Edge is Bridge iff in All Spanning Trees

Theorem
Let $G$ be a simple graph.

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

Then $e$ is a bridge in $G$ $e$ belongs to every spanning tree for $G$.

Necessary Condition
Let $e$ be a bridge.

That is, suppose the edge deletion $G - e$ is disconnected.

Let $T$ be an arbitrary spanning tree for $G$.

By definition $T$ is a connected subgraph of $G$.

If $T$ did not contain $e$, then it would also be a subgraph of $G - e$.

This contradicts the fact that $G - e$ is disconnected.

Therefore $e$ is in $T$.

Sufficient Condition
Suppose $T$ is a spanning tree for $G$.

Suppose that $T$ does not contain $e$.

Then $T$ is a subgraph of the edge deletion $G - e$.

Since $T$ is by definition connected, so is $G - e$.

Hence $e$ is not a bridge.