Beffa T., Staib F., Lott Fischer J., Lyon P.-F., Gumowski P., Marfenina O. E., Dunoyer-Geindre S., Georgen F., Roch-Susuki R., Gallaz L., Latgé J. P. (1998). Mycological control and surveillance of biological waste and compost. Medical Mycology, 36 (suppl.I), 137-145.
T. Beffa1, F. Staib 2 , J. Lott Fischer
1, P. F. Lyon 1,
P. Gumowski
3, O. E. Marfenina 4, S. Dunoyer-Geindre
3,
F. Georgen 3, R. Roch-Susuki
3, L. Gallaz 3 and J. P. Latgé
5
1Laboratoire de Microbiologie, Université , Rue Emilie-Argand 11, Neuchâtel, Switzerland; 2Brentanostrasse 26, Berlin, Germany; 3INRAAIC, Institut de Recherche Appliquées en Immunologie Clinique, Genève, Switzerland; 4Soil Biology Department, Soil Science Faculty, Moscow State University, Moscow, Russia, 5 Mycology Unit, Institut Pasteur, Paris, France
The collection and recycling of biosolids and the organic fraction
of municipal solid waste (MSW) is an important factor for the success
of the so-called "circle economy" as a component of modern waste
management policy.
Composting is one of the major treatment
processes used to transform biodegradable wastes (kitchen, garden and
industrial waste and sewage sludge) into agriculturally useful
products. Composting has several ecological and economical advantages:
a) recycling of humigenic materials, to compensate for the
important loss of humus in agricultural soils,
b) reduction of
treatment costs, compared to MSW incineration,
c) energy gain
(heat and electricity), in the case of pre-methanization,
d)
substitution of peat [31].
Composting at industrial scale can pose problems of occupational
safety, due to the occurrence of aerosols containing allergenic /
pathogenic micro-organisms and toxins.
Industrial composting has
to be a controlled process, leading to optimal hygienisation
(i.e. elimination of allergenic and pathogenic micro-organisms) and
degradation of the biowaste. However, the composting procedures
(systems and management) vary greatly and tend to be highly
empirical. In addition, the composting waste management authorities
often do not impose satisfactory plant performance criteria on the
composting industry, particularly as regards hygiene. In consequence,
accidents could occur, which would lead to the rejection of composting
as a whole. It is therefore essential to shed light on the exclusion
of risks associated with composting, i.e. the public health hazards
due to allergenic or pathogenic micro-organisms, and in particular the
mould Aspergillus fumigatus .
Below is a summation of some recent developments regarding the
recent work on hygienic aspects, in particular the presence and
dispersion of fungi (e.g. Aspergillus fumigatus ), of
biological waste and compost.
To avoid possible health risks, intervention at several levels is
necessary: education of the population about what should and should
not be put in the green waste container, optimization of the
composting process itself and the management of composting sites,
medical follow-up of compost workers and fundamental research on the
detection of medically important fungi in biowaste and
compost. Composting can be carried out at different scales: in the
backyard, in small community composts, or in large centralized
facilities. For smaller quantities of biowaste, vermicompost is an
interesting alternative. Very little is known, though, about the
hygienic aspects of this low-temperature process.
Composting : a microbiological process
Composting is
a self-heating, aerobic solid phase biodegradative process of organic
waste materials. The composting process at the microbial level
involves several interrelated factors, i.e. metabolic heat generation,
temperature, ventilation (oxygen input), moisture content, and
available nutrients. The temperature reflects both prior microbial
activity and the current rate of activity. The initial rapid increase
of temperature involves a rapid transition from a mesophilic to a
thermophilic microflora. The compost ecosystem then tends to limit
itself due to inhibitory high temperatures, resulting from excessive
heat accumulation. If good management is continuously provided
(i.e. regular aeration or frequent turning), the thermogenic stage
continues until heat production becomes lower than heat dissipation,
due to the exhaustion of easily metabolizable substrates. During the
terminal cooling or maturing phase, the amount of readily available
nutrients becomes a limiting factor, causing a decline in microbial
activity and heat output. During these temperature changes various
microbial groups succeed each other, each of which is adapted to a
particular environment.
