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Antimicrobial Agents and Chemotherapy, June 2002, p. 1934-1939, Vol. 46, No. 6
0066-4804/02/$04.00+0     DOI: 10.1128/AAC.46.6.1934-1939.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Sulfonamide Resistance in Haemophilus influenzae Mediated by Acquisition of sul2 or a Short Insertion in Chromosomal folP

Virve I. Enne,^1, Anna King,² David M. Livermore,³ and Lucinda M. C. Hall^1*

Department of Medical Microbiology, Barts and The London School of Medicine and Dentistry, Queen Mary, University of London, London E1 2AD,¹ Department of Infection, KCL, St. Thomas' Hospital Campus, London SE1 7EH,² Antibiotic Resistance Monitoring and Reference Laboratory, Central Public Health Laboratory, London NW9 5HT, United Kingdom³

Received 6 August 2001/ Returned for modification 23 November 2001/ Accepted 16 March 2002

Determinants of sulfonamide resistance were investigated in clinical isolates of Haemophilus influenzae from the United Kingdom and Kenya. The mechanism of sulfonamide resistance in H. influenzae has not previously been reported. Eight isolates requiring at least 1,024 µg of sulfamethoxazole per ml for inhibition carried the sul2 gene, a common mediator of acquired sulfonamide resistance in enteric bacteria. In other isolates with similarly high levels of resistance, the chromosomal gene encoding dihydropteroate synthase, folP, was found to carry an insertion of 15 bp together with other missense mutations relative to folP of H. influenzae strain Rd RM118 (MIC, 8 µg/ml); the folP sequence was identical in all seven such isolates investigated, although they represented three different strains by restriction pattern analysis. The 15-bp insertion was absent in isolates inhibited by sulfamethoxazole at 2 to 64 µg/ml (although these exhibited considerable divergence in folP sequence) and in highly resistant isolates carrying sul2. Transformation with a 599-bp fragment of folP containing the insertion but no other differences conferred high-level resistance on a recipient strain, confirming the role of the insertion. Other amino acid substitutions in dihydropteroate synthase may modulate the level of sulfonamide inhibition in susceptible isolates and those with more moderate levels of resistance. The two mechanisms of resistance, mediated by sul2 and modified folP, were detected in isolates from both the United Kingdom and Kenya.

Present address: Department of Pathology and Microbiology, University of Bristol, Bristol BS8 1TD, United Kingdom.