Edge is Minimum Weight Bridge iff in All Minimum Spanning Trees

Theorem
Let $G$ be an undirected network.

Let every edge of $G$ have a unique weight.

Let $e$ be an edge of $G$.

Then $e$ is a bridge of minimum weight in $G$ $e$ belongs to every minimum spanning tree of $G$.

Necessary Condition
$e$ is a bridge of minimum weight that does not belong to some minimum spanning tree $Q$.

Let $e$ be added to $Q$ to make $Q'$.

Then $e$ forms part of a unique cycle $C$ in $Q$.

Thus there exists an edge $f \in C$ such that $\map w Q < \map w {Q + e - f}$.

This contradicts the minimality of $Q$.