Microbial Volatile Organic Compounds (MVOCs) | Nigrospora species
By Karen Abella Santo-Pietro
Volatile Organic Compounds (VOC's) are chemicals with low molecular weights, high vapor pressure and low water solubility. These chemical characteristics allow VOC's to easily evaporate into the air or "off-gas". VOC's can be produced through industrial or biological processes. In the industrial setting, VOC's are commonly used or are created as by-products in the manufacture of paints, pharmaceuticals, refrigerants, petroleum fuels, household cleaners, and other products. VOC's can also be produced by microorganisms such as fungi and bacteria. During metabolism, microbes can produce these chemicals, specifically called Microbial Volatile Organic Compounds (MVOC's). This article will concentrate on MVOC's, as opposed to industrially produced VOC's, and their relevance in the indoor air quality setting.
Microbial Volatile Organic Compounds (MVOC's) are composed of low molecular weight alcohols, aldehydes, amines, ketones, terpenes, aromatic and chlorinated hydrocarbons, and sulfur-based compounds, all of which are variations of carbon-based molecules. MVOC's have a very low odor threshold, thus, making them easily detectable by smell. They often have strong odors and are responsible for the odious smells ("old cheese", dirty socks" or "locker room") associated with mold and bacterial growth.
MVOC's are products of the microbes' primary and secondary metabolism. In primary metabolism, the organism breaks down food in the environment to extract nutrients needed for the maintenance of cell structures and, in the process, creates MVOC's as by-products. In secondary metabolism, the production of MVOC's is driven by the competition for resources in a nutrient-poor environment. MVOC's produced during primary fungal metabolism include ethanol, 1-octen-3-ol, 2-octen-1-ol, and benzyl cyanide. Some fungi can produce ethanol by fermentation. Others, such as Aspergillus niger, Aspergillus flavus, and Penicillium roqueforti are able to produce 1-octen-3-ol. Low concentrations of this particular MVOC emit a mushroom-like or musty odor. Aspergillus flavus can also produce 2-octen-1-ol which has been described as "a strong musty, oily odor". The fungus Botrytis cinerea can produce benzyl cyanide which emits a grassy odor.
MVOC's produced during fungal secondary metabolism include 2-methyl-isoborneol, geosmin (1-10-dimethyl-trans-9-decalol), and terpenes. Chaetomium sp. is known to produce 2-methyl isoborneol and geosmin emitting a musty, earthy odor. Penicillium aurantiogriseum and Penicillium vulpinum growing on oat substrate have been shown to produce terpenes. The greatest occurrence of MVOC production (especially terpenes and sesquiterpenes) seems to coincide with spore formation and mycotoxin production as observed in species of Aspergillus and Penicillium. Mycotoxins differ from MVOC's in that they are relatively large molecules that are not volatile, and do not easily evaporate or "off-gas" into the air.
Information on bacterial MVOC's produced in indoor settings is limited. Studies conducted on a few bacteria, such as the actinomycetes Streptomyces griseus and Streptomyces odorifer show that they can produce geosmin, 2-methyl -isoborneol, and 3-methyl-butanol.
Why are MVOCs relevant in the indoor setting?
First, the perception of MVOC's is an indication that microbial growth is occurring. Their potential to elicit health effects remains speculative. Fungi and bacteria may survive or dominate by producing toxic chemicals, such as mycotoxins and MVOC's, to inhibit or kill their competitors. These chemicals, at the concentrations that occur at the microbial/microbial interface, can interfere with cellular processes such as DNA, RNA, and protein synthesis and membrane or enzyme functions. Extrapolating these effects to plants or animals involves a consideration of cellular resistance (or sensitivity) and dose. In the indoor environment, exposure to fungal MVOC's has been blamed for headaches, nasal irritation, dizziness, fatigue, and nausea. However, evidence is inconclusive on this point, and other factors should also be considered. A few studies have attempted to document the effects of direct exposure to MVOC's, but none have unequivocally documented a connection with any health effect at any concentration commonly measured in contaminated buildings. Although a few studies have implied a causal relationship between exposure and symptoms of disease, there are still aspects of this relationship that need to be evaluated. The specific toxic properties and concentrations of MVOC's needed to produce symptoms are still unknown.
Inspectors are particularly interested in determining whether the presence of "marker" chemicals, such as MVOC's, could equate to building contamination. Comparative analysis of MVOC levels from outdoor, indoor affected, and indoor unaffected areas using GC/MS (gas chromatography/mass spectrometry) may provide information on microbial contamination in buildings. Studies comparing the level of VOC's in indoor air and MVOC emissions from microorganisms in culture could also potentially be conducted. However, microbial growth can produce variable MVOC's depending on the substrate and the phase of fungal growth. MVOC's emitted by microbes in the field may also differ from those in lab cultures because the competition for resources that occurred in the investigated area is difficult to reproduce in the laboratory setting. Moreover, some VOC's may be from non-microbial sources, such as limonene and pinene in cleaning agents. With all these considerations in mind, more studies are needed to further current knowledge of MVOC's and their effects on human health.
