Airborne microorganism monitoring: a comparison of several methods, including a new direct counting technique

Samples of aerosol from the surrounding air were collected by forcing them to impact onto a solid nutrient medium, onto membrane filters, or onto microscope slides on microcover slips. The samples were cultivated or viewed in a fluorescence microscope by using a technique developed by us, or investigated by scanning electron microscopy. The amounts of microorganisms found by cultivation method, i.e. those forming microcolonies (CFU), were on average 85% lower than the amounts determined by the fluorescence technique. Cultivation of microorganisms trapped on Synpor filters of various pore size resulted in lower counts than conventional cultivation. Among bacteria, the genera Micrococcus, Bacillus and Corynebacterium predominated; the genera Neisseria, Actinomyces, Pseudomonas and others were also found. Of micromycetes, the genera Penicillium, Aspergillus and Cladosporium occurred most frequently in air. In the summer season, yeasts and other micromycetes prevailed, whereas in the winter season the bacteria counts were higher. The total counts of all components of the aeroplankton were higher in summer than in winter. Dust particles bounded mainly bacteria.