A large variety of mesophilic, thermotolerant and thermophilic
aerobic micro-organisms (including bacteria, actinomycetes, yeasts,
moulds and various other fungi) have been extensively reported in
composting and other self-heating organic materials [13,11,9,7]. In
function of their degradation potential and their ability to grow at
elevated temperatures, they are active at different moments during the
process.
Mesophilic microorganisms are partially killed or are poorly
active during the initial thermogenic stage (temperatures between
40-60° C), where the number and species diversity of
thermophilic/thermotolerant bacteria, actinomycetes and fungi
increases. The optimal temperature for thermophilic fungi is
40-55° C, with a maximum at 60-62° C. Fungi are killed or
are present transiently as spores at temperatures above 60° C.
Among the fungi, the mould Aspergillus fumigatus has a special
significance: due to its capacity to degrade almost all components of
organic waste (sugars, fatty acids, proteins, cellulose, pectin,
xylan, [14]), and its thermotolerance (optimal growth at 37° C,
good growth between 30 and 45° C, maximal growth at 52° C
[19]), it finds ideal proliferation conditions in young compost.
Aspergillus fumigatus conidia can survive at temperature of
55-60° C for a fairly long period. Unfortunately,Aspergillus
fumigatus is also a known opportunistic pathogen and allergen
[20].
Thermophilic bacteria are very active at 50-60° C. High
temperatures (> 60° C) are often considered to reduce
dramatically the functional biodiversity [13]. It is generally assumed
that to obtain efficient and rapid decomposition temperatures should
not be allowed to exceed 55-60° C. However, at these
temperatures the thermohygienization towards potentially pathogenic
and / or allergenic microorganisms is not guaranteed.
Recent
results demonstrated that in the composts studied temperatures between
65-80° C were usually reached during the thermogenic phase [7,
8]. We showed in these hot composts the presence of a great variety
and high numbers (10 7-1011 cells/g compost dry
weight) of aerobic high thermophilic bacteria growing at temperatures
between 60-82° C. These bacteria were present in all types of
waste (green, kitchen, sewage sludge) and industrial systems
studied. The demonstrated functional bacterial diversity during the
thermogenic phase seems to make it possible to compost at high
temperatures (65-75° C) for a longer period of time, but not
exceeding 80° C. By this means, the composting process can be
performed with a better destruction of potential human pathogens and
allergenic moulds, as well as phytopathogens and seeds.
In the newly introduced field of biological waste and composting management it should not be ignored that a variety of allergens and/or pathogens can be found in the raw material of biowaste and compost. These hazards arise from three possible sources of contamination:
a) primary pathogens of intestinal origin (bacteria, virus,
intestinal parasite cysts or eggs),
b) secondary or opportunistic
allergenic and / or pathogenic forms, mainly moulds, developed during
compost processing and stocking,
c) bacterial and fungal
allergens and toxins.
The first source is linked to the fecal pollution of the raw
material. This contamination is at its maximum for products
incorporating high amounts of urban waste water sludge or farm wastes,
lower for household refuse composts and industrially processed
composts of vegetable waste. The second hazard results from the
development of meso- and thermophilic / thermotolerant fungi and
actinomycetes which play a role in biowaste degradation and
maturation.
Who knows better than the mycologist that organic
wastes occurring in the household may serve as a nutrient substratum
for fungi, among which causative agents of infection, allergy or
intoxication may be found. The detection of the soil of potted plants
as a source of airborne fungal infections in immunosuppressed patients
in the hospital by the medical mycologist and, likewise, of various
materials specifically colonized by a variety of allergenic fungal
species in the home environment by the allergist in cooperation with
the mycologist, have drawn attention to the ecology of fungi in our
everyday life [30].
The role of A. fumigatus during the
composting process has been a subject of interest for some time
[6]. However, other fungi of medical-mycological interest have not
been routinely detected in compost facilities [26]. Also, practical
rules and standards for the control of fungi from the medical mycology
community are expected by composting industries.
