Very high resistance to amoxicillin in Streptococcus pneumoniae: an epidemiological fact or a technical issue?
To explore reproducibility of high amoxicillin minimum inhibitory concentrations (MIC(AMX) ), isolates received during 2002 and 2003 in the National Reference Laboratory of Streptococcus pneumoniae with an amoxicillin MIC of 16 microg/ml (43 strains) and 8 g/ml (12 strains) when singly determined on a routine basis in this center by agar dilution, were retested 10 times by agar dilution and microdilution following NCCLS guidelines, not only using double dilutions but also dilution steps of 2 microg/ml (i.e, 2, 4, 6, 8, 10, 12, 14 and 16 microg/ml). A significant (p<0.05) shift to a higher MIC(AMX )was obtained with microdilution vs. agar dilution. Routine MIC(AMX )of 16 microg/ml were confirmed in 0 strains by agar dilution and in 6 by microdilution, when retested. These 6 strains presented a modal MIC(AMX )value of 10 microg/ml (5 cases) and of 14 micro g/ml (1 case) when using 2 microg/ml microdilution steps. There is low reproducibility of the highest MIC(AMX )values.
Camacho
M-T
MT
Spanish Reference Pneumococcal Laboratory, Centro Nacional de Microbiología, Instituto de Salud Carlos III, Ctra. Majadahonda-Pozuelo Km. 2, 28220 Majadahonda, Madrid.
Casal
J
J
Aguilar
L
L
Gimenez
M-J
MJ
Fenoll
A
A
D000658:Amoxicillin / Q000494:pharmacology*; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D015169:Colony Count, Microbial; D004305:Dose-Response Relationship, Drug; D024881:Drug Resistance, Bacterial; D008826:Microbial Sensitivity Tests; D015203:Reproducibility of Results; D013296:Streptococcus pneumoniae / Q000187:drug effects* / Q000302:isolation & purification
D000900:Anti-Bacterial Agents; D000658:Amoxicillin