Putting food in the refrigerator should hinder the growth of bacteria, because the enzyme systems that the microbes depend upon to multiply start to slow down when the temperature is reduced. But anything above freezing isn't going to stop bacterial growth completely [source: Brewer]. In fact, refrigerators themselves can easily turn into bacteria farms.
A study of 30 typical European fridges commissioned by Microban, a maker of antimicrobial products, found that some of them contained as many as 129,000 "bacterial colony forming units" per square centimeter [source: Chowdhury]. Positively disgusting, huh? Hopefully, you scrub your fridge out often enough that it's not that bad. But even so, it's not a good idea to leave food in there for too long.
One problem is determining exactly how long is too long, since the life of individual items varies. Raw hamburger, for example, can only be stored for one to two days, while egg salad, for example, is typically safe to eat for three to five days, and bacon and sausage for a week. An unopened package of lunchmeat is good for up to two weeks [source: Foodsafety.gov].
If you're in doubt about a particular food item, consult the website StillTasty.com, which has a database compiled from government shelf-life estimates.