Photorhabdus

"glowing-rods"

Insect pathogen

Photorhabdus are the only known terrestrial bioluminescent bacteria. Most members of the Photorhabdus are however insect pathogens that live in a strict symbiotic relationship within the guts entomopathogenic Heterorhabditid nematodes. Infective juvenile nematodes search in the soil for insect prey until they encounter a suitable host.  They then scratch their way into the insect's hemocoel (an "open" blood system ) and "vomit" up Photorhabdus cells directly into the blood. The Photorhabdus then set up a lethal septicemia, secreting toxins and virulence factors that rapidly kill the insect host. The bacteria replicate rapidly and bio-convert the insect tissues into more bacteria that serve as a food source for the reproducing nematodes. It is around the time of insect death that the bioluminescence of the insect corpse can be seen.

Manduca sexta glowing with Photorhabditis           Infective nematodes "burst" free

Bioluminescence is an oxygen and energy costly process and as yet we have no good explanation as to why they do this. Theories include some unknown biochemical role, a warning to scavenging nocturnal mammals or even that it serves as a lure to temp fresh insect victims into range. When the insect resources have been exhausted, the bacteria provide the nematode with an unknown "food signal" which switches them into a developmental state known as an infective juvenile. At this point they re-package the bacteria before bursting from the insect corpse in search of fresh victims. Insects provide ideal host systems to study the interaction between bacterial pathogen and animal host.

Photorhabdus luminescens on a insect midgut under the collagen sheath..

Immunogold conjugates may be used to reveal the expression of specific virulence factors in sectioned insects. Below left shows the location of the TcaC toxin on Photorhabdus luminescens cells during infection . Below right shows Photorhabdus expressing the Jellyfish green fluorescent protein attaching to the insect midgut.

Human pathogen

While Photorhabdus have never been isolated as free living in the environment, an increasing number of clinical isolates, designated Photorhabdus asymbiotica, are being identified from human patients in the United States, Australia and recently Nepal. Clinical collaborators suggest that many other cases are misdiagnosed due to the failure of clinical microbiology laboratories to recognise this unexpected organism. 

The source of human infection is not yet known, although the etiology of the infection is consistent with an invertebrate vector. Cases occur in warm wet months, usually after rain storms, and on extremities such as the feet.  P. asymbiotica is associated with severe soft tissue and systemic infections, and has been considered an “emerging human pathogen”.

Images of a Photorhabdus asymbiotica infection kindly provided by Dr John Gerrard (Goldcoast Hospital, Queensland)

We are currently sequencing the genome of the human pathogen Photorhabdus asymbiotica ATCC43949 in collaboration with the Sanger center UK. 

http://www.sanger.ac.uk/Projects/P_asymbiotica/