Nowadays, there is urgent demand for wireless sensor network applications. In these applications, usually a base station is responsible for monitoring the entire network and collecting information. If emergency happens, it will propagate such information to all other nodes. However, quite often the message source is not a fixed node, since there may be base stations in charge of different regions or events. Therefore, how to propagate information efficiently when message sources vary from time to time is a challenging issue. None of conventional broadcast algorithms can deal with this case efficiently, since the change of message source incurs a huge implementation cost of rebuilding a broadcast tree. To deal with this difficult problem, we make endeavor in studying multiple source broadcast, in which targeted algorithms should be source-independent to serve the practical need. In this paper, we formulate the Minimum-Latency Multisource Broadcast problem. We propose a novel solution using a fixed shared backbone, which is independent of the message sources and can be used repeatedly to reduce the broadcast latency. To the best of our knowledge, our work is deemed the first attempt to design such a multisource broadcast algorithm with a derived theoretical latency upper bound.