A compound of formula (Ib) for the use in the treatment or prevention of infections and resulting diseases caused by Pseudomonas aeruginosa wherein: X 1 is CR 11 or N; X 4 is CR 14 or N; R 2 is selected from hydrogen and C 1-7 -alkyl; R 9 is hydrogen, halo or C 1-7 -alkyl; R 11 and R 14 are each individually selected from hydrogen, halogen, C 1-7 -alkyl, C 1-7 -haloalkyl, hydroxy, C 1-7 -hydroxyalkyl, C 1-7 -alkoxy, C 1-7 -haloalkoxy, -NR 24 R 25, C 1-7 -alkyl-NR 24 R 25, aryl-C 1-7 -alkyl-O-C 1-7 -alkinyl-, aryl and heteroaryl, wherein aryl and heteroaryl are optionally substituted with one, two or three substituents selected from the list of halogen, cyano, C 1-7 -alkyl C 1-7 -haloalkyl, hydroxy, C 1-7 -alkoxy, -NR 24 R 25, C 1-7 -alkyl-NR 24 R 25, -CO-NH-(CH 2) n -NR 24 R 25, -CO-NH- r-OH, -CO-NH-(CH 2) o -heterocycloalkyl, -CO-OH, -O-C 1-7 -hydroxyalkyl, -O-(CH 2) o -CO-OH, -SO 2 -C 1-7 -alkyl, -SO 2 -NR 24 R 25, heterocycloalkyl, -O-heterocycloalkyl and heterocycloalkyl substituted with C 1-7 -alkyl or oxo; R 15 is hydrogen, halogen, C 1-7 -alkyl, C 1-7 -haloalkyl, -NR 24 R 25, C 1-7 -alkyl-NR 24 R 25, hydroxy, C 1-7 -alkoxy, haloC 1-7 -alkoxy, benzyloxy-propynyl (-C≡C-CH 2 -O-benzyl), heterocycloalkyl, aryl and heteroaryl, wherein aryl is optionally substituted with one -NR 20 R 21 or heterocycloalkyl substituted with C 1-7 -alkyl R 17 is hydrogen, halogen, C 1-7 -alkyl, C 1-7 -haloalkyl, -NR 24 R 25, C 1-7 -alkyl-NR 24 R 25, hydroxy, C 1-7 -alkoxy, haloC 1-7 -alkoxy, benzyloxy-prop-1-ynyl, heterocycloalkyl, aryl and heteroaryl, wherein heterocycloalkyl is optionally substituted with one -NR 24 R 25, wherein aryl and heteroaryl are optionally substituted with one, two or three substituents selected from the list of halogen, C 1-7 -alkyl, C 1-7 -haloalkyl, C 1-7 -hydroxyalkyl, hydroxy, C 1-7 -alkoxy, -NR 24 R 25, -SO 2 -C 1-7 -alkyl, -SO 2 -NR 24 R 25, heterocycloalkyl and heterocycloalkyl substituted with C 1-7 -alkyl; R 18 is hydrogen, halogen, C 1-7 -alkyl, C 1-7 -haloalkyl, hydroxy, C 1-7 -hydroxyalkyl, C 1-7 -alkoxy, C 1-7 -haloalkoxy, -NR 24 R 25, C 1-7 -alkyl-NR 24 R 25, aryl and heteroaryl, wherein aryl and heteroaryl are optionally substituted with one, two or three substituents selected from the list of halogen, cyano, C 1-7 -alkyl C 1-7 -haloalkyl, hydroxy, C 1-7 -alkoxy, -NR 24 R 25, C 1-7 -alkyl-NR 24 R 25, -CO-NH-(CH 2) r -NR 24 R 25, -CO-NH-(CH 2) o -OH, -CO-NH-(CH 2) o -heterocycloalkyl, -CO-OH, -O-C 1-7 -hydroxyalkyl, -O-(CH 2) r -CO-OH, -SO 2 -C 1-7 -alkyl, -SO 2 -NR 24 R 25, heterocycloalkyl, -O-heterocycloalkyl and heterocycloalkyl substituted with C 1-7 -alkyl or oxo; R 20 and R 21 are hydrogen; R 24 and R 25 are each individually selected from hydrogen and C 1-7 -alkyl; n is 2, 3, 4, 5 or 6; o is 0, 1, 2, 3, 4, 5, 6, 7 or 8; r is