Plasmid-located pathogenicity determinants of Serratia entomophila, the causal agent of amber disease of grass grub, show similarity to the insecticidal toxins of Photorhabdus luminescens

Targeting of the actin cytoskeleton by insecticidal toxins from Photorhabdus luminescens

Photorhabdus luminescens produces several types of protein toxins, which are essential for participation in a trilateral symbiosis with nematodes and insects. The nematodes, carrying the bacteria, invade insect larvae and release the bacteria, which kill the insects with their toxins. Recently, the molecular mechanisms of the toxin complexes PTC3 and PTC5 have been elucidated. The biologically active components of the toxin complexes are ADP-ribosyltransferases, which modify actin and Rho GTPases, respectively. The actions of the toxins are described and compared with other bacterial protein toxins acting on the cytoskeleton.

Units of plasticity in bacterial genomes: new insight from the comparative genomics of two bacteria interacting with invertebrates, Photorhabdus and Xenorhabdus

Background

Flexible genomes facilitate bacterial evolution and are classically organized into polymorphic strain-specific segments called regions of genomic plasticity (RGPs). Using a new web tool, RGPFinder, we investigated plasticity units in bacterial genomes, by exhaustive description of the RGPs in two Photorhabdus and two Xenorhabdus strains, belonging to the Enterobacteriaceae and interacting with invertebrates (insects and nematodes).

Results

RGPs account for about 60% of the genome in each of the four genomes studied. We classified RGPs into genomic islands (GIs), prophages and two new classes of RGP without the features of classical mobile genetic elements (MGEs) but harboring genes encoding enzymes catalyzing DNA recombination (RGP_mob), or with no remarkable feature (RGP_none). These new classes accounted for most of the RGPs and are probably hypervariable regions, ancient MGEs with degraded mobilization machinery or non canonical MGEs for which the mobility mechanism has yet to be described. We provide evidence that not only the GIs and the prophages, but also RGP_mob and RGP_none, have a mosaic structure consisting of modules. A module is a block of genes, 0.5 to 60 kb in length, displaying a conserved genomic organization among the different Enterobacteriaceae. Modules are functional units involved in host/environment interactions (22-31%), metabolism (22-27%), intracellular or intercellular DNA mobility (13-30%), drug resistance (4-5%) and antibiotic synthesis (3-6%). Finally, in silico comparisons and PCR multiplex analysis indicated that these modules served as plasticity units within the bacterial genome during genome speciation and as deletion units in clonal variants of Photorhabdus.

Conclusions

This led us to consider the modules, rather than the entire RGP, as the true unit of plasticity in bacterial genomes, during both short-term and long-term genome evolution.

Nucleotide sequence of the Serratia entomophila plasmid pADAP and the Serratia proteamaculans pU143 plasmid virulence associated region

Some strains of Serratia entomophila and S. proteamaculans cause amber disease of the New Zealand grass grub Costelytra zealandica (Coleoptera: Scarabaeidae), an important pasture pest in New Zealand. The disease determinants of S. entomophila, are encoded on a 153,404-bp plasmid, termed pADAP for amber disease associated plasmid. The S. proteamaculans strain 143 (Sp143) exhibits an unusual pathotype, where only 60-70% of C. zealandica larvae infected with the bacterium succumb to disease. DNA sequence analysis of the Sp143 pU143 virulence associated region identified high DNA similarity to the pADAP sep virulence associated region, with DNA sequence variation in the sepA gene and the variable region of the sepC component. No pADAP anti-feeding prophage orthologue was detected in the Sp143 genome. The region of pADAP replication was cloned and found to replicate in S. entomophila but not in Escherichia coli. DNA sequence analysis of the plasmid pSG348 repA gene from the French isolate of Serratia grimesii, identified 93% DNA identity to the pADAP repA gene. A comparison of the pU143 virulence associated region with the completed pADAP nucleotide sequence is given.

Toxins and secretion systems of Photorhabdus luminescens

Photorhabdus luminescens is a nematode-symbiotic, gram negative, bioluminescent bacterium, belonging to the family of Enterobacteriaceae. Recent studies show the importance of this bacterium as an alternative source of insecticides, as well as an emerging human pathogen. Various toxins have been identified and characterized in this bacterium. These toxins are classified into four major groups: the toxin complexes (Tcs), the Photorhabdus insect related (Pir) proteins, the “makes caterpillars floppy” (Mcf) toxins and the Photorhabdus virulence cassettes (PVC); the mechanisms however of toxin secretion are not fully elucidated. Using bioinformatics analysis and comparison against the components of known secretion systems, multiple copies of components of all known secretion systems, except the ones composing a type IV secretion system, were identified throughout the entire genome of the bacterium. This indicates that Photorhabdus luminescens has all the necessary means for the secretion of virulence factors, thus it is capable of establishing a microbial infection.

CPDadh: a new peptidase family homologous to the cysteine protease domain in bacterial MARTX toxins

A cysteine protease domain (CPD) has been recently discovered in a group of multifunctional, autoprocessing RTX toxins (MARTX) and Clostridium difficile toxins A and B. These CPDs (referred to as CPDmartx) autocleave the toxins to release domains with toxic effects inside host cells. We report identification and computational analysis of CPDadh, a new cysteine peptidase family homologous to CPDmartx. CPDadh and CPDmartx share a Rossmann-like structural core and conserved catalytic residues. In bacteria, domains of the CPDadh family are present at the N-termini of a diverse group of putative cell-cell interaction proteins and at the C-termini of some RHS (recombination hot spot) proteins. In eukaryotes, catalytically inactive members of the CPDadh family are found in cell surface protein NELF (nasal embryonic LHRH factor) and some putative signaling proteins.

Cloning and heterologous expression of insecticidal-protein-encoding genes from Photorhabdus luminescens TT01 in Enterobacter cloacae for termite control

Enterobacter cloacae, one of the indigenous gut bacteria of the Formosan subterranean termite (Coptotermes formosanus), was genetically modified with a transposon Tn5 vector containing genes (tcdA1 and tcdB1) encoding orally insecticidal proteins from the entomopathogenic bacterium Photorhabdus luminescens subsp. laumondii TT01, a symbiont of the entomopathogenic nematode Heterorhabditis bacteriophora, for termite control. In the laboratory, termites were fed filter paper inoculated with the recombinant bacteria. The chromosomal expression of the introduced genes showed that there were insecticidal activities against termite workers and soldiers challenged with the transformed bacteria. After termites were fed recombinant bacteria, the termite mortality was 3.3% at day 5, and it increased from 8.7% at day 9 to 93.3% at day 29. All the dead termites contained the recombinant bacteria in their guts. Transfer of the recombinant bacteria occurred between donor workers (initially fed recombinant bacteria) and recipient workers (not fed). More than 20% of the recipient termites ingested recombinant bacteria within 2 h, and 73.3% of them had ingested recombinant bacteria after 12 h. The method described here provides a useful alternative for sustainable control of the Formosan subterranean termite (C. formosanus) and other social insects, such as the imported red fire ant (Solenopsis invicta).

