Bacterial Contamination of Ultrasound and Stethoscope Surfaces in Low- and High-Resource Settings

ABSTRACT.

Point-of-care ultrasound is an accurate diagnostic and monitoring tool. Its increasing affordability, portability, and versatility make it an excellent component of standard clinical evaluation alongside the stethoscope. However, like the stethoscope, ultrasound carries risks of surface contamination and potential cross-infection. In this international observational study, we compared the surface contamination of ultrasound equipment to stethoscopes in two medical centers: a tropical low-resource hospital and academic high-resource hospital. Ultrasound equipment and coupling gel had similar prevalence of microbial surface contamination compared with observed stethoscopes. Most microbes were commensal Gram-positive, but some were opportunistic and pathogenic microbes (such as Escherichia coli and Staphylococcus aureus). In conclusion, it is crucial to appreciate and reduce the risk of ultrasound device contaminations. When ultrasound is used bedside, similar to stethoscopes, conscientious hygiene measures are equally fundamental.

INTRODUCTION

Point-of-care ultrasound is a rapid, noninvasive, reproducible, cost-efficient, and accurate diagnostic and monitoring modality. It is quickly becoming the standard for primary bedside evaluation of cardiac, abdominal, or pulmonary problems, especially in resource-limited settings.¹ Technological advancements have further augmented its point-of-care potential resulting in its incorporation in medical residency programs.² There is increasing speculation of its use as a standard component of physical examination alongside, or in some contexts replacing, the stethoscope.^3–5

Stethoscopes are well-known vectors for microbial contamination responsible for nosocomial bacterial infections. Commonly used cleaning practices reduce contamination—but do not completely eliminate transmission risk.⁶ Similarly, ultrasound equipment may be a reservoir of pathogenic bacteria and a potential source of cross-contamination.⁷ However, a direct comparison to stethoscope contamination has not been reported.

This study aims to: 1) survey the prevalence of ultrasound equipment contamination; and 2) survey the prevalence of stethoscope contamination. We hypothesize that both ultrasound equipment and stethoscope show similar microbial contaminated surfaces.

MATERIALS AND METHODS

This is an international two-center prevalence study conducted in the Sint Elisabeth Hospital in Willemstad, Curaçao (Hospital 1) and in the Amsterdam University Medical Center, location VUmc, Amsterdam, The Netherlands (Hospital 2). Hospital 1 is a 160-beds peripheral hospital in a tropical low-income country. Hospital 2 is a 730-beds tertiary academic center in a high-income country. Three departments participated in each hospital: Emergency Department, Clinical Diagnostics Unit, and Intensive Care Unit 1 (ICU-1) in Hospital 1; ICU-2A, ICU-2B, and Intermediate Care Unit-C (ICU-2C) in Hospital 2. All cultures were performed by one investigator per center and before patient contact. All cultures were performed at random times of the day, during normal daytime hours and without prior notice.

Each department had one dedicated ultrasound machine. Cultures were performed on two ultrasound probes per machine, keyboard, probe cords, trolley handle, probe tray, and accompanying coupling gel. In Hospital 1, the stethoscopes belonging to the ultrasound-performing physician were cultured. In Hospital 2, each bed had a dedicated stethoscope, which were all separately cultured.

The culture samples were collected by investigators and processed by the respective microbiology laboratory per institution. The scanning surface of the ultrasound probes and resonator membrane of stethoscopes were carefully pressed against blood agar plates for approximately 10 seconds. For the coupling gel, a gel drop was smeared on a blood agar plate, and the gel bottle tip was pressed on the agar surface. The keyboard, cord (belonging to the ultrasound probe), handle (to move the ultrasound trolley), and echo tray (where probes are placed on ultrasound trolley) were sampled with (premoistened) eSwabs (Copan). The entire surface of a moistened swab was rubbed along the surface of the entire respective object, over a period of 10 seconds. The moistened swabs were subsequently plated on blood agar (Columbia Agar with 5% Sheep blood). In Hospital 1, plates were incubated for 48 hours at 37°C. Bacterial growth was assessed at 24 and 48 hours. Gram-negative and Gram-positive species were determined by Maldi Biotyper (Bruker). Colony count was semi-quantitatively assessed. The quantification of colonies was noted as “+” for growth in the first streak, “++” for growth in the second streak, and “+++” for growth in the third and/or fourth streak (based on a three-phase streaking pattern).

