Iron acquisition seems to be of particular importance for the life
cycle of P. luminescens, as this bacterium has the largest known
set of iron, heme, hemin and siderophore transporters. Within an
insect, P. luminescens needs to get access to the available
nutrients and has to deal with low-iron conditions. P. luminescens
has also been found as an opportunistic pathogen in human
Iron is essential to most bacteria and is often found at limiting
concentrations in soil and water habitats and in eukaryotic hosts.
Larval stages of the tobacco hornworm, Manduca sexta, contain a
ferritin-type iron binding protein in the hemolymph. One mechanism
that bacteria use to acquire iron from eukaryotic hosts is to
produce siderophore molecules that have high affinity for iron and
form soluble iron complexes to sequester and transfer ferric iron
into the bacterial cells. Members of the family Enterobacteriaceae
typically produce catechol and hydroxamate siderophores, some of
which are considered to be virulence factors that capture iron from
its bound form, usually as ferritin, in eukaryotic hosts.
Siderophores can also function in antibiosis; i.e., siderophores
produced by rhizobacteria can inhibit the growth of pathogenic
organisms in the rhizosphere and enhance plant growth.