Serratia entomophila inoculation causes a defect in exocytosis in Costelytra zealandica larvae

Rapid elimination of midgut luminal proteinase activity and gut clearance are the two major symptoms of amber disease in Costelytra zealandica larvae because of the three-subunit protein toxin complex produced in Serratia entomophila and Serratia proteamaculans. Quantitative PCR analysis of mRNA from the major serine proteinase gene families showed that loss of proteinase activity did not result from transcriptional downregulation. Unexpectedly, protein levels and rates of protein synthesis increased, rather than decreased, in the midgut of diseased insects. Proteomic analysis of midgut tissues showed marked differences between healthy and diseased midguts. Large increases in soluble forms of both actin and tubulin were identified from 2D-gels, together with concurrent decreases in the levels of polymeric actin-associated proteins: actin depolymerizing factor and cyclophilin. These results suggest that the Serratia toxin acts to cause degradation of the cytoskeletal network and prevent secretion of midgut gut digestive proteinases as both the actin cytoskeleton and microtubules are involved in exocytosis. Proteinases synthesized in the diseased midgut must be rapidly degraded because they do not accumulate in an inactive form.

Serine proteases identified from a Costelytra zealandica (White) (Coleoptera: Scarabaeidae) midgut EST library and their expression through insect development

Costelytra zealandica larvae are pests of New Zealand pastures causing damage by feeding on the roots of grasses and clovers. The major larval protein digestive enzymes are serine proteases (SPs), which are targets for disruption in pest control. An expressed sequence tag (EST) library from healthy, third instar larval midgut tissue was constructed and analysed to determine the composition and regulation of proteases in the C. zealandica larval midgut. Gene mining identified three trypsin-like and 11 chymotrypsin-like SPs spread among four major subgroups. Representative SPs were examined by quantitative PCR and enzyme activity assayed across developmental stages. The serine protease genes examined were expressed throughout feeding stages and downregulated in nonfeeding stages. The study will improve targeting of protease inhibitors and bacterial disruptors of SP synthesis.

Bacterial strategies to overcome insect defences

Recent genetic and molecular analyses have revealed how several strategies enable bacteria to persist and overcome insect immune defences. Genetic and genomic tools that can be used with Drosophila melanogaster have enabled the characterization of the pathways that are used by insects to detect bacterial invaders and combat infection. Conservation of bacterial virulence factors and insect immune repertoires indicates that there are common strategies of host invasion and pathogen eradication. Long-term interactions of bacteria with insects might ensure efficient dissemination of pathogens to other hosts, including humans.

Identification of a putative Mexican strain of Serratia entomophila pathogenic against root-damaging larvae of Scarabaeidae (Coleoptera)

The larvae of scarab beetles, known as "white grubs" and belonging to the genera Phyllophaga and Anomala (Coleoptera: Scarabaeidae), are regarded as soil-dwelling pests in Mexico. During a survey conducted to find pathogenic bacteria with the potential to control scarab larvae, a native Serratia sp. (strain Mor4.1) was isolated from a dead third-instar Phyllophaga blanchardi larva collected from a cornfield in Tres Marías, Morelos, Mexico. Oral bioassays using healthy P. blanchardi larvae fed with the Mor4.1 isolate showed that this strain was able to cause an antifeeding effect and a significant loss of weight. Mortality was observed for P. blanchardi, P. trichodes, and P. obsoleta in a multidose experiment. The Mor4.1 isolate also caused 100% mortality 24 h after intracoelomic inoculation of the larvae of P. blanchardi, P. ravida, Anomala donovani and the lepidopteran insect Manduca sexta. Oral and injection bioassays were performed with concentrated culture broths of the Mor4.1 isolate to search for disease symptoms and mortality caused by extracellular proteins. The results have shown that Mor4.1 broths produce significant antifeeding effects and mortality. Mor4.1 broths treated with proteinase K lost the ability to cause disease symptoms and mortality, in both the oral and the injection bioassays, suggesting the involvement of toxic proteins in the disease. The Mor4.1 isolate was identified as a putative Serratia entomophila Mor4.1 strain based on numerical taxonomy and phylogenetic analyses done with the 16S rRNA gene sequence. The potential of S. entomophila Mor4.1 and its toxins to be used in an integrated pest management program is discussed.

Evolution of the chaperone/usher assembly pathway: fimbrial classification goes Greek

Many Proteobacteria use the chaperone/usher pathway to assemble proteinaceous filaments on the bacterial surface. These filaments can curl into fimbrial or nonfimbrial surface structures (e.g., a capsule or spore coat). This article reviews the phylogeny of operons belonging to the chaperone/usher assembly class to explore the utility of establishing a scheme for subdividing them into clades of phylogenetically related gene clusters. Based on usher amino acid sequence comparisons, our analysis shows that the chaperone/usher assembly class is subdivided into six major phylogenetic clades, which we have termed alpha-, beta-, gamma-, kappa-, pi-, and sigma-fimbriae. Members of each clade share related operon structures and encode fimbrial subunits with similar protein domains. The proposed classification system offers a simple and convenient method for assigning newly discovered chaperone/usher systems to one of the six major phylogenetic groups.

Induced expression of theSerratia entomophilaSep proteins shows activity towards the larvae of the New Zealand grass grubCostelytra zealandica

Serratia entomophila and Serratia proteamaculans cause amber disease of the grass grub Costelytra zealandica (Coleoptera: Scarabaeidae). Three genes required for virulence, sepABC, are located on a large plasmid, pADAP. The translated products of the sep genes are members of the toxin complex (Tc) family of insecticidal toxins that reside in the genomes of some Enterobacteriaceae. Each of the sep genes was placed either singly or as various combinations under the control of an inducible arabinose promoter, allowing their inductive expression. Western Immunoblot confirmed that each of the Sep proteins migrated at their predicted size on sodium dodecyl sulphate-polyacrylamide gel electrophoresis gel. Bioassays of sonicated filtrates derived from the various arabinose-induced para-SEP constructs showed that only when sepA, sepB and sepC were coexpressed were amber disease symptoms observed in grass grub larvae. Fourteen days after ingestion of the Sep protein filtrate, approximately 64% of the larvae reverted from a diseased to a healthy phenotype. Redosing the revertents with a fresh Sep protein filtrate reinitiated the amber pathotype, indicating that the Sep proteins are needed to be continuously present to exert an effect.

