RAPID DETECTION OF SALMONELLAE BY MEANS OF A NEW IMPEDANCE-SPLITTING METHOD

An impedance-Splitting method is proposed for the rapid detection of s almonellae in foods. The measuring system, BacTrac(TM) 4100, permits t he registration of changes, caused by bacterial metabolism, not only o f the impedance of the culture medium but also of changes in the ionic layers at the measuring electrodes, which has advantages in case of h igh salt concentrations. These changes are expressed as percentage dec reases of the initial values, M-value and E-value, respectively. Food samples were pre-enriched 14 to 16 h at 37-degrees-C in peptone water by addition of mannitol, which facilitated the detection of salmonella e on selective culture media. Following this, 0.1 ml of the preenrichm ent culture was transferred to 9.9 ml of Impedance-Splitting Salmonell ae (ISS) medium which consisted of magnesium chloride (hydrated), mala chite green oxalate, novobiocin, phosphate buffer, mannitol, peptone a nd yeast extract. Despite the high magnesium chloride concentration in this medium, salmonellae produced changes of the E-value up to 105%, while the changes in M-values were limited to a few percent. The imped ance changes were automatically recorded during incubation in the meas uring system for up to 22 h at 40-degrees-C, and the time required to exceed a threshold value of 15% (E reaction time) was evaluated. Compa rative testing of the ISS method with standard cultural analysis of 25 0 unknown food samples showed high sensitivity and selectivity in dete cting salmonellae. From all of the 122 Salmonella-positive samples, th e 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 tetra thionate brilliant green medium (64). Six samples were false positive by Enterobacter cloaceae. One strain each of Salmonella enteritidis PT 8 and Salmonella panama were not recorded. The ISS method is very suit able as a screening test, all the more since a negative investigation result will be obtained within 38 h. In view of the practicability, th is method is superior to the enzyme-immunological and molecular-biolog ical procedures.