Pasteurella multocida is a Gram-negative bacterial pathogen that is
the primary causative agent of a range of economically important diseases in
animals, including fowl cholera in chickens, atrophic rhinitis in pigs and
haemorrhagic septicaemia in cattle. Hfq is an RNA-binding protein that acts to
modulate the action of small regulatory RNA (sRNA) molecules. Hfq normally acts
as an RNA chaperone that facilitates the interaction of sRNAs with their target
mRNAs, altering either the rate of mRNA translation or the stability of the
mRNA. In this way Hfq and sRNAs act together to regulate gene expression. To
determine whether sRNAs play an important role in gene regulation in P. multocida, we constructed an hfq
mutant in the highly virulent P. multocida strain VP161. The mutant
displayed normal growth in liquid BHI broth but displayed reduced colony size
and a less mucoid appearance on solid BHI medium. Hyaluronic acid assays showed
that the production of capsule was significantly reduced in the mutant;
complementation with an intact hfq restored capsule production. The hfq
mutant showed reduced in vivo survival in mice and reduced virulence in
chickens when inoculated via the intratracheal route. We compared the
transcriptomes of the wild-type and hfq mutant strains by RNA-seq; 129
genes were differentially expressed in the hfq mutant, with 111 genes
up-regulated and 18 genes down-regulated (expression ratio > 2-fold and FDR
< 0.05). The majority of the genes in the capsule biosynthesis locus were
significantly down-regulated in the hfq mutant, consistent with the
reduced capsule production measured by the hyaluronic acid assays. Taken
together, these data show that the action of Hfq is critical for full P. multocida virulence and capsule
production. Furthermore, they strongly suggest that sRNAs are important
regulators of virulence genes in P. multocida.