Use of the stationary phase inducible promoters, spv and dps, to drive heterologous antigen expression in Salmonella vaccine strains

Oral vaccination with recombinant Lactobacillus plantarum encoding Trichinella spiralis inorganic pyrophosphatase elicited a protective immunity in BALB/c mice

Background

Trichinellosis is a serious zoonotic disease distributed around the world. It is needed to develop a safe, effective and feasible anti-Trichinella vaccine for prevention and control of trichinellosis. The aim of this study was to construct a recombinant Lactobacillus plantarum encoding Trichinella spiralis inorganic pyrophosphatase (TsPPase) and investigate its immune protective effects against T. spiralis infection.

Methodology/Principal findings

The growth of recombinant L. plantarum was not affected by TsPPase/pSIP409-pgsA′ plasmid, and the recombinant plasmid was inherited stably in bacteria. Western blot and immunofluorescence assay (IFA) indicated that the rTsPPase was expressed on the surface of recombinant L. plantarum. Oral vaccination with rTsPPase induced higher levels of specific serum IgG, IgG1, IgG2a and mucosal secretory IgA (sIgA) in BALB/c mice. ELISA analysis revealed that the levels of IFN-γ and IL-4 released from spleen, mesenteric lymph nodes and Peyer’s patches were evidently increased at 2–4 weeks following vaccination, compared to MRS (De Man, Rogosa, Sharpe) medium control group (P < 0.05). Immunization of mice with rTsPPase exhibited a 67.18, 54.78 and 51.91% reduction of intestinal infective larvae, adult worms and muscle larvae at 24 hours post infection (hpi), 6 days post infection (dpi) and 35 dpi, respectively (P < 0.05), and the larval molting and development was significantly inhibited by 45.45% at 24 hpi, compared to the MRS group.

Conclusions

TsPPase plays a crucial role in T. spiralis molting and development, oral vaccination with rTsPPase induced a significant local mucosal sIgA response and systemic Th1/Th2 immune response, and immune protection against T. spiralis infection in BALB/c mice.

In the previous study, a Trichinella spiralis inorganic pyrophosphatase (TsPPase) was expressed and its role in larval molting and development was observed. In this study, a recombinant TsPPase/pSIP409-pgsA′ plasmid was constructed and transferred into Lactobacillus plantarum NC8, the rTsPPase was expressed on the surface of recombinant L. plantarum NC8. Oral immunization of mice with rTsPPase DNA vaccine elicited a high level of specific serum IgG, IgG1, IgG2a and mucosal secretory IgA (sIgA). The levels of IFN-γ and IL-4 released from spleen, mesenteric lymph nodes and Peyer’s patches were evidently increased at 2–4 weeks following vaccination. Immunization of mice with rTsPPase showed a significant reduction of intestinal infective larvae, adult worms and muscle larvae, and intestinal larval molting and development was significantly suppressed. The results indicated that oral vaccination with rTsPPase elicited a significant local mucosal sIgA response and specific systemic Th1/Th2 immune response, and an obvious protective immunity against T. spiralis infection.

Regulated Delayed Shigella flexneri 2a O-antigen Synthesis in Live Recombinant Salmonella enterica Serovar Typhimurium Induces Comparable Levels of Protective Immune Responses with Constitutive Antigen Synthesis System

Shigella flexneri (S. flexneri), a leading cause of bacillary dysentery, is a major public health concern particularly affecting children in developing nations. We have constructed a novel attenuated Salmonella vaccine system based on the regulated delayed antigen synthesis (RDAS) and regulated delayed expression of attenuating phenotype (RDEAP) systems for delivering the S. flexneri 2a (Sf2a) O-antigen.

Methods: The new Salmonella vaccine platform was constructed through chromosomal integration of the araC P_BAD lacI and araC P_BAD wbaP cassettes, resulting in a gradual depletion of WbaP enzyme. An expression vector, encoding Sf2a O-antigen biosynthesis under the control of the LacI-repressible P_trc promoter, was maintained in the Salmonella vaccine strain through antibiotic-independent selection. Mice immunized with the vaccine candidates were evaluated for cell-mediate and humoral immune responses.

