A Surface Monitoring Refresher: Contact Plates and Swabs
Monitoring the microbial flora of environmental surfaces, manufacturing plants, and equipment is an important stage in achieving Good Manufacturing Practices in facilities handling pharmaceuticals, medical devices, food or cosmetics.
Pharmaceutical and medical device manufacturing facilities have stringent sterility requirements for surface sampling in cleanrooms and isolators. Methods to monitor the environmental flora have been described using either contact plates1 or by swabbing technique.2-4
The contact plate method is recommended when quantitative data are sought from flat, impervious surfaces. Contact plates are filled so that the media forms a dome. The nutrient medium used in the contact plate may also contain a neutralizing agent. The surface of the media is pressed against the surface being tested. The resulting sampled area for a 50 mm plate is approximately 25 cm2. The plates are incubated for the required amount of time, and colonies, if present, are then counted.
Disadvantages of the contact plate method are that this method is not suitable for sampling of irregular surfaces, confluent growth of microorganisms can occur, and the media residue must be removed from the sampling site.
Swabbing (Figures 1-4) is employed for equipment and irregular surfaces where contact plates are not suitable. The swab method may also be used on flat surfaces, provided a template is used to define the sample size.2 Types of swabs that may be used for the swabbing technique include cotton, synthetics, and calcium alginate materials with the appropriate diluent.
The swab method involves obtaining the sample by rubbing a sterile swab, moistened with a non-nutritive medium in several directions over a standardized sample area. The swab is then placed into a specified amount of rinse solution and agitated to transfer the microorganisms present on the swab into the solution. The collection medium may be tested by a most probable number method, membrane filtration method, or direct plating method.
Disadvantages of the swab method are that sampling and technique can affect the results and that the method requires manipulation to culture the sample. Swabs are designed for hard-to-reach places, and can fit easily into equipment recesses, nooks, and crevices. After collection of the sample, it is recommended that a standard membrane filtration of the rinse solution be conducted.
Extra measures should be taken to assure the highest levels of sterility for environmental monitoring. One should look for a Sterility Assurance Level (SAL) of 10-6, which makes the swabs ideal for the stringent sterility requirements of surface sampling in cleanrooms, isolators and pharmaceutical and medical device laboratories. The swabs should be double wrapped, gamma-irradiated, and performance-validated after exposure to the vaporized hydrogen peroxide atmosphere used during isolator facility decontamination cycles. Swabs should conform to ISO standards4 and have a certificate of analysis.
1 Griffiths, W. E., Contact slides for use in environmental hygiene studies. Environ. Health. 87:36-37, 1978.
2 “Fundamentals of an Environmental Monitoring Program,” PDA J. Pharm. Sc. and Technol., Supp. TR13, 55:19-20, 2001.
3 Cleanroom Housekeeping: operating and monitoring procedures, IEST-RP-CC018.3. Institute of Environmental Sciences and Technology, Mount Prospect, IL, 2000.
4 Cleanrooms and associated controlled environments: biocontamination control (ISO 14698-1.2), International Organization for Standardization, Geneva, 2002