N.22 – Microbial monitoring in closed environment: general suggestions


Viable particles are bacteria, yeast and moulds living microscopic organisms present in the air and on surfaces of the environment. Their testing usually includes the monitoring of

– air,
– surfaces,
– staff.


Viable air sampling

Gravitational or settling Petri or Contact plates containing sterile growth media are used for sampling.

Petri dish – The Petri dish are passively expose to the environment for 60 minutes. Viable micro-organisms that settle on the medium surface grow when the plates are incubated. Settling plates are cost effective but they do not directly correlate to the microbial contamination of environment because they do not provide a quantitative measurement of air contamination.

Air sampler – The active monitoring instrument measure an exact volume of air with a quantifiable number of viable micro-organisms. Contamination can therefore be measured per cubic meter of air (=1000 litres).


Viable surface sampling

Contact plate – Contact plates (=RODAC) are similar to Petri dishes containing sterile growth medium. The agar protrudes above the sides of the plate. The convex Contact plate is pressed against flat, regular surface to be sampled. Any viable micro-organism present on that surface will adhere to the agar and grow after proper incubation.

Contact plates are not appropriate for irregular surfaces.

Swab – Sterile swabs are used for sampling of uneven surface like tubing, equipment, corner, etc. The swabbing can be a qualitative or quantitative technique. A known flat area (traditionally 10×10 cm) is swabbed using a back and forth technique that is repeated after 90° swab rotation. The same technique is used on uneven surfaces.

The swab is then streaked onto the agar surface of a Petri dish for identification or transferred to Stomacher for microbial population counting.

Sponge – Sterile sponges are used for sampling large surfaces. The sample site is pressed by the humidified sponge adopting aseptic technique to avoid contamination by the hands. The sponge is then treated in a bag by Stomacher. The obtained dilution is then tested.


Viable staff monitoring

Personnel working in a controlled contamination environment should be monitored for germ contamination to evaluate the gowning proficiency. Sampling sites are hands, gloves and gowns. Maxi Contact plates are used to dynamically monitor the staff hands immediately before and after a critical process.


Sampling locations

Sampling locations are selected based on potential product exposure areas, equipment design, validation data, processing parameters.

The frequency of sampling depends on product and quality requirements and based on trending results.

A sampling plan, reported in a Standard Operating Procedure, should describe the procedure, the sampling sites, the sampling frequency, the static and dynamic situation.


Action/Alert levels

Each company or laboratory should establish “alert and action” level based on previous environmental monitoring data. The exceeding the alert level should cause the increasing sampling intensity or frequency in the affected area.

The consequent “corrective actions” will have the goal to return a facility to acceptable limits of viable micro-organisms as quickly as possible. These actions should include a determination of what caused the viable counts to be above acceptable limits and how that will be rectified in the future.



F. McAteer – Verifying accurate environmental monitoring of viable particles – CleanRooms December 2006.