The present invention is directed to compounds which are beta-lactamase inhibitors. The compounds and their pharmaceutically acceptable salts are useful in combination with beta-lactam antibiotics, for the treatment of bacterial infections, including infections caused by drug resistant organisms, including multi-drug resistant organisms. The present invention includes compounds according to Formula (I): or a pharmaceutically acceptable salt thereof, wherein the values of R1, R2, R3, R4, R5 and R6 are described herein.