Here are two good approaches to managing corporate Twitter accounts. (Note: neither of these involve interns.)
Strategy 1: Have a team, but also appoint people in functional units to be responsible for bridging knowledge gaps throughout the company. Add a crisis response protocol for the team to follow. Ensure the typical tone is markedly different than the crisis tone.
Strategy 2: Have an all-hands, anyone who wants to, giant force of tweeters, but formally train and acculturate them to how to behave on behalf of the company. Be transparent and flat.
(The third strategy, that of Whole Foods, is not fully described. They only include the Whole Foods HQ tweeter, not the stores, nor do they discuss the interaction between the stores and HQ, which is the tricky bit.)