Conventional and rapid analytical microbiology 195 2 Fig 8.2 Calibration curve showing changes in conductance detection time with bacterial total viable count(Tvc) Enterobacteriaceae Cousins and marlatt 1990, Petitt 1989), Pseudomonas (Banks et al. 1989), Yersinia enterocolitica(Walker 1989)and yeasts( Connolly et al., 1988), E coli(Druggan et al. 1993), Campylobacter(Bolton and Powell 993). In the future, the number of types of organism capable of being detected will undoubtedly increase. Considerable research is currently being done on media for the detection of Listeria and media for other organisms will follow Most of the electrical methods described above involve the use of direct measurement,i.e. the electrical changes are monitored by electrodes immersed in the culture medium. Some authors have indicated the potential for indirect conductance measurement(Owens et al. 1989)for the detection of microorgan- isms. This method involves the growth medium being in a separate compartment to the electrode within the culture cell. The liquid surrounding the electrode is a gas absorbent, e.g. potassium hydroxide for carbon dioxide. The growth medium is inoculated with the sample and, as the microorganisms grow, gas is released This is absorbed by the liquid surrounding the electrode, causing a change in conductivity, which can be detected This technique may solve the problem caused by microorganisms that oduce only small conductance changes in conventional direct conductance cells. These organisms, e.g. many yeast species, are very difficult to detect usingEnterobacteriaceae (Cousins and Marlatt 1990, Petitt 1989), Pseudomonas (Banks et al. 1989), Yersinia enterocolitica (Walker 1989) and yeasts (Connolly et al., 1988), E.coli (Druggan et al. 1993), Campylobacter (Bolton and Powell 1993). In the future, the number of types of organism capable of being detected will undoubtedly increase. Considerable research is currently being done on media for the detection of Listeria, and media for other organisms will follow. Most of the electrical methods described above involve the use of direct measurement, i.e. the electrical changes are monitored by electrodes immersed in the culture medium. Some authors have indicated the potential for indirect conductance measurement (Owens et al. 1989) for the detection of microorgan￾isms. This method involves the growth medium being in a separate compartment to the electrode within the culture cell. The liquid surrounding the electrode is a gas absorbent, e.g. potassium hydroxide for carbon dioxide. The growth medium is inoculated with the sample and, as the microorganisms grow, gas is released. This is absorbed by the liquid surrounding the electrode, causing a change in conductivity, which can be detected. This technique may solve the problem caused by microorganisms that produce only small conductance changes in conventional direct conductance cells. These organisms, e.g. many yeast species, are very difficult to detect using Fig. 8.2 Calibration curve showing changes in conductance detection time with bacterial total viable count (TVC). Conventional and rapid analytical microbiology 195