Already at the XI ISHAM Congress 1991 in Montreal, Beffa et al. reported on the presence of A. fumigatus in municipal composting facilities and the possible risks of exposure to A. fumigatus for compost workers and neighbors [6]. At the same congress, Staib presented a report on "Pathogenic fungi in human dwellings". He drew attention to the biological waste container as a new source of indoor fungal growth [30]. On the occasion of the XII ISHAM Congress 1994 in Adelaide, there was a report by Staib on "Health risks from decay of organic wastes and compost: Proposals for mycological control" [29]. From the viewpoint of a medical mycologist and microbiologist, 12 proposals were made on a variety of topics: education of the population; the biological waste container; waste processing and fungi of medical interest; composting and fungi, and the role of A. fumigatus ; immunological control respecting exposure to fungi of medical interest; the need for mycological departments in reference institutions for waste and composting microbiology and their tasks e.g. elaboration of appropriate methods of mycological analysis, procedures for testing the decomposability of newly developed environmentally friendly materials by fungi of medical interest; precautions to avoid spore inhalation; surveillance of wildlife animals and pests in the environment of composting plant; and an international exchange of knowledge gained in this field and the results obtained.
There is an increasing interest in specific proposals intended for
the citizens and authorities responsible for biological waste and
composting management. In the editorial of the Zentralblatt fü r
Bakteriologie, Staib made the following remark: "Unless the subject of
fungi in biowaste and compost has been thoroughly examined and
publicly discussed by medical-mycological societies and the ISHAM, no
rules, standards or regulations should be applied in practice"
[27]. Therefore, our special thanks go to the convenor of the XIII
ISHAM Congress, Professor L. Polonelli, who corroborated the need for
a symposium dealing with the topic of mycological control and
surveillance of biological wastes and compost.
In addition to the foregoing basic considerations, some examples from everyday
life will illustrate the need for medical mycological control as follows:
Independently of the elaboration of a mycological surveillance
program with the assistance of ISHAM, and in addition to information
from offices of local government, education of the public should start
in biology classes in schools. In 1990/91, the Mycology Unit of the
Robert Koch Institute in Berlin held lectures with practical
demonstrations before members of the German Society for Teachers of
Biology. A teaching program for schools dealing with moulds,
biological waste and composting was elaborated by these teachers and
published in their professional journal [24]. We hope that such
education will show the young generation why mycological departments
are important in institutions of medical microbiology.
After a 3-4-month exposure of four HIV-positive men to Cryptococcus neoformans (C.n.), aspergilli, Mucoraceae and Dematiaceae found in pigeon droppings and waste, only one of them who exhibited a low count of 50 CD4 lymphocytes / µ L (normal count 650-1250 / µ L) fell sick and had a systemic course of cryptococcosis, but the others with higher CD4 counts, remained free of C. n.and free of infections by the other opportunistic fungi they had been exposed to [4]. This example shows that various types of specific immunodeficiency can be decisive for opportunistic infections.
Cases of cutaneous histoplasmosis in badgers in Germany, i.e. outside of endemic areas of histoplasmosis, have raised the question if the use of biological wastes as feed by badgers and accidental microfocus of Histoplasma capsulatum (H.c.) may be a source of infection [16]. Research on this topic is under way.
A new field of applied mycology will develop from the proposal No. 11, "In procedures designed to test the decomposability of newly developed environmentally friendly household materials, fungi of medical interest should be included along with other test organisms" [27].
Recently, Dill et al. reported that gardeners in a large German horticultural establishment developed inflammatory toxic reactions on their fingers. They had been exposed to compostable flower pots manufactured from recycled paste-board. A heavy growth of Stachybotrys chartarum (a well known mycotoxin producing fungus) developed on these pots [12]. This observation proves the recommendations made in the above mentioned proposal No. 11. It has to be noted that so far in Germany an incident of this kind has to be considered by various government institutions, e.g. the Federal Institute for Testing of Materials, the Federal Environmental Agency, the Federal Institute for Health Protection of Consumers and Veterinary Medicine, and the Robert Koch Institute - Federal Institute for Infectious and Non-Communicable Diseases. Generally, the division of responsibility has resulted in problems.
