jennyema - the bad bug causing problems in beef that is getting all the press is a rare strain of E. coli
(Escherichia coli) - E. coli O157:H7.
Of course it's not found in solid pieces of meat like a London Broil - it's mainly in ground beef. Here's a place to read more: http://people.ku.edu/~jbrown/ecoli.html
Keeping the surface of the meat coated with wine provides an acidic environment tha bacteria don't like. Actually - the cut surface of any beef will contain some bacteria that is not inside the meat. Cooking to an internal temp of 200-F will kill the nasty little bugs the acid in the wine didn't kill on the surface of the meat.
LEFSElover - sorry I was having a cognative lapse the other night .... I was thinking about the way Mom used to marinade meat in an open dish ... not the way I do it these days. The way I do it - put the meat in a 1-gallon zip top bag, pour in the marinade, squeeze out all the air, zip it shut, fold the bag over to reduce the size of the bag to the size of the meat ... then turn every hour or so. That way - the meat should always be in contact with the wine.