selected from 1, 2, 3 and 4; and Y is -CH 2 - or -CO-; or a pharmaceutically acceptable salt thereof. X 1 is CR 11 or N; X 4 is CR 14 or N; R 2 is selected from hydrogen and C 1-7 -alkyl; R 9 is hydrogen, halo or C 1-7 -alkyl; R 11 and R 14 are each individually selected from hydrogen, halogen, C 1-7 -alkyl, C 1-7 -haloalkyl, hydroxy, C 1-7 -hydroxyalkyl, C 1-7 -alkoxy, C 1-7 -haloalkoxy, -NR 24 R 25, C 1-7 -alkyl-NR 24 R 25, aryl-C 1-7 -alkyl-O-C 1-7 -alkinyl-, aryl and heteroaryl, wherein aryl and heteroaryl are optionally substituted with one, two or three substituents selected from the list of halogen, cyano, C 1-7 -alkyl C 1-7 -haloalkyl, hydroxy, C 1-7 -alkoxy, -NR 24 R 25, C 1-7 -alkyl-NR 24 R 25, -CO-NH-(CH 2) n -NR 24 R 25, -CO-NH- r-OH, -CO-NH-(CH 2) o -heterocycloalkyl, -CO-OH, -O-C 1-7 -hydroxyalkyl, -O-(CH 2) o -CO-OH, -SO 2 -C 1-7 -alkyl, -SO 2 -NR 24 R 25, heterocycloalkyl, -O-heterocycloalkyl and heterocycloalkyl substituted with C 1-7 -alkyl or oxo; R 15 is hydrogen, halogen, C 1-7 -alkyl, C 1-7 -haloalkyl, -NR 24 R 25, C 1-7 -alkyl-NR 24 R 25, hydroxy, C 1-7 -alkoxy, haloC 1-7 -alkoxy, benzyloxy-propynyl (-C≡C-CH 2 -O-benzyl), heterocycloalkyl, aryl and heteroaryl, wherein aryl is optionally substituted with one -NR 20 R 21 or heterocycloalkyl substituted with C 1-7 -alkyl R 17 is hydrogen, halogen, C 1-7 -alkyl, C 1-7 -haloalkyl, -NR 24 R 25, C 1-7 -alkyl-NR 24 R 25, hydroxy, C 1-7 -alkoxy, haloC 1-7 -alkoxy, benzyloxy-prop-1-ynyl, heterocycloalkyl, aryl and heteroaryl, wherein heterocycloalkyl is optionally substituted with one -NR 24 R 25, wherein aryl and heteroaryl are optionally substituted with one, two or three substituents selected from the list of halogen, C 1-7 -alkyl, C 1-7 -haloalkyl, C 1-7 -hydroxyalkyl, hydroxy, C 1-7 -alkoxy, -NR 24 R 25, -SO 2 -C 1-7 -alkyl, -SO 2 -NR 24 R 25, heterocycloalkyl and heterocycloalkyl substituted with C 1-7 -alkyl; R 18 is hydrogen, halogen, C 1-7 -alkyl, C 1-7 -haloalkyl, hydroxy, C 1-7 -hydroxyalkyl, C 1-7 -alkoxy, C 1-7 -haloalkoxy, -NR 24 R 25, C 1-7 -alkyl-NR 24 R 25, aryl and heteroaryl, wherein aryl and heteroaryl are optionally substituted with one, two or three substituents selected from the list of halogen, cyano, C 1-7 -alkyl C 1-7 -haloalkyl, hydroxy, C 1-7 -alkoxy, -NR 24 R 25, C 1-7 -alkyl-NR 24 R 25, -CO-NH-(CH 2) r -NR 24 R 25, -CO-NH-(CH 2) o -OH, -CO-NH-(CH 2) o -heterocycloalkyl, -CO-OH, -O-C 1-7 -hydroxyalkyl, -O-(CH 2) r -CO-OH, -SO 2 -C 1-7 -alkyl, -SO 2 -NR 24 R 25, heterocycloalkyl, -O-heterocycloalkyl and heterocycloalkyl substituted with C 1-7 -alkyl or oxo; R 20 and R 21 are hydrogen; R 24 and R 25 are each individually selected from hydrogen and C 1-7 -alkyl; n is 2, 3, 4, 5 or 6; o is 0, 1, 2, 3, 4, 5, 6, 7 or 8; r is selected from 1, 2, 3 and 4; and Y is -CH 2 - or -CO-;