Do you enjoy lox on your bagel? A refreshing ceviche in the hot summer weather? New research published this week in Applied and Environmental Microbiology highlights the importance of proper storage of salmon and other meats that require no cooking prior to eating.
The research, published by Silin Tang et al out of Dr. Teresa Bergholz’s lab, focused on cold-smoked salmon as a model of ready-to-eat seafood. During its preparation, cold-smoked salmon isn’t heated to a temperature high enough to kill microbes, leaving it vulnerable to Listeria monocytogenes growth. Exposure of meat to L. monocytogenes is particularly dangerous, since this bacterium can grow even at refrigeration temperatures (ideally 1.6°C, or 35°F – minimally below 40°F).
The scientists wanted to examine what genes L. monocytogenes turns on and off when growing on salmon compared to a laboratory medium with similar salt, pH, and temperature conditions. Many experiments have been run looking at L. monocytogenes gene expression, but most of these have been conducted using laboratory medium – a great way to identify genes used in a single variable but not reflective of the environment the bacteria will actually experience. Experiments that have looked at expression on different food sources have had to deal with different growth rates on foodstuffs versus lab medium.
Fortunately, L. monocytogenes grew similarly well on salmon and in traditional lab medium. This meant the experimental time points would yield a similar number of bacteria, and possible confounding factors would be minimized. Using RNA-seq, the scientists found about 150 genes diffentially expressed in salmon-grown Listeria: 88 with increased expression and 61 with decreased expression, relative to growth on lab medium. What kinds of genes?
Most of the expression differences were related to metabolic preferences. The bacteria grown on salmon had more expression of genes related to utilization of ethanolamine and 1,2-propanediol, probably important for growth on the high-fat food source. These bacteria also had more sugar transporter expression as well as expression of genes in the pentose phosphate pathway, suggesting the bacteria are able to utilize a wide variety of carbohydrate sources available in salmon
One standout group of genes was not metabolically-linked: a set of five genes known to be regulated by a virulence regulator, PrfA. One of these genes, a phospholipase, may digest salmon cell membranes, making available lipid components like ethanolamine for bacterial growth through the upregulated metabolic pathways.
Listeria infections are a serious cause of foodborne illness (1600 hospitalizations and 260 deaths annually in the US) – you may remember several recalls related to Listeria contamination in the past year. Fear of these outbreaks is one reason New York now requires all fish to be frozen before serving. The research from Tang et. al. aims to understand how bacteria grow on foodstuffs like cold-smoked salmon. Applications from understanding bacterial metabolics on salmon may generate better precautionary measures in food preparation or be useful in identifying contaminated items before outbreaks begin!
-- Julie Wolf