Staphylococcus aureus small colony variants are resistant to the antimicrobial peptide lactoferricin B.
OBJECTIVE
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.
Samuelsen
Orjan
O
Department of Medical Microbiology, University Hospital of North Norway, Tromsø, Norway. orjan.samuelsen@unn.no
Haukland
Hanne Husom
HH
Kahl
Barbara C
BC
von Eiff
Christof
C
Proctor
Richard A
RA
Ulvatne
Hilde
H
Sandvik
Kjersti
K
Vorland
Lars H
LH
D000900:Anti-Bacterial Agents / Q000378:metabolism / Q000494:pharmacology; D003593:Cytoplasm / Q000737:chemistry; D024881:Drug Resistance, Bacterial; D006427:Hemin / Q000235:genetics; D007781:Lactoferrin / Q000378:metabolism / Q000494:pharmacology*; D008564:Membrane Potentials; D008826:Microbial Sensitivity Tests; D046529:Microscopy, Electron, Transmission; D016253:Microscopy, Immunoelectron; D009154:Mutation; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000235:genetics / Q000254:growth & development; D013936:Thymidine / Q000235:genetics; D024483:Vitamin K 3 / Q000378:metabolism
D000900:Anti-Bacterial Agents; C082171:lactoferricin B; D024483:Vitamin K 3; D006427:Hemin; D007781:Lactoferrin; D013936:Thymidine