SIGNIFICANCE OF PREINCUBATION TEMPERATURE AND INOCULUM CONCENTRATION ON SUBSEQUENT GROWTH OF LISTERIA-MONOCYTOGENES AT 14-DEGREES-C

The influence of the bacterial concentration of an inoculum (10(1) or 10(3) cfu ml(-1)) of two strains of Listeria monocytogenes (Scott A: s erotype 4b and V7: serotype 1) and one strain of L. innocua (Lin 11), and the time and temperature at which the inoculum was stored (cold st orage: 4 degrees C for 4 weeks, or without cold storage: -20 degrees C before immediate transfer), and the temperature at which cells were p re-incubated (30 degrees C and 14 degrees C) on subsequent growth in R ichard's broth at 14 degrees C was investigated. Richard's broth at a pH 5.9 was used to simulate potential growth in soft cheese (camembert type) and an incubation temperature of 14 degrees C was used to simul ate storage-temperature ripening of cheese. Enumeration of the number of viable cells was by plate count method, except where viable cell nu mbers were less than 10(3) cfu ml(-1), when the MPN (Most Probable Num ber) technique was used. With cold storage and an inoculum of 10(3) cf u ml(-1) (high bacterial concentration) the pre-incubation temperature s (30 degrees C and 14 degrees C) did not significantly influence the subsequent growth curve: there was no significant lag (less than 21 h) and cell numbers peaked in about 8.5 d. However, with cold storage an d an inoculum of 10(1) cfu ml(-1) (low bacterial concentration) and a preincubation temperature of 30 degrees C a significant shift in the g rowth curve was observed over that pre-incubated at 14 degrees C, with the appearance of a lag of about 7.7 d. At a pre-incubation temperatu re of 14 degrees C with the low inoculum concentration, there was a me asurable lag of about 1 d. Without cold storage and a pre-incubation t emperature of 30 degrees C, there was a lag time of 2.3 d. Storage con ditions, pre-incubation temperature and inoculum concentration therefo re appear to influence the subsequent growth curve. Importantly, howev er, the growth curves for cultures from inocula, pre-incubated at eith er 30 degrees C or 14 degrees C, appeared to involve two distinct valu es of the exponential growth rate (k) : the initial portion of the gro wth curve described by a low value of k and the subsequent portion by a consistently and significantly greater value. The appearance of two distinct growth phases was reproduced in further data determined for a ll the studied strains of the microorganism. Further study to explain these unexpected and reproducible findings is being conducted.