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

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The possibility that long term in vitro chilled storage may result in sub-lethal damage to Listeria monocytogenes cells was investigated by comparing growth of chill-stored (starvation at 4 °C) and fresh cultures on selective and non-selective media. Growth of freshly grown cells was minimally (3-8%) affected by selective LSAMM agar compared with non-selective Brain Heart Infusion agar. In contrast, numbers of chill-stored strains were reduced by greater than 99% after direct plating on the same selective and non-selective media. Furthermore, chill-stored strains were able to grow in standard selective broth (Listeria Selective broth and Fraser broth) only if undiluted inocula (approximately 105-106 cfu ml−1) were used, whereas they were capable of growth in Brain Heart Infusion broth even when the lowest dilutions were used (approximately 101 cfu ml−1). The potential public health consequences of this finding for the isolation of Listeria monocytogenes from foods is considered.

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