Infection and Immunity, June 2000, p. 3210-3218, Vol. 68, No. 6
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Departamento de Microbiología, Universidad de Navarra, Pamplona, Spain1; Forschungszentrum Borstel, Borstel, Germany2; and Escuela de Medicina Veterinaria, Universidad Nacional, Heredia, Costa Rica3
Received 20 December 1999/Returned for modification 8 February 2000/Accepted 9 March 2000
The outer membrane (OM) of the intracellular parasite
Brucella abortus is permeable to hydrophobic probes and
resistant to destabilization by polycationic peptides and EDTA. The
significance of these unusual properties was investigated in a
comparative study with the opportunistic pathogens of the genus
Ochrobactrum, the closest known Brucella
relative. Ochrobactrum spp. OMs were impermeable to
hydrophobic probes and sensitive to polymyxin B but resistant to EDTA.
These properties were traced to lipopolysaccharide (LPS) because (i)
insertion of B. abortus LPS, but not of Escherichia coli LPS, into Ochrobactrum OM increased its
permeability; (ii) permeability and polymyxin B binding measured with
LPS aggregates paralleled the results with live bacteria; and (iii) the
predicted intermediate results were obtained with B. abortus-Ochrobactrum anthropi and E. coli-O. anthropi
LPS hybrid aggregates. Although Ochrobactrum was sensitive
to polymyxin, self-promoted uptake and bacterial lysis occurred without
OM morphological changes, suggesting an unusual OM structural rigidity.
Ochrobactrum and B. abortus LPSs showed no
differences in phosphate, qualitative fatty acid composition, or acyl
chain fluidity. However, Ochrobactrum LPS, but not B. abortus LPS, contained galacturonic acid. B. abortus and Ochrobactrum smooth LPS aggregates had similar size and
zeta potential (
12 to
15 mV). Upon saturation with polymyxin, zeta potential became positive (1 mV) for Ochrobactrum smooth
LPS while remaining negative (
5 mV) for B. abortus smooth
LPS, suggesting hindered access to inner targets. These results show
that although Ochrobactrum and Brucella share a
basic OM pattern, subtle modifications in LPS core cause markedly
different OM properties, possibly reflecting the adaptive evolution of
B. abortus to pathogenicity.
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