Sub-lethal damage of Listeria monocytogenes after long-term chilled storage at 4 degrees C

The possibility that long term in vitro chilled storage may result in sub-l ethal damage to Listeria monocytogenes cells was investigated by comparing growth of chill-stored (starvation at 4 degrees C) and fresh cultures on se lective and non-selective media. Growth of freshly grown cells was minimall y (3-8%) affected by selective LSAMM agar compared with non-selective Brain Heart Infusion agar. In contrast, numbers of chill-stored strains were red uced by greater than 99% after direct plating on the same selective and non -selective media. Furthermore, chill-stored strains were able to grow in st andard selective broth (Listeria Selective broth and Fraser broth) only if undiluted inocula (approximately 10(5)-10(6) cfu ml(-1)) were used, whereas they were capable of growth in Brain Heart Infusion broth even when the lo west dilutions were used (approximately 10(1) cfu ml(-1)). The potential pu blic health consequences of this finding for the isolation of Listeria mono cytogenes from foods is considered.