In connection with the observations of the various opportunistic pathogens in HIV-positive persons by Arasté h et al., it was found that infections by Cryptococcus neoformans together with Mycobacterium avium intracellulare (M.a.i.) occurred if there was a CD4 lymphocyte count as low as 3-20 / µ L [5]. Epidemiologically, this predisposition is of interest since in Berlin, Wittstatt found mycobacteriosis to be present in 7.1 % of 1067 psittacine home birds autopsied during the 1988-1996 period. In a third of these cases, M.a.i. was the causative agent (which was also excreted in droppings) [32]. It seems therefore logical to exclude droppings of home birds from the biological waste container, according to proposal No. 12 [27]. Compatible C. neoformans strains could be isolated from biological waste containing bird droppings. The perfect stage with basidiospore formation on bird filtrate agar could be reached already after 48 hours at 26° C [28]. Based on this observation, it is suggested that all types of morphogenesis of medically interesting fungi and other microorganisms may occur under the various growth conditions in biowaste.
A HIV-positive male person who had a CD4 lymphocyte count of 30 / µ L, was diagnosed with histoplasmosis as the first opportunistic infection. He had disposed of the crusts of the skin lesions loaded with H. c. by putting them into the biological waste container [16]. He had done so as a daily routine over weeks. Thus, attention is drawn on the possible presence of microfoci of H.c.outside endemic areas, i.e. sources of infection for persons handling wastes and operating compost processing installations as well as for wildlife animals living in the vicinity of waste handling and composting plant.
Positive epidemiological evidence is the best argument to conclude from potential hazard to actual risk. However, precise scientific data are not available at the moment. Emphasis must be placed on possible biases, which may lead to some underestimation of long-term effects. In fact, many compost workers consider their job only as temporary, while they look for less hard, better paid and more highly regarded work. This makes long-term surveys difficult, many people vanishing before the end of the study. In addition, as soon as some possibly compost-related clinical symptoms are observed, doctors in charge of occupational medicine for the plant immediately request that the employee changes his work to an unexposed environment.
In a preliminary study, the occupational risk factors for workers in
municipal composting facilities had been evaluated [17].
As part of a 3 year multidisciplinary pilot follow-up study that began
in 1992 in a large composting facility in the Geneva area, we have
studied the evolution of the immune response towards a panel of
Aspergillus fumigatus antigens in a group of ten exposed
compost workers (CW) recruited on a voluntary basis. This facility was
visited twice per year and each CW was submitted to: i) lung function
tests, ii) laboratory investigations, iii) non-invasive
microbiological samplings.
Spirographic measures (Vitalograph®) consistently showed a transient weak to moderate decrease of the lung functions after work periods of over 3-4 hours on compost piles. However, no significant deterioration has appeared during the 3 years of the survey.
A.fumigatus immunity was assessed with a purified precipitate (F27) from the culture filtrate, and various mycelial and purified extracts for both Ig-specific ELISA tests and lymphocyte transformation tests (LTT). The cellular and specific IgG, IgM, IgA and IgE responses toward A.fumigatus antigens were compared to the responses observed in 10 A.fumigatus allergic patients (AP) and in 10 normal non-exposed subjects non-allergic to A.fumigatus (CP). LTT responses were significantly increased in CW and AP subjects in comparison to the responses observed in CP. No statistically significant differences could be observed between CW and AP. Specific IgG and IgA responses were increased in CW compared to CP. Again, no statistically significant differences could be observed between CW and AP. With IgM specific Ab, no differentiation between the three groups could be made (no statistical difference). IgE were present in all AP and could be detected in low amounts in most of the CW, with a seasonal fluctuation, though all CW were asymptomatic. No IgE could be found in CP. In newly engaged CW, an hyperimmunity state towards A.fumigatus equivalent to that of long-term exposed CW appeared after 9-18 months. The appearance of this immunity was essentially related to the frequency of the exposure A.fumigatus isolates: Culture swabs (nose and/or ear external ducts) were positive for A.fumigatus in > 90% of CW after direct work on compost piles over 2-3 hours. After stopping work positive isolates were found in 45% after 15 hours, in 15-20% after 24 hours and were all negative after 48 hours.
