With the latest issue of Consumer Reports confirming that the produce industry is still selling pre-washed, bagged salad greens that contain problematic levels of bacteria, it’s appropriate to review why this will no doubt lead to another outbreak of E. coli O157:H7, Salmonella or Listeria.
Since 1993, at least 20 Escherichia coli O157:H7 outbreaks have been traced to California-grown leafy greens – primarily lettuce and spinach. Most of these outbreaks involved packaged product that, despite several chlorinated washes, contained sufficient numbers of E. coli O157:H7 to cause infection at the time of consumption.
Although the exact E. coli contamination routes for these products remain unknown, a federal study published in September 2008 indicates that plant tissue damage from processing and also in the field can promote significant multiplication of E. coli O157:H7 over a short time.
The research findings by the Produce Safety and Microbiology Research Unit of USDA’s Agricultural Research Service in California suggest that harvesting and processing are critical control points in the prevention or reduction of E. coli O157:H7 contamination of lettuce.
The research assessed the role of plant tissue damage (mechanical or diseased-induced lesions) in the contamination of leafy greens with E. coli O157:H7 and the growth of the pathogen.
Within only 4 hours after inoculation, the population sizes of E. coli O157:H7 increased 4.0-, 4.5-, and 11-fold on lettuce leaves that were mechanically bruised, cut into large pieces, and shredded into multiple pieces, respectively.
During the same time, E. coli O157:H7 population sizes increased only two-fold on leaves that were left intact after harvest.
Also, the population size of E. coli O157:H7 was 27 times greater on young leaves affected by soft rot due to infection by Erwinia chrysanthemi than on healthy middle-aged leaves. Confocal microscopy revealed that leaf tip burn lesions harbored dense populations of E. coli cells both internally and externally. Investigation of the cut lettuce stems showed that the pathogen grew 11-fold over 4 hours of incubation after its inoculation.