An assessment of the potential of herbivorous insect gut bacteria to develop competence for natural transformation

Whereas the capability of DNA uptake has been well established for numerous species and strains of bacteria grown in vitro, the broader distribution of natural transformability within bacterial communities remains largely unexplored. Here, we investigate the ability of bacterial isolates from the gut of grass grub larvae (Costelytra zealandica (White); Coleoptera: Scarabaeidae) to develop natural genetic competence in vitro. A total of 37 mostly species-divergent strains isolated from the gut of grass grub larvae were selected for spontaneous rifampicin-resistance. Genomic DNA was subsequently isolated from the resistant strains and exposed to sensitive strains grown individually using established filter transformation protocols. DNA isolated from wild-type strains was used as a control. None of the 37 isolates tested exhibited a frequency of conversion to rifampicin-resistance in the presence of DNA at rates that were significantly higher than the rate of spontaneous mutation to rifampicin-resistance in the presence of wild-type DNA (the limit of detection was approximately < 1 culturable transformant per 10(9) exposed bacteria). To further examine if conditions were conducive to bacterial DNA uptake in the grass grubs gut, we employed the competent bacterium Acinetobacter baylyi strain BD413 as a recipient species for in vivo studies. However, no transformants could be detected above the detection limit of 1 transformant per 10(3) cells, possibly due to low population density and limited growth of A. baylyi cells in grass grub guts. PCR analysis indicated that chromosomal Acinetobacter DNA remains detectable by PCR for up to 3 days after direct inoculation into the alimentary tract of grass grub larvae. Nevertheless, neither transforming activity of the DNA recovered from the alimentary tract of grass grubs larvae nor competence of bacterial cells recovered from inoculated larvae could be shown.

Acute oral toxicity of Yersinia pseudotuberculosis to fleas: implications for the evolution of vector-borne transmission of plague

Yersinia pestis diverged from Yersinia pseudotuberculosis</= 20 000 years ago, during which time it evolved to be transmitted by fleas. In comparing the ability of these closely related species to infect the rat flea Xenopsylla cheopis, we found that Y. pseudotuberculosis, unlike Y. pestis, is orally toxic to fleas. Fleas showed signs of acute toxicity, including diarrhoea, immediately after feeding on blood containing Y. pseudotuberculosis in response to protein toxin(s) produced by the bacteria. Adherence of Y. pseudotuberculosis to the midgut and large intracellular vacuoles in midgut epithelial cells were detected during the first 24 h after infection. The insect pathogen Photorhabdus luminescens and its TcdA1 and TcdB1-TccC1 insecticidal toxin complexes were similarly toxic to fleas, implicating the toxin complex (tc) genes also present in Yersinia species. However, the Y. pestis and Y. pseudotuberculosis TcaAB and TcaC-TccC proteins were non-toxic to fleas, and Y. pseudotuberculosis mutants deleted of tc genes retained acute toxicity. Our results indicate that loss of one or more insect gut toxins was a critical step in the recent evolution of flea-borne transmission in the genus Yersinia. Changes in the tc insecticidal genes do not appear to have been responsible, but may have had other effects on Yersinia-flea interactions.

Expression and insecticidal activity of Yersinia pseudotuberculosis and Photorhabdus luminescens toxin complex proteins

Photorhabdus luminescens toxin complex (Tc) has been characterized as a potent three-component insecticidal protein complex. Homologues of genes encoding P. luminescens Tc components have been identified in several other enterobacteria and in Gram-positive bacteria, showing these genes are widespread in bacteria. In particular, tc gene homologues have been identified in Yersinia enterocolitica, Yersinia pseudotuberculosis and Yersinia pestis and may have a role in Y. pestis evolution. Y. enterocolitica tc genes have been shown to be active against Manduca sexta larvae. Here, we demonstrate that expression optimization is essential to obtain bioactive P. luminescens Tc proteins and demonstrate that TcaAB and TcdB + TccC are stand-alone toxins against a M. sexta insect model. Moreover, we report that Y. pseudotuberculosis IP32953 Tc proteins are also toxic to M. sexta larvae but do not cross-potentiate as P. luminescens Tc components.

Identification and type III-dependent secretion of the Yersinia pestis insecticidal-like proteins

Plague, or the Black Death, is a zoonotic disease that is spread from mammal to mammal by fleas. This mode of transmission demands that the causative agent of this disease, Yersinia pestis, is able to survive and multiply in both mammals and insects. In recent years the complete genome sequence of a number of Y. pestis strains have been determined. This sequence information indicates that Y. pestis contains a cluster of genes with homology to insecticidal toxin encoding genes of the insect pathogen Photorhabdus luminescens. Here we demonstrate that Y. pestis KIM strains produced the encoded proteins. Production of the locus-encoded proteins was dependent on a gene (yitR) encoding a member of the LysR family of transcriptional activators. Evidence suggests the proteins are type III secretion substrates. N terminal amino acids (100 to 367) of each protein fused to an epitope tag were secreted by the virulence plasmid type III secretion type. A fusion protein comprised of the N-terminus of YipB and the enzymatic active component of Bordetella pertussis adenylate cyclase (Cya) was translocated into both mammalian and insect cells. In conclusion, a new class of Y. pestis type III secreted and translocated proteins has been identified. We hypothesize that these proteins function to promote transmission of and infection by Y. pestis.

Structural Characterisation of the Insecticidal Toxin XptA1, Reveals a 1.15 MDa Tetramer with a Cage-like Structure

A recently identified class of proteins conferring insecticidal activity to several bacteria within the Enterobacteriaceae family have potential for control of commercially important insect pests. Here, we report the first purification, biophysical characterisation and 3-D structural analysis of one of the toxin components, XptA1, from Xenorhabdus nematophila PMFI296 to a resolution of 23 A. Membrane binding studies indicate that the three-component toxin system has a different mode of action from that of proteins from Bacillus thuringiensis (Bt). Biophysical characterisation of XptA1 suggests a mechanism of action of XptA1 whereby it first binds to the cell membrane forming a structure with a central cavity and forms a complex with its partners XptB1 and XptC1 producing the full insecticidal toxin. The structure of XptA1 is shown by a combination of electron microscopy, ultracentrifugation and circular dichroism spectroscopy to be a 1.15 MDa tetramer with a cage-like structure. Each of the four symmetry-related subunits has three well-defined domains and a longitudinal twist with one end narrower than the other. One third of the residues of XptA1 are alpha-helical and it is suggested the subunits associate partly via an alpha-helical coiled-coil interaction. XptA1 itself shows the same secondary structure at neutral pH and in an alkaline environment up to pH10.5. This pH tolerance indicates that the folded XptA1 can pass through the midgut of Lepidopteran insects susceptible to the insecticidal toxin complex. This implies therefore that its folded structure is important for its biological activity.

Isolation and characterization of the Serratia entomophila antifeeding prophage

The Serratia entomophila antifeeding prophage (Afp) is thought to form a virus-like structure that has activity towards the New Zealand grass grub, Costelytra zealandica. Through the trans based expression of AnfA1, an RfaH - like transcriptional antiterminator, the Afp, was able to be induced. The expressed Afp was purified and visualized by electron microscopy. The Afp resembled a phage tail-like bacteriocin, exhibiting two distinct morphologies: an extended and a contracted form. The purified Afp conferred rapid activity towards C. zealandica larvae, causing cessation of feeding and a change to an amber colouration within 48 h postinoculation, with increased dose rates causing larval mortality.

