It is widely known that pathogenic E. coli germs have ability to survive inside fresh vegetable produce such as spinach, lettuce and sprouts, where they are protected against disinfection. This ability may be a key factor explaining the transmission of the E. coli strain in the dreadful (and still ongoing) German outbreak.
A new article has just come out dissecting how related EHEC E. coli bacteria attach to plant surfaces. They have remarkable complex adhesion abilities that enable the deadly microbes to persist inside plant tissue cavities and on plant surfaces. These consist of various different surface protein hooks that are given names like pili, curli, and flagella.
Although we know that EHEC and EAEC bacteria are a bit different to one another, this new work on EHEC E. coli teaches us a lot about the possible routes of transmission of the German HUS germ– for example through seeds.
A scientific summary on the mechanisms of the EHEC germ attachment to plants is given below. The paper provides access to the latest research on this topic.
(The term STpEAEC which means Shigatoxin producing EAEC is now being used to describe the German outbreak strain; see previous GMO Pundit post for the latest news about STpEAEC)
Surface structures involved in plant stomata and leaf colonization by Shiga-toxigenic Escherichia coli O157:H7
Zeus Saldaña, Ethel Sánchez, Juan Xicohtencatl-Cortes, Jose Luis Puente and Jorge A. Girón
Fronteers in Microbiology Volune 2 Art 119 Original Research Article published: 27 May 2011 doi: 10.3389/fmicb.2011.00119
Shiga-toxigenic Escherichia coli (STEC) O157:H7 uses a myriad of surface adhesive appendages including pili, flagella, and the type 3 secretion system (T3SS) to adhere to and inflict damage to the human gut mucosa. Consumption of contaminated ground beef, milk, juices, water, or leafy greens has been associated with outbreaks of diarrheal disease in humans due to STEC.
The aim of this study was to investigate which of the known STEC O157:H7 adherence factors mediate colonization of baby spinach leaves and where the bacteria reside within tainted leaves. We found that STEC O157:H7 colonizes baby spinach leaves through the coordinated production of curli, the E. coli common pilus, hemorrhagic coli type 4 pilus, flagella, and T3SS.
Electron microscopy analysis of tainted leaves revealed STEC bacteria in the internal cavity of the stomata, in intercellular spaces, and within vascular tissue (xylem and phloem), where the bacteria were protected from the bactericidal effect of gentamicin, sodium hypochlorite or ozonated water treatments. We confirmed that the T3S escN mutant showed a reduced number of bacteria within the stomata suggesting that T3S is required for the successful colonization of leaves. In agreement, non-pathogenic E. coli K-12 strain DH5α transformed with a plasmid carrying the locus of enterocyte effacement (LEE) pathogenicity island, harboring the T3SS and effector genes, internalized into stomata more efficiently than without the LEE.
This study highlights a role for pili, flagella, and T3SS in the interaction of STEC with spinach leaves. Colonization of plant stomata and internal tissues may constitute a strategy by which STEC survives in a nutrient-rich microenvironment protected from external foes and may be a potential source for human infection.
Keywords: plant colonization, stomata, STEC, O157:H7, pathogenesis, spinach, T3SS, pili
Hat-tip to Kate Howell.
been illegally dumped next to feedlots. In addition, run-off water from neighboring fields, which is collected in furrows and sometimes used to irrigate alfalfa fields, could carry manure to the fields. Three of the four farmers reported at least occasionally seeing deer in their fields. In fact, one had fields next to a wildlife refuge and reported that deer were in the fields every day. Contaminated alfalfa plants, cattle manure, or deer feces could be picked up by the thresher during harvesting and contaminate the seeds. No evidence was found for bacterial contamination at the seed processor.
own in waste-amended soil.
Isolation of Salmonella from alfalfa seed and demonstration of impaired growth of heat-injured cells in seed homogenates.
Int J Food Microbiol. 2003 May 15;82(3):245-53.
Liao CH, Fett WF.
