Separation and Concentration of Pathogens from Foods
The ability to detect (presence or absence) bacterial pathogens at the level of one viable cell in 25 g of food within 30 min would permit, for example, rejection of shipments at a loading bay, diversion of materials to no, mild, or complete processing, even monitoring of Hazard Analysis Critical Control Points (HACCP). But current identification kits and systems are mainly immunological, DNA hybridization, or conductance-based, needing levels of at least 105 organisms/mL for reliable detection. To reach 105/mL from 1 cell/25 g means a concentration gain of about 107. This is 22 doublings-or the familiar overnight enrichment. DNA amplification methods cannot be applied directly to 25-g food samples. But assuming we could satisfactorily extract the target microbe into a sample aliquot suitable for the Polymerase Chain Reaction and amplify it without noise, a 5-min PCR cycle would take 2 hr to achieve a concentration gain of, say 106, plus the time needed to detect the amplified product. So PCR-type reactions are potential candidates for rapid tests. Immunomagnetic separation methods, combined with PCR,
seem to be exploding in popularity at the moment, though I am not convinced of their ultimate potential and ultimately I believe physical or chemical ways of separating pathogens from foods will provide the neces sary concentration gains for detection faster than DNA amplifications.