Insecticidal toxins from Photorhabdus bacteria and their potential use in agriculture

Most of the insecticidal toxins used in agriculture come from a single bacterium Bacillus thuringiensis or 'Bt'. Here we review our work on the array of toxins produced by Photorhabdus and Xenorhabdus bacteria that are symbiotic with entomopathogenic nematodes, and discuss their potential for use in agriculture as alternatives to Bt. Despite the fact that both Photorhabdus and Xenorhabdus are introduced directly into the insect blood stream by their nematode vectors, they produce a range of toxins with both oral and injectable insecticidal activity. The toxin complexes (Tc's) are large orally active toxins that are displayed on the outer surface of the bacterium. They require three components (A-C) for full toxicity and one 'A' component has been successfully expressed in transgenic Arabidopsis to confer insect resistance. One such group of Tc's, the PirAB binary toxins, have oral activity against mosquitoes and some caterpillar pests. Their mode of action is not known but they show significant sequence similarity to a recently described neurotoxin beta-leptinotarsin-h isolated from the blood of the Colorado potato beetle. Other toxins such as 'makes caterpillars floppy' (Mcf) and proteins encoded by the 'Photorhabdus virulence cassettes' (PVCs) only show injectable activity. Mcf1 promotes apoptosis in a wide range of cells and appears to mimic mammalian BH3 domain-only proteins in the mitochondrion whereas the mode of action of the PVCs remains undetermined. The likely biological reasons for the massive functional redundancy in Photorhabdus insecticidal toxins are discussed.

Virulence of Serratia strains against Costelytra zealandica

Strains of Serratia spp. showed a high level of virulence when injected into the hemocoel of larvae Costelytra zealandica, with Serratia entomophila, S. plymuthica, and S. marcescens showing significantly higher virulence than S. proteamaculans. Toxicity was independent of the amber disease-causing plasmid pADAP, suggesting a generalized Serratia toxin.

Identification, typing, and insecticidal activity of Xenorhabdus isolates from entomopathogenic nematodes in United Kingdom soil and characterization of the xpt toxin loci

Xenorhabdus strains from entomopathogenic nematodes isolated from United Kingdom soils by using the insect bait entrapment method were characterized by partial sequencing of the 16S rRNA gene, four housekeeping genes (asd, ompR, recA, and serC) and the flagellin gene (fliC). Most strains (191/197) were found to have genes with greatest similarity to those of Xenorhabdus bovienii, and the remaining six strains had genes most similar to those of Xenorhabdus nematophila. Generally, 16S rRNA sequences and the sequence types based on housekeeping genes were in agreement, with a few notable exceptions. Statistical analysis implied that recombination had occurred at the serC locus and that moderate amounts of interallele recombination had also taken place. Surprisingly, the fliC locus contained a highly variable central region, even though insects lack an adaptive immune response, which is thought to drive flagellar variation in pathogens of higher organisms. All the X. nematophila strains exhibited a consistent pattern of insecticidal activity, and all contained the insecticidal toxin genes xptA1A2B1C1, which were present on a pathogenicity island (PAI). The PAIs were similar among the X. nematophila strains, except for partial deletions of a peptide synthetase gene and the presence of insertion sequences. Comparison of the PAI locus with that of X. bovienii suggested that the PAI integrated into the genome first and then acquired the xpt genes. The independent mobility of xpt genes was further supported by the presence of xpt genes in X. bovienii strain I73 on a type 2 transposon structure and by the variable patterns of insecticidal activity in X. bovienii isolates, even among closely related strains.

Occurrence of sep insecticidal toxin complex genes in Serratia spp. and Yersinia frederiksenii

Some strains of Serratia entomophila and S. proteamaculans cause amber disease of the grass grub Costelytra zealandica (Coleoptera: Scarabaeidae). Three genes required for virulence, sepABC, are located on a large plasmid, pADAP. Sequence analysis suggests that the sepABC gene cluster may be part of a horizontally mobile region. This study presents evidence for the putative mobility of the sep genes of pADAP. Southern blot analysis showed that orthologues of the sep genes reside on plasmids within S. entomophila, S. liquefaciens, S. proteamaculans, and a plasmid from Yersinia frederiksenii. Three plasmids hybridized to the pADAP sep virulence-associated region but not the pADAP replication and conjugation regions. Subsequent DNA sequence analysis of the Y. frederiksenii sep-like genes, designated tcYF1 and tcYF2, showed that they had 88% and 87% DNA identity to sepA and sepB, respectively. These results indicate that the sep genes are part of a discrete horizontally mobile region.

Prevalence and genetic analysis of plasmid-mediated quinolone resistance determinants QnrA and QnrS in Enterobacteriaceae isolates from a French university hospital

The spread of plasmid-mediated quinolone resistance determinants QnrA and QnrS was evaluated in a collection of 186 extended-spectrum beta-lactamase (ESBL)-positive enterobacterial isolates from 2002 to 2005 and 185 nalidixic acid-resistant strains isolated during the first 6 months of 2005 at the Bicêtre hospital, France. Out of these 186 ESBL-positive isolates, 2.2 and 1.6% carried a QnrA1 and a QnrS1 determinant, respectively. The ESBLs associated with QnrA1 were VEB-1, SHV-12, and CTX-M-1, whereas those associated with QnrS1 were TEM-52, SHV-12, and CTX-M-1. Among the 185 nalidixic acid-resistant strains isolated in 2005, 0.5 and 2.7% had a QnrA1 determinant and a QnrS1 determinant, respectively. The genetic environments of the qnrA1 gene differed but were always associated with sul1 type integrons. In contrast, qnrS1 genes were not embedded in class 1 integrons but located often (but not systematically) downstream of the insertion sequence ISEcl2 on plasmids that often carried a novel beta-lactamase gene, bla(LAP-1). This is the first study identifying the QnrS resistance determinant in Europe and indicating that this determinant might also be widespread.

Erwinia carotovora Evf antagonizes the elimination of bacteria in the gut of Drosophila larvae

Erwinia Virulence Factor (Evf) has been identified in Erwinia carotovora carotovora 15 (Ecc15) as a virulence factor that promotes colonization of the Drosophila larval gut and provokes the triggering of a systemic immune response. Here we have analysed how Evf promotes persistence and colonization of bacteria inside the larval gut. Erwinia evf mutants do not persist in immune-deficient Drosophila, indicating that Evf does not act by counteracting immunity. The results indicated that Evf is not a toxin because various gram-negative bacteria expressing evf can persist without affecting viability of Drosophila larvae. Evf did not appear to be a factor antagonizing a host-specific reaction because in vitro assays failed to reveal detoxifying enzymatic activities against various compounds thought to contribute to the hostile environment of the gut. These findings were corroborated by the observation that Evf is not required for survival in midgut organ cultures. By contrast, bacteria expressing evf allow persistence in trans of bacteria lacking evf indicating that Evf promotes the accumulation of gram-negative bacteria in the anterior midgut by affecting gut physiology.

