Meat – Is it bad to marinate meat for too long

I can't see any reason for the marinating itself to make any difference. If it's safe to leave the (un-marinated) meat in the same conditions for the same length of time, then it's safe to marinate it for that long.

Five days in the refrigerator is definitely stretching it for chicken - usually no more than a few days is recommended, and that's assuming it was fresh when you bought it (not a day before the sell-by date). Even if it's safe, it's not going to be very good. I've accidentally left raw chicken in the refrigerator and forgotten about it before, and it started to smell "off" after about 4 days. I hate to say it, but I wouldn't use it at this point.

As for the steak, you're probably okay, because you only have surface bacteria, but I would cook it well and right away. Again, most cooks I know will recommend no more than a few days in the fridge.

P.S. I've heard people say that weak acids such as lime juice "preserve" the meat, but never from a reliable source; even if it worked, every marinade is different and it would be nearly impossible to predict the exact amount of time it preserves for. You have no idea how much bacteria existed at the outset, and even if the marinade somehow helped to inhibit growth of new bacteria, the "spoiled-ness" of raw meat doesn't come from the bacteria itself but from the toxins they leave behind. It's not a requirement for the bacteria to multiply in order for the food to spoil, if enough existed in the first place.

Unless somebody has it on good authority that marinating makes any significant difference, I refuse to put any stock in that bit of folk wisdom and recommend that others be equally skeptical.

This works by increasing the porousness of the meat inside the bag. When a vacuum is created, there is a natural tendency for matter to occupy that space. It does this by increasing the amount of space between particles, otherwise known as density. This increases the size of the microscopic holes in the meat, and thus effectively increases the surface area in which the marinade can contact the meat. More surface area means by marinade sticks to your food.

If you'd like to see the effects of vacuum pressure on foods greatly exagerrated, put a marshmallow in a vacuum seal bag and see what happens. :-)