COMPARISON OF LAG TIMES OBTAINED FROM OPTICAL-DENSITY AND VIABLE COUNT DATA FOR A STRAIN OF PSEUDOMONAS-FRAGI

Values were obtained for the lag times of a strain of the meat spoilag e bacterium Pseudomonas fragi under a variety of cultural conditions b y analyzing optical density (OD) and viable count (VC) data using the Gompertz equation. Lag times produced from OD data were shorter than t hose derived from VC data. Observations showed that during the lag pha se, cell length increased before the cells began to divide, explaining the apparent earlier resolution of lag phase observed with the OD mea surement technique. By linear regression the general equation: Log (VC Lag time) = A + B.Log (OD Lag time) where A = 0.344 and B = 0.868 was derived to interconvert OD lag times and VC lag times. This equation was applied to predicted lag times derived from models that had been c onstructed using optical density data and that are used to predict gro wth kinetics of cold-tolerant pathogens on foods. With this adjustment , the models demonstrated an improved predictive ability when compared to measured values for growth of these pathogens on foods.