Prevalence of local immune response against oral infection in a Drosophila/Pseudomonas infection model

Pathogens have developed multiple strategies that allow them to exploit host resources and resist the immune response. To study how Drosophila flies deal with infectious diseases in a natural context, we investigated the interactions between Drosophila and a newly identified entomopathogen, Pseudomonas entomophila. Flies orally infected with P. entomophila rapidly succumb despite the induction of both local and systemic immune responses, indicating that this bacterium has developed specific strategies to escape the fly immune response. Using a combined genetic approach on both host and pathogen, we showed that P. entomophila virulence is multi-factorial with a clear differentiation between factors that trigger the immune response and those that promote pathogenicity. We demonstrate that AprA, an abundant secreted metalloprotease produced by P. entomophila, is an important virulence factor. Inactivation of aprA attenuated both the capacity to persist in the host and pathogenicity. Interestingly, aprA mutants were able to survive to wild-type levels in immune-deficient Relish flies, indicating that the protease plays an important role in protection against the Drosophila immune response. Our study also reveals that the major contribution to the fly defense against P. entomophila is provided by the local, rather than the systemic immune response. More precisely, our data points to an important role for the antimicrobial peptide Diptericin against orally infectious Gram-negative bacteria, emphasizing the critical role of local antimicrobial peptide expression against food-borne pathogens.

Normal feeding and digestion involves the ingestion of many microorganisms. Many are innocuous, some are commensal, and others may be pathogenic. Eukaryotes have thus evolved complex mechanisms to detect, control, and if necessary, eliminate intestinal microbes. Insects are no exception, and the fruit fly, Drosophila, employs a physical barrier within the intestinal lumen and the peritrophic matrix, and an innate immune response which exhibits similarities to the mammalian counterpart. Pseudomonas entomophila was identified as a novel entomopathogenic bacterium that can infect and colonize the gut of Drosophila. In this paper, Liehl et al. describe one specific secreted virulence factor of P. entomophila, the zinc metalloprotease, AprA, which they demonstrate to be required for defense against the host gut epithelial immune response. AprA defends P. entomophila against the Drosophila antimicrobial peptides, produced by the gut innate immune response. P. entomophila aprA mutants are attenuated for virulence in wild-type Drosophila but are equally infective as wild-type bacteria in immune-deficient mutant flies that do not express these antimicrobial peptides. Although secreted proteases have previously been described as a potentially important defense against host immune proteins, this is one of the rare examples of an in vivo demonstration of such a specific role against insect antimicrobial peptides.

Low temperature-induced insecticidal activity of Yersinia enterocolitica

The insecticidal toxin complexes (Tcs) are produced by several Enterobacteriaceae associated with insects, such as Photorhabdus luminescens, Serratia entomophila and Xenorhabdus nematophilus. Genome sequences revealed tc-like genes in Yersinia spp., but insecticidal activity of this genus associated with the toxins has not been described. Through the search for genes upregulated at low growth temperatures in Yersinia enterocolitica strain W22703, a genomic island of 19 kb termed tc-PAI(Ye) with homologues of the toxin genes tcaA, tcaB, tcaC and tccC was identified. Southern blot and polymerase chain reaction (PCR) analysis of 34 strains demonstrated that the tc-PAI(Ye) is present in biovars 2, 3 and 4, but neither in biovars 1A and 1B, nor in five Yersinia species apathogenic in humans. Using the luxCDABE operon as reporter, the expression of the toxin genes was shown to be completely repressed in cells cultured at 37 degrees C, and to increase by 4.6 orders of magnitude when the growth temperature was decreased gradually to 10 degrees C. These data provide the first indication that temperature is a critical parameter for induction or repression of tc gene transcription. Whole-cell extracts of Y. enterocolitica strain W22703 cultivated at 10 degrees C, but not at 30 degrees C, led to insect mortality when fed to Manduca sexta larvae, in contrast to an insertional tcaA mutant. Overall the results suggest that the tc-PAI(Ye) could play an important role in the transmission and survival of pathogenic Y. enterocolitica strains outside mammalian hosts.

Photorhabdus virulence cassettes confer injectable insecticidal activity against the wax moth

Two recently sequenced genomes of the insect-pathogenic bacterium Photorhabdus and a large Serratia entomophila plasmid, pADAP, have phage-related loci containing putative toxin effector genes, designated the "Photorhabdus virulence cassettes" (PVCs). In S. entomophila, the single plasmid PVC confers antifeeding activity on larvae of a beetle. Here, we show that recombinant Escherichia coli expressing PVC-containing cosmids from Photorhabdus has injectable insecticidal activity against larvae of the wax moth. Electron microscopy showed that the structure of the PVC products is similar to the structure of the antibacterial R-type pyocins. However, unlike these bacteriocins, the PVC products of Photorhabdus have no demonstrable antibacterial activity. Instead, injection of Photorhabdus PVC products destroys insect hemocytes, which undergo dramatic actin cytoskeleton condensation. Comparison of the genomic organizations of several PVCs showed that they have a conserved phage-like structure with a variable number of putative anti-insect effectors encoded at one end. Expression of these putative effectors directly inside cultured cells showed that they are capable of rearranging the actin cytoskeleton. Together, these data show that the PVCs are functional homologs of the S. entomophila antifeeding genes and encode physical structures that resemble bacteriocins. This raises the interesting hypothesis that the PVC products are bacteriocin-like but that they have been modified to attack eukaryotic host cells.

Homologues of insecticidal toxin complex genes in Yersinia enterocolitica biotype 1A and their contribution to virulence

Yersinia enterocolitica is an enteric pathogen that consists of six biotypes: 1A, 1B, 2, 3, 4, and 5. Strains of the latter five biotypes can carry a virulence plasmid, known as pYV, and several well-characterized chromosomally encoded virulence determinants. Y. enterocolitica strains of biotype 1A lack the virulence-associated markers of pYV-bearing strains and were once considered to be avirulent. There is growing epidemiological, clinical, and experimental evidence, however, to suggest that some biotype 1A strains are virulent and can cause gastrointestinal disease. To identify potential virulence genes of pathogenic strains of Y. enterocolitica biotype 1A, we used genomic subtractive hybridization to determine genetic differences between two biotype 1A strains: an environmental isolate, Y. enterocolitica IP2222, and a clinical isolate, Y. enterocolitica T83. Among the Y. enterocolitica T83-specific genes we identified were three, tcbA, tcaC, and tccC, that showed homology to the insecticidal toxin complex (TC) genes first discovered in Photorhabdus luminescens. The Y. enterocolitica T83 TC gene homologues were expressed by Y. enterocolitica T83 and were significantly more prevalent among clinical biotype 1A strains than other Yersinia isolates. Inactivation of the TC genes in Y. enterocolitica T83 resulted in mutants which were attenuated in the ability to colonize the gastrointestinal tracts of perorally infected mice. These results indicate that products of the TC gene complex contribute to the virulence of some strains of Y. enterocolitica biotype 1A, possibly by facilitating their persistence in vivo.

