Membrane localization of the S1 subunit of pertussis toxin in Bordetella pertussis and implications for pertussis toxin secretion

Contribution of a Novel Pertussis Toxin-Like Factor in Mediating Persistent Otitis Media

Chronic otitis media (COM) is the long-term infection and inflammation of the middle ears typically caused by upper respiratory tract pathogens that are able to ascend the Eustachian tube. Our understanding of contributing factors is limited because human otopathogens cannot naturally colonize or persist in the middle ears of mice. We recently described a natural COM in mice caused by Bordetella pseudohinzii and proposed this as an experimental system to study bacterial mechanisms of immune evasion that allow persistent infection of the middle ear. Here we describe a novel pertussis toxin (PTx)-like factor unique to B. pseudohinzii, apparently acquired horizontally, that is associated with its particularly efficient persistence and pathogenesis. The catalytic subunit of this toxin, PsxA, has conserved catalytic sites and substantial predicted structural homology to pertussis toxin catalytic subunit PtxA. Deletion of the gene predicted to encode the catalytic subunit, psxA, resulted in a significant decrease in persistence in the middle ears. The defect was not observed in mice lacking T cells, indicating that PsxA is necessary for persistence only when T cells are present. These results demonstrate the role of a novel putative toxin in the persistence of B. pseudohinzii and its generation of COM. This PsxA-mediated immune evasion strategy may similarly be utilized by human otopathogens, via other PTx-like toxins or alternative mechanisms to disrupt critical T cell functions necessary to clear bacteria from the middle ear. This work demonstrates that this experimental system can allow for the detailed study of general strategies and specific mechanisms that otopathogens use to evade host immune responses to persist in the middle ear to cause COM.

Secretion of Pertussis Toxin from Bordetella pertussis

Production and secretion of pertussis toxin (PT) is essential for the virulence of Bordetella pertussis. Due to the large oligomeric structure of PT, transport of the toxin across bacterial membrane barriers represents a significant hurdle that the bacteria must overcome in order to maintain pathogenicity. During the secretion process, PT undergoes a two-step transport process. The first step involves transport of the individual polypeptide chains of PT across the inner membrane utilizing a generalized secretion pathway, most likely the bacterial Sec system. The second step involves the use of a specialized apparatus to transport the toxin across the outer membrane of the bacterial cell. This apparatus, which has been termed the Ptl transporter and which is unique to the PT secretion pathway, is a member of the type IV family of bacterial transporters. Here, the current understanding of the PT secretion process is reviewed including a description of the Ptl proteins that assemble to form the transporter, the general structure of type IV transporters, the known similarities and differences between canonical type IV substrate transport and Ptl-mediated transport of PT, as well as the known sequence of events in the assembly and secretion of PT.

Acquisition and Adaptation of Ultra-small Parasitic Reduced Genome Bacteria to Mammalian Hosts

The first cultivated representative of the enigmatic phylum Saccharibacteria (formerly TM7) was isolated from humans and revealed an ultra-small cell size (200-300 nm), a reduced genome with limited biosynthetic capabilities, and a unique parasitic lifestyle. TM7x was the only cultivated member of the candidate phyla radiation (CPR), estimated to encompass 26% of the domain Bacteria. Here we report on divergent genomes from major lineages across the Saccharibacteria phylum in humans and mammals, as well as from ancient dental calculus. These lineages are present at high prevalence within hosts. Direct imaging reveals that all groups are ultra-small in size, likely feeding off commensal bacteria. Analyses suggest that multiple acquisition events in the past led to the current wide diversity, with convergent evolution of key functions allowing Saccharibacteria from the environment to adapt to mammals. Ultra-small, parasitic CPR bacteria represent a relatively unexplored paradigm of prokaryotic interactions within mammalian microbiomes.

