Filamentous fungi and yeasts on mattresses covered with different encasings

ARTICLE

The efficacy of synthetic mattress encasings in the reduction of mites on mattresses has been demonstrated by various authors [1-4]. The clinical benefit of this reduction is still under discussion: some authors report an improvement of the asthmatic symptoms [2, 5, 6] whereas other investigators failed to demonstrate positive clinical effects [4, 7, 8]. However, there is little doubt that reduction of allergen exposure results in an improvement of allergic symptoms in sensitised patients. Obviously mite allergen avoidance cannot be achieved by the use of mattress encasings alone, but requires several means to reduce the total allergen exposure in the domestic environment [9-11].

There is increasing evidence that besides mite allergens, filamentous fungi and yeasts in the domestic environment have a strong impact on the severity of asthma [12]. Tariq et al. reported 981 children studied of whom 6% reacted positively to Alternaria alternata and Cladosporium herbarum. This sensitisation was positively correlated with the diagnosis of asthma, eczema or rhinitis [13]. Garrett et al. demonstrated that exposure to Penicillium spp. is significantly associated with asthma in children, whereas Aspergillus spores are a risk factor for atopy. However, both correlations were not very strong and no further significant associations between atopy and fungal exposure were found [14]. Peat et al. recently reviewed the effects of moulds in the home on respiratory health [15]. They concluded that the increasing risk of allergic children due to mould exposure is fairly small with an odds ratio in the range of 1.5-3.5. A recent study analysed data from 1,614 children in 10 different countries to assess the correlation between self-reported moulds in the homes and respiratory symptoms. The authors found a significantly higher period prevalence of cough and upper respiratory symptoms in children living in houses with reported moulds compared with "dry" houses [16]. Similar results are reported by other authors [17, 18]. Other investigators confirm that the sensitisation to moulds in otherwise atopic subjects increases the risk of symptomatic allergic disease [19]. In contrast to fungal allergens, bacterial antigens might even have a protective effect via exposure to endotoxin release, a hypothesis which is currently being discussed [20, 21].

However, it seems to be useful to investigate methods which may achieve a reduction of the fungal growth in the dwelling environment. In contrast to the bedding, which may easily be cleaned and disinfected by washing, the reduction of fungal growth in the mattress by common cleaning methods (e.g. removal of dust with a vacuum cleaner) is almost impossible.

Meanwhile an increasing number of hospitals apply synthetic encasings to protect the mattresses from any contamination. These encasings may be useful in the domestic environment, because they can be removed and washed in a common washing mashine. However, at present no data are available concerning the fungal flora beneath these encasings. This question is important, because the removal of the encasings may result in a considerable contamination of the ambient air (and, thereby, the bedding).

In the event of increased fungal growth beneath these synthetic encasings their application can hardly be recommended for usage by sensitized patients.

Materials and methods

Study design

Seventy-five participating volunteers were asked which encasing material (cotton/polyurethane) they would prefer. Thirty-four chose cotton encasings, 32 synthetic encasings. Nine people had no specific wishes and were therefore randomly assigned to the groups. Group A (n = 37) received complete cotton mattress encasings, group B (n = 38) complete synthetic mattress encasings, which consisted of a polyester micro fibre with a polyurethane surface layer (Pro-Tex^® by Germed Corp, Schwarzenbek, Germany). This synthetic mattress encasing is permeable to water vapor but impermeable to water droplets and particles > 3 mum. All participants received new foam mattresses. All participants were at least 18 years old and gave written consent to the study. They agreed to refrain from cleaning or removing the encasing during the investigation. In addition, the participants had to refrain from washing the blankets or pillows, only the pillow slips and the covers of the blankets were allowed to be removed and washed during the investigation.

Residence at a different location (e.g. during vacation) was allowed to a max. of 30 days during the 12 month study period. Exclusion criteria were known asthmatic disease or sensitisation to house dust mite allergens and application of acaricides or insecticides in the dwelling environment, because this could confound the natural growth of microorganisms in the bedding.

Sampling

Dust samples from the mattress surfaces were collected after careful removal of the particular encasing immediately after delivery of the mattresses and after 3, 6 and 12 months. A vacuum cleaner (Rowenta Super Compact RS-007) was equipped with a high-grade steel filter clamp containing sterile gelatine filters (Sartorius Corp., Göttingen, Germany) which allowed us to collect dust particles > 2.0 mum. Vacuuming was carried out for 3 min by moving the filter clamp across a diagonal line of the mattress surface. The assessed surface was 820 cm².

The filter clamp was disinfected after each sampling procedure. Filters were transported in sterile tubes.

Assessment of fungi

After arrival in the laboratory 5.0 ml Sabouraud bouillon (pH 5.8) was added to the tubes to dissolve the gelatine filter. The solution was vortexed for 30 s and four samples (two samples with 1.0 ml and two samples with 0.1 ml respectively) were taken from the tube and plated on agar plates containing malt agar (Oxoid). The 1 ml plate was only considered if no growth was detected on the 0.1 ml plate; this procedere was necessary because we did not know about the fungal load of the samples in advance. The agar plates were incubated aerobically at 20° C and at 37° C to provide for optimal growth. After 48 h visible colonies of grown microorganisms were counted. The weighted arithmetic mean was calculated from the different dilutions. Each agar plate was assessed counting all colonies which showed typical signs of fungal growth. Additionally, fungi which were obtained during the final sampling, after 12 months, were identified (generic or species level). These agar plates were incubated for 7 days. The complete method including the sampling procedure and the incubation of the fungi had been validated in preliminary trials to ensure a reliable reproducibility.