The main objective of the compost research at the laboratory of microbiology of the University of Neuchâ tel is to optimize the thermogenic phase of the composting process, in order to maximize the hygienisation (elimination of allergenic and pathogenic micro-organisms). In this way, an efficient degradation of the organic matter and a good maturation of the compost is ensured, and the recolonisation with pathogenic / allergenic microorganisms and the phytotoxicity hazards in the maturation phase are avoided. The research is supported by the Swiss National Foundation (Priority Program Biotechnology, module 5B, biosafety research), and several compost industries.
It should enable the provision of guidelines to collectives, industries and private citizens for the establishment of composting locations, for the choice of the composting system, and for its management, in order to warrant a good composting process with a high hygienic security.
The composting systems studied belong to the
following types of installations:
- open-air triangular windrows (elongated heaps),
- boxes with automatic aeration and/or turning, roofed or in a closed hall,
- closed bioreactors.
In
some composting facilities, very wet and nutrient rich materials, such as
kitchen waste or sewage sludge, are treated by methanization before composting.
Undoubtedly, the biggest potential biohazard associated with
composting is A. fumigatus, an opportunistic pathogen and
allergen [2,10,15,22]. It was detected in all composting types
investigated. Up to 106-107 A. fumigatus
CFU (colony forming unit) were measured in gDW (gram dry weight)
compost, and up to 106 A. fumigatus CFU per
m3 of air were measured at sites where compost was
processed.
Research
at our laboratory has shown that the presence and abundance of
Aspergillus fumigatus in
the composts and in the air can be taken as a bio-indicator for the presence
and dispersal of other potentially pathogenic microorganisms and particles.
Temperatures up to 80° C have been observed in the center of
composting heaps. These elevated temperatures are generally considered
sufficient for the elimination of pathogenic microorganisms. Extensive
measurements carried out at our laboratory showed, however, that
important temperature gradients can exist in the compost mass, and
that temperatures in the outer or lower zones can be up to 30-40°
C below the core temperature. A. fumigatus proliferates or
persists in these cooler parts. The destruction of A. fumigatus
spores, which are very heat tolerant, requires quite high temperatures
(> 65° C) [3]. Also, recolonisation of the compost by
A. fumigatus after the thermogenic phase is frequently
observed, although concentrations do not reach the initial values. The
extent of the recolonisation depends on the compost maturity, which
itself depends on the organic matter content of the starting material
[25].
An intensive management of the compost is necessary for good
hygienisation. The following parameters have to be chosen carefully to
assure a correct composting process: composition of the initial
substrate (C:N ratio around 30 with a good structure), adjustment of
humidity, frequent mixing to redistribute microorganisms and substrate
and to bring material to the hot center of the heap, adjustment of
aeration (frequency, duration), duration of the process long enough to
avoid recolonisation during maturation, or direct use of fresh compost
without storage.
But, it is not only the composting process itself that is important, but also how the material is treated before and after it: development of A. fumigatus happens already in the home, when biodegradable waste is stocked for an extended period of time. To avoid this, waste collection has to happen at short intervals (not longer than 1 week between collections). Once the material gets to the composting site, it should be processed immediately, and not let sit on big heaps for days. This will also help to control odours. Special care should also be given to the finished compost. If possible, treatment should be continued, although at more infrequent intervals, until the compost is used (mostly for agricultural purposes). If for shortage of space this cannot be done, the compost should be stored unsifted in big heaps which are not too large, to allow a minimal natural ventilation.
When comparing the results from different sites that compost in open-air triangular windrows, it seems that there are two main factors that influence the proliferation of A. fumigatus: the turning frequency, and the degree of humidity of the compost. These two parameters are in fact coupled: effective watering of the compost is only possible during turning.