A PolyQ Membrane Protein of Vibrio cholerae Acts as the Receptor for Phage Infection

Bacteriophage VP1 is a typing phage used for the phage subtyping of Vibrio cholerae O1 biotype El Tor, but the molecular mechanisms of its receptor recognition and the resistance of its host to infection are mostly unknown. In this study, we aimed to identify the host receptor and its role in resistance in natural VP1-resistant strains. Generating spontaneous resistance mutations and genome sequencing mutant strains found the polyQ protein VcpQ, which carries 46 glutamine residues in its Q-rich region, to be responsible for infection by VP1. VcpQ is a membrane protein and possibly forms homotrimers. VP1 adsorbed to V. cholerae through VcpQ. Sequence comparisons showed that 72% of natural VP1-resistant strains have fewer glutamines in the VcpQ Q-rich stretch than VP1-sensitive strains. This difference did not affect the membrane location and oligomer of VcpQ but abrogated VP1 adsorption. These mutant VcpQs did not recover VP1 infection sensitivity in a V. cholerae strain with vcpQ deleted. Our study revealed that the polyQ protein VcpQ is responsible for the binding of VP1 during its infection of V. cholerae and that glutamine residue reduction in VcpQ affects VP1 adsorption to likely be the main cause of VP1 resistance in natural resistant strains. The physiological functions of this polyQ protein in bacteria need further clarification; however, mutations in the polyQ stretch may endow V. cholerae with phage resistance and enhance survival against VP1 or related phages.IMPORTANCE Receptor recognition and binding by bacteriophage are the first step for its infection of bacterial cells. In this study, we found the Vibrio cholerae subtyping phage VP1 uses a polyQ protein named VcpQ (V. cholerae polyQ protein) as the receptor for VP1 infection. Our study reveals the receptor's recognition of phage VP1 during its adsorption and the VP1 resistance mechanism of the wild resistant V. cholerae strains bearing the mutagenesis in the receptor VcpQ. These mutations may confer the survival advantage on these resistant strains in the environment containing VP1 or its similar phages.

An In-Silico Sequence-Structure-Function Analysis of the N-Terminal Lobe in CT Group Bacterial ADP-Ribosyltransferase Toxins

The C3-like toxins are single-domain proteins that represent a minimal mono-ADP-ribosyl transferase (mART) enzyme with a simple model scaffold for the entire cholera toxin (CT)-group. These proteins possess a single (A-domain) that modifies Rho proteins. In contrast, C2-like toxins require a binding/translocation partner (B-component) for intoxication. These are A-only toxins that contain the E-x-E motif, modify G-actin, but are two-domains with a C-domain possessing enzymatic activity. The N-domain of the C2-like toxins is unstructured, and its function is currently unknown. A sequence-structure-function comparison was performed on the N-terminal region of the mART domain of the enzymatic component of the CT toxin group in the CATCH fold (3.90.210.10). Special consideration was given to the N-domain distal segment, the α-lobe (α₁–α₄), and its different roles in these toxin sub-groups. These results show that the role of the N-terminal α-lobe is to provide a suitable configuration (i) of the α₂–α₃ helices to feature the α3-motif that has a role in NAD⁺ substrate binding and possibly in the interaction with the protein target; (ii) the α₃–α₄ helices to provide the α_3/4-loop with protein-protein interaction capability; and (iii) the α₁-N_tail that features specialized motif(s) according to the toxin type (A-only or A-B toxins) exhibiting an effect on the catalytic activity via the ARTT-loop, with a role in the inter-domain stability, and with a function in the binding and/or translocation steps during the internalization process.

Humanised monoclonal antibodies neutralise pertussis toxin by receptor blockade and reduced retrograde trafficking

Pertussis toxin (PTx) is a major protective antigen produced by Bordetella pertussis that is included in all current acellular vaccines. Of several well‐characterized monoclonal antibodies binding this toxin, the humanised hu1B7 and hu11E6 antibodies are highly protective in multiple in vitro and in vivo assays. In this study, we determine the molecular mechanisms of protection mediated by these antibodies. Neither antibody directly binds the B. pertussis bacterium nor supports antibody‐dependent complement cytotoxicity. Both antibodies, either individually or as a cocktail, form multivalent complexes with soluble PTx that bind the FcγRIIb receptor more tightly than antibody alone, suggesting that the antibodies may accelerate PTx clearance via immune complex formation. However, a receptor binding assay and cellular imaging indicate that the main mechanism used by hu11E6 is competitive inhibition of PTx binding to its cellular receptor. In contrast, the main hu1B7 neutralising mechanism appears to be inhibition of PTx internalisation and retrograde trafficking. We assessed the effects of hu1B7 on PTx retrograde trafficking in CHO‐K1 cells using quantitative immunofluorescence microscopy. In the absence of hu1B7 or after incubation with an isotype control antibody, PTx colocalizes to organelles in a manner consistent with retrograde transport. However, after preincubation with hu1B7, PTx appears restricted to the membrane surface with colocalization to organelles associated with retrograde transport significantly reduced. Together, these data support a model whereby hu11E6 and hu1B7 interfere with PTx receptor binding and PTx retrograde trafficking, respectively.

