Insertion of fluorescent fatty acid probes into the outer membranes of thepathogenic spirochaetes Treponema pallidum and Borrelia burgdorferi

The authors examined the ability of octadecanoyl (C-18), hexadecanoyl (C-16 ) and dodecanoyl (C-12) fatty acid (FA) conjugates of 5-aminofluorescein (O AF, 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 phospholi pid bilayers with the acyl chain within the hydrophobic interior of the api cal leaflet and the hydrophilic fluorescein moiety near the phospholipid he ad 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. burgdor feri than into those of T, pallidum, although both organisms were completel y labelled at concentrations at or below 2 mug ml(-1). Intact spirochaetes were labelled with OAF but not with antibodies against known periplasmic an tigens, thereby confirming that the probe interacted exclusively with the s pirochaetal OMs, Separate experiments in which organisms were cooled to 4 d egreesC (i.e. below the OM phase-transition temperatures) indicated that la belling with OAF was due to insertion of the probe into the OMs, B, burgdor feri, but not T, pallidum, was labelled by relatively high concentrations o f 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 ba cterial counterparts. The data also raise the intriguing possibility that t hese two pathogenic spirochaetes obtain long-chain FAs, nutrients they are unable to synthesize, by direct permeation of their OMs.