Amoebas may vomit E. coli on your greens
02.may.08
New Scientist
Ewen Callaway
http://www.newscientist.com/article/dn13827-amoebas-may-vomit-ie-colii-on-your-greens.html?DCMP=ILC-
hmts&nsref=news2_head_dn13827
Harmless protozoa that live on grocery store greens can shelter deadly food
pathogens like E. coli and Salmonella.
A laboratory study has found that food pathogens survive being eaten by
protozoa living on spinach and lettuce. The temporary asylum might help
bacteria stick onto leafy greens or resist efforts to kill them before
packaging.
Whether the shelter the protozoa provide contributes to pathogen outbreaks,
however, remains to be seen.
A team led by microbiologist Sharon Berk, of Tennessee Technological
University in Cookeville, fed lab-grown bacterial pathogens to protozoa
found on grocery store produce. Her team did not find the pathogenic
bacteria on the supermarket veggies.
When Berk's team then examined the protozoa, they discovered the bacteria
alive and well in their stomachs.
Apparently the pathogens upset the protozoa's digestion, though. A day after
mixing E. coli O157:H7 and Salmonella with protozoa, the team noticed that
many of the bacteria had been "vomited" up into round clumps.
When the researchers added these clumps to pulverized spinach, the E. coli
cells tripled in number after just a few hours.
Berk says she does not know whether protozoa are responsible for E. coli
outbreaks, like that in 2006 that killed three and sickened hundreds of
people who had eaten tainted spinach.
However, she says food safety researchers ought to now add pathogen-eating
protozoa – which might prove more difficult to wash off of greens – to their
list of possible dangers.
"I don't think you are every going to get them all off. Amoebae – they can
be like glue," she adds.
Maria Brandl at the US Department of Agriculture in Albany, California, who
collaborated on the study, previously found that the clumps – or pellets –
of E. coli are resistant to the low levels of bleach used to wash many
greens after they have been harvested.
But linking protozoa to outbreaks of food borne pathogens like E. coli might
be a tall order. "Frankly, we are far from showing 'aha this is the
culprit'. I don't know if you can ever demonstrate that the pellet is
responsible for outbreaks," she says.
Against huge odds, Berk's lab may have come close. During the 2006 spinach
outbreak, a student in her lab got sick after eating a few leaves of baby
spinach. The packaged greens were among the bags recalled by producers.
"We kept that bag and we did find amoebae in there," she says, but they did
not check for E. coli O157:H7. So the "smoking spinach bag" for the
protozoan link to food pathogens remains to be found.