Characterization of Individual Human Antibodies That Bind Pertussis Toxin Stimulated by Acellular Immunization

Despite high vaccination rates, the incidence of whooping cough has steadily been increasing in developing countries for several decades. The current acellular pertussis (aP) vaccines all include the major protective antigen pertussis toxin (PTx) and are safer, but they appear to be less protective than infection or older, whole-cell vaccines. To better understand the attributes of individual antibodies stimulated by aP, we isolated plasmablast clones recognizing PTx after booster immunization of two donors. Five unique antibody sequences recognizing native PTx were recovered and expressed as recombinant human IgG1 antibodies. The antibodies all bind different epitopes on the PTx S1 subunit, B oligomer, or S1-B subunit interface, and just one clone neutralized PTx in an in vitro assay. To better understand the epitopes bound by the nonneutralizing S1-subunit antibodies, comprehensive mutagenesis with yeast display provided a detailed map of the epitope recognized by antibodies A8 and E12. Residue R76 is required for antibody A8 binding and is present on the S1 surface but is only partially exposed in the holotoxin, providing a structural explanation for A8's inability to neutralize holotoxin. The B-subunit-specific antibody D8 inhibited PTx binding to a model receptor and neutralized PTx in vitro as well as in an in vivo leukocytosis assay. This is the first study, to our knowledge, to identify individual human antibodies stimulated by the acellular pertussis vaccine and demonstrates the feasibility of using these approaches to address outstanding issues in pertussis vaccinology, including mechanisms of accelerated waning of protective immunity despite repeated aP immunization.

Investigating pertussis toxin and its impact on vaccination

Whooping cough, caused by Bordetella pertussis, remains a major global health problem. Each year around 40 million of pertussis cases resulting in 200,000-400,000 annual deaths occur worldwide. Pertussis toxin is a major virulence factor of B. pertussis. Murine studies have shown its importance in bacterial colonization and in immunomodulation to evade innate or adaptive immunity. The toxin is composed of an A protomer expressing ADP-ribosyltransferase activity and a B oligomer, responsible for toxin binding to target cells. The toxin is also a major protective antigen in all currently available vaccines. However, vaccine escape mutants with altered toxin expression have recently been isolated in countries with high vaccination coverage illustrating the need for improved pertussis vaccines.

Assembly and mechanisms of bacterial type IV secretion machines

Type IV secretion occurs across a wide range of prokaryotic cell envelopes: Gram-negative, Gram-positive, cell wall-less bacteria and some archaea. This diversity is reflected in the heterogeneity of components that constitute the secretion machines. Macromolecules are secreted in an ATP-dependent process using an envelope-spanning multi-protein channel. Similar to the type III systems, this apparatus extends beyond the cell surface as a pilus structure important for direct contact and penetration of the recipient cell surface. Type IV systems are remarkably versatile in that they mobilize a broad range of substrates, including single proteins, protein complexes, DNA and nucleoprotein complexes, across the cell envelope. These machines have broad clinical significance not only for delivering bacterial toxins or effector proteins directly into targeted host cells, but also for direct involvement in phenomena such as biofilm formation and the rapid horizontal spread of antibiotic resistance genes among the microbial community.

ClubSub-P: Cluster-Based Subcellular Localization Prediction for Gram-Negative Bacteria and Archaea

The subcellular localization (SCL) of proteins provides important clues to their function in a cell. In our efforts to predict useful vaccine targets against Gram-negative bacteria, we noticed that misannotated start codons frequently lead to wrongly assigned SCLs. This and other problems in SCL prediction, such as the relatively high false-positive and false-negative rates of some tools, can be avoided by applying multiple prediction tools to groups of homologous proteins. Here we present ClubSub-P, an online database that combines existing SCL prediction tools into a consensus pipeline from more than 600 proteomes of fully sequenced microorganisms. On top of the consensus prediction at the level of single sequences, the tool uses clusters of homologous proteins from Gram-negative bacteria and from Archaea to eliminate false-positive and false-negative predictions. ClubSub-P can assign the SCL of proteins from Gram-negative bacteria and Archaea with high precision. The database is searchable, and can easily be expanded using either new bacterial genomes or new prediction tools as they become available. This will further improve the performance of the SCL prediction, as well as the detection of misannotated start codons and other annotation errors. ClubSub-P is available online at http://toolkit.tuebingen.mpg.de/clubsubp/

