Ranking risk: E. coli O157:H7 and Salmonella growth on leafy greens

The objective of this work was to study the growth potential of E. coli O157:H7 and Salmonella spp. in leafy vegetable extracts at different temperature conditions.

lettuce.nov10Cocktails of five strains of E. coli O157: H7 and of S. enterica were used. Inoculated aqueous vegetable extracts were incubated at 8, 10, 16 and 20°C during 21 days. Microbial growth was monitored using Bioscreen C® . In spinach extract, results showed that for E. coli O157:H7 and Salmonella significant differences (p<0.05) for μabs (maximum absorbance rate) were obtained. For both pathogens, growth in chard was slightly lower. In contrast, iceberg lettuce and parsley showed the lowest values of μabs , below 0.008 h-1 . The coefficients of variance (CoV) calculated for the different replicates evidenced that at low temperature (8 °C) a more variable behaviour of both pathogens is expected (CoV > 180%).

This study provides evidence that aqueous extracts from vegetable tissues can result in distinct growth niche producing different response in various types of vegetables.

Finally, these results can be used as basis to establish risk rankings of pathogens and leafy vegetable matrices with relation to their potential growth.

Assessing the growth of Escherichia coli O157:H7 and Salmonella in spinach, lettuce, parsley and chard extracts at different storage temperatures.

J Appl Microbiol. [Epub ahead of print]

Posada-Izquierdo G, Del Rosal S, Valero A, Zurera G, Sant’ Ana A, Alvarenga VO, Pérez-Rodríguez F



Listeria uses alternative metabolic pathways to grow on cold salmon

Listeria monocytogenes grows on refrigerated smoked salmon by way of different metabolic pathways from those it uses when growing on laboratory media. This discovery could lead to reduced incidences of foodborne illness and death, said principal investigator Teresa Bergholz. The research appeared in Applied and Environmental Microbiology, a journal of the American Society for Microbiology.

listeria4In the study, the investigators showed that L. monocytogenes grows on cold smoked salmon by using different metabolic pathways to obtain energy from those it uses on laboratory media, even when the media was modified to have the same salt content and pH as the salmon. To grow on the salmon, the bacterium upregulates genes that enable it to use two compounds from cell membranes — ethanolamine and propanediol — as energy sources.

L. monocytogenes, as well as Salmonella, are known to use those same genes to grow within a host — in the gastrointestinal tract, and on macrophages. “There may be ways we can use this information to control the pathogen both in foods as well as in infected people,” said Bergholz, assistant professor in the Department of Veterinary and Microbiological Sciences at North Dakota State University, Fargo. “Understanding how a foodborne pathogen adapts to environmental stresses it encounters on a specific food could allow food microbiologists to develop inhibitors of metabolic or stress response pathways that are necessary for the pathogen to grow or survive on that product.”

“The information may also enable improved risk assessments, as virulence of a pathogen may be affected considerably by the stress responses and/or metabolic pathways used to survive on the food,” said Bergholz.

Bergholz noted that ready to eat products typically have very low levels of contamination with L. monocytogenes, and that the bacterium must be able to grow on the product during refrigerated storage in order to reach an infectious dose. “In many cases, the addition of organic acids will slow or stop the growth of this pathogen on ready to eat meats and seafood.”

Chinese watermelons explode

Watermelons are exploding in China the same way David Letterman used to drop them out of windows.

An investigative report by China Central Television found farms in Jiangsu province were losing acres of fruit to overuse of a chemical that helps fruit grow faster, causing a rash of exploding watermelons in eastern China.