strategies are typically reported as part of investigations and infection control initiatives during MDRO outbreaks [4,5], reports on the contamination status of sinks and sink drains in non-outbreak contexts are limited. Therefore, it is unclear when such contaminations occur, how long they persist and what kinds of colonising organisms are present during non-outbreak periods. This study determined what contaminations occur after sink and drain installation, when and for how long, as well as differences in contamination patterns depending on the sink use purpose, in a non-outbreak setting.

Methods

The study was conducted from November 2022 to October 2023 at Tokyo Medical University Hospital (a 905-bed educational tertiary hospital in Tokyo, Japan). New two sinks (sinks A and B) and drainpipes replaced existing sinks on the north and south sides of a general internal medicine hospital ward. The sinks were inside staff station and outside of patient rooms and were used only by medical staff and not by patients. The sinks were for use before and after patient care, mainly for handwashing with soap, and not for discarding patient waste or nutrients. Although the exact usage frequency of the sinks is unknown, 10 nurses and 2–3 nursing assistants provide patient care during the day, and four nurses provided patient care at night, washing their hands with running water during the care process. A third single sink (sink C) and drainpipe were installed in the medical staff break area on a different, non-medical floor in the same building for non-medical uses such as handwashing, brushing teeth and washing lunch boxes.

A 6-mm hole was made 5 cm below the sink base in each drainpipe for bacterial sampling inside the drainpipes. Samples were collected by rubbing a 5-cm² scrub sampler (3M™ Environmental Scrub Sampler with 10-mL Wide Spectrum Neutralizer, 3M Company, St. Paul, MN, USA) four times along the sink basin and drain and using sterile swabs (Swab with Test Tube, J.C.B. Industry Limited) to scrub the taps and insides of the drainpipes (through the 6-mm holes) four times. Water samples were not obtained. Sampling was performed on days 1 (the day of installation) and 3 in week 1, in week 2 and monthly thereafter until month 12.

The samples were then cultured in tryptic soy broth agar (Becton Dickinson, USA) for 24 h and then sub-cultured to MacConkey agar (SHIMADZU, Japan) and Columbia nalidixic acid agar (Becton Dickinson, USA). Bacterial identification was performed by matrix-assisted laser desorption and ionisation. Among the tested microbes, pathogens (pathogenic microbes) were defined as Enterobacterales, Pseudomonas aeruginosa, Stenotrophomonas maltophilia, Acinetobacter baumannii and Staphylococcus aureus. The detected pathogens were subjected to drug susceptibility tests. MDRO screening was not performed on patients admitted to this ward during the study period. The sink basins were cleaned daily by cleaning staff using detergent as part of the hospital’s general routine, and the drainpipes were heated by a wire-wrapped heater to >60 °C for 20 min 1–2 times daily.

Results

The pathogenic bacterial colonisation patterns (what, where, when and for how long) are shown in Table I and Figure 1. The detected pathogens included P. aeruginosa, Citrobacter freundii and S. maltophilia. Other than pathogenic microbes, coagulase-negative Staphylococcus, Pseudomonas sp., Acinetobacter sp., Agrobacterium sp., Candida sp., Naganishia sp., Bacillus subtilis and Microbacterium sp. were noted. Intermittent colonisation by the pathogenic microbes was observed, wherein colonisation occurred once, disappeared and then reappeared. The positive rates of pathogens among samples from taps, sink basins, drains and drainpipes were 6.7% (2/30), 0% (0/30), 6.7% (2/30) and 56.7% (17/30), respectively, in sinks in medical areas; they were 0% (0/15), 6.7% (1/15), 0% (0/15) and 6.7% (1/15), respectively, in sinks in non-medical areas. Pathogenic microbes tended to be more prevalent in the drainpipes of sinks. Additionally, pathogenic microbes were more frequently and persistently observed in the staff station (used clinically) than in the break room (used non-clinically).

Table I.

Pathogenic bacterial colonisation patterns (what, where, when and for how long)

Sink	Part of sink	1st week (day 1)	1st week (day 3)	2nd week	1st month	2nd month	3rd month	4th month	5th month	6th month	7th month	8th month	9th month	10th month	11th month	12th month
A	Tap			S. maltophilia
	Basin
	Drain										P. aeruginosa C. freundii
	Drainpipe		P. aeruginosa S. maltophilia	S. maltophilia	S .maltophilia			P. aeruginosa		P. aeruginosa	P. aeruginosa C. freundii	P. aeruginosa			P. aeruginosa S. maltophilia	P. aeruginosa S. maltophilia
B	Tap									P. aeruginosa S. maltophilia
	Basin
	Drain									S. maltophilia
	Drainpipe			S. maltophilia	S. maltophilia			P. aeruginosa S. maltophilia		P. aeruginosa S. maltophilia		P. aeruginosa S. maltophilia		S. maltophilia	S. maltophilia	P. aeruginosa
C	Tap
	Basin											S. maltophilia
	Drain
	Drainpipe		S. maltophilia

Notes: S. maltophilia; Stenotrophomonas maltophilia, P. aeruginosa; Pseudomonas aeruginosa, C. freundii; Citrobacter freundii.

Figure 1.

Sink diagram showing pathogenic bacterial colonisation patterns. S. maltophilia; Stenotrophomonas maltophilia, P. aeruginosa; Pseudomonas aeruginosa, C. freundii; Citrobacter freundii.