The ins and outs of pertussis toxin

Pertussis toxin, produced and secreted by the whooping cough agent Bordetella pertussis, is one of the most complex soluble bacterial proteins. It is actively secreted through the B. pertussis cell envelope by the Ptl secretion system, a member of the widespread type IV secretion systems. The toxin is composed of five subunits (named S1 to S5 according to their decreasing molecular weights) arranged in an A-B structure. The A protomer is composed of the enzymatically active S1 subunit, which catalyzes ADP-ribosylation of the α subunit of trimeric G proteins, thereby disturbing the metabolic functions of the target cells, leading to a variety of biological activities. The B oligomer is composed of 1S2:1S3:2S4:1S5 and is responsible for binding of the toxin to the target cell receptors and for intracellular trafficking via receptor-mediated endocytosis and retrograde transport. The toxin is one of the most important virulence factors of B. pertussis and is a component of all current vaccines against whooping cough.

Expression and purification of HtpX-like small heat shock integral membrane protease of an unknown organism related to Methylobacillus flagellatus

The M48 conserved family of peptidases contains a single catalytic zinc ion tetrahedrally co-ordinated by two histidines within an HEXXH motif. The proteases of this class are generally toxic to the cell and thus difficult to express and purify. Here, we report the expression and purification of the small HtpX-like heat shock metalloprotease from an unknown organism related to the obligate methylotrophic anaerobic bacterium, Methylobacillus flagellatus. The protease was expressed in the Escherichia coli vector - pT7. Optimization of expression was done to increase the yield and solubility of the expressed protein. Improved refolding procedures from inclusion bodies of pT7 E. coli system were devised to get the protease in an active and stable form. The protease was purified to near homogeneity in its active form from the refolded proteins of the inclusion bodies by a two-step (cation exchange followed by gel filtration) high performance liquid chromatography (HPLC). The purified protease was active on zymography and casein hydrolysis assays. The activity of the protease was found to be optimum at pH 7.4 and at a temperature of 37 degrees C but significant activity was also retained at higher temperatures of 45-50 degrees C. Centrifugal fractionation showed that it is a membrane localized endopeptidase. The methods described here can serve as guidelines to express and purify other homologues of M48 family of proteases for functional and structural studies.

Secretion of heterologous proteins in Bacillus subtilis can be improved by engineering cell components affecting post-translocational protein folding and degradation

AIMS

To explore the potential to enhance secretion of heterologous proteins in Bacillus subtilis by engineering cell factors affecting extracytoplasmic protein folding and degradation.

METHODS AND RESULTS

Bottleneck components affecting the extracytoplasmic phase of protein secretion were genetically engineered and their effects on the secretion of 11 industrially interesting heterologous proteins were studied by Western blotting and enzymatic assays. Overproduction of PrsA lipoprotein enhanced the secretion of alpha-amylase of Bacillus stearothermophilus (fourfold) and pneumolysin (1.5-fold). Increasing the net negative charge of the cell wall because of lack of the d-alanine substitution of anionic cell wall polymers enhanced the secretion of pneumolysin c. 1.5-fold. Decreasing the level of HtrA-type quality control proteases caused harmful effects on growth and did not enhance secretion. Pertussis toxin subunit, S1 was found to be a substrate for HtrA-type proteases and its secretion was dependent on these proteases.

CONCLUSIONS

Secretion of heterologous proteins can be enhanced by engineering components involved in late stages of secretion in a protein-dependent manner.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study revealed both possibilities and limitations of modulating the post-translocational phase of secretion as a means to improve the yield of heterologous proteins.

Analysis of subassemblies of pertussis toxin subunits in vivo and their interaction with the ptl transport apparatus

Pertussis toxin (PT) has an AB(5) structure that is typical of many bacterial protein toxins; however, this toxin is more complex than many toxins since it is composed of five different subunit types, subunits S1 to S5. Little is known about how PT assembles in vivo and how and when it interacts with its secretion apparatus, known as the Ptl transporter. In order to better understand these events, we expressed subsets of the genes encoding the S1, S2, and/or S4 subunits of PT in strains of Bordetella pertussis that either did or did not produce the Ptl proteins. We found evidence to suggest that certain subassemblies of the toxin, including subassemblies consisting of the S1 subunit and incomplete forms of the B oligomer, can form in vivo, at least transiently. These results suggest that the B oligomer of the toxin does not need to completely form before interactions between the S1 subunit and B-oligomer subunits can occur in vivo. All subassemblies localized primarily to the membrane fraction of the cell. Moreover, we found that Ptl-mediated secretion occurs in a strain that produces S1 and an incomplete complement of B-oligomer subunits. These results indicate that subassemblies of the toxin consisting of the S1 subunit and a partial B oligomer can interact with the Ptl system.

