Influence of testing methodology on the tigecycline activity profile against presumably tigecycline-non-susceptible Acinetobacter spp.
OBJECTIVE
To compare the tigecycline activity profile against Acinetobacter spp. by Etest versus broth microdilution in isolates with high Etest MIC.


METHODS
Acinetobacter spp. isolates with tigecycline MICs of >or=0.5 mg/L determined by commercially developed Etests strips (January 2006 to July 2007) in five Spanish hospitals were considered. Values were rounded to the nearest upper double-dilution. Susceptibility by broth microdilution following CLSI (formerly NCCLS) recommendations, as the reference method, was determined in a central laboratory. BSAC breakpoints were used: susceptible <or=1 mg/L; intermediate = 2 mg/L; and resistant >2 mg/L.


RESULTS
One hundred and forty-eight isolates were collected: 12 isolates with a tigecycline Etest MIC of 0.5 mg/L, 14 with 1 mg/L, 86 with 2 mg/L, 31 with 4 mg/L and 5 with 8 mg/L. Isolates with Etest MICs of 0.5-1 mg/L showed the same values by broth microdilution. Among isolates with Etest MICs of 2 mg/L, only 5.8% of strains showed the same value by both methods (88.4% showed values that were one or two dilutions lower by microdilution). None of the 36 isolates with Etest MICs of 4-8 mg/L showed the same value by both methods, with values at least two dilutions lower by microdilution. Weak correlation (R = 0.238; P <or= 0.001) was found between both methods. All 26 Etest susceptible isolates, 80/86 (93.0%) Etest intermediate and 32/36 (88.9%) Etest resistant strains were susceptible by microdilution.


CONCLUSIONS
Caution should be taken in interpreting Etest MICs of >or=2 mg/L for Acinetobacter spp. since strains with Etest MICs of 2-4 mg/L are susceptible when tested by microdilution. False non-susceptibility by Etest may exclude tigecycline as a therapeutic option in a field where multiresistance is the rule.
Casal
M
M
Microbiology Department, Hospital Universitario Reina Sofía, Cordoba, Spain.
Rodríguez
F
F
Johnson
B
B
Garduno
E
E
Tubau
F
F
de Lejarazu
R Ortiz
RO
Tenorio
A
A
Giménez
M J
MJ
Bartolomé
R
R
Garcia-Rey
C
C
Aguilar
L
L
García-Escribano
N
N
D000150:Acinetobacter / Q000187:drug effects* / Q000302:isolation & purification; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D003951:Diagnostic Errors; D024881:Drug Resistance, Bacterial; D006761:Hospitals; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods*; D008911:Minocycline / Q000031:analogs & derivatives* / Q000494:pharmacology; D013030:Spain; D000078304:Tigecycline
D000900:Anti-Bacterial Agents; D000078304:Tigecycline; D008911:Minocycline