Inoculum effect of b-lactams and killing curves determination in AmpC-producing (DHA-1) Klebsiella pneumoniae isolates

Abstract number: P1052

Cerna H., Bergerova T., Urbaskova P., Hrabak J.

Objectives: An inoculum size is a critical point in treatment of infections caused by extended-spectrum b-lactamase- and AmpC-producing bacteria. A treatment failure was described in many cases of infections due to these bacteria. An inoculum effect determined by standard dilution method is not an appropriate model of the infection process in organism. The aim of this study is to compare the inoculum effect determination on b-lactams by a standard microdilution method and the description of dynamic of bacterial population of AmpC-producing Klebsiella pneumoniae after addition these antibiotics.

Methods: Four DHA-1 producing Klebsiella pneumoniae strains (two inducible and two with high level of expression) were used in this study. MIC and inoculum effect was determined by a classical microdilution broth method (CLSI). Dynamic of bacterial population – bacterial count was measured by direct plating samples after addition antibiotic in brain-heart infusion.

Results: Using standard inoculum (105 CFU per ml), the strains producing inducible AmpC were determined as non-resistant to cefotaxime (CTX) (MIC = 1 mg/l), ceftazidime (CAZ) (MIC = 4 mg/ml) and cefepime (FEP) (MIC = 2 mg/l) according to EUCAST criteria. When the inoculum was 106 CFU/ml, the strains were resistant to all tested b-lactams (e.g. MIC of CTX was 256 mg/ml, MICs of CAZ and FEP were 1024 mg/ml). The strain with a high-level expressed AmpC was not resistant only to cefepime (MIC = 2 mg/l). MIC of CTX was 64 mg/l and of CAZ 128 mg/l. Killing curves showed different times required for an inactivation of antibiotics. For the strains producing inducible AmpC, decreasing count of bacteria were observed in CTX for 6 hrs in a concentration of 1 mg/l, at least for 7 hrs in a concentration of 2 mg/l, in CAZ for 3 hrs in a concentration of 1 mg/ml, for 6 hrs in a concentration of 8 mg/l and in FEP for 3 hrs in a concentration of 1 mg/l and at least for 7 hrs in a concentration of 8 mg/l.

Conclusions: This work showed that the time required for hydrolyzing antibiotics by b-lactamases under the MIC is shorter than the time recommended for MIC reading by standard method. Therefore the data obtained using microdilution method do not exactly described the situation in the organism and the inoculum effect could be misinterpreted due to lack of antibiotic in a well of microtiter plate.

This work was supported by a research project MSM 0021620819.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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