Spore counts in compost were effected at an intensive managed
facility, where the windrows were turned daily with a specialized
machine, and compared to those of an extensively
(i.e. non-intensively) managed facility where turnings were performed
only every third week with a front end loader. Although the core
temperatures measured in the windrows during sampling were about the
same (70° C to 77° C for the fresh composts and 63° C
to 70° C for the composts of several weeks of age), the much
lower mould counts at the intensive managed facility indicated that
there, through the frequent turnings, the entire compost had been
submitted to those high temperatures. At the extensively managed
facility, where the turnings were infrequent, only the center had been
heated up, while the surface stayed much cooler, furthering the
proliferation of A. fumigatus [21].
These experiments
showed that it is not in fact the core temperature of a compost heap
that is decisive for the elimination of moulds and other pathogenic or
allergenic microorganisms. Rather this can only be achieved by
frequent turnings so that temperatures of 65° C are reached in
the whole compost material.
Measurements of spore concentrations in the air 2m behind a turning
machine clearly showed that lower numbers of A. fumigatus in
the compost, due to intensive management of the windrows (more
frequent turnings and water addition) resulted in a lesser spore load
of the air.
Although concentrations in the direct vicinity of
turning of shredding machines were quite high (up to 107
CFU per m3), exposure of the people working in the place
was often much less, because by operating the machines, they were
normally standing at the side, in front or above them. In the open
air, a rapid dilution of spores was observed: already in 10m distance
from the turning machine, A. fumigatus concentrations were
100-1000 times reduced, and in 500m distance from the site, even
downwind, no more A. fumigatus spores were measured (0- 20
CFU/m3) than in places far away from composting
installations.
When
composting is carried out in boxes, a new parameter for the control of the
process has to be taken into consideration: aeration. Air is blown into the
compost to ensure sufficient supply of oxygen to the aerobic microflora. At the
same time, moisture is transported out of the compost by the air stream, and
heat is withdrawn through evaporative cooling.
We
examined in our studies four different installations that use box composting.
One problem encountered by this type of process is an excessive depth of the
composting material, boxes were usually 2-4 m high filled with compost.
Turning of these big masses of compost was slow, and turning frequencies were
accordingly low. This led to an inhomogeneous temperature distribution,
vertical as well as horizontal, and consequently to an inconsistent composting
process, and an insufficient thermohygienization. At one site that had no
aeration system, almost no degradation of organic matter was observed during
the passage in the boxes. In those heaps, A. fumigatus
concentration was low, most probably due to the low oxygen content in the
compost. But as soon as the material was stored in big heaps outside the boxes,
strong heating occurred, and high concentrations of A. fumigatus
(up to 105 CFU/gDW) were measured on the
surface of the heaps, as in extensively managed open air windrows.
A. fumigatus spore concentrations in installations where the
boxes were only roofed were similar to open air windrow
sites. Emission of spores during turning depended a lot on the
construction of the turning machine: more gentle mixing evoked less
spore dispersion. Also, transport of spores by the wind seemed to be
hindered by the box walls and the roof.
Composting
inside closed buildings led to elevated concentrations of spores inside the
rotting hall. Spore counts were permanently > 1000 / m
3.
Even if the turning was supposed to be fully automated, the frequent presence
of maintenance staff in the hall was necessary (high corrosion, clogging
of the aeration system, sampling for process control). Outside the rotting
hall, spore concentrations were minimal.
Composting in a bioreactor allows in principle the highest degree of
control over the process. All input and output parameters can be
easily monitored, analyzed and in function adjusted. On the other
hand, visual control of the compost and sampling is often not
possible. As in box composting, problems of homogeneity of the
material were observed, and drying out of the compost through
suboptimal aeration was frequent. Thermohygienization in bioreactors
was often insufficient, in spite of quite high core temperatures due
to a good thermal insulation of the reactors.
Composting in
bioreactors generated no spore emission at all inside the building
where the reactors are located, except when loading and emptying of
the reactor.
When looking at A. fumigatus concentrations in combined
methanization and composting installations, one has to look at the two
processes separately. Methanization can be carried out at two
different temperature ranges: under mesophilic ( 40° C) or
thermophilic (55-65° C) conditions. For hygienic reasons, the
latter is often preferred.
