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Antimicrobial Agents and Chemotherapy, November 2008, p. 3922-3927, Vol. 52, No. 11
0066-4804/08/$08.00+0     doi:10.1128/AAC.00341-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Role of ampD Homologs in Overproduction of AmpC in Clinical Isolates of Pseudomonas aeruginosa{triangledown}

Amber J. Schmidtke and Nancy D. Hanson*

Department of Medical Microbiology and Immunology and Center for Research in Anti-Infectives and Biotechnology, Creighton University School of Medicine, Omaha, Nebraska

Received 11 March 2008/ Returned for modification 16 May 2008/ Accepted 29 August 2008

AmpD indirectly regulates the production of AmpC β-lactamase via the cell wall recycling pathway. Recent publications have demonstrated the presence of multiple ampD genes in Pseudomonas aeruginosa and Escherichia coli. In the prototype P. aeruginosa strain, PAO1, the three ampD genes (ampD, ampDh2, and ampDh3) contribute to a stepwise regulation of ampC β-lactamase and help explain the partial versus full derepression of ampC. In the present study, the roles of the three ampD homologs in nine clinical P. aeruginosa isolates with either partial or full derepression of ampC were evaluated. In eight of nine isolates, decreased RNA expression of the ampD genes was not associated with an increase in ampC expression. Sequence analyses revealed that every derepressed isolate carried mutations in ampD, and in two fully derepressed strains, only ampD was mutated. Furthermore, every ampDh2 gene was of the wild type, and in some fully derepressed isolates, ampDh3 was also of the wild type. Mutations in ampD and ampDh3 were tested for their effect on function by using a plasmid model system, and the observed mutations resulted in nonfunctional AmpD proteins. Therefore, although the sequential deletion of the ampD homologs of P. aeruginosa can explain partial and full derepression in PAO1, the same model does not explain the overproduction of AmpC observed in these clinical isolates. Overall, the findings of the present study indicate that there is still an unknown factor(s) that contributes to ampC regulation in P. aeruginosa.

* Corresponding author. Mailing address: Department of Medical Microbiology and Immunology, Creighton University, 2500 California Plaza, Omaha, NE 68178. Phone: (402) 280-5837. Fax: (402) 280-1875. E-mail: ndhanson{at}creighton.edu

{triangledown} Published ahead of print on 8 September 2008.

Antimicrobial Agents and Chemotherapy, November 2008, p. 3922-3927, Vol. 52, No. 11
0066-4804/08/$08.00+0     doi:10.1128/AAC.00341-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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