Source
US Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, PA 19038, USA. cliao@arserrc.gov
Abstract
Three major foodborne outbreaks of salmonellosis in 1998 and 1999 were linked to the consumption of raw alfalfa sprouts. In this report, an improved method is described for isolation of Salmonella from alfalfa seed lots, which had been implicated in these outbreaks. From each seed lot, eight samples each containing 25 g of seed were tested for the presence of Salmonella by the US FDA Bacteriological Analytical Manual (BAM) procedure and by a modified method applying two successive pre-enrichment steps. Depending on the seed lot, one to four out of eight samples tested positive for Salmonella by the standard procedure and two to seven out of eight samples tested positive by the modified method. Thus, the use of two consecutive pre-enrichment steps led to a higher detection rate than a single pre-enrichment step. This result indirectly suggested that Salmonella cells on contaminated seeds might be injured and failed to fully resuscitate in pre-enrichment broth containing seed components during the first 24 h of incubation. Responses of heat-injured Salmonella cells grown in buffered peptone water (BPW) and in three alfalfa seed homogenates were investigated. For preparation of seed homogenates, 25 g of seeds were homogenized in 200 ml of BPW using a laboratory Stomacher and subsequently held at 37 degrees C for 24 h prior to centrifugation and filtration. While untreated cells grew at about the same rate in BPW and in seed homogenates, heat-injured cells (52 degrees C, 10 min) required approximately 0.5 to 4.0 h longer to resuscitate in seed homogenates than in BPW. This result suggests that the alfalfa seed components or fermented metabolites from native bacteria hinder the repair and growth of heat-injured cells. This study also shows that an additional pre-enrichment step increases the frequency of isolation of Salmonella from naturally contaminated seeds, possibly by alleviating the toxic effect of seed homogenates on repair or growth of injured cells.
Detection of Salmonella by indicator agar media and PCR as affected by alfalfa seed homogenates and native bacteria
Significance and Impact: The results generated from this study will aid the seed distributors, sprout growers and public health officials to identify and recall the Salmonella-contaminated seed lots to be used for sprout production.
C.-H. Liao and L.M. Shollenberger Letters in Applied Microbiology 2003, 36, 152–15
US Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, Wyndmoor, PA, USA
Dr. Tribe,
One of the things that struck me the other day in following this outbreak is that this is really nothing new. In going through my ‘archives’, I found a few things from 10-12 years ago where it was found that E. coli O157 can remain viable in soil for greater than 4 months (DL Jones, Soil Use and Management, 15:(2) June 1999.) This bacterium has also been detected inside surface sterilized leaves of lettuce (Solomon et al, Applied and Environmental Microbiology 68: 2002.) And according to the CDC, a multistate outbreak of E. coli O157 was linked to alfalfa sprouts grown from contaminated seeds in 1997 (Breuer et al, Emerging Infectious Disease, 7:(6) Nov-Dec, 2001.)
Thanks. I have posted text and links from these and several related papers in the main posting. Please keep feeding any helpful information so we can make this a good information resource
Just in:
German News-Channel n-tv reports on its website that “a definitive transfer of STpEAEC from a human (unaware of her illness!) to customers of a catering service has taken place in Hesse – either by contact or by contamination of used tools to prepare meals”
http://www.n-tv.de/panorama/Mensch-uebertraegt-EHEC-auf-Essen-article3604761.html
This is especially interesting because STpEAEC is still “in the game” in thousands of unaware infected people across Germany (or maybe even the world?)
Therefore I still don’t dismiss bioterror as a potential initial origin.
Best regards
PS: Robert-Koch-Institute currently informs that an infectious potential of STpEAEC through excrements of infected people could exist for more than 4 weeks even for months to come and they started a study – just now (go figure!):
http://www.rki.de/cln_178/nn_467482/DE/Home/Info-HUS.html
These are very serious developments. They are so serious I’m going to wait until I get firm expert confirmation befor posting about them on the main page.
It is very unlikely that this germ would have been selected by a bioterrorist.
My earlier post warning of persistent spread internationally was too understated. WHO will have to be more definite about management of this risk.
Are we talking E. coli on seeds, or in seeds?