(6R,7S)-7-[2-[(S)-2-Amino-2-carboxyethylthio]acetylamino]-7-methoxy-3-[(1-methyl-1H-tetrazol-5-yl)thiomethyl]-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid 2-sodium salt