JAC Advance Access originally published online on December 11, 2007
Journal of Antimicrobial Chemotherapy 2008 61(2):465-466; doi:10.1093/jac/dkm473
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Letters to the Editor |
Comment on: Acinetobacter spp. and time–kill studies
Division of Infectious Diseases, Department of Internal Medicine, Korea University College of Medicine, Seoul, Republic of Korea
* Corresponding author. Tel: +82-2-2626-3050; Fax: +82-2-866-1643; E-mail: heejinmd{at}medimail.co.kr
Keywords: Acinetobacter baumannii , killing kinetics , antimicrobial interactions
We thank you for the opportunity to reply to the comments by Lopardo1 on our recent article.2
First, as for the concentration of imipenem and colistin (each at 1x MIC), there was no difference in the concentration between microdilution and time–kill studies; antibiotic dosage was adjusted according to the volumes of bacterial suspensions. It should be mentioned that substantial regrowth of Acinetobacter baumannii isolates at 24 h using 1x MIC or even higher antibiotic concentrations has been also described by Owen et al.3
With regard to the titres in the time–kill assay (Figures 1 and 2 in Song et al.2), Lopardo points out that most Acinetobacter spp. isolates reach not more than 109 cfu/mL in the stationary phase. In our opinion, however, there was no chance of carry-over while undertaking 10-fold dilutions. We undertook 10-fold dilutions cautiously to avoid carry-over and repeated experiments if the results did not show sequential, 10-fold decreases in colony counts with the serial dilutions. We are not sure why there is such a difference between previous reports and ours. We hypothesize that this might be due to conditions such as the small bacterial size, the negative post-antibiotic effect and structural changes related to the fitness cost of multidrug resistance.4 Previously, James et al.4 reported that Acinetobacter spp. were reverted to small coccoid forms (
1 µm in diameter) from bacillary morphology in the low-nutrient stationary phase, which were also aggregated. On the assumption that Acinetobacter isolates were in the small coccoid form (volume of one bacterial cell 
0.2 µm3) of low-nutrient stationary phase, the concentrations of bacterial suspensions would reach 1013–1014 cfu/mL. However, further studies would be required to justify our high titres in time–kill assays.
Finally, we did not describe the MICs of rifampicin and sulbactam in the time–kill synergy tests, considering that they were in the non-susceptible ranges. The MICs of rifampicin were 4 mg/L in four cases (second, fourth, seventh and eighth isolates) and 8 mg/L in the others. In the case of sulbactam, the MICs in most cases were 32 mg/L except one (second isolate), which showed intermediate sulbactam resistance (MIC of 16 mg/L). On the assumption that imipenem and colistin were more active against multidrug-resistant A. baumamii isolates compared with sulbactan and rifampicin, respectively, individual time–kill curves of sulbactam and rifampicin were not taken, which might be the limitation of our study.
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1 Lopardo HA. Acinetobacter spp. and time–kill studies. J Antimicrob Chemother (2007) 61:464.
2
Song JY, Kee SY, Hwang IS, et al. In vitro activities of carbapenem/sulbactam combination, colistin, colistin/rifampicin combination and tigecycline against carbapenem-resistant Acinetobacter baumannii. J Antimicrob Chemother (2007) 60:317–22.
3
Owen RJ, Li J, Nation RL, et al. In vitro pharmacodynamics of colistin against Acinetobacter baumannii clinical isolates. J Antimicrob Chemother (2007) 59:473–7.
4
James GA, Korber DR, Caldwell DE, et al. Digital image analysis of growth and starvation responses of a surface-colonizing Acinetobacter sp. J Bacteriol (1995) 177:907–15.
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