Statistical evaluation

The U-test was used to assess the significance of differences. The statistical evaluation as well as the plotting were carried out with the program SPSS for Windows.

Results

Five volunteers had to be excluded from the study, two because they did not accept the foam mattress, and three because they were absent when the sampling had to be carried out. Therefore the final size of each group was 35. There were no relevant differences concerning sex and age distribution in both groups (group A consisted of 22 females and 13 males, median age 38 years; group B comprised 19 females and 16 males, median age 42 years).

During the first week after delivery of the new mattresses and encasings the dust samples were collected from 14 participants with cotton encasings (= group A) and 12 with synthetic encasings (= group B) to ensure that no differences between the microbial bioburden of the new mattresses or encasings exist. These results are presented in Figures 1 and 2 "month 1". We examined only a portion because differences in the initial bioburden were very unlikely since all mattresses and encasings belonged to the same batch. Sampling results after three, six, and 12 months are also presented in Figures 1 and 2.

Except for the fungi grown at 37° C, significantly higher counts were obtained for group A after three months. Six and 12 months after the start of the study, the counts of fungi grown either at 20° C or at 37° C were significantly higher in the group using the cotton encasings (group A) compared to the synthetic ones (group B).

The predominant fungal taxa traced in the final dust sampling after 12 months are listed in table I. In both groups species of the genus Penicillium were found most frequently. On the surface of the mattresses covered with cotton encasings 16 different genera including 8 samples with mycelia sterilia and 6 with unknown yeasts were counted. Aspergillus spp. were also frequently recovered from group B samples. A total of (at least) 20 different species were counted (table I).

Discussion

The focus of the investigation was the assessment of fungal growth since fungi are important in allergic disease.

However, our results indicate that mattresses covered by cotton encasings are more rapidly colonised by fungi than mattresses with synthetic encasings. Of course, this does not necessarily apply to all synthetic encasings, since it is known that synthetic encasings are different in terms of permeability to particles and water [22].

We did not take samples from the surfaces of the encasings for two reasons: firstly, the microbial counts obtained by agar contact plates from the encasings are affected by the sedimentation of particles and are therefore a weak parameter for the microbial colonisation of the mattresses. Secondly, a reduction of the microbial colonisation of the encasings (independant from the specific encasing material) can be easily achieved by washing at temperatures > 60° C. However, if the mattress is colonised by a microbial agent responsible for allergic symptoms in the user of the bedroom, this allergen will contaminate the indoor air with every change of the encasing or bed spread. A standardised cleaning or disinfection of the mattress on the other hand is almost impossible under domestic conditions. Therefore it is important to know if it is possible to reduce the microbial colonisation of the mattresses with specific encasing materials.

Several investigators have assessed the influence of fungal exposure in the domestic environment on the development of allergic diseases, since moulds are almost ubiquitous in our dwellings. The generally low correlations between fungal exposure and positive skin tests may be due to poor sensitivity in the assays because many test series contain relatively few taxa, resulting in an insufficient sensitivity [23, 24].

The predominant fungi we found after 12 month are species of the genera Penicillium, Aspergillus, Scopulariopsis and Cladosporium. Most of them are typical moulds of the domestic environment, particularly in house dust. Katz et al. assessed the indoor mould levels in 59 houses; they reported that the most common isolated genus was Aspergillus followed by Penicillium, Alternaria and Cladosporium [19]. The prevalent fungus in the study by Ishii et al. was Penicillium which was identified in every dust sample, followed by Cladosporium, Trichoderma and Aspergillus [25]. Wassenaar isolated fungi of the genus Aspergillus, Penicillium and Wallemia most frequently [26].

Since the synthetic encasings used in our investigation minimize the penetration of fungal allergens and allow for wet surface cleaning or disinfection, the number of allergens can be reduced significantly.

As a result we can make two recommendations:

Firstly, patients who are allergic to microbial allergens (e.g. moulds or yeasts) should not use cotton encasings at all, because they hardly reduce the microbial colonisation of the mattress, even if they are washed or disinfected frequently. However, these patients should not use synthetic encasings as a sole means of allergen avoidance, but as one part of an allergen avoidance regimen in the domestic environment. Additional steps that should be taken to achieve a maximum of allergen avoidance are suggested by other authors [27, 28].

Secondly, cotton encasings should be removed periodically (e.g. every third month) and washed at temperatures of at least 60° C because they are colonised by fungi quickly. The latter recommendation does not follow from our study, and further investigations are necessary to give clear instructions on how frequently removal and cleaning should be carried out. However, since we observed the first significant difference regarding the fungal growth after 3 months, this time interval seems to be appropriate.

CONCLUSION

We thank the participants for their cooperation within the 12 month study and K. Rane (West Lafayette) for critically proofreading the manuscript.

Article accepted on 27/7/01

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