Research article

Comparative genomics of the emerging human pathogen Photorhabdus asymbiotica with the insect pathogen Photorhabdus luminescens

Paul Wilkinson^{* 1} , Nicholas R Waterfield^{* 2} , Lisa Crossman³ , Craig Corton³ , Maria Sanchez-Contreras² , Isabella Vlisidou² , Andrew Barron³ , Alexandra Bignell³ , Louise Clark³ , Douglas Ormond³ , Matthew Mayho³ , Nathalie Bason³ , Frances Smith³ , Mark Simmonds³ , Carol Churcher³ , David Harris³ , Nicholas R Thompson³ , Michael Quail³ , Julian Parkhill³ and Richard H ffrench-Constant¹

¹School of Biosciences, University of Exeter in Cornwall, Penryn, TR10 9EZ, UK

²Department of Biology and Biochemistry, University of Bath, Bath, BA2 7AY, UK

³Pathogen Sequencing Unit, The Wellcome Trust Sanger Institute, Hinxton, Cambridge, CB10 1SA, UK

author email corresponding author email* Contributed equally

BMC Genomics 2009, 10:302doi:10.1186/1471-2164-10-302

Published:	7 July 2009

Abstract

Background

The Gram-negative bacterium Photorhabdus asymbiotica (Pa) has been recovered from human infections in both North America and Australia. Recently, Pa has been shown to have a nematode vector that can also infect insects, like its sister species the insect pathogen P. luminescens (Pl). To understand the relationship between pathogenicity to insects and humans in Photorhabdus we have sequenced the complete genome of Pa strain ATCC43949 from North America. This strain (formerly referred to as Xenorhabdus luminescens strain 2) was isolated in 1977 from the blood of an 80 year old female patient with endocarditis, in Maryland, USA. Here we compare the complete genome of Pa ATCC43949 with that of the previously sequenced insect pathogen P. luminescens strain TT01 which was isolated from its entomopathogenic nematode vector collected from soil in Trinidad and Tobago.

Results

We found that the human pathogen Pa had a smaller genome (5,064,808 bp) than that of the insect pathogen Pl (5,688,987 bp) but that each pathogen carries approximately one megabase of DNA that is unique to each strain. The reduced size of the Pa genome is associated with a smaller diversity in insecticidal genes such as those encoding the Toxin complexes (Tc's), Makes caterpillars floppy (Mcf) toxins and the Photorhabdus Virulence Cassettes (PVCs). The Pa genome, however, also shows the addition of a plasmid related to pMT1 from Yersinia pestis and several novel pathogenicity islands including a novel Type Three Secretion System (TTSS) encoding island. Together these data suggest that Pa may show virulence against man via the acquisition of the pMT1-like plasmid and specific effectors, such as SopB, that promote its persistence inside human macrophages. Interestingly the loss of insecticidal genes in Pa is not reflected by a loss of pathogenicity towards insects.

Conclusion

Our results suggest that North American isolates of Pa have acquired virulence against man via the acquisition of a plasmid and specific virulence factors with similarity to those shown to play roles in pathogenicity against humans in other bacteria.