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PUBLIC RELEASE DATE:
24-Apr-2008

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Contact: Carrie Slijepcevic
cslijepcevic@asmusa.org
American Society for Microbiology
@ASMnewsroom

Tips from the Journals of the American Society for Microbiology

Protozoa May Enable Food-Borne Pathogens on Leafy Vegetables

Protozoa found on lettuce and spinach may sequester harmful food-borne pathogens ultimately contributing to their survival on produce surfaces say researchers from Tennessee Technological University, Cookeville and the Produce Safety and Microbiology Research Unit, Albany, California. They report their findings in the April 2008 issue of the journal Applied and Environmental Microbiology.

Several outbreaks of food-borne illnesses attributed to Escherichia coli O157:H7 and Salmonella enterica have received national attention in recent years. The Centers for Disease Control and Prevention reported that fresh produce was the most significant source of food-borne illness in 2005. Protozoa are single-celled organisms whose main function is bacterial consumption. They are commonly found in the natural microflora of plants and several species of amoebae have been associated with fresh salad vegetables. The recent occurrence of multiple outbreaks has encouraged researchers to further examine the interaction between food-borne pathogens and protozoa.

In the study protozoa (Glaucoma sp., Colpoda steinii, and Acanthamoeba palestinensis) as well as the soil-borne strain, Tetrahymena pyriformis, were cultured from store-bought spinach and lettuce and washed and allowed to graze on green fluorescent protein- or red fluorescent protein-labeled enteric pathogens including E. coli O157:H7, S. enterica, and Listeria monocytogenes. They were then monitored for their ability to sequester the bacteria and for vesicle production (food vacuoles released by protozoa offering a means of protection to some bacteria). Results showed Glaucoma produced vesicles with all bacterial strains and Tetrahymena also displayed vesicle production, but only of E. coli O157:H7 and S. enterica, not L. monocytogenes. Further studies of E. coli O157:H7 following vesicle production revealed that 4 hours after the addition of spinach extract, the bacteria had multiplied and escaped the vesicles. C. steinii did not produce any vesicles from any of the pathogens.

"The presence of protozoa on leafy vegetables and their sequestration of enteric bacteria in vesicles indicate that they may play an important role in the ecology of human pathogens on produce," say the researchers.

(P. Gourabathini, M.T. Brandl, K.S. Redding, J.H. Gunderson, S.G. Berk. 2008. Interactions between food-borne pathogens and protozoa isolated from lettuce and spinach. Applied and Environmental Microbiology, 74. 8: 2518-2525.)


Leaf Age May Contribute to Contamination of Lettuce with E. coli and Salmonella

A new study presents the first evidence that harmful pathogens frequently linked with food-borne illnesses are more commonly found on younger inner leaves than on older outer leaves of romaine lettuce. The researchers from the Produce Safety and Microbiology Research Unity, Albany, California and the University of California, Berkley report their findings in the April 2008 issue of the journal Applied and Environmental Microbiology.

Lettuce (Lactuca sativa) is the fresh produce item most commonly implicated in epidemics of food-borne illness, while Escherichia coli O157:H7 and Salmonella enterica are the most frequently attributed bacterial agents. Although previous studies have focused on E. coli O157:H7 colonization on cut or shredded lettuce leaves, little is known of its ability to colonize whole lettuce leaves in both pre- and post-harvest environments.

In the study researchers investigated the growth of E. coli O157:H7 and S. enterica on romaine lettuce leaves both pre- and post-harvest. The increased population size of E. coli O157:H7 on young lettuce plants ranged from 16- to 100-fold in the presence of warm temperatures and free water on the leaves. The increase in population size also varied significantly with leaf age, however the colonization was consistently 10-fold higher on the young (inner) leaves than on the middle leaves. Growth rates of S. enterica were found to be similarly leaf age dependent. Both bacterial pathogens also displayed higher population rates on younger leaves than on middle leaves harvested from mature lettuce heads.

"Our results indicate that leaf age and nitrogen content contribute to shaping the bacterial communities of preharvest and postharvest lettuce and that young lettuce leaves may be associated with a greater risk of contamination with E. coli O157:H7.

(M.T. Brandl, R. Amundson. 2008. Leaf age as a risk factor in contamination of lettuce with Escherichia coli O157:H7 and Salmonella enterica. Applied and Environmental Microbiology, 74. 8: 2298-2306.)


New Method Simultaneously Tests for Fifteen Respiratory Viruses

A new test capable of simultaneously detecting 15 respiratory viruses may allow for quicker diagnosis and treatment of potentially deadly respiratory infections in children and adults worldwide. The researchers from The Netherlands report their findings in the April 2008 issue of the Journal of Clinical Microbiology.

Acute respiratory infections (RTIs) are responsible for high rates of morbidity and mortality throughout the world, however the cause remains inconclusive in over half of all reported cases. Respiratory viruses are believed to be among the pathogens contributing to a significant number of undiagnosed infections, but clinical presentation of patients with RTIs are generally not pathogen specific. While cell culture is still considered the "gold standard" for laboratory detection of respiratory viruses, slow response time and low sensitivity are suboptimal for routine detection practices.

In the study a new multiparameter test called RespiFinder was developed to detect 15 respiratory viruses in one reaction. Researchers then used the RespiFinder, as well as the cell culture method and a respiratory syncytial virus (RSV)-specific immunochromatography assay (ICA) to examine 144 clinical samples and compared the results. In most cases the RespiFinder was as effective or nearly as effective (more than 95%) as the cell culture, or "gold standard", in both sensitivity and specificity. Only test results for rhinovirus and RSV came in under 95%. In addition, compared to the RSV-specific ICA, the RespiFinder showed a specificity and sensitivity of approximately 80%.

"The RespiFinder assay provides a user-friendly and high-throughput tool for the simultaneous detection of 15 respiratory viruses with excellent overall performance statistics," say the researchers.

(M. Reijans, G. Dingemans, C.H. Klaassen, J.F. Meis, J. Keijdener, B. Mulders, K. Eadie, W. van Leeuwen, A. van Belkum, A.M. Horrevorts, G. Simons. 2008. RespiFinder: a new multiparameter test to differentially identify fifteen respiratory viruses. Journal of Clinical Microbiology, 46. 4: 1232-1240.)

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