Physiological and biochemical studies on psychrotolerance in Listeria monocytogenes

Listeria monocytogenes is known to survive and multiply at refrigeration temperatures. Although this behaviour is of concern to the food industry, detailed knowledge of how the organism can sustain growth at these low temperatures is limited. Previous studies have indicated that changes in fatty acid content and carbohydrate uptake rates may be factors in the ability of the organism to grow at low temperatures (Puttman et al., 1993; Annous et al., 1995, Wilkins et al., 1972). However, these studies used cells grown in batch culture using a complex medium so that the effects of growth rate could not be separated from those of temperature. In addition, it is known that microorganisms are able to acquire lipids from complex growth media (Ratledge and Wilkinson, 1985). In order to overcome these problems, the physiological effects of changes in the growth temperature were investigated using cells of L. monocytogenes grown in a chemostat at a single growth rate using a suitable defined medium. Six defined media were compared for their ability to sustain sequential growth of Listeria monocytogenes in batch culture. The most suitable of these (Trivett and Meyer, 1971) was then used to culture L. monocytogenes NCTC 7973 at a range of growth temperatures between 30°C and 10°C in a chemostat. Fatty acid studies, using continuously cultured cells, indicated that decreasing the growth temperature resulted in a reduction in the amount of anteiso-17:0 fatty acid with a concomitant increase in the levels of a number of smaller chain fatty acids.A qualitative polar lipid study revealed that phosphatidylglycerol and diphosphatidylglycerol, an unidentified phospholipid and two glycolipids were present at all growth temperatures examined. A third glycolipid was seen only at 10°C, the lowest growth temperature examined. A study of carbohydrate uptake using continuously cultured cells indicated that growth at 10°C caused a significant increase in the uptake of 2-deoxy-D-glucose at the highest assay temperature (30°C) only. However, it was also found that assay temperature, rather than growth temperature was the major factor in the rate and amount of 2-deoxy-D-glucose taken up by L. monocytogenes NCTC 7973.