The use of norfloxacin 10 mcg as a screening disc to predict fluoroquinolone resistance in Streptococcus pneumoniae

Abstract number: 1134_03_199

Borén C., Smyth R.W., Hryniewicz W., Kahlmeter G., Sadowy E.


The use of sensitive screen discs for the detection of antibiotic class resistance is becoming increasingly important. A nalidixic acid disc (30 mcg) has been successfully used for the detection of fluoroquinolone (FQ) resistance in Enterobacteriaceae, Haemophilus spp and Neisseria spp. Cefpodoxime has been used for the detection of ESBL and most recently cefoxitin for the detection of MRSA. We compared the sensitivity of norfloxacin (10 mcg) and ciprofloxacin (1 and 5 mcg) discs for the detection of fluoroquinolone resistance in a collection of FQ sensitive and resistant Streptococcus pneumoniae.


A collection of S. pneumoniae strains with (from Poland) and without (from Sweden) fluoroquinolone resistance mechanisms were tested with antibiotic discs, norfloxacin 10 mcg and ciprofloxacin 1 and 5 mcg, from Oxoid UK. Mueller–Hinton Agar (Oxoid, UK) with 5% defibrinated sheep blood and IsoSensitest Agar (Oxoid,UK) with 5% defibrinated horse blood and NAD were used. Mueller–Hinton agar was investigated with confluent and semi-confluent inoculum while ISA was tested with a semi-confluent inoculum. Ciprofloxacin MIC-values (E-test, AB Biodisk, Sweden) were determined on ISA. All plates were incubated at 35–37 C in 5% carbon dioxide for 18–24 h before measuring the inhibition zones. Strains exhibiting ciprofloxacin MIC-values of 0.125–2.0 mg/L were considered part of the wild type MIC distribution as defined by the EUCAST epidemiological cut-off value (


Strains with MIC >= 32 mg/L were reliably detected with all methods and discs. Most difficult to reliably separate from the wild type distribution were strains with MIC 4.0 mg/L. Irrespective of agar and inoculum the norfloxacin 10 mcg disc more easily distinguished strains with MIC 4 mg/L from strains of <=2 mg/L. Ciprofloxacin 1 mcg was slightly inferior to the norfloxacin disc but much more reliable than ciprofloxacin 5 mcg.


Using methods based on NCCLS and BSAC/SRGA methodologies, norfloxacin 10 mcg and ciprofloxacin 1 mcg were preferable to ciprofloxacin 5 mcg in the detection of fluoroquinolone resistance in S. pneumoniae. We suggest that fluoroquinolone resistance surveillance is performed with one of these discs and that ciprofloxacin MIC-determination is performed on strains identified by the screen test.

Session Details

Date: 01/08/2007
Time: 00:00-00:00
Session name: XXIst ISTH Congress
Location: Oxford, UK
Presentation type:
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