The trees would not take up the microbes through their roots but there are several problems that could arise from growing fruit trees over a drain field. As the waste is expelled from the septic tank, it still contains microoganisms such as e coli and fecal coliforms. The high levels of these microbes are usually killed off as they come into contact with the soil and especially as the soil dries out. There may, however, be excessive levels of these microbes in the soil above what would be considered normal. If the water doesn't drain properly from the drain field, then there could be waste water accumulating on the soil surface. Besides drowning the trees, the excess water on the surface may evaporate instead of draining from the site, leaving excessive microbes on the soil surface. Most of these would die off as the soil dries out, but some may survive and then get blown from the soil surface and get deposited on the fruit. The fruit can become contaminated and must be washed thoroughly or disposed of to prevent ingesting any of the microoganisms. If there is plenty of cover crop such as grasses on top of the drain field, then the amount of blow material can be minimized. There are probably positive testimonials about growing edible products over drain fields, but it may be only a matter of time before something happens and the fruit becomes contaminated. It's something that can be avoided altogether by not growing edible food from a drain field site. I have had to dig up one too many drain fields in my life that were damaged by plant roots. Besides being expensive, it can be pretty nasty. It can be difficult enough to get these things to drain properly, much less trying to get them to function properly with trees growing over them.