Advancements In Rapid Testing
Innovative technologies designed to safeguard products via sophisticated analyses yield immediate results bringing new levels of efficiency to food-safety programs.
Food processing companies face a number of challenges in continuing to supply products that maintain an excellent level of food safety. New government standards, increased consumer awareness, a more globally competitive business environment, and emerging pathogens all create additional hurdles to the efficient processing and distribution of safe products. Previous methodologies that rely on one or two days of enrichment to provide results before product can be released are not able to provide adequate turn around time for today’s processing environment. Between no-tolerance policies for certain pathogens, requirements of HACCP (hazard analysis and critical control point) plans and end-customer documentation requirements, it is critical to know as soon as possible when problems arise. Fortunately, many rapid and/or automated procedures have been devised to alleviate these burdens.
Improvements in existing procedures
Many new technologies are being applied to existing methods to make them faster, more selective, more sensitive, or even all three. The great benefit of these types of approaches is that no additional equipment is necessary, only an additional step or a replacement step is added. An example of this type of solution is the increased usage of immunomagnetic separation (IMS) along with a traditional enzyme-linked immunosorbent assay (ELISA). The use of antibody-coated magnetic beads can reduce pathogen-enrichment times from days to hours, allowing a standard ELISA test to proceed more rapidly. Similar techniques include filtration or centrifugation as a means to concentrate the organisms of interest to a level high enough to be detected by more conventional means.
Another approach uses special media in the confirmation of a presumptive to more rapidly identify an organism of interest. Media containing chromogenic substances have been developed that produce specifically colored colonies for targets like Listeria monocytogenes, Salmonella, or E. coli O157:H7. Use of these media can substantially reduce the time required to confirm or reject a presumptive result, as time-consuming biochemical confirmations are not generally needed if a typical colony is observed. Newer, simpler approaches
Of course, several new technologies focus on replacing an existing technology with one that is faster, simpler, or more sensitive. Many of these require investment in brand new equipment, training for personnel, and usage of entirely new media and methods. Fortunately, there are several technologies that require only minimal changes to present methods yet still provide savings in time and effort. Currently popular lateral flow devices offer a very simple way of reducing assay time. Although many of these kits entail the use of specific enrichment media, their enrichment times are generally significantly reduced (often to just eight hours), and actual test time is very short (typically 10 minutes, as opposed to approximately two hours for a typical ELISA test). Assays are typically simple to perform, as everything is contained within the device itself, rather than requiring a technician to perform the various steps of an ELISA. Interpretation is also rapid, with a positive result usually shown as a dark line in a white field.
Many simplified technologies for the assessment of viable cell counts are available as well as environmental or sanitation monitoring. Several companies produce products to assess aerobic, anaerobic, coliform, E. coli, Enterobacteriaceae, Staphylococcus aureus, or even Listeria counts either more rapidly (i.e. reduced incubation time) or more simply (i.e. color changes and/or fluorescence) than more conventional methods.
ATP bioluminescence systems are becoming more common as an indicator of proper sanitation. Most of these systems rely on a swab designed for use by non-technical personnel immediately after sanitation. The swab is generally inserted into a reader, which in turn displays a quantitative result. The immediate result is an indicator as to whether that particular area requires additional sanitation or is free of biological contaminants.
The largest savings in time, and greatest gains in efficiency, come from solutions based around entirely new technologies, and therefore require obtaining entirely new pieces of equipment. Although these approaches typically involve an investment of money to purchase the equipment and time to train on it, they generally result in significantly reduced time to obtain results, much greater sensitivity, and greater specificity (i.e., a reduced number of false presumptives).
Many of the Polymerase Chain Reaction (PCR) based methods on the market today fall into this category. Typically, the actual input from the technician is minimal, and consists of prepping the sample for use with the instrument, along with programming the instrument through a computer interface.
Generally, these automated methods require the purchase of a thermocycler/detector to run the reaction and assay the results. Due to the DNA (genetic material) amplification of the PCR reaction, these methods are able to detect extremely low levels of the target pathogen, even after short incubation times (as low as four to eight hours, in some cases).
However, a highly trained operator is required, as minute contaminants (known as amplicons) accidentally introduced into the system can wreak havoc on the assay. Despite this, the automation of the system reduces the technician load considerably — the “set-it-and-forget-it” concept.
Other “set-it-and-forget-it” types of assays automate the ELISA process, requiring only the initial incubation and sample setup. These systems may not result in faster result times for individual samples, but typically the reduction in direct technician involvement allows for more samples to be completed in a given period of time.
Automation also has the advantage of eliminating the interpretive quality of many ELISA kits, which can require the user to judge visually the amount of color change in the sample produced by the procedure. Automated methods typically interpret presumptive or negative results based on spectrophotometric methods (i.e., if the absorbance at a given wavelength of light is above a certain cutoff, the sample is presumptive positive for the target).
Additional automated procedures are available for other portions of the confirmation process. Many manufacturers produce biochemical identification systems that greatly reduce the time to identify the biochemical characteristics of isolates, thereby reducing the amount of time needed to confirm a presumptive result. Some systems can even compare isolates on the basis of DNA homology.
This methodology allows a processor to determine, for example, not only if a portion of the plant contains Listeria monocytogenes, but whether the Listeria isolated from the loading dock is genetically related to the one isolated on the product line, and therefore represents the same contaminant or two separate contaminating events.
Another set of equipment-mediated solutions involves automated plating and/or plate counting. Spiral platers can significantly reduce the time needed to prepare aerobic plate counts, and, due to the reduction in concentration from the center to the edge of a spiral plate, fewer dilutions need to be plated for each sample. An automated plate reader allows a technician to obtain faster results from spiral or conventionally prepared plates than manual plate counting, as well as increasing repeatability between technicians. In some cases, automated plate readers can even distinguish different colored colonies, allowing separate counts of different organisms on chromogenic agars.
Finally, a major advantage of all of these automated methods comes in the field of information management. Most Laboratory Information Management Systems (LIMS) allow for the transfer of information from these automated processes into a central computer system. A LIMS system can permit the automatic reporting of test results, greatly reducing both the time it takes to produce the necessary paperwork, and the chance of transcription error in creating it. LIMS systems also assist with trending and tracking of laboratory results and can be used with networked automated systems to provide “real-time” feedback.
The future of rapid testing
As the food processing environment continues to require faster and faster turn around times, and more emerging pathogens in a wider range of products require monitoring, more and faster analytical methods will be developed. In the future, many of the methods discussed above are likely to be combined; resulting in even greater reduced testing times. Alternately, much work is being focused on the development of biosensors that could be placed in a food processing system for the direct detection of pathogens in the food matrix. Although there has been great progress in this area, notably in the detection of metabolic products given off by certain pathogens, currently specific pathogens cannot be detected in real time, resulting in a continued need for “test-and-hold” procedures.
To summarize, while industrial, consumer and governmental pressures are combining to require faster and faster response times, several avenues for using rapid methodologies exist. Although the greatest gains are seen from approaches that require large investments of training and capital, many require only minor departures from more traditional practices, and require little or no additional equipment or training.
Until additional technologies to allow real-time detection of organisms of interest are sufficiently developed, food processors will continue to rely on these rapid methods for the assurance of safe food products. NP
Bill Centrella is a research microbiologist for Food Safety Net Services Ltd. in San Antonio, Texas.