Purpose: This study’s purpose was to investigate strain-to-strain growth variation in growth of L. monocytogenes on glucose.
Methods: L. monocytogenes strains Scott A and EGD-e were grown in TSB and then in a defined medium (DM) with 10 mM glucose. Late exponential phase cultures in DM were used as inoculum for both tubes and 96-well plates which were incubated at 37°C. Glucose concentrations ranged from 100 mM to 62.5 µM for 96-well plates and 13.8 mM, 55.5 µM, and 555 nM for tubes. Counts of colony forming units (CFU) on tryptic soy agar from tubes were used to calculate growth kinetics.
Results: Plate CFU counts from 3 biological replicates demonstrated differences in growth rates between EGD-e and Scott A for 555 nM only (0.26 ± 0.02 h-1 versus 0.18 ± 0.01 h-1; P < 0.05). Both strains exhibited stationary phase for 13.8 mM but transitioned quickly into death phase at 55.5 µM. In 96-well plates, EGD-e in glucose above 5 mM demonstrated traditional exponential growth and transition into stationary phase. Whereas, Scott A also demonstrated rapid growth and death without a discernable stationary phase. The slope of death often shouldered with two stages, the second steeper than the first.
Significance: These data suggest that growth rates of L. monocytogenes are affected by both strain and glucose concentration. Thus some strains may influence the outgrowth of others, potentially accounting for different recovery frequencies from different environments.