The broadly insecticidal Photorhabdus luminescens toxin complex a (Tca): activity against the Colorado potato beetle, Leptinotarsa decemlineata, and sweet potato whitefly, Bemisia tabaci

Toxin complex a (Tca), a high molecular weight insecticidal protein complex produced by the entomopathogenic bacterium Photorhabdus luminescens, has been found to be orally toxic to both the Colorado potato beetle, Leptinotarsa decemlineata, and the sweet potato whitefly, Bemisia tabaci biotype B. The 48 hour LC50 for Tca against neonate L. decemlineata was found to be 2.7 ppm, and the growth of 2nd instar L. decemlineata exposed to Tca for 72 hours was almost entirely inhibited at concentrations above 0.5 ppm. B. tabaci was highly susceptible to Tca as well; newly emerged nymphs that were artificially fed Tca developed poorly, or not at all. Tca concentrations between 0.1 and 0.2 ppm reduced the number of nymphs reaching the second instar by 50%. In addition, a preparation of Tca missing two prominent subunits, TcaAii and TcaAiii, was found to be at least as toxic to L. decemlineata and B. tabaci as Tca itself, indicating that the activity of Tca is not dependant on the presence of these subunits at the time of ingestion.

Potentiation and cellular phenotypes of the insecticidal Toxin complexes of Photorhabdus bacteria

The toxin complex (tc) genes of bacteria comprise a large and growing family whose mode of action remains obscure. In the insect pathogen Photorhabdus, tc genes encode high molecular weight insecticidal toxins with oral activity against caterpillar pests. One protein, TcdA, has recently been expressed in transgenic plants and shown to confer insect resistance. These toxins therefore represent alternatives to toxins from Bacillus thuringiensis (Bt) for deployment in transgenic crops. Levels of TcdA expression in transgenic plants were, however, low and the full toxicity associated with the native toxin was not reconstituted. Here we show that increased activity of the toxin TcdA1 requires potentiation by either of two pairs of gene products, TcdB1 and TccC1 or TcdB2 and TccC3. Moreover, these same pairs of proteins can also cross-potentiate a second toxin, TcaA1B1. To elucidate the likely functional domains present in these large proteins, we expressed fragments of each 'toxin' or 'potentiator' gene within mammalian cells. Several domains produced abnormal cellular morphologies leading to cell death, while others showed specific phenotypes such as nuclear translocation. Our results prove that the Tc toxins are complex proteins with multiple functional domains. They also show that both toxin genes and their potentiator pairs will need to be expressed to reconstitute full activity in insect-resistant transgenic plants. Moreover, they suggest that the same potentiator pair will be able to cross-potentiate more than one toxin in a single plant.

Invertebrates as a source of emerging human pathogens

Despite their importance, little is known about the origins of many emerging human pathogens. However, given the age and current predominance of invertebrates, it is likely that bacteria-invertebrate interactions are not only a present source of human pathogens but have also shaped their evolution. Pathogens of invertebrate and unicellular organisms represent an extensive reservoir of bacterial strains equipped with virulence factors that evolved to overcome the innate immune responses of their hosts. This reservoir might represent a source of new human pathogenic strains and might also foster the spread of novel virulence factors into existing human commensal or pathogenic bacteria. This article examines the available evidence for this concept by examining pairs of closely related bacteria, one of which is benign, but insect associated, and one of which is a human pathogen.

Cloning Serratia entomophila antifeeding genes--a putative defective prophage active against the grass grub Costelytra zealandica

Serratia entomophila and Serratia proteamaculans (Enterobacteriaceae) cause amber disease in the grass grub Costelytra zealandica (Coleoptera: Scarabaeidae), an important pasture pest in New Zealand. Larval disease symptoms include cessation of feeding, clearance of the gut, amber coloration, and eventual death. A 155-kb plasmid, pADAP, carries the genes sepA, sepB, and sepC, which are essential for production of amber disease symptoms. Transposon insertions in any of the sep genes in pADAP abolish gut clearance but not cessation of feeding, indicating the presence of an antifeeding gene(s) elsewhere on pADAP. Based on deletion analysis of pADAP and subsequent sequence data, a 47-kb clone was constructed, which when placed in either an Escherichia coli or a Serratia background exerted strong antifeeding activity and often led to rapid death of the infected grass grub larvae. Sequence data show that the antifeeding component is part of a large gene cluster that may form a defective prophage and that six potential members of this prophage are present in Photorhabdus luminescens subsp. laumondii TTO1, a species which also has sep gene homologues.

Global features of sequences of bacterial chromosomes, plasmids and phages revealed by analysis of oligonucleotide usage patterns

Background

Oligonucleotide frequencies were shown to be conserved signatures for bacterial genomes, however, the underlying constraints have yet not been resolved in detail. In this paper we analyzed oligonucleotide usage (OU) biases in a comprehensive collection of 155 completely sequenced bacterial chromosomes, 316 plasmids and 104 phages.

Results

Two global features were analyzed: pattern skew (PS) and variance of OU deviations normalized by mononucleotide content of the sequence (OUV). OUV reflects the strength of OU biases and taxonomic signals. PS denotes asymmetry of OU in direct and reverse DNA strands. A trend towards minimal PS was observed for almost all complete sequences of bacterial chromosomes and plasmids, however, PS was substantially higher in separate genomic loci and several types of plasmids and phages characterized by long stretches of non-coding DNA and/or asymmetric gene distribution on the two DNA strands. Five of the 155 bacterial chromosomes have anomalously high PS, of which the chromosomes of Xylella fastidiosa 9a5c and Prochlorococcus marinus MIT9313 exhibit extreme PS values suggesting an intermediate unstable state of these two genomes.

Conclusions

Strand symmetry as indicated by minimal PS is a universally conserved feature of complete bacterial genomes that results from the matching mutual compensation of local OU biases on both replichors while OUV is more a taxon specific feature. Local events such as inversions or the incorporation of genome islands are balanced by global changes in genome organization to minimize PS that may represent one of the leading evolutionary forces driving bacterial genome diversification.

A Novel Secreted Protein Toxin from the Insect Pathogenic Bacterium Xenorhabdus nematophila

The bacterium Xenorhabdus nematophila is an insect pathogen that produces several proteins that enable it to kill insects. Screening of a cosmid library constructed from X. nematophila strain A24 identified a gene that encoded a novel protein that was toxic to insects. The 42-kDa protein encoded by the toxin gene was expressed and purified from a recombinant system, and was shown to kill the larvae of insects such as Galleria mellonella and Helicoverpa armigera when injected at doses of around 30-40 ng/g larvae. Sequencing and bioinformatic analysis suggested that the toxin was a novel protein, and that it was likely to be part of a genomic island involved in pathogenicity. When the native bacteria were grown under laboratory conditions, a soluble form of the 42-kDa toxin was secreted only by bacteria in the phase II state. Preliminary histological analysis of larvae injected with recombinant protein suggested that the toxin primarily acted on the midgut of the insect. Finally, some of the common strategies used by the bacterial pathogens of insects, animals, and plants are discussed.

