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JAC Advance Access originally published online on February 1, 2008
Journal of Antimicrobial Chemotherapy 2008 61(3):509-514; doi:10.1093/jac/dkm523
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© The Author 2008. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Original research

Characterization of extended-spectrum β-lactamase-producing isolates of Haemophilus parainfluenzae

Stephen G. Tristram1,*, Marthinus J. Pitout2, Karen Forward3, Sarah Campbell4, Scott Nichols1 and Ross J. Davidson3,4

1 School of Human Life Sciences, University of Tasmania, Launceston, Tasmania 7250, Australia 2 University of Pretoria, Pretoria, South Africa 3 Dalhousie University, Halifax, Canada 4 Queen Elizabeth II Health Sciences Centre, Halifax, Canada

Received 9 September 2007; returned 28 November 2007; revised 20 November 2007; accepted 10 December 2007

* Corresponding author. Tel: +61-3-63-243323; Fax: +61-3-63-243658; E-mail: stephen.tristram{at}utas.edu.au

Objectives: To characterize the β-lactam resistance mechanisms of two clinical isolates of cefotaxime-resistant Haemophilus parainfluenzae recovered from patients in South Africa.

Methods: The relatedness of isolates and plasmids was assessed using PFGE and restriction enzyme analysis, respectively. Plasmid-mediated and chromosomally integrated blaTEM genes and ftsI genes were sequenced, and the plasmid-mediated blaTEM-15 was used to transform a range of control organisms.

Results: The two isolates were found to be unique according to PFGE, but had an identical 3.7 kb plasmid encoding a TEM-15 β-lactamase. Both isolates also had substitutions in penicillin binding protein 3 (PBP3) consistent with substitutions known to exist in β-lactamase-negative ampicillin-resistant (BLNAR) strains of Haemophilus influenzae. The cefotaxime MICs for control strains of H. influenzae, H. parainfluenzae and BLNAR H. influenzae transformed with the plasmid-mediated blaTEM-15 were 1.0, 1.0 and 4.0 mg/L, respectively, compared with 16.0 and 8.0 mg/L, respectively, for the two parent H. parainfluenzae.

Conclusions: The high-level cefotaxime resistance in the H. parainfluenzae isolates was due to a combination of a plasmid-mediated TEM-15 extended-spectrum β-lactamase with altered PBP3 probably contributing. Other contributing resistance mechanisms could not be excluded.

Keywords: plasmid , ftsI , TEM , ESBL , PBP3 , promoter , penicillin binding protein


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