Duplex (thermotroph-psychrotroph) quadrant plates: convenient, error-avoiding tools for monitoring of HACCP-contained food lines and for epidemiological investigations under conditions of military or other constraints.

A set of two "two-tier" (thermotroph-psychrotroph) single quadrant plates (QPs) was developed previously to allow convenient enumeration of numbers of colony-forming units of most pertinent pathogenic bacteria or marker bacteria in foods. These include Listeria monocytogenes, Staphylococcus aureus, Bacillus cereus, thermotrophic and psychrotrophic Enterobacteriaceae, Clostridium spp., and Enterococcus spp. As the QPs had given excellent results in monitoring samples of marketed food products potentially involved in food-transmitted illnesses, the approach was tested for practicability under military deployment and other constraints. Three approaches were envisaged: (i) validating lapse-free adherence to meticulously codified good military catering practices; (ii) acceptance/rejection testing of locally procured foods or meals; and (iii) employing rapid culture in support of evidence obtained by microscopy in attempts to identify foods involved in infectious or toxic disease outbreaks occurring in the field. The method was found to be elegant, avoiding confusion when larger number of specimens were to be screened, as well as easy to teach to staff with little or no training in microbiology, and it provided entirely reliable results. For use outside the laboratory, preparation of food macerates by use of shake flasks containing glass or plastic beads and peptone saline as a substitute for stomaching was found acceptable, though the shake flask technique led to slightly diminished colony counts. Results obtained with incubation times shortened to ca. 12 h could be relied on only when the results were alarmingly positive, but not when the colony counts at the 12-h point did not yet indicate a reason for concern.