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Corresponding author at: Department of Medical Sciences, Frank H. Netter MD, School of Medicine at Quinnipiac University, Hamden, CT 06518, United States. Tel.: +1 203 582 6475; fax: +1 203 582 1418.
Clinical Microbiology Laboratory, Yale New Haven Hospital, New Haven, CT 06510, United StatesDepartment of Medical Sciences, Frank H. Netter MD, School of Medicine at Quinnipiac University, Hamden, CT 06518, United StatesDepartment of Laboratory Medicine, Yale School of Medicine, New Haven CT 06510, United States
Contaminated nebulizers are a potential source of bacterial infection but no single method is universally accepted for disinfection. We hypothesized that baby-bottle steam sterilizers effectively disinfect home nebulizers.
Methods
Home nebulizers were inoculated with the common CF respiratory pathogens methicillin resistant Staphylococcus aureus, Burkholderia cepacia, Haemophilus influenzae, mucoid and non mucoid Pseudomonas aeruginosa, and Stenotrophomonas maltophilia. The nebulizers were swabbed for bacterial growth, treated with either the AVENT (Philips), the NUK Quick & Ready (Gerber) or DRY-POD (Camera Baby) baby bottle steam sterilizer and reswabbed for bacterial growth.
Results
All steam sterilizers were effective at disinfecting all home nebulizers. Viable bacteria were not recovered from any inoculated site after steam treatment, under any conditions tested.
Conclusions
Steam treatment is an effective disinfection method. Additional studies are needed to confirm whether these results are applicable to the clinical setting.
Home nebulizer therapy is an integral part of treatment regimens for patients with Cystic Fibrosis (CF). Benefits include the delivery of therapies such as antibiotics to the site of infection while reducing the systemic side effects. The risk of bacterial colonization of home nebulizers varies depending on the study but several studies report that home nebulizers used by asthmatics or CF patients may become colonized with bacteria [
]. This is not surprising as bacterial pathogens such as Pseudomonas aeruginosa survive in water and can colonize both plastic surfaces and human lungs via the formation of bacterial biofilms [
The recognition that bacterial colonization of home nebulizers is a potential risk for respiratory infection has led experts to examine many different methodologies for disinfection. These include cleaning with 2.0–3.5% acetic acid, soaking with water, washing with soap and water (either tap or sterile), 70–90% ethanol or isopropyl alcohol, 3% hydrogen peroxide, or 0.5% hypochlorite [
]. Ideally, the method used to clean and disinfect nebulizers, needs to be simple and efficient as to not add to the growing treatment burden that could significantly compromise patient adherence to therapy. Tai et al. reported that soaking a nebulizer for 10 min in water followed by a rinse was more effective at removing contaminated Escherichia coli than either soaking or rinsing alone [
]. Sterile water was not superior to tap water in this study and bacteria were recovered from most sites even after soaking and rinsing. Rosenfeld et al. found that aggressive tap water rinse sterilized 17/19 nebulizers inoculated with Staphylococcus aureus and mucoid and non-mucoid P.aeruginosa [
]. Reychler compared five methods of disinfection, hypochlorite solution, 3.5% acetic acid, 0.5% Hexanios, 0.5% washing detergent, and a dishwasher, using facemasks and mouthpieces inoculated with common CF pathogens (S. aureus, P. aeruginosa, Stenotrophomonas maltophilia, Burkholderia cenocepacia, and Alcaligenes xylosoxydans). The authors found that all were effective except acetic acid for the treatment of S. aureus [
]. In a separate study Reychler et al. found that environmental organisms but not the CF pathogens, methicillin sensitive S. aureus or S. maltophilia, were cleared from CF patient nebulizers with 0.5% hypochlorite [
]. Given that rinsing or soaking in tap water is efficacious in disinfecting home equipment in previous studies, it is likely that home steam-sterilizers, commonly used and sold for baby bottles, will also be effective. Brief exposure to steam (3 s) can effectively decontaminate a variety of surfaces and eradicate >99.5% of an existing bacterial biofilm [
]. Steam-sterilization is recommended to disinfect the Altera® nebulizer that delivers inhaled aztreonam (http://www.cff.org/treatments/Therapies/Respiratory/Cayston/). Importantly, repeated steam sterilization treatments do not impair the in vitro function of the eFlow® rapid nebulizer [
]. The procedure is fast, straightforward, and easy to perform making it an ideal method for disinfection. However, there is little published data on whether steam sterilization effectively disinfects home nebulizers. Therefore we sought to examine the effectiveness of three different commercially available baby-bottle steam sterilizers for their ability to disinfect nebulizers inoculated in vitro with respiratory pathogens commonly isolated from CF patients.
