Viability of Fungal Spores

Most of the microbial IAQ sampling methods collect fungal spores and hyphae. Spores are minute propagative units (propagules) that function as seeds, usually with some food reserve, but not containing a preformed embryo. The fungal spores that are important in IAQ investigations are generally between 2.0 and 10 mm in length. The mechanism by which spores are produced is the basis for classifying fungi. This is why identification using culture analysis is the primary method of choice. Spore types that are important in IAQ investigations include ascospores, basidiospores, conidia, and sporangiospores. The major functions of fungal spores are maintaining the populations, spreading genetic variability, and extending their distribution range. Spores are dispersed mostly by air, but can also be dispersed by water, arthropods, larger animals, plant seeds or propagating materials, and human activities. Because fungi have the ability to produce massive numbers of spores, fungal spores are among the most common microbial particles in outdoor and indoor environments.

Many factors such as water, nutrition, temperature, light, and pH can determine the viability of a fungal spore, as well as influence the process of a fungal spore developing into a sporulating colony, as illustrated in Figure 5.1. Understanding of these factors may help both laboratory and investigators design a suitable culture method to maximize recovery of fungi.

Fungal spores can remain viable for a short period, for weeks or as long as many years, depending on the fungal species, spore types, and storage conditions.1 It appears that under the same conditions, chlamydospores and resting spores survive longer than do other types of spores; ascospores, longer than basidiospores; and smaller-sized conidia (e.g., of Aspergillus or Penicillium), longer than dark-colored, larger spores (e.g., Stachybotrys, Ulocladium), although this kind of generalization is often challenged. Most spore longevity data are derived from experimental records, and those laboratory conditions may not reflect field conditions. These data should be used only as an estimation of spore longevity. Indoor fungi are mostly in an open environment that has many factors affecting the survi-vability of fungi, such as desiccation, temperature, chemicals, and radiation. The common belief is that the number of viable fungal spores reported as CFUs

Fig. 5.1. To develop into a sporulating colony, a viable spore must undergo a process consisting of spore activation, germination, growth, and sporulation. Many factors such as water, nutrition, temperature, light, and pH can influence the process.

represents only about 1% of total spores.6 Because there are so many variables, it is not realistic to estimate the survival rate of fungal spores in an indoor environment.

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