Department of Microbiology, City Hospital, Dudley Road, Birmingham B18 7QH, UK
Sir,
Ertapenem is a synthetic, parenteral, 1-ß methyl-carbapenem that is structurally related to ß-lactam antibiotics and has in vitro activity against Gram-positive and Gram-negative aerobic and anaerobic organisms.1 In this communication, we describe a method for assessing the susceptibility of ertapenem based on the British Society for Antimicrobial Chemotherapy (BSAC) standardized methods for determining MIC2 and disc susceptibility testing.3 Detection of ß-lactam resistance (including penicillins, cephalosporins, carbapenems and combinations of ß-lactam and ß-lactamase inhibitors) in staphylococci is best achieved by using methicillin, oxacillin or cefoxitin as an indicator antibiotic, not by testing the ß-lactam antibiotic that is to be used therapeutically.3 Staphylococci were therefore not included in the study.
A total of 893 organisms were studied (see Table 1), including appropriate ATCC and NCTC strains (Escherichia coli NCTC 10418; E.coli ATCC 25922; Haemophilus influenzae NCTC 11931; H. influenzae ATCC 49247; Streptococcus pneumoniae ATCC 49619; Bacteroides fragilis NCTC 9343). MIC determinations and disc susceptibility testing were undertaken using the BSAC methodology.24 Data were analysed using the BSAC susceptible breakpoints5 shown in Table 1.
|
The susceptibility of each organism was categorized by comparing the ertapenem MIC with the MIC range for the wild susceptible population. This information was then used to analyse the zone diameter data. Table 1 summarizes the MIC and zone diameter breakpoints (BPs) for each of the organism groups tested. MIC and zone diameter ranges for the control strains were: E.coli NCTC 10418, 0.0040.015 mg/L (3541 mm); E.coli ATCC 25922, 0.0080.03 mg/L (3539 mm); H. influenzae NCTC 11931, 0.060.25 mg/L (3038 mm); H. influenzae ATCC 49247, 0.251 mg/L (2534 mm); S. pneumoniae ATCC 49619, 0.060.25 mg/L (3540 mm); B. fragilis NCTC 9343, 0.120.5 mg/L (3539).
All of the E. cloacae, S. pneumoniae and B. fragilis with ertapenem MICs higher than the wild susceptible population were detected by disc susceptibility testing using the interpretative criteria recommended.
This study has shown that a 10 µg ertapenem disc is suitable for testing by BSAC methodology, and those organisms with reduced susceptibility can be detected using the criteria suggested. However, the large zones of inhibition observed for the susceptible population may preclude multiple disc testing on one plate.
Footnotes
* Corresponding author. Tel: +44-121-507-5693; Fax: +44-121-551-7763; E-mail: jenny.andrews{at}swbh.nhs.uk
References
1 . INVANZ annotated prescribing information. Merck & Co. Incorporated, Hoddesdon, UK.
2
.
Andrews, J. M. (2001). Determination of minimum inhibitory concentrations. Journal of Antimicrobial Chemotherapy 48, Suppl. S1, 516.
3
.
Andrews, J. M. (2001). BSAC standardized disc susceptibility testing method. Journal of Antimicrobial Chemotherapy 48, Suppl. S1, 4357.
4
.
King, A. (2001). Recommendations for susceptibility tests on fastidious organisms and those requiring special handling. Journal of Antimicrobial Chemotherapy 48, Suppl. S1, 7780.
5 . British Society for Antimicrobial Chemotherapy. (2003). BSAC disc diffusion method for antimicrobial susceptibility testing. Version 2.1.4 May 2003. [Online.] http://www.bsac.org.uk (29 July 2003, date last accessed).
|