Pantoea ananatis is a ubiquitous organism found in almost every environment on
earth. It has been implicated in diseases of a wide range of agronomic crops
worldwide, including onion, maize, rice and pineapple, as well as a human disease. In
South Africa, P. ananatis causes blight and dieback of Eucalyptus, resulting in severe
losses of this important forestry resource. Nevertheless, little is known about the
pathogenicity mechanisms utilised by this pathogen to cause disease in this host.
The whole genome of a highly virulent Eucalyptus-pathogenic P. ananatis strain,
LMG20103, was sequenced. This genome sequence was subsequently mined to
identify a vast array of genes encoding putative pathogenicity determinants.
Comparative genomics revealed that it has evolved to be able to thrive in a wide range
of environments and that this strain carries pathogenicity determinants that may allow
it to infect hosts in both the animal and plant Kingdom. Interestingly, no Type II and
III secretion systems, which form a major part of the pathogenicity arsenal of many
plant pathogenic bacteria are present in P. ananatis. However, three loci on the
genome encode three distinct copies of the Type VI secretion system, which has
recently been demonstrated to play an important role in diseases caused by many
plant- and animal-pathogenic bacteria. In silico analysis of these secretion systems
showed that they likely secrete several pathogenicity effectors which may have a role
in P. ananatis infection of both plant and animal hosts. Another putative
pathogenicity determinant identified from the genome, the exopolysaccharide
ananatan, was experimentally demonstrated to play a role in disease expression on
both onion seedlings and pineapple fruit. This was done through the production of a
library of mutants which encompasses all the genes on the P. ananatis genome.
Genome sequencing enabled the identification of all the putative pathogenicity factors
of P. ananatis LMG20103 and the use of the mutant library and post-genomic
techniques has and will allow the functional characterization of many of these
pathogenicity determinants. By this means, the mechanisms underlying the disease
caused by P. ananatis on Eucalyptus and other hosts can be better understood. With
this information, more directed and effective strategies for the control of this pathogen
and its diseases can be developed.