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Development and Structure |
The Bacterial STD Branch, Centers for Disease Control and Prevention, Mailstop D-13, 1600 Clifton Rd, Atlanta, GA 30333, USA1
The Center for Microbial Pathogenesis2 and the Departments of Medicine3 and Genetics and Developmental Biology4, the University of Connecticut Health Center, Farmington, CT 06030, USA
Author for correspondence: David L. Cox. Tel: +1 404 639 3446. Fax: +1 404 639 3976. e-mail: dlc6{at}cdc.gov
The authors examined the ability of octadecanoyl (C18), hexadecanoyl (C16) and dodecanoyl (C12) fatty acid (FA) conjugates of 5-aminofluorescein (OAF, HAF and DAF, respectively) to insert into the outer membranes (OMs) of Treponema pallidum, Borrelia burgdorferi and Escherichia coli. Biophysical studies have demonstrated that these compounds stably insert into phospholipid bilayers with the acyl chain within the hydrophobic interior of the apical leaflet and the hydrophilic fluorescein moiety near the phospholipid head groups. Consistent with the known poor intrinsic permeability of the E. coli OM to hydrophobic compounds and surfactants, E. coli was not labelled with any of the FA probes. OAF inserted more readily into OMs of B. burgdorferi than into those of T. pallidum, although both organisms were completely labelled at concentrations at or below 2 µg ml-1. Intact spirochaetes were labelled with OAF but not with antibodies against known periplasmic antigens, thereby confirming that the probe interacted exclusively with the spirochaetal OMs. Separate experiments in which organisms were cooled to 4 °C (i.e. below the OM phase-transition temperatures) indicated that labelling with OAF was due to insertion of the probe into the OMs. B. burgdorferi, but not T. pallidum, was labelled by relatively high concentrations of HAF and DAF. Taken as a whole, these findings support the prediction that the lack of lipopolysaccharide renders T. pallidum and B. burgdorferi OMs markedly more permeable to lipophilic compounds than their Gram-negative bacterial counterparts. The data also raise the intriguing possibility that these two pathogenic spirochaetes obtain long-chain FAs, nutrients they are unable to synthesize, by direct permeation of their OMs.
Keywords: flow cytometry, fatty acid metabolism, membrane fluidity
Abbreviations: DAF, dodecanoylaminofluorescein; FA, fatty acid; HAF, hexadecanoyl-aminofluorescein; MFI, mean fluorescence intensity; OAF, octadecanoylaminofluorescein; OM, outer membrane; OspA, outer surface protein A; TpCM, T. pallidum cultivation medium
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