a Epidemic Intelligence Service, International Child Survival and Emerging Infections Program Support Activity, Division of Parasitic Diseases, National Center for Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia; b Epidemiology and Surveillance Division, National Immunization Program, Centers for Disease Control and Prevention, Atlanta, Georgia, USA and c Pneumococcal Diseases Research Unit, Department of Clinical Microbiology and Infectious Diseases, University of Witwatersrand and the South African Institute for Medical Research, Johannesburg, South Africa
Sir,
To strengthen global monitoring of antibiotic resistance rates among clinical isolates of Streptococcus pneumoniae and Haemophilus influenzae, the World Health Organization has drafted recommendations for surveillance in developing countries.1 One of the proposals is to replace the gold standard microbroth dilution (MBD) method with the Etest for the purpose of determining the MICs of isolates that are identified as resistant by the disc diffusion test; compared with the MBD method, the Etest is easier to perform and requires less expertise. The objectives of the present study were to evaluate the Etest in the field and to compare the results with those obtained with the MBD method.
Isolates of S. pneumoniae and H. influenzae were recovered from nasopharyngeal specimens collected from 371 ill children at two outpatient clinics in Bangui, Central African Republic (CAR) during a 1995 antimicrobial resistance survey.2 The strains were screened in the National Laboratory in Bangui for resistance to co-trimoxazole and either penicillin (S. pneumoniae isolates) or ampicillin (H. influenzae isolates) by the KirbyBauer disc diffusion method; a 1 µg oxacillin disc was used to detect resistance to penicillin. The medium used for determining the susceptibilities of the pneumococcal isolates was MuellerHinton agar (Oxoid, Basingstoke, UK) supplemented with either 5% sheep blood (for penicillin) or 5% lysed horse blood (for co-trimoxazole) and that used for susceptibility testing of the H. influenzae isolates was Haemophilus Sensitivity Test medium (Oxoid). MICs for all strains categorized as non-susceptible by the disc diffusion method were determined by the Etest method (AB Biodisk, Solna, Sweden) according to the manufacturer's instructions; the media used were the same as those used for disc testing and all plates were incubated overnight at 3537°C in an ambient atmosphere. These strains were also referred to the South African Institute for Medical Research in Johannesburg where MICs were determined by a MBD method recommended by the National Committee for Clinical Laboratory Standards (NCCLS).3 For all three methods, the isolates were categorized as susceptible, intermediate susceptible or resistant according to interpretative criteria of the NCCLS.3 If an MIC obtained with the Etest fell outside intervals covered by NCCLS criteria, the organism was assigned to a category that corresponded to the closest NCCLS interval.
Altogether, 272 strains of S. pneumoniae and 73 of H. influenzae were screened. The results obtained with the Etest agreed closely with those obtained with the MBD method (Table), concordance being observed for 88.6% and 87.2% of pneumococcal isolates in respect of penicillin and co-trimoxazole, respectively and for 100% of H. influenzae isolates in respect of co-trimoxazole; as only one isolate of H. influenzae exhibited reduced susceptibility to ampicillin, the MIC for this strain was not determined with the Etest. Minor errors4 associated with the use of the Etest were noted for 11 (12.5%) of 88 pneumococcal isolates tested for susceptibility to penicillin, for six (12.8%) of 47 pneumococcal isolates tested for susceptibility to cotrimoxazole and for none of 12 H. influenzae isolates tested for susceptibility to co-trimoxazole. There were no major or very major errors. The sensitivities and specificities of the Etest, in terms of identifying pneumococci with reduced susceptibilities, were 87.5 and 89.1%, respectively for penicillin and 94.1 and 90% respectively for co-trimoxazole; the corresponding values for H. influenzae and co-trimoxazole were 100 and 100% respectively. Overall resistance rates for the isolates, including those found to be susceptible by the disc diffusion method and therefore not requiring further testing, were similar, irrespective of whether they were based on MICs determined by the Etest or the MBD method, i.e. 10.3% versus 8.8% of pneumococci tested for susceptibility to penicillin, 7% versus 6.3% of pneumococci tested for susceptibility to co-trimoxazole and 12.3% versus 12.3% of H. influenzae isolates tested for susceptibility to co-trimoxazole.
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Notes
* Correspondence address. Centers for Disease Control and Prevention, Mailstop F22, 4770 Buford Highway, Atlanta, GA 30341-3724, USA. Tel: +1-770-488-3588; Fax: +1-770-488-4203; E-mail: axr9{at}cdc.gov
References
1 . World Health Organization. (1994). Draft version of the manual for the national surveillance of antimicrobial resistance of Streptococcus pneumoniae and Haemophilus influenzae: epidemiological and microbiological methods. WHO Programme for the Control of Acute Respiratory Infections.
2 . Centers for Disease Control and Prevention. (1997). Antibiotic resistance among nasopharyngeal isolates of Streptococcus pneumoniae and Haemophilus influenzaeBangui, Central African Republic, 1995. Morbidity and Mortality Weekly Report 46, 624.[Medline]
3 . National Committee for Clinical Laboratory Standards. (1994). Performance Standards for Antimicrobial Susceptibility TestingFifth Informational Supplement: Approved Standard M100-S5. NCCLS, Villanova, PA.
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6 . Jorgensen, J. H., Howell, A. W. & Maher, L. A. (1991). Quantitative antimicrobial susceptibility testing of Haemophilus influenzae and Streptococcus pneumoniae by using the E-test. Journal of Clinical Microbiology 29, 10914.[ISI][Medline]