The combination of elevated temperatures and acidic conditions led to
a drastic reduction of moulds. The sludge that resulted from the
methanization process was either pressed, and subjected 'as is' to a
short composting process under aerobic conditions, or was mixed and
co-composted with shredded wood. The conditions for fungal growth were
enhanced through the input of air.
As composting is mostly carried out in boxes or bioreactors, if good
mixing of the material is not guaranteed, a strong recolonisation of
the compost can occur.
Five of the installations examined were equipped with
biofilters. Composting was either carried out in boxes in a closed
hall or in bioreactors. Spore counts carried out after the filter
yielded results of a few up to several 100 CFU / m 3 of
air. Taking into consideration that A. fumigatus concentration
measured inside the rotting hall were usually a few 1000 CFU or much
higher during turning of the composts, it seemed that the biofilters
were reducing the amount of spores dispersed into the environment,
although they were not completely holding them back.
Biofilters
are usually laid out for the elimination of bad odors, and not for the
retention of microorganisms.
Earth-worm compost (vermicompost) is advertised as an effective, low
price, "natural and ecologically clean" fertilizer. Vermicomposting is
used in some industrial biowaste systems, at animal enterprises, in
small farms, and in private gardens. The application of earth-worm
compost increases crop production of vegetables (tomatoes, cucumbers,
salads), and improves the growth of flowers and indoor plants. Thus,
these composts are attractive for many citizens. In the former Soviet
Union for example, millions of town people have a small suburban
garden-plot, and are interested in their own cheap and "clean"
vegetable production. These gardeners do not only use commodity
vermicompost, but try to make their own vermicomposting
systems. Vermicomposting is also beneficial for farmers, because of
the possibility to use both the compost, and the earth-worms for
poultry feeding. Nevertheless, very little is known about the health
risks from fungal growth in earth-worm composts.
We examined
different types of earth-worm composting systems, where compost was
produced from biowaste, pulp trash, mushroom farm waste, and poultry
and pig manure. Some of these systems are primitive, and not
controlled technologically, but are often preferred because of their
cheapness. Earth-worms of the species Eisenia fetida (Sav.)
were used in the composting process.
Temperatures in the composting environment were between 20 - 25° C. The structure of microfungal communities was described for the species composition, their frequency of occurrence (%) and abundance. To compare microfungal communities in different vermicomposts and in composts without earth-worms, the original program for cluster analysis BIOMATRIX v.2.2 [23] was used. To determine the incidence of microfungi in the air of composting environments, open Petri dishes were exposed (at the height 1,5 m) for 15 and 30 min for sedimentation plating.
We found that microfungal communities in vermicompost differed in several respects from similar communities in compost without earth-worms, namely in their diversity, species composition, and abundance. The overall diversity of microfungal species in compost without worms was somewhat higher than in vermicompost. This difference was observed distinctly at the late stages of the composting process. The simplification was due mainly to a reduction in the number of rare microfungal species. It was very important that the abundance and frequency of occurrence of some microfungi of medical interest was, as a rule, somewhat higher in vermicompost than in compost without earth-worms. This trend was observed for representatives of genera Aspergillus (A. fumigatus, A. niger, A. flavus), Fusarium (F. oxysporum, F. moniliforme), and Chrysosporium spp. The highest level of abundance was determined in the environment of vermicompost based on poultry manure.
Simultaneously to the elimination of some dark-colored fungi - Cl. cladosporioides, Al. alternata, Doratomyces stemonitis, dark-colored sterile mycelium appeared. The distinction tended to be significant after 40 days of composting, when a clear decrease in overall species diversity occurred.
In some vermicomposts a distinct difference in Aspergillus species distribution between different layers of compost was observed.
