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JAC Advance Access originally published online on November 15, 2005
Journal of Antimicrobial Chemotherapy 2005 56(6):1126-1129; doi:10.1093/jac/dki385
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© The Author 2005. 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

Staphylococcus aureus small colony variants are resistant to the antimicrobial peptide lactoferricin B

Ørjan Samuelsen1,2,*, Hanne Husom Haukland1,2, Barbara C. Kahl3, Christof von Eiff3, Richard A. Proctor4, Hilde Ulvatne1, Kjersti Sandvik1 and Lars H. Vorland2,5

1 Department of Medical Microbiology, University Hospital of North Norway, Tromsø, Norway; 2 Department of Microbiology and Virology, University of Tromsø, Tromsø, Norway; 3 Institute of Medical Microbiology, University Hospital of Münster, Münster, Germany; 4 Department of Medicine and Medical Microbiology/Immunology, University of Wisconsin Medical School, Madison, Wisconsin, USA; 5 Northern Norway Regional Health Authority, Bodø, Norway

Received 4 August 2005; returned 15 September 2005; revised 28 September 2005; accepted 28 September 2005

* Corresponding author. Tel: +47-77627043; Fax: +47-77627015; E-mail: orjan.samuelsen{at}unn.no

Objectives: To determine whether Staphylococcus aureus small colony variants (SCVs) are resistant to the antimicrobial peptide lactoferricin B. To assess if deficiency in transmembrane potential, a common characteristic of SCVs that are haemin- or menadione-auxotrophs, affects the uptake of the peptide into the bacterial cytoplasm.

Methods: A broth microdilution technique was used for susceptibility testing to determine the MIC of lactoferricin B for SCVs with three different auxotrophisms (haemin, menadione or thymidine) and their isogenic parent strains. Both clinical isolates and genetically defined mutants were used. The internalization of lactoferricin B in a hemB mutant and the respective parent strain was studied using transmission electron microscopy and immunogold labelling.

Results: All SCVs showed reduced susceptibility to lactoferricin B irrespective of their auxotrophy compared with their isogenic parent strains. The MIC for all SCVs was >256 mg/L, whereas the MICs for the parent strains ranged from 16–256 mg/L. Surprisingly, the hemB mutant contained significantly more lactoferricin B intracellularly than the respective parent strain.

Conclusions: The resistance mechanism of SCVs towards the antimicrobial peptide lactoferricin B is presumably caused by the metabolic changes present in SCVs rather than by a changed transmembrane potential of SCVs or reduced uptake of the peptide.

Keywords: cellular uptake , electron microscopy , immunolabelling , transmembrane potential , metabolic resistance , antimicrobial resistance , resistance mechanism


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