I've always cycled tanks by just adding flake food, and after a few days in the tank the flakes are always covered in fungus.
While it's still in the water you can tell it's fungus by the fact that the white-ish hazy "ball" is actually composed of radiating fungal hyphae when examined under low magnification (but the hyphae will all mush together and look like a blob of gel when you take the food out of the water). In contrast, bacteria will look like a clear, uniform gelatinous mass under low magnification even when still under water.
I've never had any problem with the fungus (it just breaks the food down into waste products same as fish would if they were there) - fungi are always present in your tank, but will only be visible (as hyphae) when there is food available (e.g., fish food, dead fish, damaged fins).
I've never had any visible bacterial growth on the food - not saying it's impossible, but the only thing I've ever seen growing on my food is fungus. As soon as the food source is gone the fungus will go dormant again. Depending on the size and sturdiness of the flakes this process could take a few days to a week or more, during which time the "ball" of hyphae will continue to expand - but you can speed the process up by breaking the flakes into small bits before adding them to the tank.
Last edited by Jules; 07-20-2012 at 05:52 PM.