Table 2

Antimicrobial susceptibilities of isolates producing CTX-M-type ESBL from mastitic cows

Antimicrobial agentaGroup 1Group 2Group 3
MIC range (μg/ml)MIC50 (μg/ml)MIC90 (μg/ml)Breakpoint (μg/ml; mm)bNo. of isolates (%)MIC range (μg/ml)No. of resistant isolates (%) (B)hMIC range (μg/ml)No. of resistant isolates (%) (C)h
SusceptibleIntermediateResistant (A)h
Ampicilline>512>512>512≥320049 (100)>51210 (100)>5126 (100)
Cefazoline>256>256>256≥320049 (100)>25610 (100)>2566 (100)
Cefuroximee>256>256>256≥32c0049 (100)>25610 (100)>2566 (100)
Ceftazidime≤2 to 64≤2≤2≥16c45 (91.8)3 (6.1)1 (2.0)**B16–6410 (100)**A,C≤20**B
CAZ/CLA≤0.25 to >8≤0.250.50.5 to >8≤0.25 to 0.5
Cefotaxime≤16 to 25664128≥4c-g4 (8.2)g45 (91.8)512 to >51210 (100)128 to >5126 (100)
CTX/CLA≤1 to >8≤1≤1≤1 to 8≤1
Cefpodoxime≤8 to >64>64>64≥161 (2.0)048 (98.0)>6410 (100)>646 (100)
Ceftriaxone64 to >512512>512≥4c0049 (100)>51210 (100)512 to >5126 (100)
Ceftiofure64 to >512>512>512≥80049 (100)>51210 (100)>5126 (100)
Cefquinomee16 to >128128>128d>128>128
Cefepime≤8 to 64≤8≤8≥32c43 (87.8)3 (6.1)3 (6.1)**B,C>6410 (100)**A≤8 to >644 (66.6)**A
Cefmetazole≤4 to >32≤4≤4≥64c47 (95.9)02 (4.1)≤4 to 320≤4 to >321 (16.6)
Moxalactam≤8 to >32≤8≤8≥64c48 (98.0)01 (2.0)≤80≤80
Imipenem≤1≤1≤1≥1649 (100)00≤10≤10
Meropenem≤2 to 4≤2≤2≥4c48 (98.0)01 (2)≤20≤20
Aztreonam≤8 to >64≤816≥16cg43 (87.8)g6 (12.2)**B, *C64 to >6410 (100)**A,*C≤8 to 323 (50)*A,B
Gentamicine≤2 to >16≤2≤2≥1646 (93.9)d3 (6.1)≤2 to >161 (10.0)≤20
Amikacin≤4 to >16≤4≤4≥6448 (98.0)1 (2.0)g≤4 to 80≤40
OTETe≤4 to >16≤4>16≥1624 (49.0)11 (22.4)14 (28.6)*B≤4 to >167 (70.0)*A≥166 (100)
SXTe≤0.5/9.5 to >4/76≤0.5/9.54/76≥4/7642 (85.7)d7 (14.3)**B,*C4/76 to >4/767 (70.0)**A≤0.5/9.5 to >4/763 (50)*A
Enrofloxacine≤0.25 to 2≤0.252≥245 (91.8)2 (4.1)2 (4.1)≤0.25 to >21 (10.0)≤0.25 to 0.50
Ciprofloxacin≤0.5 to 2≤0.5≤0.5≥4c46 (93.9)3 (6.1)0*B≤0.5 to >21 (10.0)*A≤0.5 to 10
Cefoxitinf≤14 mmc48 (98.0)1 (2.0)0*B,**CS to R1 (10.0)*AS to R1 (16.6)**A
Kanamycine,f≤13 mm38 (77.6)1 (2.0)10 (20.4)*B, CS to R5(50.0)*AS to R4 (66.6)*A
CHLf≤13 mm41 (83.7)08 (16.3)S to R4 (40.0)S to R3 (50)
Levofloxacinf≤13 mmc49 (100)00*BS to R1(10.0)*AS0
  • a Assessed by the broth microdilution or disk diffusion method for 41 K. pneumoniae and 6 K. oxytoca isolates producing CTX-M-2 and 2 K. pneumoniae isolates producing CTX-M-14 (group 1), 10 E. coli isolates producing CTX-M-15 (group 2), and other Enterobacteriaceae isolates producing CTX-M-2 or CTX-M-14 (group 3; CTX-M-2: E. coli, n = 1; C. koseri, n = 2; E. aerogenes, n = 1; E. coli, CTX-M-14, n = 2). Abbreviations: CAZ/CLA, ceftazidime/clavlanic acid; CTX, cefotaxime; OTET, oxytetracycline; SXT, trimethoprim/sulfamethoxazol; CHL, chloramphenicol.

  • b Breakpoint for resistance in accordance with CLSI document M31-A3 (2008) (15) for veterinary pathogens.

  • c Breakpoint for resistance in accordance with CLSI document M100-S21 (2011) (20) for isolates from human infections with Enterobacteriaceae.

  • d Breakpoint for resistance or intermediate is not defined in CLSI documents M31-A3 (2008) (15) and M100-S21 (2011) (20).

  • e Antimicrobial agent approved for cattle in Japan; the other 17 antimicrobials in this table are unapproved for cattle.

  • f Susceptibilities were tested by disc diffusion method according to CLSI documents M31-A3 (2008) (15) and M100-S21 (2011) (20).

  • g Isolates in the susceptible, intermediate, and resistant categories could not be differentiated.

  • h Significant differences were determined by the χ2 test for comparison with the group indicated by the capital letter (A, B, or C): *, P < 0.05; **, P < 0.01.