There is absolutely no excuse for a screener (or supervisor!) whining about changing gloves. What they need to remember is that wearing gloves is a two-way street. Sure, the gloves are there to protect us, no doubt about that, but the gloves are also there to protect the passenger from us (and other passengers). Maybe it's my germaphobia coming into play here, maybe it's my dental school background, but I change my gloves quite a bit, admittedly not between every passenger (then I use antibacterial gel on my gloves), but between every few. There is no excuse for not changing gloves and there's no excuse for not having any around.