Rapid Detection of Salmonellae by Means of a New Impedance-Splitting Method

An Impedance-Splitting method is proposed for the rapid detection of salmonellae in foods. The measuring System, BacTrac^™ 4100, permits the registration of changes, caused by bacterial metabolism, not only of the impedance of the culture medium but also of changes in the ionic layers at the measuring electrodes, which has advantages in case of high salt concentrations. These changes are expressed as percentage decreases of the initial values, M-value and E-value, respectively. Food samples were pre-enriched 14 to 16 h at 37°C in peptone water by addition of mannitol, which facilitated the detection of salmonellae on selective culture media. Following this, 0.1 mi of the preenrichment culture was transferred to 9.9 ml of Impedance-Splitting Salmonellae (ISS) medium which consisted of magnesium chloride (hydrated), malachite green oxalate, novobiocin, phosphate buffer, mannitol, peptone and yeast extract. Despite the high magnesium chloride concentration in this medium, salmonellae produced changes of the E-value up to 100%, while the changes in M-values were limited to a few percent. The impedance changes were automatically recorded during incubation in the measuring system for up to 22 h at 40°C, and the time required to exceed a threshold value of 15% (E reaction time) was evaluated. Comparative testing of the ISS method with standard cultural analysis of 250 unknown food samples showed high sensitivity and selectivity in detecting salmonellae. From all of the 122 Salmonella -positive samples, the largest number (119) was obtained by the ISS method, as compared to that obtained by conventional testing with the selenite-cystine (106), Rappaport Vassiliadis soya (95), Rappaport Vassiliadis (92) and tetrathionate brilliant green medium (64). Six samples were false positive by Enterobacter cloaceae . One strain each of Salmonella enteritidis PT8 and Salmonella panama were not recorded. The ISS method is very suitable as a screening test, all the more since a negative investigation result will be obtained within 38 h. In view of the practicability, this method is superior to the enzyme-immunological and molecular-biological procedures.