Given an ultragraph in the sense of Tomforde, we construct a topological
quiver in the sense of Muhly and Tomforde in such a way that the universal C*-algebras
associated to the two objects coincide. We apply results of Muhly and Tomforde for
topological quiver algebras and of Katsura for topological graph C*-algebras to study the
K-theory and gauge-invariant ideal structure of ultragraph C*-algebras.