1. Disc susceptibility testing for Burkholderia cepacia complex
50th ICAAC
Sept , 2010
Boston
Disc susceptibility testing for Burkholderia cepacia complex
K PITMAN, M WOOTTON, L DAVIES, VE DANIEL, RA HOWE
Introduction
Burkholderia cepacia complex (Bcc) is a group of nine closely related environmental “species” that can cause serious infections in cystic fibrosis. While CLSI has specific criteria for Bcc disc testing for a limited number of agents, users of BSAC or EUCAST methods would normally use “pseudomonas” criteria. This study examines the reliability of these disc criteria.
Methods (cont)
Respective criteria were used to categorise MIC and zone data as Sensitive/ Intermediate/ Resistant (S/I/R). S/I/R categories from disc results were compared to categorical results from ISO MIC testing and assigned errors defined as minor (MIN) (ISO S or I reported as I or R by disc) major (MAJ) (ISO I or R reported as S or I by disc),or very major (V MAJ) (ISO R reported as S by disc).
Figure 2: MIC vs zone diameter data for piperacillin/tazobactam
Results
Percentage errors for each disc method are shown in Table 2. Significant errors were found for most antimicrobials tested for each method.
Choice of reference MIC method influences apparent performance of each method, as MICs can be very different according to method (Figure 1)
Methods
An international set of 100 genetically characterised Bcc including all clinically important genomovars was tested (Table 1).
Table 1: Genomovars tested
Table 2: Percentage errors of each disc method
BSAC (75/10 mcg disc)
EUCAST (30/6 mcg disc)
Results (cont)
Performance was particularly poor for piperacillin/tazobactam; Figure 2 shows a poor relationship between MIC and zone diameter for this agent.
Reference susceptibility results were established by the ISO microbroth MIC method using Mueller-Hinton broth and the BSAC agar dilution MIC method using Isosensitest agar. Disc testing was performed in triplicate by BSAC (Isosensitest agar) and EUCAST methods (Mueller-Hinton agar) against meropenem (MEM), ceftazidime (CAZ), amikacin (AMI), piperacillin/tazobactam (PTZ), and ciprofloxacin (CIP). Zone diameters were measured using the BIOMIC Microbiology Video Reader System (Giles Scientific, New York, USA).
Conclusions
Disc testing using EUCAST or BSAC methods and pseudomonas criteria gives unreliable results. Analysis of MIC/zone distributions suggests that disc testing may not be appropriate for this organism.
Figure 1: MIC distributions by different methods 12
Travel grant kindly funded by:
Specialist Antimicrobial Chemotherapy Unit, Public Health Wales, Cardiff, UK