Comparison of rapid and traditional methods
PharmTech: How do rapid methods differ from traditional microbial-detection methods?
Ganatra: All the traditional methods require the growth of microorganisms in media. Scientists need to examine cultures visually to
check for microorganisms. Because of our limited vision, humans can see those microbes only when growth reaches a high number
of colony-forming units. Rapid methods typically use markers that can be detected by an instrument. Many instruments detect
those markers even at a low number of colony-forming units. This ability significantly reduces the time to detection.
Cloak: Traditional methods can be slow, labor-intensive, and subjective. Rapid methods usually target an alternative analyte such
as a DNA or RNA sequence unique to the organism. They tend to be easier to use and provide a much faster time to result. This
differential is similar across the board, regardless of whether the target is bacterial, fungal, or viral.
Williams: Rapid methods may be more sensitive and objective in detecting bacteria and fungi compared with the traditional methods.
For viruses, special detection methods are used because viruses are unable to proliferate on the substrate used for bacterial
or fungal growth.
PharmTech: Do rapid methods require special equipment?
Rook: Rapid methods do require special equipment, which ranges from handheld to capital equipment.
Cloak: The type, quantity, and cost of instrumentation required varies, depending upon the rapid method in question. The manufacturers
of rapid methods generally provide all the components necessary to the run the test.
Daane: Some equipment such as luminometers are compact instruments that fit on the laboratory bench and are designed for industrial
microbiology. Other systems require a dedicated room or the purchase of multiple modules for volume testing, which takes up
valuable bench space.
PharmTech: Is there a quantitative or qualitative difference between the results that rapid and traditional methods achieve?
Rook: Qualitative or quantitative differences can exist, based on each particular technique. This is where validation of the rapid
method can get quite complex. Comparing traditional results, in colony-forming units, to a new method that detects bacteria
in terms of relative light units can be challenging.
Ganatra: In traditional methods, detection is highly dependent on growth conditions that do not allow all the microbes to proliferate.
Rapid methods target metabolic markers, so they can also detect viable but nonculturable microorganisms. So, rapid methods
generally give higher counts than traditional methods.
Cloak: Qualitative or quantitative result differences depend on the specific methods in question and also the application of those
methods. In most cases, the rapid method performance is benchmarked against performance requirements of traditional methods.
However, some rapid methods can provide more sensitive and specific results in a shorter timeframe than the more conventional
Daane: Quantitative tests are useful when contamination is present, but are useless otherwise since there's nothing to count. That's
why a qualitative, absence–presence screening makes sense. It provides rapid results, including confirmation of negative results
within 24 hours, that a company can act on to save time and money. Rapid quantitative tests such as flow-cytometry methods
are currently limited to filterable liquids only. Other product types require sample preparation such as dilution, enrichment,
and incubation that limits the results to that of a more expensive qualitative test.
PharmTech: Are certain methods best for bacteria or for yeast?
Williams: Each rapid microbial-detection method has its own strengths, which depend on the type of testing being performed and the
product being tested. One rapid method may be ideally suited for a particular product or test, but may not be applicable to
Rapid methods that are useful for detecting bacteria usually are also valuable for detection of yeast and mold as well. But
specific methods are required for virus detection because viruses grow differently. Bacterial or eukaryotic cells are used
as a growth substrate for viruses.