In Hospital 2, plates were incubated for 48 hours at 37°C. Bacterial growth was assessed at 24 and 48 hours. Determination of species was based on colony morphology. Possible pathogenic bacteria (e.g., Staphylococcus aureus, hemolytic streptococci, and Gram-negative bacteria) were determined by VITEK-MS (Biomerieux). For all S. aureus isolates in both hospitals, oxacillin screening was used to detect resistant species. The semi-quantification of colonies was noted as “+” for 1–10 colony forming units (cfu), “++” for 11–100 cfu, and “+++” for >100 cfu. Cultured microbes were classified as coagulase-negative staphylococci, other commensal Gram-positive flora, Gram-negative bacteria, and S. aureus; representing increasingly common pathogenicity.

RESULTS

A total of 63 ultrasound and stethoscopes surfaces were cultured on six departments. Of the 63 cultures, 50 (79%) resulted in bacterial growth. Thirty (48%) cultures were coagulase-negative staphylococci (S. hominis, S. epidermidis, S. caprae, S. capitis, and S. cohnii). Nine (14%) cultures found commensal Gram-positive floras. Finally, five (8%) Gram-negative pathogenic bacteria and 6 (10%) S. aureus were identified (also shown in Supplemental Table 1). Figure 1 shows the distribution of cultured microbes across different settings.

Figure 1.

Distribution of microbial contamination type across different settings. If multiple microbes were cultured, the most commonly pathogenic was reported in the figure. Percentages in figure were rounded to integers. CNS refers to coagulase-negative staphylococci. Other Gram-positive refers to commensal Gram-positive flora. This figure appears in color at www.ajtmh.org.

The ultrasound culture results are shown in Table 1. Both in Hospital 1 and 2, cultures of ultrasound probes on ICUs had either no growth or growth of commensal Gram-positive bacteria such as coagulase-negative staphylococci. In both hospitals, cords, keyboards, handles, and coupling gel contained Gram-negative bacteria and S. aureus. The stethoscope culture results, shown in Table 2, reveal both opportunistic and pathogenic microbes. Supplemental Table 2 shows the specific Gram-positive bacterial findings per object and department for Hospital 1.

Table 1.

Culture results of point-of-care ultrasound equipment in Hospital 1 and Hospital 2

Hospital 1 (Willemstad, Curaçao)
	Emergency department	Growth	Clinical diagnostics unit	Growth	Intensive care unit	Growth
Probe 1	Moraxella osloensis (GN)	+	CG+	++	No growth	−
	CG+	+	–	–	–	–
Probe 2	CG+	++	Enterococcus faecalis	+++	CG+	+
	–	–	CNS	+++	–	–
Keyboard	CNS	+	CNS	+	S. aureus (MSSA)	+
	–	–	–	–	CNS	+
Cords	No growth	+	S. aureus (MSSA)	+	No growth	−
	–	+	CNS	+	–	–
Handle	CNS	+	CNS	+	S. aureus (MSSA)	++
Tray	CNS	+	No growth	−	No Growth	−
Coupling gel	Corynebacterium minutissimum	+++	No growth	−	CNS	+

Hospital 2 (Amsterdam, The Netherlands)
	Intensive care unit 1	Growth	Intensive care unit 2	Growth	Intermediate care unit	Growth
Probe 1	CNS	+++	No growth	–	CNS	+
	Bacillus species	+	–	–	–	–
Probe 2	CNS	+++	No growth	–	CNS	+
Keyboard	CNS	+++	CNS	+	No growth	–
	Bacillus species	+	–	–	–	–
Handle	CNS	+++	CNS	++	CNS	++
	–	–	Yeast	+	–	–
Cords	E. coli (GN)	+++	CNS	+	CNS	+
	Sphingomonas paucimobilis (GN)	+++	–	–	–	–
	Pantoea agglomerans (GN)	+++	–	–	–	–
Tray	No growth	–	CNS	++	No growth	–
Coupling gel	Burkholderia cepacia (GN)	+++	Yeast	+	CNS	+++

CG+ = commensal Gram-positive flora; E = Escherichia; GN = Gram-negative; MSSA = methicillin-sensitive Staphylococcus aureus; CNS = coagulase-negative staphylococci. Growth was quantified as colony forming units (cfu): “+” for 1–10 cfu, “++” for 11–100 cfu, and “+++” for > 100 cfu.

TABLE 2.

Culture results of stethoscopes in both centers

Hospital 1	Physician’s stethoscope	Growth	Physician’s stethoscope	Growth	Bedside stethoscope	Growth
1–3	CG+	+	CG+	+	CNS	+
	–	+	–	+	Bacillus species	+

Hospital 2	Bedside stethoscope	Growth	Bedside stethoscope	Growth	Bedside stethoscope	Growth
1–3	S. aureus (MSSA)	++	CNS	++	CNS	++
	Bacillus spp.	+	–	+	–	–
4–6	S. aureus (MSSA)	+	S. aureus (MSSA)	+	CNS	++
	CNS	++	CNS	++	–	–
7–9	CNS	++	CNS	++	CNS	+
10–12	CNS	+	CNS	+	CNS	+
13–15	CNS	++	CNS	+	CNS	++
	–	–	–	–	Pseudomonas luteola	+
16–18	CNS	+	CNS	+	No growth	–

CG+ = commensal Gram-positive flora; MSSA = methicillin-sensitive Staphylococcus aureus; S = Staphylococcus; CNS = coagulase-negative staphylococci (unspecified). First column indicates the number of the cultures. Growth was quantified as colony forming units (cfu): “+” for 1–10 cfu, “++” for 11–100 cfu, and “+++” for > 100 cfu.

