A compound of structural formula I: or a pharmaceutically acceptable salt thereof, for use in treating or preventing bacterial infection; in combination with a β-lactam antibiotic, the bacterial infection being a methicillin resistant Staphylococcus bacterial infection; wherein A is selected from the group consisting of: phenyl, and pyridyl; X is selected from the group consisting of: N, and CH; R 1 is selected from the group consisting of: hydrogen, and -C 1-6 alkyl, wherein alkyl is unsubstituted or substituted with one to five substituents selected from -C 1-6 alkyl; R 2 is -C 5-8 cycloalkyl, wherein cycloalkyl is unsubstituted or substituted with one to five substituents selected from R a, or R 1 and R 2 together with the nitrogen atom to which they are attached form a monocyclic or bicyclicC 3-10 cycloheteroalkyl ring, wherein the cycloheteroalkyl ring is unsubstituted or substituted with one to five substituents selected from -C 1-6 alkyl, R 3 is selected from the group consisting of: -C 3-10 cycloalkyl, -(CH 2) n -C 2-6 cycloheteroalkyl, -aryl, and -heteroaryl, wherein each CH 2, cycloalkyl, cycloheteroalkyl, aryl and heteroaryl is unsubstituted or substituted with one to four substituents selected from R b; R 4 is hydrogen, R 5 is selected from the group consisting of: hydrogen, halogen, -C 1-6 alkyl, -OC 1-6 alkyl, -C 1-6 alkyl-OH, -CN, -CHF 2, -CF 3, -(CH 2) q -NR c R d, -C 3-6 cycloalkyl, -(CH 2) s -C 2-5 cycloheteroalkyl, -aryl, and -heteroaryl, wherein each CH 2, alkyl, cycloalkyl, cycloheteroalkyl, aryl and heteroaryl is unsubstituted or substituted with 1-3 substituents selected from R g; each R a is independently-C 1-6 alkyl, each R b is independently selected from the group consisting of: halogen, -C 1-6 alkyl, -OC 1-6 alkyl, -CHF 2, -CO 2 C 1-6 alkyl, -(CH 2) p -NHCO 2 C 1-6 alkyl, heteroaryl, wherein each alkyl, and heteroaryl is unsubstituted or substituted with 1-4 substituents selected from: halogen, -C 1-6 alkyl, and -OC 1-6 alkyl, R c and R d are each independently selected from the group consisting of: hydrogen, and C 1-6 alkyl, each R g is independently selected from the group consisting of: halogen, and -C 1-6 alkyl, wherein each alkyl, is unsubstituted or substituted with one to three substituents selected from halogen; each n is independently 0, 1 or 2; each p is independently 0, 1 or 2; each q is independently 0, 1 or 2; and each s is independently 0, 1 or 2. or a pharmaceutically acceptable salt thereof, for use in treating or preventing bacterial infection; in combination with a β-lactam antibiotic, the bacterial infection being a methicillin resistant Staphylococcus bacterial infection; wherein A is selected from the group consisting of: phenyl, and pyridyl; phenyl, and pyridyl; X is selected from the group consisting of: N, and CH; N, and CH; R 1 is selected from the group consisting of: hydrogen, and -C 1-6 alkyl, hydrogen, and -C 1-6 alkyl, wherein alkyl is unsubstituted or substituted with one to five substituents selected from -C 1-6 alkyl; R 2 is -C 5-8 cycloalkyl, wherein cycloalkyl is unsubstituted or substituted with one to five substituents selected from R a, or R 1 and R 2 together with the nitrogen atom to which they are attached form a monocyclic or bicyclicC 3-10 cycloheteroalkyl ring, wherein the cycloheteroalkyl ring is unsubstituted or substituted with one to five substituents selected from -C 1-6 alkyl, R 3 is selected from the group consisting of: -C 3-10 cycloalkyl, -(CH 2) n -C 2-6 cycloheteroalkyl, -aryl, and -heteroaryl, -C 3-10 cycloalkyl, -(CH 2) n -C 2-6 cycloheteroalkyl, -aryl, and -heteroaryl, wherein each CH 2, cycloalkyl, cycloheteroalkyl, aryl and heteroaryl is unsubstituted or substituted with one to four substituents selected from R b; R 4 is hydrogen, R 5 is selected from the group consisting of: hydrogen, halogen, -C 1-6 alkyl, -OC 1-6 alkyl, -C 1-6 alkyl-OH, -CN, -CHF 2, -CF 3, -(CH 2) q -NR c R d, -C 3-6 cycloalkyl, -(CH 2) s -C 2-5 cycloheteroalkyl, -aryl, and -heteroaryl, hydrogen, halogen, -C 1-6 alkyl, -OC 1-6 alkyl, -C 1-6 alkyl-OH, -CN, -CHF 2, -CF 3, -(CH 2) q -NR c R d, -C 3-6 cycloalkyl, -(CH 2) s -C 2-5 cycloheteroalkyl, -aryl, and -heteroaryl, wherein each CH 2, alkyl, cycloalkyl, cycloheteroalkyl, aryl and heteroaryl is unsubstituted or substituted with 1-3 substituents selected from R g; each R a is independently-C 1-6 alkyl, each R b is independently selected from the group consisting of: halogen, -C 1-6 alkyl, -OC 1-6 alkyl, -CHF 2, -CO 2 C 1-6 alkyl, -(CH 2) p -NHCO 2 C 1-6 alkyl, heteroaryl, halogen, -C 1-6 alkyl, -OC 1-6 alkyl, -CHF 2, -CO 2 C 1-6 alkyl, -(CH 2) p -NHCO 2 C 1-6 alkyl, heteroaryl, wherein each alkyl, and heteroaryl is unsubstituted or substituted with 1-4 substituents selected from: halogen, -C 1-6 alkyl, and -OC 1-6 alkyl, R c and R d are each independently selected from the group consisting of: hydrogen, and C 1-6 alkyl, hydrogen, and C 1-6 alkyl, each R g is independently selected from the group consisting of: halogen, and -C 1-6 alkyl, halogen, and -C 1-6 alkyl, wherein each alkyl, is unsubstituted or substituted with one to three substituents selected from halogen; each n is independently 0, 1 or 2; each p is independently 0, 1 or 2; each q is independently 0, 1 or 2; and each s is independently 0, 1 or 2.