2. Methods
2.1 Bacterial strains and growth conditions
Table 1 lists the strains inoculated onto the nebulizers to test for disinfection. Bacteria were grown overnight on blood agar plates (Remel, Lenexa, KS), inoculated into Trypticase Soy Broth (TSB) (Remel, Lenexa, KS) at a density of 0.5 McFarland. Ten microliters of this suspension was used to inoculate the nebulizers. To determine the pre-exposure inoculum, the 0.5 McFarland suspension of each bacterial strain was serially diluted, the diluted bacterial suspensions inoculated on blood agar plates for 48 h at 37 °C, and the colony forming units recorded (Table 1).
Table 1Bacterial strains tested for steam-treatment.
For all conditions three different nebulizers, the Pari LC Plus®. eFlow® rapid, and eFlow Altera®, were inoculated with each of the above bacterial strains in three different locations for each individual experiment (Fig. 1). Initially, the disinfection of both unassembled and fully assembled nebulizers was compared. Assembled nebulizers were inoculated prior to assembly and then put together prior to steam treatment. Once we determined that there was no difference in bacterial recovery comparing assembled with the unassembled nebulizers (data not shown), all remaining experiments were performed on fully assembled nebulizers.
Fig. 1Bacterial inoculation sites. The nebulizers Pari LC Plus® (A) and eFlow®rapid (B) were disassembled for inoculation and then reassembled prior to steam treatment for experiments after the initial comparison of assembled and disassembled steam treatments. The Altera® was inoculated as shown for the eFlow® rapid.
Three different conditions were tested: 1) Dry samples: The nebulizer inoculated with the 10 μl bacterial suspensions was air dried in a hood for 30 min and then was subjected to steam sterilization treatment. 2) Wet samples: The nebulizer with the 10 μl bacterial suspensions was immediately placed in the sterilizer. 3) Sputum samples: A pool of de-identified discarded sputum that had grown only normal flora recovered from three unknown CF patients was vortexed and 0.5 ml was transferred to a microfuge tube. Since the specimens were pooled de-identified sputum being discarded by the clinical microbiology laboratory, this study meets the criteria as being exempt from review by the Yale Human Investigations Committee. Ten microliters of 0.5 McFarland bacterial suspension was transferred to the sputum containing microfuge tube and 10 μl of each seeded sputum was inoculated to the three different sites on each nebulizer. An un-inoculated sputum sample was used as a control in these experiments. The sputum contained normal flora (Fig. 2) so the amount of bacteria recovered from the inoculated sputum was determined using the 4 quadrant semi-quantitative streaking method commonly used in clinical microbiology laboratories. With this method 1+ represents bacterial growth in the first quadrant only, 2+ in the first and second quadrant, 3+ the first three quandrants, and 4+ all streaked quadrants.
Fig. 2Pretreatment growth of bacteria from de-identified pooled CF sputum inoculated with P. aeruginosa. The chocolate agar (A) and blood agar plates (C) show both the laboratory inoculated P. aeruginosa and the mixed normal flora. The colistin nalidixic acid agar plate (B) that selects for gram positive organisms shows only mixed normal flora (e.g. white colonies) and no P. aeruginosa while the MacConkey agar plate (D) that is selective for gram negative bacterium shows only P. aeruginosa. No bacteria were recovered after steam treatment from any plates.
In a separate set of experiments we used mucoid P. aeruginosa and S. aureus with a 5.0 McFarland suspension, performed serial dilutions to quantitate the bacterial amount, and either inoculated the nebulizer directly or seeded 100 μl of sputum with 10 μl each of the higher bacterial inoculum and performed the experiments as described above. Additional experiments with 5.0 MacFarland mucoid P. aeruginosa and S. aureus included both 24 and 48 h bacterial incubation times to allow for potential biofilm formation prior to steam treatment. For 24 and 48 h incubation experiments, the water rinsing method of Rosenfeld et al. was performed as described to determine how steam treatment compared with a published disinfection protocol [
Three different steam sterilizers, NUK Quick 'n Ready Steam Sterilizer (Gerber) the AVENT 3-in-1 Electric Steam Sterilizer (Philips) and the DRY-POD Sterilizer (Camera Baby) were used per the manufacturer's instructions for all experiments. Nebulizers were steam treated for approximately 10 min in each machine using tap water. The nebulizers were allowed to cool for 10 min prior to removal from the steam sterilizer. The previously inoculated sites were wiped with a sterile cotton swab dipped in 0.45% saline that was then used to streak a blood agar plate, the plate incubated at 37 °C and examined for growth at 48 h. Each steam sterilizer was used for each nebulizer under each of the three conditions (dry, wet and sputum) tested for each bacterial strain.