DISCUSSION

The main findings of this observational study on microbial contamination of ultrasound equipment and stethoscopes in low and high-resource settings are: 1) Ultrasound probes, but also cords, keyboards, handles, and coupling gel contained Gram-negative bacteria and S. aureus (jointly 17% of ultrasound cultures); 2) Stethoscopes cultured in the same setting contained similar fraction of Gram-negative bacteria and S. aureus (19% of stethoscope cultures). These results underline the fact that both ultrasound equipment with coupling gel and stethoscopes carry the risk for cross-contamination.

Point-of-care ultrasound’s portability, repeatability, and noninvasiveness—characteristics that it shares with the stethoscope—have made it a principal clinical tool and an extension of routine physical examination. It can be repeated frequently, for a large variety of indications and across a large group of patients. For both instruments, these characteristics produce an increased risk of contamination and cross-transmission of microbes. The current study reports slightly higher growth and commensal microbes on stethoscopes. However, prevalence of opportunistic and pathogenic microbes, those responsible for (most) infections, is similar to ultrasound equipment and stethoscopes. Finally, results show a slightly higher prevalence of pathogenic and opportunistic microbes in the tropical low-resource hospital than the academic high-resource hospital.

Previous research have shown the presence of microbes on ultrasound probes as the vector for nosocomial infections, albeit with a lower contamination prevalence.⁸ This may be caused by the involvement of participating departments in the design of the aforementioned investigation, which may produce a (disinfection-associated) performance bias. Moreover, only the ultrasound probes, which are likely cleaned more regularly, were cultured. Machine surfaces, just like other inanimate objects, may be a reservoir for nosocomial pathogens as they are cleaned or disinfected much more rarely.^9,10 This investigation reaffirms the potential contamination of surfaces outside of the ultrasound probes. When performing an ultrasound examination, it is crucial to avoid contact among any part of the ultrasound equipment (aside from the probe) and the patient’s environment unless thorough (before and after) disinfection is performed. Moreover, clinician hand hygiene between patient (or environment) contact and equipment contact is crucial.

Most notably, coupling gel in Hospital 1 contained a colony of Corynebacterium minutissimum, often associated with erythrasma; a superficial skin infection especially pathogenic in warm climates and obese patients with diabetes type 2. Contamination of coupling gel may potentially pose a risk for a large part of the population presenting to Hospital 1’s Emergency Department. Similarly, in Hospital 2, coupling gel revealed bacterial contamination with Burkholderia cepacia. Previous literature described how B. cepacia from coupling gel used during central venous catheter insertion resulted in a massive outbreak in a tertiary hospital.¹¹ Thus, in all settings, it is imperative that coupling gel containers do not contact either patient or probe surface. To this end, individually packaged, single-use ultrasound gel may provide further mitigation of cross-contamination.¹²

The risk for contamination can be further compounded in low-resource settings, where more costly diagnostic and monitoring tools may be scant or even unavailable. This leads to increased overall bedside handling and, if proper disinfection protocols are not in place, increased cross-contamination. Previous research has demonstrated how relatively simple, but consistently performed, cleaning procedures with alcohol or other cleaning agents can almost completely eliminate opportunistic and pathogenic microbes from both ultrasound and stethoscope surfaces.¹³

This study has several limitations: it has a small sample size and does not report actual cross-contamination or resulting infections rates. However, this is the first study surveying both ultrasound and stethoscope contamination across different clinical and geographical settings.

CONCLUSION

The rise of point-of-care ultrasound as a diagnostic instrument introduces new challenges in hygiene and infection prevention. The current study found that both stethoscopes and ultrasound equipment harbor commensal, opportunistic, and pathogenic bacteria. Machine surfaces, such as tray, handle, cords, keyboards, and coupling gel containers may be colonized and should therefore, not contact patient or probe surface. This report underlines the importance of careful cleaning of ultrasound equipment and stethoscope, as well as accompanying clinician hand hygiene. Ultrasound is often heralded as the stethoscope of the future, but, if not conscientiously used, may become the contaminator of the future.

Note: Supplemental tables appear at www.ajtmh.org.