Differential expression of CTX-M-15 beta-lactamase between two major Escherichia coli strains in the UK

Abstract number: p508

Karisik  E., Ellington  M.J., Pike  R., Livermore  D.M., Woodford  N.


E. coli with CTX-M beta-lactamases are a major problem in the UK, with several CTX-M-15-producing outbreak strains as well as many sporadic producers. One outbreak strain (A) is widespread across England, and a second (D) is prevalent only at a single centre, where A is also frequent. Strain A generally has lower-level cephalosporin resistance than strain D and we investigated the basis of this difference.


Three representatives each of strains A and D were investigated; each strain A representative was from a different centre. Plasmids were transformed into E. coli DH5alfa. MICs were determined by the British Society for Antimicrobial Chemotherapy method. Culture sonicates were used for isoelectric focusing and to assay cefotaximase specific activity. RNA was extracted from clinical isolates and used for reverse-transcriptase (RT) PCR to assess expression of blaCTX-M-15. The region upstream of blaCTX-M-15 was sequenced in strain A representatives.


Two typical strain A isolates (A1 and A3) required lower cefotaxime (16–32 mg/L vs. >64 mg/L) and ceftazidime (2–4 mg/L vs. 32–64 mg/L) MICs compared with the three strain D isolates (D1–D3) and with the third strain A representative (A2). Cefotaximase specific activity was up to 15-fold lower in strain A1 and A3 than in D1, but only 2-fold lower in the case of A2. CTX-M-15-producing transformants derived from all three strain A representatives (including A2) required lower cephalosporin MICs compared with those derived from the strain D representatives, and all had significantly lower cefotaximase specific activity. CTX-M-15 was hardly detectable by electrofocusing in isolates A1 and A3 but was obvious in A2 and in D1–D3; an OXA-1 band was equally intense in all six isolates. Similarly, RT-PCR showed a blaCTX-M-15 band of lower intensity for isolates A1 and A3 than for A2 or D1–D3, whilst the blaOXA-1 band was equally intense in all. All three strain A isolates had an IS26 element between blaCTX-M-15 and its normal promoter, provided by ISEcp1; DNA sequencing indicated no differences immediately upstream of blaCTX-M-15 in A1–A3.


Expression of CTX-M-15 beta-lactamase in strain A was generally lower than in strain D, explaining the lower cephalosporin resistance. IS26, between blaCTX-M-15 and its normal promoter, may be responsible for this lower expression but, it also appears that trans-acting factors may up-regulate CTX-M-15 expression, as in isolate A2.

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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