We found that vermicompost application did not increase the abundance of problematic moulds in the compost-amended soils. The formation of mycobiota in vermicompost are influenced by biotic and abiotic factors: type and concentration of organic substances, temperature, pH, microbial interactions, and earth-worm influence. It is known [1] that earth-worm activity usually leads to the neutralization of the pH of soils and substrata, in which they are living. As previously mentioned, the vermicomposting conditions are characterized by rather high temperatures and neutral pH. This ecological situation is known to be often preferable for the growth of some opportunistic microfungi, for example representatives of the genera Aspergillus and Fusarium. Very little is known about the influence of some biotic factors (microbial-invertebrate interactions) on the microfungal community formation in vermicomposts. The significance of the interaction between fungi and the earth-worms had been demonstrated in our experiments. We found that some changes in microfungal communities' composition in vermicompost could be connected to the feeding preferences of worms. In was shown experimentally that the earth-worm E. fetida did prefer dark-colored fungi ( Cl. cladosporioides, Al. alternata) as a food. On the contrary, the substrata with addition of mycelium of Aspergillus species (A. niger) were not attractive for earth-worms. In the investigations of the earth-worms feeding capacities it was also observed that microfungal growth was drastically changed after fungal spores had passed through the earth-worm intestine. The spores of dark-colored fungi were the most sensitive to the influence of worms digestion. For Al. alternata and Cl. cladosporioides, a clear increase of spore germination time, decrease of germination level and mycelial growth was observed in earth-worms excretes. In contrast, no changes in the life cycle of A. niger were found after the earth-worms had been fed with A. niger spores. Germination and mycelial growth are strongly influenced by the microflora, in particular by the mechanism of fungistasis. The fungistasis is caused by microbial products. In the experiments with microfungi and bacteria isolated from earth-worms and vermicompost it was shown that vermicompost colonization by fungi can be partly suppressed or activated by the initial bacterial population [18]. The bacteria of the genus Bacillus, which were isolated as common components of the gut-microbiota of E. fetida and from vermicompost, inhibited spore germination, mycelial growth and gave rise to microcycle conidiation for Cl. cladosporioides, T. harzianum, and Al. alternata. On the other hand, the metabolites from bacteria of the genera Pseudomonas and Spirillium, which were isolated from vermicompost, activated spore germination and mycelial growth of some fungal species, including A. niger.
In conclusion, I would like to draw attention to the high possibility of the formation of specific microfungal communities in vermicomposts. It should be stressed that vermicompost microfungal communities often include species from the genera Aspergillus, Fusarium, and Chrysosporium, which are of priority interest to human medicine. The specificity of microfungal communities formation in vermicompost may have some ecological reasons: 1) the support of special conditions for the growth of the earth-worm population during the vermicomposting process, and 2) the earthworm - microfungal interactions. The presence of problematic moulds should become a point of mycological control in vermicompost during the composting process (particularly during the active composting phase), in the prepared compost, and in the vermicomposting environment.
It is hoped that the content of this paper will contribute to the ongoing discussion on a mycological surveillance program for biowaste treatment and composting, and complete the proposals presented previously [10, 29]. However, further multidisciplinary studies involving engineers, biologists/microbiologists, allergy specialists/epidemiologists, and competent authorities, are necessary to establish definitive recommendations on the management of biowaste and compost, and on the control of its hygienic and agronomic quality.
The aspects that should be investigated or improved in priority are the following:
Research at the University of Neuchâtel is supported by the Swiss National Science Foundation (Priority Program Biotechnology, Module 5B (biosafety research), Grants no. 5002-038921 and 5002-049917) and several industries active in the field of composting. The project title is: "Composting of organic wastes, optimization of the thermogenic phase to overcome the hygienic and public health hazards".
The co-convenors were T. Beffa and F.Staib
The contributors of this symposium were: F. Staib, Comments on proposals for the avoidance of health risks posed by fungi from biowaste and compost; T. Beffa, Composting: a microbiological process; J. Lott Fischer, P.-F. Lyon and T. Beffa, Elimination or reduction of pathogenic fungi during composting, in function of the type of system and management; O. E. Marfenina, Earth-worm compost and fungi; P. Gumowski, S. Dunoyer-Geindre, F. Georgen, R. Roch-Susuki, L. Gallaz and J. P. Latgé, Follow-up of compost and waste workers : evolution of immune response to compost dust and Aspergillus fumigatus.
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