S. maltophilia was consistently detected in both the tap and drainpipes in sink A at the same time in the second week. Additionally, S. maltophilia was consistently detected in the tap, drain and drainpipe in sink B at the same time in the sixth month. P. aeruginosa was consistently detected in the tap and drainpipe in sink B at the same time in the sixth month. P. aeruginosa and C. freundii were consistently detected in both the drain and drainpipe in sink A at the same time in the seventh month. The susceptibility testing of these pathogens revealed no MDROs. Although no MDRO patient screening tests were conducted, no MDROs were also detected in clinical culture samples from patients based on clinical symptoms.

Discussion

This study revealed intermittent pathogenic microbial contamination in newly replaced handwashing sinks in a non-outbreak hospital setting. The study’s strength lies in its observations of microbial colonisation over time, beginning from the installation of new sinks. The colonisation period tended to be longer in drainpipes and shorter in sink drains or taps. Interestingly, pathogenic microbes tended to be more prevalent in drainpipes and were more frequently and persistently observed in sinks in medical areas than in sinks in staff break areas, even during non-outbreak settings.

Common threads in previous MDRO transmission research have included sink contamination during outbreaks and outbreak responses. Contamination during outbreaks tends to be resistant to infection control strategies and thus persistent [3]. On the other hand, reports during non-outbreak periods are limited but divided into persistent and transient sink contamination [6]. In this study, intermittent colonisation by pathogenic microorganisms was confirmed during a non-outbreak period. Although the underlying reason remains unclear, the effectiveness of daily sink disinfection or drainpipe heating may play a role. Furthermore, if handwashing with detergent was performed immediately before microbial sample collection, detergents may have been mixed into the collected samples, possibly affecting the results, because this study was conducted in a real-world daily usage setting. The sampling scrubbers and sponges were moistened with a neutralising solution, but its performance against the detergent is not known. In addition, although a various Enterobacterales bacteria have been reported in previous sink-mediated transmissions, it is unclear why no Enterobacterales bacteria other than Citrobacter sp. were detected in this study.

The colonisation patterns of pathogenic microbes changed over time, but the colonisation period tended to be longer in drainpipes. This suggests that pathogenic microbes primarily settle in drainpipes and then spread to drains, whereas tap contamination occurs due to splashback [2,4]. Pathogens washed away after daily patient care might also contaminate the taps and drains. Additionally, we suspect that errors in the cleaning procedure may have caused tap contamination from the drains. Previous studies have reported that microbial contamination of cleaning equipment was associated with nosocomial transmission [7,8]. Our hospital’s cleaning procedure involves beginning with the tap above the sink and moving downward to the sink basin and drain; however, in reality, cleaners have been observed using the same cleaning equipment to clean the tap after cleaning the sink drain.

In this study, sinks in medical areas were more likely than sinks in non-medical areas to be colonised by pathogenic microorganisms. Previous outbreak reports have shown sinks used by medical staff and patients to be the main infection source [4,5], although the transmission of multi-drug-resistant Acinetobacter through taps in cafeterias outside of medical areas has been reported [9]. Additionally, another study reported that handwashing sinks were significantly more likely than medical-area sinks to be contaminated by carbapenemase-producing Enterobacterales [10]. Staff should use alcoholic hand rubs immediately after patient care, following World Health Organization guidelines. Contrary to guidelines, some medical staff members may wash their hands mainly with soap instead of using alcohol, resulting in contaminated sinks in medical areas.

This study has several limitations. First, the sinks were installed in general internal medicine wards. Conducting similar studies in intensive care or surgical wards and determining any differences will be interesting. Second, this study focussed on newly installed sinks and did not compare the microbial colonisation of sinks that had already been installed at the same time. Third, because tap water sampling was not performed, the possibility cannot be ruled out that the water itself was contaminated, subsequently contaminating the taps and sinks. Fourth, the study did not analyse the relationship between detected pathogens from sinks and clinical healthcare-associated infections. Future research will clarify this point. Fifth, the sink usage frequency remains unclear. Sinks in staff rooms might be more consistently contaminated by frequent usage than those in break rooms. Finally, same pathogens were defined based on the species. Genotyping would be needed to confirm similarity between isolates in further research.

Pathogenic microbes persisted for several months in the sinks used clinically for hand hygiene in the ward staff station compared with that in in non-clinically used sinks in the break room. Therefore, awareness of the higher risk associated with parts of the sink is required to mitigate healthcare-associated pathogen transmission.

CRediT authorship contribution statement

I. Nakamura: Writing – original draft, Project administration, Funding acquisition, Formal analysis, Data curation. Y. Miura: Writing – review & editing, Methodology. T. Shimodaira: Writing – review & editing, Methodology, Data curation. A. Okugawa: Writing – review & editing, Methodology, Data curation. H. Watanabe: Writing – review & editing, Project administration.

Consent for publication

Not applicable.

Availability of data and materials

The dataset supporting the conclusions of this article is included in the article.

Ethics statement

This study was approved by the Ethical Committee of Tokyo Medical University (approval number: T2020-0078).

Funding sources

This work was funded by JSPS KAKENHI (grant number: 25K13687).

Conflict of interest statement

The authors declare that they have no competing interests.