Peripheral sequences of the Serratia entomophila pADAP virulence-associated region

Some strains of the Enterobacteriaceae Serratia entomophila and Serratia proteamaculans cause amber disease in the grass grub, Costelytra zealandica (Coleoptera: Scarabaeidae), an important pasture pest in New Zealand. The genes responsible for this disease reside on a large, 155-kb plasmid designated amber disease-associated plasmid (pADAP). Herein, we report the DNA sequencing of approximately 50 kb upstream and 10 kb downstream of the virulence-encoding region. Based on similarity with proteins in the current databases, and potential ribosome-binding sites, 63 potential ORFs were determined. Eleven of these ORFs belong to a type IV pilus cluster (pilL-V) and a further eight have similarities to the translated products of the plasmid transfer traH-N genes of the plasmid R64. In addition, a degenerate 785-nt direct repeat flanks a 44.7-kb region with the potential to encode three Bacillus subtilis Yee-type proteins, a fimbrial gene cluster, the sep virulence-associated genes and several remnant IS elements.

Interactions of insecticidal toxin gene products from Xenorhabdus nematophilus PMFI296

Four genes on a genomic fragment from Xenorhabdus nematophilus PMFI296 were shown to be involved in insecticidal activity towards three commercially important insect species. Each gene was expressed individually and in combinations in Escherichia coli, and the insecticidal activity of the lysates was determined. The combined four genes (xptA1, xptA2, xptB1, and xptC1), in E. coli, showed activity towards Pieris brassicae, Pieris rapae, and Heliothis virescens. The genes xptA1, xptB1, and xptC1 were involved in expressing activity towards P. rapae and P. brassicae, while the genes xptA2, xptB1, and xptC1 were needed for activity towards H. virescens. When each of these three genes was expressed individually in E. coli and the cell lysates were used in insect assays or mixed and then used, insecticidal activity was detected at a very low level. If the genes xptB1 and xptC1 were expressed in the same E. coli cell and this cell lysate was mixed with cells expressing xptA1, activity was restored to P. rapae and P. brassicae. Similarly mixing XptB1/C1 lysate with XptA2 lysate restored activity towards H. virescens. Individual gene disruptions in X. nematophilus PMFI296 reduced activity to insects; this activity was restored by complementation with cells expressing either xptA1 or xptA2 for their respective disruptions or E. coli expressing both xptB1 and xptC1 for individual disruptions of either of these genes. The genes xptA2, xptC1, and xptB1 were expressed as an operon in PMFI296 and inactivation of xptA2 or xptC1 resulted in silencing of downstream gene(s), while xptA1 was expressed as a single gene. Therefore, the two three gene product combinations interact with each other to produce good insecticidal activity.

Extracellular secretion of the virulence plasmid-encoded ADP-ribosyltransferase SpvB in Salmonella

Nontyphoid Salmonella enterica requires the plasmid-encoded spv genes to establish successful systemic infection in experimental animals. The SpvB virulence-associated protein has recently been shown to contain the ADP-ribosyltransferase domain. SpvB ADP-ribosilates actin and depolymerizes actin filaments when expressed in cultured epithelial cells. However, spontaneous secretion or release of SpvB has not been observed under in vitro growth conditions. In the present study we investigated the secretion of SpvB from Salmonella using in vitro and in vivo assay systems. We showed that SpvB is secreted into supernatant from Salmonella strains that contain the cloned spvB gene on a plasmid when they grew in intracellular salts medium (ISM), a minimal medium mimicing the intracellular iron concentrations of eukaryotic cells. A series of mutant SpvB proteins revealed that an N-terminal region of SpvB located at amino acids 1-229 was sufficient to promote secretion into extracellular milieu. Confocal immunofluorescence microscopy also demonstrated efficient localization of the N-terminal domain of SpvB(1-360) tagged with biotinylated peptide within infected host cell cytosol but not truncated SpvB(1-179) fusion protein. In addition, mutations that inactivate genes within Salmonella pathogenicity island 1 or Salmonella pathogenicity island 2 that encode type III secretion systems (TTSS) could secrete the SpvB protein into the culture medium. These results indicate that SpvB protein is transported from the bacteria and into the host cytoplasm independent of TTSS.

Use of the green fluorescent protein to monitor the fate of Serratia entomophila causing amber disease in the New Zealand grass grub, Costelytra zealandica

A series of constitutive green fluorescent protein (pGFPuv) derivatives of the bacterium Serratia entomophila (Enterobacteriaceae) were constructed, allowing the fate of cells causing amber disease ingested by the New Zealand grass grub (Costelytra zealandica, Coleoptera: Scarabaeidae) to be monitored. Examination of tissue and contents of the alimentary tract over time from ingestion, under fluorescence microscopy, revealed that the major site of S. entomophila colonisation in the grass grub is intestinal particulate matter. Visual examinations showed that wild type pathogenic strain persisted in high numbers in the grass grub intestinal tract, notably in the area of the hindgut, but the S. entomophila pADAP-free strain 5.6RC and the pADK mutant derivatives (pADK-4, -10, -13) that gave a non-feeding without gut clearance phenotype, were unable to colonise the gut. The indiscriminate colonisation of the intestinal tract particulate matter by pathogenic bacteria, rather than the colonisation of a specific site of activity, suggests that the bacterial toxins are induced and released from the bacteria while they live freely in the grass grub intestinal tract.

Genome plasticity in Yersinia pestis

Yersinia pestis, the causative agent of bubonic plague, emerged recently (<20000 years ago) as a clone of Yersinia pseudotuberculosis. There is scant evidence of genome diversity in Y. pestis, although it is possible to differentiate three biovars (antiqua, mediaevalis or orientalis) based on two biochemical tests. There are a few examples of restriction fragment length polymorphisms (RFLPs) within Y. pestis; however, their genetic basis is poorly understood. In this study, six difference regions (DFRs) were identified in Y. pestis, by using subtractive hybridization, which ranged from 4.6 to 19 kb in size. Four of the DFRs are flanked by insertion sequences, and their sequences show similarity to bacterial genes encoding proteins for flagellar synthesis, ABC transport, insect toxicity and bacteriophage functions. The presence or absence of these DFRs (termed the DFR profile) was demonstrated in 78 geographically diverse strains of Y. pestis. Significant genome plasticity was observed among these strains and suggests the acquisition and deletion of these DNA regions during the recent evolution of Y. pestis. Y. pestis biovar orientalis possesses DFR profiles that are different from antiqua and mediaevalis biovars, reflecting the recent origins of this biovar. Whereas some DFR profiles are specific for antiqua and mediaevalis, some DFR profiles are shared by both biovars. Furthermore, the progenitor of Y. pestis, Y. pseudotuberculosis (an enteric pathogen), possesses its own DFR profile. The DFR profiles detailed here demonstrate genome plasticity within Y. pestis, and they imply evolutionary relationships among the three biovars of Y. pestis, as well as between Y. pestis and Y. pseudotuberculosis.