The versatile bacterial type IV secretion systems

Bacteria use type IV secretion systems for two fundamental objectives related to pathogenesis--genetic exchange and the delivery of effector molecules to eukaryotic target cells. Whereas gene acquisition is an important adaptive mechanism that enables pathogens to cope with a changing environment during invasion of the host, interactions between effector and host molecules can suppress defence mechanisms, facilitate intracellular growth and even induce the synthesis of nutrients that are beneficial to bacterial colonization. Rapid progress has been made towards defining the structures and functions of type IV secretion machines, identifying the effector molecules, and elucidating the mechanisms by which the translocated effectors subvert eukaryotic cellular processes during infection.

Temporal expression of pertussis toxin and Ptl secretion proteins by Bordetella pertussis

Pertussis toxin is an AB(5) toxin comprised of protein subunits S1 through S5. The individual subunits are secreted by a Sec-dependent mechanism into the periplasm, where the toxin is assembled. The Ptl type IV secretion system mediates secretion of assembled toxin past the outer membrane. In this study, we examined the time course of protein expression, toxin assembly, and secretion as a function of the bacterial growth cycle. Logarithmic growth was observed after a 1-h lag phase. Secreted toxin was first observed at 3 h. Secretion continued throughout the logarithmic growth phase and decreased as the culture entered the stationary phase after about 24 h. On a per cell basis, toxin secretion occurred at a constant rate of 3 molecules/min/cell from 2 to 18 h. More of toxin subunits S1, S2, and S3 were produced than were secreted, resulting in periplasmic accumulation. Periplasmic S1, S2, and S3 were found to be soluble in the periplasm, as well as membrane associated. About one-half of the periplasmic S1, S2 and S3 subunits were incorporated into holotoxin. Secretion component PtlF was present at a low level at time zero, and the level increased between 2 and 24 h from 30 to 1,000 molecules per cell; however, the initial level of PtlF, 30 molecules per cell, supported maximal secretion. The accumulation of both periplasmic toxin and secretion components suggests that translation rates exceed the rate of secretion and that secretion, not toxin and Ptl complex assembly, is rate limiting.

Reduced glutathione is required for pertussis toxin secretion by Bordetella pertussis

The abilities of cysteine-containing compounds to support growth of Bordetella pertussis and influence pertussis toxin transcription, assembly, and secretion were examined. Cysteine is an essential amino acid for B. pertussis and must be present for protein synthesis and bacterial growth. However, cysteine can be metabolized to sulfate, and high concentrations of sulfate can selectively inhibit transcription of the virulence factors, including pertussis toxin, via the BvgAS two-component regulatory system in a process called modulation. In addition, pertussis toxin possesses several disulfide bonds, and the cysteine-containing compound glutathione can influence oxidation-reduction reactions and perhaps disulfide bond formation. Bacterial growth was not observed in the absence of a source of cysteine. Oxidized glutathione, as a sole source of cysteine, also did not support bacterial growth. Cysteine, cystine, and reduced glutathione did support bacterial growth, and none of these compounds caused modulation at the concentrations tested. Similar amounts of periplasmic pertussis toxin were detected regardless of the source of cysteine; however, in the absence of reduced glutathione, pertussis toxin was not efficiently secreted. Addition of the reducing agent dithiothreitol was unable to compensate for the lack of reduced glutathione and did not promote secretion of pertussis toxin. These results suggest that reduced glutathione does not affect the accumulation of assembled active pertussis toxin in the periplasm but plays a role in efficient pertussis toxin secretion by the bacterium.

