Background
One of the primary ways in which bacteria & other microbial agents are spread in the environment, farms, food processing plants, abattoirs, butchers, bakeries, groceries, food outlets, kitchens, restaurants and food testing laboratories is through aerosol transmission. The classical way to determine the airborne bacterial load of a facility is to use passive sampling. This type of testing relies on time for the aerosols to land on open agar plates strategically placed throughout the facility. Although this technique delivers very reliable results it can be time consuming and can cost the facility valuable productive time while the testing is being performed.
There is an alternative method, Active sampling, which uses a devise to capture the aerosols quickly and efficiently. Active sampling can greatly reduce the time required to sample a facility for airborne bacterial load.
Passive Sampling
A typical procedure for passive sampling would be to place open petri dishes for 1 hour at least 1 meter above the floor and approximately 1 meter from the walls or any other major obstacles. Typically the plan of open plates would be established and repeated on a regular basis e.g. once per week.
The results of this testing plan would be used to establish a trend analysis to determine if the facility was maintaining an acceptable airborne microbial load or if some remedial action may be required to decrease the general airborne bacterial load.
Active Sampling
Unlike passive sampling where a number of agar plates are placed throughout the facility at the same time and then all left to sit for 1 hour, active sampling requires an air sampling instrument. This type of instrument is designed to hold an open agar plate and to sample the air by drawing it through the device at a controlled rate over the surface of the agar plate. Once a suitable volume of air to samples is established this device would be moved around the facility and sampling performed in each of the identified sampling spots previously used for routine passive sampling.
Establishing a Baseline for airborne bacterial load
To establish a baseline the operator should test the passive & active sampling method when there are no staff working in the area. Using non-selective total viable count plates, place the active sampling device beside the open passive agar plate and run 500L of air through the device. Repeat the process.
The two active & Passive plates should be incubated one set at room temperature and the other set at 35-37oC. After 24 and 48 hours count the colonies on the plates. The counts achieved will help to determine how much air should be sampled by the active air sampler to reflect the results achieved by the conventional settle plates.
Setting the Active Sampling Air Volume to Sample
It is important to establish the degree of confidence that the active sampling is truly reflecting the results that would be achieved using settle plates. Only when the test are performed in an area with no staff working can this be achieved.
Adjust the volume sampled to reflect the colony count achieved by the settle plates then you are ready to use active sampling with or without staff working.
Conditioned & Controlled Ventilation System (CCVS)
The difference between testing with staff working and testing in an empty facility is called “Conditioned & Controlled Ventilation System” (CCVS) (1). If staff are working during sampling the results achieved will definitely be affected by staff just by the disruption to the airflow and stirring up of dust & dirt as they move through the facility. The hygiene practices of the staff will also have a significant effect of the Conditioned Ventilation System test results.
Reference
- Napoli et al, BMC Public Health 2012, 12:594
Title: Air sampling procedures to evaluate microbial contamination: a comparison between active and passive methods in operating theatres.