Here's what they said:
We tested for total coliforms and for other bacteria, including enterococcus, that are better indicators of fecal contamination. Federal action limits exist for indicator organisms in water, raw meat, milk, and some processed foods, but not produce. Those organisms are typically used to gauge possible pathogen contamination.
Several industry experts we consulted suggested that for leafy greens, an unacceptable level of total coliforms or enterococcus is 10,000 or more colony forming units per gram (CFU/g) or a comparable estimate. In our tests, 39 percent of samples exceeded that level for total coliforms and 23 percent for enterococcus.
My suggestion: If you want mixed greens, buy them direct from the farmer, or grow them yourself. Otherwise, buy your lettuce by the head, the way your parents did. Wash it yourself. Really, it isn't that hard.