Journal of Bacteriology, August 1999, p. 5024-5032, Vol. 181, No. 16
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
-Glucosides

Grupo de Patogénesis Molecular Bacteriana, Unidad de Microbiología e Inmunología, Facultad de Veterinaria, Universidad Complutense, 28040 Madrid, Spain,1 and Lehrstuhl für Mikrobiologie, Theodor- Boveri-Institut für Biowissenschaften, Universität Würzburg, 97074 Würzburg, Germany2
Received 16 March 1999/Accepted 31 May 1999
The
-glucoside cellobiose has been reported to specifically
repress the PrfA-dependent virulence genes hly and
plcA in Listeria monocytogenes NCTC 7973. This
led to the hypothesis that
-glucosides, sugars of plant origin, may
act as signal molecules, preventing the expression of virulence genes
if L. monocytogenes is living in its natural habitat
(soil). In three other laboratory strains (EGD, L028, and 10403S),
however, the effect of cellobiose was not unique, and all fermentable
carbohydrates repressed hly. This suggested that the
downregulation of virulence genes by
-glucosides is not a specific
phenomenon but, rather, an aspect of a global regulatory mechanism of
catabolite repression (CR). We assessed the effect of carbohydrates on
virulence gene expression in a panel of wild-type isolates of L. monocytogenes by using the PrfA-dependent phospholipase C gene
plcB as a reporter. Utilization of any fermentable sugar
caused plcB repression in wild-type L. monocytogenes. However, an EGD variant was identified in which,
as in NCTC 7973, plcB was only repressed by
-glucosides.
Thus, the regulation of L. monocytogenes virulence genes by
sugars appears to be mediated by two separate mechanisms, one
presumably involving a CR pathway and another specifically responding
to
-glucosides. We have identified in L. monocytogenes a
4-kb operon, bvrABC, encoding an antiterminator of the BglG
family (bvrA), a
-glucoside-specific enzyme II permease component of the phosphoenolpyruvate-sugar phosphotransferase system
(bvrB), and a putative ADP-ribosylglycohydrolase
(bvrC). Low-stringency Southern blots showed that this
locus is absent from other Listeria spp. Transcription of
bvrB was induced by cellobiose and salicin but not by
arbutin. Disruption of the bvr operon by replacing part of
bvrAB with an interposon abolished the repression by
cellobiose and salicin but not that by arbutin. Our data indicate that
the bvr locus encodes a
-glucoside-specific sensor that
mediates virulence gene repression upon detection of cellobiose and
salicin. Bvr is the first sensory system found in L. monocytogenes that is involved in environmental regulation of
virulence genes.
Present address: Institut für Hygiene un Mikrobiologie, 97080 Würzburg, Germany.
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