John Gill Commentary Acts 28:5

"Howbeit he shook off the creature into the fire, and took no harm." — Acts 28:5 (ASV)

And he shook off the beast into the fire
Having held it a while, and as being master of it, and as not being afraid of it, though it was the ready way to provoke it to fasten on him again:

and felt no harm ;
it having not bit him, nor infected him with its poison; and hereby was fulfilled what our Lord promised to his disciples, (Mark 16:18) ;