THE USE OF AN AUTOMATED GROWTH ANALYZER TO MEASURE RECOVERY TIMES OF SINGLE HEAT-INJURED SALMONELLA CELLS

A new approach to the study of recovery times of single heat-injured S almonella cells is described. It comprises the generation of a standar d heat-injured culture, serial dilution of this culture to near extinc tion, inoculation of the serial dilutions across many microtitre plate s and measurement of the subsequent recovery and growth using an autom ated turbidometric analyser. Lag times for individual cells were estim ated from turbidity data using a model that accurately extrapolated th e growth curve back to the starting inoculum level. Lag times were com pared using a number of different commercially available pre-enrichmen t media. The most typical result was a very broad distribution of lag times at the single cell inoculum level, with many values in excess of 20 h. Even at an inoculum level 10-fold higher, lag times for some in jured cells were estimated to be > 10 h. More significantly, it was fo und that some media recovered more injured cells than others and vice versa. Between the worst and best media there were as many as 3 log(10 ) cycles difference in the number of cells recoverable. No trends were apparent linking choice of medium with performance. The implications of these findings, in relation to traditional and rapid methodology, a re discussed.