Antimicrobial tests to find the effectiveness of treatment -2


Agar Plate test
Procedure: Sample is maintained on an agar already containing, or inoculated with, an organism for 18-24 hours (bacteria), or for upto 4 weeks (fungi). Agar normally contains a nutrient but samples can be tested with a nutrient-free agar to see if they are a viable carbon source
Evaluation Visual (growth in contact area and halo)
Advantages Relatively quick. Can be used for testing with bacteria or fungi or mixed spores. Can be modified to cope with different materials e.g. samples with low diffusion rates (plastics, textiles treated with hydrophobic chemicals) can be held on the agar for 24 hours prior to incubation to allow biocide to diffuse out.
Disadvantages Size of halo can indicate highly effective biocide or more usually high diffusion rate
Accuracy Good (average of 4 samples) variation of more than 1mm in halo size is significant. Ratings of growth on sample can be less accurate
Soil burial test
Procedure: The most severe test. Samples are buried for upto 28 days (longer for plastics and coated  materials)
Evaluation Measure loss of tensile strength or loss of weight. Control sample must rot completely in about 7 days.
Disadvantages Long lime-consuming and expensive
Accuracy Average of 9/10 samples +/- 10%
Saturated Atmosphere test
Evaluation Visual assessment (using a microscope)
Disadvantages Long time of incubation
Accuracy +/- 10%
Count test
Procedure: Sample are inoculated with microorganisms and left in contact for 0, 6, 12, 18 and 24 hours at 37°C. Organisms are then extracted and counted.
Evaluation Number of colony forming units (CFU) counted after 24 hours and compared with the number at time 0 hours.
Advantages Relatively quick. Can distinguish between a biocide and a biostat
Disadvantages Only suitable for bacteria. Time consuming and expensive. With some fabrics micro-organisms can hide producing invalid results
Accuracy Average of 4 samples +/-30-40%