1. Ammann, Harrier M. 1998. Microbial Volatile Organic Compounds. Pp. 26-1-26-17. Bioaerosols: Assessment and Control.
2. Burge, Harriet A. 1996. Health effects of biological contaminants. Pp. 171-178. Indoor air and human health.
3. Walinder, R., Ernstgard, L., Johanson, G., Norback, D., Venge, P., Wieslander, G. 2005. Acute effects of a fungal volatile compound Environmental health perspectives 113(12): 1775-1778.
By Gregorio Delgado
As indicated by its name, the genus Nigrospora, or “black spore” in Latin, is very easy to identify under the microscope by its unicellular, black, shiny, ovoid to ellipsoidal and horizontally flattened asexual spores (conidia), often with an equatorial colorless line or germ slit (Ellis, 1971). According to the U.S. Department of Agriculture Fungal Database, six species of anamorphic fungi (asexual fungal states) share these distinctive spore features. They are separated on the basis of spore diameter, which can vary from 10 to 30 μm. Other defining characteristics are the presence of colorless to brown reproductive hyphae or conidiophores (branches), which produce swollen, flask-shaped spore-bearing cells with a single spore conidium at the inflated apex. Their teleomorphs (sexual phases) are known to be included in Khuskia, a genus of ascomycetes. In culture, Nigrospora species grow rapidly on MEA, producing at first a white, cottony mycelium that becomes progressively darker to gray or black as more and more spores are formed. As seen from the reverse side of the plate, colonies are black.
Rarely found growing in indoor environments, members of Nigrospora are widespread and mainly distributed in tropical and subtropical countries but also in temperate regions. They can be isolated from air, soil or dead plant debris. They may also act as plant pathogens on numerous hosts, some of which have economic relevance like corn, cotton, rice, bananas, apples and beans. Webster (1952) showed that Nigrospora sphaerica has a violent spore discharge mechanism, called ballistosporosis, that causes its spores to be forcibly projected laterally to a maximum distance of 6.7 cm and up to 2 cm vertically. This ballistosporic spore projection was shown to be due to a sudden discharge of liquid through a fine opening in the pedicel (branch) apex and is compared in efficiency to that of the basidiomycete Agaricus and the zygomycete Entomophthora. Increased wind speed enhances the dispersal of large fungal spores like Nigrospora that are commonly found on outdoor spore trap samples.
The EMLab™ MoldRange data (Figure 1) shows the recovery rate and spore levels of Nigrospora detected outside during the year in the United States. The gray bars indicate the frequency of occurrence of spores at every month corresponding to the numbers on the left axis (from 0 to 1 = 100%), so as an example a 0.2 frequency would indicate that Nigrospora was present in 20% of the outdoor samples collected that month. The red, green and purple lines indicate Nigrospora spore concentrations when they are recovered in the air. Represented are the 2.5, 50 and 97.5 percentile levels of spores/m3, again, when the fungi is recovered, plotted against the numbers on the right axis (from 0 to 400 spores/m3). According to the graph, Nigrospora spores are typically present outdoors throughout the whole year at a consistent but low density of less than 25 spores per cubic meter on the occasions (50% of the time) that they are detected. However, upper spore levels tend to be higher from late summer to fall (August to October), when the highest counts can reach levels of 350 spores/m3 or more.
Figure 1: Frequency of detection and spore density by month.
The gray bars represent the frequency of detection, from 0 to 1 (1=100%), graphed against the left axis. The red, green, and purple lines represent the 2.5, 50, and 97.5 percentile airborne spore densities, when recovered, graphed against the right hand axis. (Source: EMLab MoldRange data. Total sample size for this graph: 39,878.)
Involvement of Nigrospora in human infections is extremely rare. The most common health effect is Type I allergic response, with seasonal rhinitis (hay fever) or asthma. A few cases of corneal inflammation (keratitis) and skin lesions caused by Nigrospora sps. have been reported (de Hoog et al., 2000). No mycotoxin production is known at this time, although a Metabolite A produced from Nigrospora oryzae has been found to show weak antibiotic properties and mild toxicity to brine shrimp and chick embryos, but not to be toxic to mice or rats at the levels tested (Wilson et. al, 1986). This fungus also produces aphidicolin, a drug that effectively inhibits cell division and has antiparasitic and antiviral properties (Kayser et. al, 2001). Another product based on Nigrospora sphaerica extracts was recently patented as a cosmetic skin benefit agent, claiming to have comparable and/or demonstrably better skin lightening activity than other known skin lightening agents.
1. de Hoog, G. S., J.Guarro, J.Gene & M.J. Figuerras (2000): Atlas of Clinical Fungi. 2nd. Edition. CBS/Universitat Rovira I Virgili, Reus.
2. Ellis, M. B. (1971). Dematiaceous Hyphomycetes. Commonw. Mycol. Inst., Kew.
3. Kayser, O., A. F. Kiderlen, S. Bertels & K. Siems (2001): Antileishmanial activities of Aphidicolin and its semisynthetic derivatives. Antimicrobial Agents and Chemotherapy 45 (1): 288-292.
4. Webster, J. (1952): Spore projection in the hyphomycete Nigrospora sphaerica.The New Phytologist 5 (2): 229-235.
5. Wilson M.E., N.D. Davis & U.L. Diener (1986): A toxic metabolite of Nigrospora oryzae (Berk and Br.) Petch. Mycopathologia 95(3):133-138.
6. Fungal Databases, Systematic Botany and Mycology Laboratory
This article was originally published on April 2006.