The use of cefadroxil in primary susceptibility testing to screen for cephalosporin resistance in Enterobacteriaceae, including ESBLs

Abstract number: 1134_01_41

Smyth R.W., Borén C., Kahlmeter G


Bacterial resistance to second and third generation cephalosporins, including ESBLs, is an increasing problem even in the community. Within Enterobacteriaceae a number of different betalactam resistance mechanisms have evolved over recent years. TEM and SHV mutants, and CTX-M types, collectively known as ESBLs, have emerged. A low strength cefpodoxime disc is probably the best method for sensitive (but non-specific) detection of strains with these mechanisms. The alternative would be to use both cefotaxime and ceftazidime. However, either strategy would for most laboratories require the use of an extra plate in Enterobacteriaceae from community acquired urinary tract infections (CA-UTI) and most laboratories have not considered this to be worthwhile. Recent epidemics with CTX-M:s in the community emphasise the need for this procedure. We describe the use of a first generation cephalosporin as a surrogate sensitive but non-specific screen method for ESBLs. This has the advantage of being a valid alternative in the treatment of uncomplicated CA-UTI.


Disc diffusion with antibiotic discs from Oxoid UK, semi-confluent inoculum and overnight incubation at 35–37 C on IsoSensitest Agar (Oxoid, UK). Antibiotics, zone breakpoints and disc strengths are given in the Table. 52 strains (E. coli, Klebsiellae, C. freundii and E. cloacae), each showing one or more resistance mechanism, were tested. 33 strains were ESBL producers of which 21 of type CTX-M, nine of SHV type, two of TEM type and one with an unidentified ESBL. The remaining strains consisted of ten with AmpC, three K1 hyperproducers, three showing cefuroxime resistance and three of Imp+ type.


  Number of strains correctly identified by
nCDR 30 mcg <14 mmCAZ 10 mcg <23 mmCTX 5 mcg <24 mmCPO 10 mcg <21 mm 
All strains5245354849
ESBL strains3329233233
ESBL of CTX-M type2121112121


Cefpodoxime identified 49 of 52 Enterobacteriaceae with cephalosporin resistance, including all ESBL strains. Cefadroxil detected 45 of 52 Enterobacteriaceae with cephalosporin resistance including all 21 of the ESBL producers of CTX-M type. A low content cefotaxime disc detected 92.3% of all resistant strains including 96.9% of ESBL producers. Ceftazidime 10 mcg disc detected only 67.3% overall and was not helpful in detection of CTX-M resistance. However, a combination of cefadroxil and ceftazidime would have identified all 33 ESBL producers in this collection.

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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