Photorhabdus luminescens is a nematode-symbiotic, gram negative, bioluminescent bacterium, belonging to the family of Enterobacteriaceae. It is part of the Photorhadbus genus, known to have three bacterial species: Photorhabdus luminescens, Photorhabdus temperata and Photorhabdus asymbiotica [1]. The first two species are nematode-symbiotic, while the third one has been isolated from human wounds, suggesting a role of this bacterium as an emerging human pathogen.

P. luminescens has a complex life cycle. It is symbiotic to entomopathogenic nematodes of the family Heterorhabditidiae [1,2], colonizing in the intestine. Heterorhabditis attacks insects at the stage of larvae. Once the nematode, in its infective juvenile stage, enters the bloodstream of the target insect, the bacteria are released and secrete toxins that rapidly kill the insect. Enzymes are also produced from the bacteria that lead to the decomposition of the carcass, providing both the bacteria and the nematodes with enough food for survival and reproduction. Then, new infective juveniles colonized with the bacterium grow, before emerging from the carcass into the soil. Bactericidal products of P. luminescens prevent infections of the carcass by other bacteria.

In order to combine symbiosis with the nematode and pathogenicity with the larva of the insect, P. luminescens must produce factors that can do both. The pathogenicity of the bacterium is the result of the presence of pathogenicity islands on its chromosome. In these islands there is a large number of genes that encode for toxins, enzymes, bacteriocins and antibiotics. Secretion of these proteins may happen with the use of different secretion systems, such as the type III secretion system. Genomic islands also encode for genes that are responsible for symbiosis and nematode growth.

Another characteristic of P. luminescens is that it undergoes phase variation [1,3]. There are two phenotypic variants of the bacterium. Phase I or primary variant is found at the infective juvenile stage of the nematode life cycle. At this point, the bacterium produces dyes, antibiotics, lipases, proteases and bioluminescence (due to the lux operon). Phase II or secondary variants are the result of in vitro cultures, after extended incubation, probably as a response to environmental stress. These variants lack or have reduced levels of the properties of the phase I variants. However, both of the variants show similarities to their pathogenicity.

P. luminescens produces a number of toxins. Tc toxins [4] are a new class of insecticidal toxins that have been shown to demonstrate both oral and injectable activity with results against the Colorado potato beetle, Leptinotarsa decemlineata and Bemisia tabaci [5], while other toxins like Mcf promote apoptosis in a variety of cells including mammalian ones. This states the potential use of the P. luminescens toxins as a replacement of the Bt toxins (Bacillus thuringiensis toxins) in the agriculture of transgenic crops infected by insects resistant to Bt toxins [6]. On the other hand, the fact that human infections with Photorhabdus species have been stated before in the USA and Australia [7,8], and the similarity of the tc genes of P. luminescens with its corresponding genes from Yersinia pestis [9], the causative agent of plague, can raise some serious questions on the possibility of a new human pathogen emerging.

In order to be able to infect its host and survive, P. luminescens must be capable of producing a wide range of proteins, including toxins. The complete genomic analysis of this organism has revealed that it indeed possesses a lot of genes encoding for toxins, proteases and lipases [10]. Four pathogenicity islands were also identified in the P. luminescens genome [11,12]; three of them contain genes that encode for toxins, while the last one encodes for a type III secretion system.

In P. luminescens, toxins are classified into four major groups: the toxin complexes (Tcs) with a high similarity to the ones found in Y. pestis genome [9], the Photorhabdus insect related (Pir) proteins, the “makes caterpillars floppy” (Mcf) toxins and the Photorhabdus virulence cassettes (PVC) [6,13].

Bacteria depend upon their communication with the extracellular environment for survival and interaction with other cells. As such, proteins that are synthesized in the cytoplasm must be transferred to the extracellular space (protein secretion). However, bacterial cells are surrounded by a cell envelope that includes the cell (cytoplasmic) membrane and a cell wall, surrounding it. In Gram negative bacteria, protein secretion involves the crossing of the newly-synthesized protein through two sets of membrane, the inner membrane (IM) and the outer membrane (OM), as well as the space spanning between them, namely the periplasmic space, leading to the development of elaborate secretion mechanisms [30,31].

Secretion can be performed in one or two steps, dividing the secretion pathways in two categories: (i) one-step or Sec-independent pathways, where the protein is secreted through a channel spanning both membranes and the periplasm and (ii) two-step or Sec-dependent pathways, where the protein is initially transferred into the periplasm across the IM with the help of a protein complex embedded in it (Sec translocase), then crosses the periplasm and subsequently is transferred through the OM into the extracellular environment. Up to date, at least seven types of protein secretion pathways have been recognized in Gram-negative bacteria.

P. luminescens is a nematode-symbiotic bacterium with a complex life cycle. As such, it needs to coordinate symbiosis and pathogenicity, thus the production of factors that can assist in both is necessary. Various toxins have been found in the genome of the bacterium, with both oral and injectible activities. These toxins are categorized into four groups; some of them are part of the pathogenicity islands identified in the P. luminescens genome.

Tcs are encoded by PAI I and show no similarity to other known proteins; they show high oral activity. The Photorhabdus insect related (Pir) toxins are binary proteins that are similar to the δ-endotoxins of B. thuringiensis. Mcf1 and Mcf2 are encoded by PAI II and act upon injection; these toxins have similarities to the C. difficile toxin, as well as RTX-like toxins. Finally, PVCs show homology to antifeeding genes in S. entomophila and encode proteins resembling types of bacteriocins.

Secretion of these proteins may happen through different secretion systems. Bioinformatics analysis against components of all known secretion systems revealed the existence of multiple copies of various secretion systems components throughout the genome of the bacterium (Table 2 and Table 3).

All, but type IV secretion systems are part of the “armory” of P. luminescens for survival and pathogenicity. This indicates that the bacterium possesses all the necessary means for the secretion of virulence factors, thus it is capable to establish a microbial infection.

Both type III and IV are used for the direct delivery of effectors in the host cell, using similar structures, thus assisting in processes such as immune system evasion during infection establishment [23]. This is important during the life cycle of the bacterium for its survival. Since both of the aforementioned systems could serve the same purpose, the presence of two type III secretion systems could counteract/justify the absence of a type IV system. On the other hand, type VI secretion systems (formerly known as type IVB) have also been identified; type VI secretion system is yet another system with similar secretion apparatus and could also justify the absence of a type IV secretion system.

Multiple copies of other secretion systems could be attributed to the variety of toxins, virulence factors and other proteins necessary for survival and infestation, secreted from P. luminescens. Further analysis however is necessary in order to identify the exact secretion mechanisms of the known toxins, as well as their exact role in establishing infection. Finally, the information presented in review could help in the identification of new secreted toxins through the identified secretion systems.