One thing nobody seems to have mentioned is that it's not necessarily the bacteria themselves that are harmful.
Yes, any stray bacteria will almost certainly be killed each time you cook, but bacteria produce protein toxins as they grow, and these toxins are extremely dangerous, even lethal at relatively low doses.
In fact, the only reason bacterial poisoning (i.e. salmonella or e.coli) is a concern in the first place is because the bacteria can produce those same toxins while in your stomach and digestive tract. But at least when you've ingested the bacteria themselves, your immune system has a chance to react and try to get rid of it before the level of toxicity reaches lethal levels; if you ingest the toxins themselves, your body may not have the same opportunity.
Protein and LPS toxins, unlike the bacteria themselves, cannot be killed with direct heat because there is simply nothing to kill. They can be destroyed, certainly, but it takes a lot more heat to do so than it takes to cook the food - you'd basically have to incinerate it.
The good thing is that generally these toxins are water-soluble and are easily removed with sufficient soap and hot water. Running something through a dishwasher cycle is basically guaranteed to kill the bacteria and wash off any toxins.
But if you leave a pan or grill sitting around with a bacterial colony growing on it for a week, then the bacteria are no longer your primary concern; the toxins they've left behind are. Plop a piece of meat on top of it and those water-soluble toxins will get baked on or absorbed just like salt. Cooking the meat will have done absolutely nothing to get rid of them.
Do yourself a favour and wash your grills and cookware, so you can avoid serious health complications or costly lawsuits. You can burn off grills as bikeboy mentions, but don't just leave them sitting around potentially contaminated.