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.
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A team led by microbiologist , 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鈥檚 team then examined the protozoa, they discovered the bacteria alive and well in their stomachs.
Sore tums
Apparently the pathogens upset the protozoa鈥檚 digestion, though. A day after mixing E. coli O157:H7 (a harmful form of the bug) and Salmonella with protozoa, the team noticed that many of the bacteria had been 鈥渧omited鈥� 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 .
However, she says food safety researchers ought to now add pathogen-eating protozoa 鈥� which might prove more difficult to wash off of greens 鈥� to their list of possible dangers.
鈥淚 don鈥檛 think you are every going to get them all off. Amoebae 鈥� they can be like glue,鈥� she adds.
鈥楽moking bag鈥�
at the US Department of Agriculture in Albany, California, who collaborated on the study, previously found that the clumps 鈥� or pellets 鈥� 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. colimight be a tall order. 鈥淔rankly, we are far from showing 鈥榓ha this is the culprit鈥�. I don鈥檛 know if you can ever demonstrate that the pellet is responsible for outbreaks,鈥� she says.
Against huge odds, Berk鈥檚 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.
鈥淲e kept that bag and we did find amoebae in there,鈥� she says, but they did not check for E. coli 0157:H7. So the 鈥渟moking spinach bag鈥� for the protozoan link to food pathogens remains to be found.
Journal reference: (DOI: 10.1128/aem.02709-07)
