N.17 – Nosocomial Infections: Aspergillosis air sampling

TRIO

Nosocomial-acquired aspergillosis typically occurs in the setting of treatment for leukaemia or other haematological malignancy. As Aspergillus species are easily found in the environment, it is accepted that aspergillosis disease can be a consequence of exogenous acquisition of the fungus.

Aspergillus spores in the air

There are several recommendations from different authors about the maximum number of CFU / cubic metre of Aspergillus in the air of protective isolation suites where compromised patients are confined. Such number varies from 5 to 0,1 CFU/cubic metre. Similar figures are valid for surfaces.

Volume of air to be sampled

Enumeration of airborne germs requires the measurement of the sample air volume by a microbiological air sampler. It is therefore possible to express the number as a concentration per unit volume of air, or where viability is of interest, as CFU / m3 (Colony Forming Units per cubic metre of air). The suggested volume of air is 1000 litres (1 cubic metre) but initially and in absence of any insight into expected airborne concentration, appropriate sampling volume should be selected through trials and errors.

Period of sampling

Longer the period of sampling, larger the volume of air, more numerous the site of sampling, higher the probability to collect Aspergillus spores in the real concentration. It is therefore advisable to adopt battery operated portable instruments with multiple aspirating heads and use the “interval sampling” option that is standard in air samplers.

Microbiological Air Sampler

Impaction onto agar surfaces in a single stage instrument is the more common method. The air stream is directed towards the surface of a single agar filled plate. The air and the particles it carries accelerate on entering the instrument through a restricted inlet. The restricted inlet is a perforated head with a number of uniform diameter holes. The rapid change in the air direction as it approaches the collecting surface at a right angle, causes particles to be thrown from the stream to impact on the agar surface, positioned below the perforated plate, with an efficiency which depends on the velocity of the air and the size of the particles. The advantage of direct impaction onto an agar surface lies in the fact that the agar plates can be incubated without further treatment. This means that colonies grow directly from collected viable airborne particles. Growth of colonies on an agar surface permits their identification using gross colony morphology and microscopic techniques.

Biological Aspects

Aspergillus spores share with most fungal spores a robustness and ability to resist impact damage and desiccation. Nonetheless they need a correct water activity value, pH and temperature for growth. The minimum accepted value is a Aw around 0.78 with an optimum of around 0.97. The relative humidity for spore germination on nutrient agar is 64-84%. A suggested medium is Czapek-Dox Agar.

Reference

Morris, M. Kokki, M. Richardson – Methods of sampling Aspergillus spores in air.