Growth Kinetics and Cell Morphology of Listeria monocytogenes Scott A as Affected by Temperature, NaCl, and EDTA†,‡

Abstract

Growth kinetics and morphological characteristics of Listeria monocytogenes Scott A grown under stress conditions induced by increasing levels of NaCl and EDTA were studied as a function of temperature. L. monocytogenes Scott A was inoculated into brain heart infusion broth (pH 6) at 19, 28, 37, and 42°C. Test cultures contained NaCl (at concentrations of 4.5, 6.0, and 7.5%) or EDTA (at concentrations of 0.1, 0.2, and 0.3 mM); control cultures contained 0.5% NaCl. Growth curves were fitted from plate count data by the Gompertz equation, and growth kinetics parameters were derived. Stationary-phase cells were examined by scanning and transmission electron microscopy. Generation times (GTs) and lag phase duration times (LPDs) increased as additive levels were increased. The bacterium grew at all NaCl levels. At 37 and 42°C, growth was slow in media containing 7.5% NaCl, and no growth occurred in media containing 0.3 mM EDTA. Temperature was a major factor in certain stress conditions that led to cell elongation and loss of flagella. Cells in control media at 28°C grew as short rods (0.5 by 1.0 to 2.0 μm), while at 42°C most cells were 4 to 10 times as long. Higher levels of NaCl at higher temperatures resulted in longer and thicker cells. At 28°C, 0.1 mM EDTA had little effect on growth kinetics and morphology; however, 0.3 mM EDTA caused a sixfold increase in GT and LPD and loss of flagellae, with most cells being two to six times as long as normal. Cell length did not correlate with growth kinetics. The results of this study suggest that the effect of altered morphological characteristics of L. monocytogenes cells grown under stress on the virulence and subsequent survival of these cells should be investigated.