As a mock control, an inoculated nebulizer was placed in each sterilizer (turned off) for 10 min and then removed. A sterile swab was dipped in sterile 0.45% saline and each area previously inoculated with bacteria was vigorously swabbed individually and plated on separate blood agar plates as described above for the treated nebulizers.
3. Results
We were initially interested in whether steam-sterilization would work equally well comparing an unassembled nebulizer with a fully assembled nebulizer. To test this we used non-mucoid P. aeruginosa as the test organism with each nebulizer. After nebulizer bacterial inoculation under dry conditions and steam treatment with either the NUK or AVENT machines, we did not recover viable bacteria from either the unassembled or fully assembled nebulizer (data not shown). Thus all remaining experiments were performed on assembled nebulizers.
The bacterial inoculums for the initial nebulizer experiments as determined by serial dilutions of the bacterial suspensions are listed in Table 1. After steam sterilization treatment with each of the three machines we did not recover any viable bacterium from any of the nebulizers tested, regardless of the bacterium used or whether the sample was wet or left to dry (Table 2). We hypothesized that the presence of sputum might provide some protection from the effects of steam treatment so we inoculated de-identified pooled CF sputum with the bacterial pathogens. We still did not recover viable bacteria after steam treatment, regardless of the strain, machine, or nebulizer (Table 3, Fig. 2). The normal flora in the pooled CF sputum samples also was not recovered post-treatment suggesting upper respiratory flora is also killed by steam treatment. We then decided to substantially increase the inoculum for mucoid P. aeruginosa and S. aureus, both in the presence and absence of sputum to reach a number where viable bacteria might be recovered after steam-treatment. Three ×107S. aureus or 2.6×107 mucoid P. aeruginosa were inoculated onto the nebulizer and 1.5×106S. aureus or 1.3×106 mucoid P. aeruginosa mixed in de-identified pooled CF sputum were inoculated onto the nebulizers, left for 30 min, and subjected to steam treatment. Again no viable bacteria were recovered after steam exposure compared with the mock treatment where bacterial growth was present (data not shown).
*Results were identical for all three baby-bottle steam sterilizers tested, all inoculation sites, and for both wet (immediate treatment) and dry (30 minute drying in hood) conditions. Estimated inoculums are in Table 1. # These bacterial suspensions were left on the nebulizers 24 h at room temperature before swabbing and treatment. 1+ represents 2–95 colonies depending on the inoculum site. 4+ represents >100,000 bacterial colony forming units. N.G.=No growth.
Table 3Sputum does not protect bacteria from steam sterilization*.
Pari LC Plus®
eFlow® rapid
Altera®
Treatment
Time
Mock
Steam
Mock
Steam
Mock
Steam
H. influenzae
30 min
3+
N.G.
3+
N.G.
3+
N.G.
B. cepacia
30 min
3+
N.G.
3+
N.G.
3+
N.G.
S. maltophilia
30 min
3+
N.G.
3+
N.G.
3+
N.G.
S. aureus (MRSA)
30 min
3+
N.G.
3+
N.G.
3+
N.G.
S. aureus (MSSA)
24 h
3+
N.G.
3+
N.G.
3+
N.G.
48 h
3+
N.G.
3+
N.G.
3+
N.G.
Mucoid
30 min
3+
N.G.
3+
N.G.
3+
N.G.
P. aeruginosa
24 h
1–2+
N.G.
2+
N.G.
2+
N.G.
48 h
1–2+
N.G.
1–2+
N.G.
1–2+
N.G.
P. aeruginosa
30 min
3+
N.G.
3+
N.G.
3+
N.G.
*Results were identical for all three steam sterilizers tested and all inoculation sites. Mixed normal flora was present in each sample that was not recovered after steam sterilization. N.G.=No growth.
Since mucoid P.aeruginosa has a propensity to form biofilms on plastic, we next examined the effectiveness of steam treatment when bacteria were left on the nebulizers for either 24 or 48 h prior to treatment both in the presence and absence of CF sputum. Interestingly, mucoid P. aeruginosa from a liquid suspension had decreased viability after 24 h on the nebulizers prior to steam treatment (Table 2). This is in contrast to S. aureus where 3+ organisms were recovered from every nebulizer at each inoculation site (Table 2). When mucoid P. aeruginosa was added to de-identified pooled CF sputum, and inoculated on the nebulizers, both the P. aeuginosa and normal flora remained viable after 24 and 48 h, confirming that the sputum enhanced bacterial viability (Table 3). After steam treatment no organisms were recovered from any inoculation site from any nebulizer with or without sputum, confirming that steam treatment works even after organisms survive on the plastic for up to 48 h (Table 2, Table 3).
