1-Azabicyclo[3.2.0]hept-2-ene-2-carboxylic acid,6-[(1R)-1-hydroxyethyl]-4-methyl-7-oxo-3-[[1-[1-oxo-3-(3-pyridinyl)propyl]-3-azetidinyl]thio]-, monosodium salt, (4R,5S,6S)-