Genomewide identification of proteins secreted by the Hrp type III protein secretion system of Pseudomonas syringae pv. tomato DC3000

The ability of Pseudomonas syringae pv. tomato DC3000 to be pathogenic on plants depends on the Hrp (hypersensitive response and pathogenicity) type III protein secretion system and the effector proteins it translocates into plant cells. Through iterative application of experimental and computational techniques, the DC3000 effector inventory has been substantially enlarged. Five homologs of known avirulence (Avr) proteins and five effector candidates, encoded by genes with putative Hrp promoters and signatures of horizontal acquisition, were demonstrated to be secreted in culture and/or translocated into Arabidopsis in a Hrp-dependent manner. These 10 Hrp-dependent outer proteins (Hops) were designated HopPtoC (AvrPpiC2 homolog), HopPtoD1 and HopPtoD2 (AvrPphD homologs), HopPtoK (AvrRps4 homolog), HopPtoJ (AvrXv3 homolog), HopPtoE, HopPtoG, HopPtoH, HopPtoI, and HopPtoS1 (an ADP-ribosyltransferase homolog). Analysis of the enlarged collection of proteins traveling the Hrp pathway in P. syringae revealed an export-associated pattern of equivalent solvent-exposed amino acids in the N-terminal five positions, a lack of Asp or Glu residues in the first 12 positions, and amphipathicity in the first 50 positions. These characteristics were used to search the unfinished DC3000 genome, yielding 32 additional candidate effector genes that predicted proteins with Hrp export signals and that also possessed signatures of horizontal acquisition. Among these were genes encoding additional ADP-ribosyltransferases, a homolog of SrfC (a candidate effector in Salmonella enterica), a catalase, and a glucokinase. One ADP-ribosyltransferase and the SrfC homolog were tested and shown to be secreted in a Hrp-dependent manner. These proteins, designated HopPtoS2 and HopPtoL, respectively, bring the DC3000 Hrp-secreted protein inventory to 22.

Restriction map of the Serratia entomophila plasmid pADAP carrying virulence factors for Costelytra zealandica

Some strains of the Enterobacteriaceae Serratia entomophila and S. proteamaculans cause amber disease in the grass grub Costelytra zealandica (Coleoptera: Scarabaeidae), an important pasture pest in New Zealand. The virulence determinants of the disease reside on a large plasmid designated pADAP (amber disease-associated plasmid). A BamHI, EcoRI, and HindIII restriction cleavage map of pADAP was constructed by means of cloning restriction fragments. Each fragment was mapped, and neighboring fragments of mapped clones were systematically isolated from libraries using DNA probes constructed from previously cloned fragments. Through the use of sniff sequencing from the distal ends of a number of pADAP subclones the location of putative IS elements and genes involved in replication and conjugation were identified and assigned on the map. The location of the amber disease virulence-associated region was also mapped. The final map of pADAP spans 155 kb, 40 kb larger than the previous estimate.

Oral toxicity of Photorhabdus luminescens W14 toxin complexes in Escherichia coli

Previous attempts to express the toxin complex genes of Photorhabdus luminescens W14 in Escherichia coli have failed to reconstitute their oral toxicity to the model insect Manduca sexta. Here we show that the combination of three genes, tcdA, tcdB, and tccC, is essential for oral toxicity to M. sexta when expression in E. coli is used. Further, when transcription from native toxin complex gene promoters is used, maximal toxicity in E. coli cultures is associated with the addition of mitomycin C to the growth medium. In contrast, the expression of tcdAB (or the homologous tcaABC operon) with no recombinant tccC homolog in a different P. luminescens strain, K122, is sufficient to confer oral toxicity on this strain, which is otherwise not orally toxic. We therefore infer that P. luminescens K122 carries a functional tccC-like homolog within its own genome, a hypothesis supported by Southern analysis. Recombinant toxins from both P. luminescens K122 and E. coli were purified as high-molecular-weight particulate preparations. Transmission electron micrograph (TEM) images of these particulate preparations showed that the expression of tcdAB (either with or without tccC) in E. coli produces visible approximately 25-nm-long complexes with a head and tail-like substructure. These data are consistent with a model whereby TcdAB constitutes the majority of the complex visible under TEM and TccC either is a toxin itself or is an activator of the complex. The implications for the potential mode of action of the toxin complex genes are discussed.

Pathobiology of amber disease, caused by Serratia Spp., in the New Zealand grass grub, Costelytra zealandica

Amber disease in the New Zealand grass grub (Costelytra zealandica) is caused by some strains of Serratia entomophila or Serratia proteamaculans (Enterobacteriaceae). When treated with pathogenic isolates, larvae ceased feeding within 48 h, developed an amber coloration after 72 h, and entered a long chronic phase without feeding. An acute dose of 2-4 x 10(4) pathogenic bacteria was sufficient to produce disease in 50% of treated larvae. Time to death was directly related to temperature. At 15 degrees C, infected larvae remained in a chronic, nonfeeding state for more than 4 months prior to death. Nonpathogenic isolates, lacking the disease-causing plasmid (pADAP), had no effect on either feeding or disease. Twenty-four hours after ingestion, bacteria were found predominantly in the hindgut and growth occurred primarily within the fermentation chamber and in the head section of the larvae. Nonpathogenic strains did not multiply in treated larvae. Treatment of diseased larvae with antibiotic eliminated Serratia cells from the insects but did not result in restoration of feeding or the dark gut characteristic of the healthy larva.

Measuring virulence factor expression by the pathogenic bacterium Photorhabdus luminescens in culture and during insect infection

During insect infection Photorhabdus luminescens emits light and expresses virulence factors, including insecticidal toxin complexes (Tcs) and an RTX-like metalloprotease (Prt). Using quantitative PCR and protein assays, we describe the expression patterns of these factors both in culture and during insect infection and compare them to the associated bacterial growth curves. In culture, light and active Prt protease are produced in stationary phase. Tca also appears in stationary phase, whereas Tcd is expressed earlier. These patterns seen in a culture flask are strikingly similar to those observed during insect infection. Thus, in an infected insect, bacteria grow exponentially until the time of insect death at approximately 48 h, when both light and the virulence factors Prt protease and Tca are produced. In contrast, Tcd appears much earlier in insect infection. However, at present, the biological significance of this difference in timing of the production of the two toxins in unclear. This is the first documentation of the expression of Tcs and Prt in an insect and highlights the malleability of Photorhabdus as a model system for bacterial infection.