Mechanism of association of adenylate cyclase toxin with the surface of Bordetella pertussis: a role for toxin-filamentous haemagglutinin interaction

Adenylate cyclase (AC) toxin from Bordetella pertussis is unusual in that, unlike most other members of the repeats-in-toxin family that are released into the extracellular milieu, it remains associated with the bacterial surface. In this study, we investigated the nature of the association of this toxin with the surface of B. pertussis. AC toxin was extracted from crude outer membrane preparations of B. pertussis with 8 M urea, but only partially with alkaline sodium carbonate and not at all with octylglucoside, suggesting that denaturation of the toxin is necessary for its removal from the membrane. B. pertussis mutants lacking filamentous haemagglutinin (FHA) released significantly more AC toxin into the medium, and AC toxin association with the bacterial surface was partially restored by expression of FHA from a plasmid, suggesting a role for FHA in surface retention of AC toxin. AC toxin distribution was unaffected by the absence of pertactin, or full-length lipopolysaccharide, or a defect in secretion of pertussis toxin. Using overlay and immunoprecipitation, we found that a direct physical association can occur between AC toxin and FHA. Combined, these findings suggest that FHA may play a role in AC toxin retention on the surface of B. pertussis and raise the possibility of an involvement of adherence mediated by FHA in delivery of AC toxin from the bacterium to the target cell.

Bacterial conjugation: a two-step mechanism for DNA transport

Bacterial conjugation is a promiscuous DNA transport mechanism. Conjugative plasmids transfer themselves between most bacteria, thus being one of the main causal agents of the spread of antibiotic resistance among pathogenic bacteria. Moreover, DNA can be transferred conjugatively into eukaryotic host cells. In this review, we aim to address several basic questions regarding the DNA transfer mechanism. Conjugation can be visualized as a DNA rolling-circle replication (RCR) system linked to a type IV secretion system (T4SS), the latter being macromolecular transporters widely involved in pathogenic mechanisms. The scheme 'replication + secretion' suggests how the mechanism would work on the DNA substrate and at the bacterial membrane. But, how do these two parts come into contact? Furthermore, how is the DNA transported? T4SS are known to be involved in protein secretion in different organisms, but DNA is a very different macromolecule. The so-called coupling proteins could be the answer to both questions by performing a dual role in conjugation: coupling the two main components of the machinery (RCR and T4SS) and actively mediating DNA transport. We postulate that the T4SS is responsible for transport of the pilot protein (the relaxase) to the recipient. The DNA that is covalently linked to it is initially transported in a passive manner, trailing on the relaxase. We speculate that the pilus appendage could work as a needle, thrusting the substrate proteins to cross one or several membrane barriers into the recipient cytoplasm. This is the first step in conjugation. The second step is the active pumping of the DNA to the recipient, using the already available T4SS transport conduit. It is proposed that this second step is catalysed by the coupling proteins. Our 'shoot and pump' model solves the protein-DNA transport paradox of T4SS.

Bacterial secrets of secretion: EuroConference on the biology of type IV secretion processes

Type IV secretion systems (TFSS) mediate secretion or direct cell-to-cell transfer of virulence factors (proteins or protein-DNA complexes) from many Gram-negative animal, human and plant pathogens, such as Agrobacterium tumefaciens, Bartonella tribocorum, Bordetella pertussis, Brucella suis, Helicobacter pylori, Legionella pneumophila and Rickettsia prowazekii, into eukaryotic cells. Bacterial conjugation is also classified as a TFSS-like process mediating the spread of broad-host-range plasmids between Gram-negative bacteria such as RP4 and R388, which carry antibiotic resistance genes. Genetic, biochemical, cell biological and structural biology experiments led to significant progress in the understanding of several aspects of TFSS processes. X-ray crystallography revealed that homologues of the A. tumefaciens inner membrane-associated proteins VirB11 and VirD4 from H. pylori and R388, respectively, may form channels for substrate translocation or assembly of the transmembrane TFSS machinery. Biochemical and cell biological experiments revealed interactions between components of the periplasmic core components VirB8, VirB9 and VirB10, which may form the translocation channel. Analysis of A. tumefaciens virulence proteins VirE2 and VirF suggested that the periplasmic translocation route of the pertussis toxin from B. pertussis may be more generally valid than previously anticipated. Secretion and modification of toxins from H. pylori and L. pneumophila profoundly affect host cell metabolism, thus entering the discipline of cellular microbiology. Finally, results from genome sequencing projects revealed the presence of up to three TFSS in a single organism, and the analysis of their interplay and adaptation to different functions will be a future challenge. TFSS-carrying plasmids were discovered in different ecosystems, suggesting that genetic exchange may speed up their evolution and adaptation to different cell-cell interactions.