Rosenfeld et al. have shown that vigorous rinsing in water alone may be sufficient to disinfect nebulizers and this protocol was used as a control in experiments with 24 h bacterial incubation times [
]. With this protocol no mucoid P. aeruginosa was recovered from any nebulizer from any inoculum site +/− sputum. S. aureus inoculated without sputum was recovered only from the PARI LC Plus nebulizer with 14 colonies, 1 colony, and 1 colony found at respective inoculum sites. The other nebulizers had no S. aureus growth after water rinsing at all sites (data not shown). In contrast, when S. aureus was added to sputum on the nebulizer for 24 h prior to rinsing, bacteria were recovered at 8/9 inoculation sites with growth ranging from 1+ to 3+ (data not shown).
4. Discussion
Many different methods have been proposed, tested, and recommended for the disinfection of home nebulizers [
]. The ideal method should be effective and user-friendly, without altering the ability of the machine to deliver the necessary medications. It is clear that nebulized therapies add a level of complexity and time to CF treatment plans resulting in a significant treatment burden. Moreover, increased treatment burdens are often associated with a decrease in adherence to treatment regimens [
]. More importantly, in CF decreased adherence has been shown to correlate with poorer lung health, therefore a fast reliable method of cleaning is extremely important to this patient cohort [
]. A rapid and easy disinfection method would potentially improve adherence to daily disinfection and be consistent with other proposed strategies to reduce the treatment burden [
Currently steam sterilization is one of many options recommended for the disinfection of home respiratory equipment and this in vitro data suggests that it is an effective method to disinfect home nebulizers with respect to common CF pathogens. Importantly we only compared steam sterilization to washing with water and not other techniques published in the literature so we cannot confirm if these other methods would have been as effective as steam sterilization under the conditions tested. However, compared with previously published similar studies, steam sterilization performs as well or better than other disinfection methods [
]. We found no difference in mucoid P. aeruginosa recovery comparing steam treatment and rinsing with water with both being equally effective. We hypothesize this is because the rinsing method allows for an overnight drying step before swabbing for growth and our data suggest that the mucoid P. aeruginosa in this study is sensitive to dessication. Alternatively, more S. aureus survived 24 h on the nebulizer and while water rinsing was generally effective in the absence of sputum, when mixed with sputum S. aureus was recovered from virtually all sites after rinsing compared with steam treatment where no organisms were recovered.
Increasing the inoculum to 107P. aeruginosa or S. aureus did not result in the recovery of viable bacteria after steam treatment, regardless of the brand of baby bottle steam sterilizer used, the location of the bacterial inoculum, or the type of nebulizer. While we did not recover viable bacteria after steam “sterilization”, our experiments are not rigorous enough to determine if the nebulizers were truly sterile, only that there was at least 5× log killing in all experiments. We cannot exclude small amounts of viable persister bacteria that were not picked up by swabs and were below the limit of detection. Ultrasound may enhance the ability to recover these organisms but is not routine practice in most clinical microbiology laboratories.
This study supports recommendations to use steam sterilization as a method for disinfection. While fully assembled nebulizers are disinfected in vitro with this method, it does not replace the need to break down the nebulizer for routine cleaning to remove any drug residue. The studied nebulizers were subject to multiple exposures with the same inoculum sites being steam treated multiple times by each machine (n=35 exposures/machine). The number of steam treatments a single machine can perform before it is no longer effective remains unknown. This is important because there are currently no controls available for patients to ensure that the machine worked properly during each run. Additionally, these are in vitro experiments and while we hypothesize that steam sterilization will be effective when applied to actual patient nebulizers, this requires additional clinical studies. These experiments are set up in the laboratory under tightly controlled conditions with characterized strains and do not reflect the microbial variability present in the CF population. It is possible that steam sterilization may be less effective against individual patient strains of bacterial pathogens with mutations that confer resistance to steam treatment or other pathogens not tested here (e.g. atypical mycobacterium). These are active areas of investigation.
Acknowledgments
This work was supported by a contract to the Yale New Haven Hospital Microbiology Laboratory from the Cystic Fibrosis Foundation.
References
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