(6R)-3-(Acetoxymethyl)-7α-[[(R)-5-amino-5-carboxy-1-oxopentyl]amino]-8-oxo-5-thia-1-azabicyclo[4.2.0]octa-2-ene-2-carboxylic acid/potassium,(1:x) salt

[6R-[6alpha,7beta(R*)]]-3-(acetoxymethyl)-7-[(5-amino-5-carboxypentanoyl)amino]-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid, potassium salt