Though not being a microbiologist, my idea is that you have to know if the distribution of bacteria is quite homogeneous over the plant or not. If not you have to consider different parts, e.g. different heights of the plants (e.g. leaves from ground, middle part, upper part) or with lettuce outer and inner plant parts or different organs (e.g. root/stem/leaves/flowers). It depends surely how bacteria are entering the plant or if they only occur on the surface, or if only edible plant parts are considered.
Anyway there should be established protocols available.
perhaps stuff like this:
Collins, C.H. (2004). Collins and Lyne’s microbiological methods. 8th ed., New York, NY: Arnold
FDA Bacteriological analytical manual. 8th ed. AOAC International, Gaithersburg, Md.
One must presume that long and short arguments contribute to the same end. - Epicurus
...except casandra's that belong to the funniest, most interesting and imaginative (or over-imaginative?) ones, I suppose.