Results: In the presence of exogenous arabinose, the Salmonella vaccine strain synthesized native Salmonella LPS as a consequence of WbaP expression. Moreover, arabinose supported LacI expression, thereby repressing Sf2a O-antigen production. In the absence of arabinose in vivo, native Salmonella LPS synthesis is repressed whilst the synthesis of the Sf2a O-antigen is induced. Murine immunization with the Salmonella vaccine strain elicited robust Sf2a-specific protective immune responses together with long term immunity.

Conclusion: These findings demonstrate the protective efficacy of recombinant Sf2a O-antigen delivered by a Salmonella vaccine platform.

New technologies in developing recombinant attenuated Salmonella vaccine vectors

Recombinant attenuated Salmonella vaccine (RASV) vectors producing recombinant gene-encoded protective antigens should have special traits. These features ensure that the vaccines survive stresses encountered in the gastrointestinal tract following oral vaccination to colonize lymphoid tissues without causing disease symptoms and to result in induction of long-lasting protective immune responses. We recently described ways to achieve these goals by using regulated delayed in vivo attenuation and regulated delayed in vivo antigen synthesis, enabling RASVs to efficiently colonize effector lymphoid tissues and to serve as factories to synthesize protective antigens that induce higher protective immune responses. We also developed some additional new strategies to increase vaccine safety and efficiency. Modification of lipid A can reduce the inflammatory responses without compromising the vaccine efficiency. Outer membrane vesicles (OMVs) from Salmonella-containing heterologous protective antigens can be used to increase vaccine efficiency. A dual-plasmid system, possessing Asd+ and DadB+ selection markers, each specifying a different protective antigen, can be used to develop multivalent live vaccines. These new technologies have been adopted to develop a novel, low-cost RASV synthesizing multiple protective pneumococcal protein antigens that could be safe for newborns/infants and induce protective immunity to diverse Streptococcus pneumoniae serotypes after oral immunization.

Salmonella vaccine vectors displaying delayed antigen synthesis in vivo to enhance immunogenicity

We have developed a regulated delayed antigen synthesis (RDAS) system for use in recombinant attenuated Salmonella vaccine (RASV) strains to enhance immune responses by reducing the adverse effects of high-level antigen synthesis. This system includes a chromosomal repressor gene, lacI, expressed from the arabinose-regulated araC PBAD promoter. LacI serves to regulate expression from a plasmid promoter, Ptrc, that directs antigen synthesis. In the presence of arabinose LacI is produced, which binds to Ptrc, blocking antigen synthesis. In vivo, an arabinose-poor environment, the concentration of LacI decreases with each cell division, allowing increased antigen synthesis. To optimize the system and for comparison, we altered the lacI ribosome-binding site, start codon, and/or codon content to construct RDAS strains chi9095, chi9959, and chi9241, synthesizing from low to high levels of LacI, respectively, and non-RDAS strain chi9555 as a control. We evaluated this system with two test antigens, the green fluorescent protein for initial in vitro assessment and the Streptococcus pneumoniae PspA protein for validation of our system in mice. All RASV strains expressing PspA generated high antilipopolysaccharide antibody titers, indicating that expression of lacI did not interfere with the capacity to induce an immune response. Strain chi9241 induced significantly higher anti-PspA IgG and IgA antibody titers than strain chi9555, which expressed PspA constitutively. Anti-PspA antibody titers were inversely correlated to the level of LacI synthesis. Strain chi9241 also induced significantly greater protective efficacy against challenge with virulent S. pneumoniae. These results suggest that regulated delayed antigen synthesis is useful for improving immunogenicity of RASV strains.

Bacterial antigen expression is an important component in inducing an immune response to orally administered Salmonella-delivered DNA vaccines

Background

The use of Salmonella to deliver heterologous antigens from DNA vaccines is a well-accepted extension of the success of oral Salmonella vaccines in animal models. Attenuated S. typhimurium and S. typhi strains are safe and efficacious, and their use to deliver DNA vaccines combines the advantages of both vaccine approaches, while complementing the limitations of each technology. An important aspect of the basic biology of the Salmonella/DNA vaccine platform is the relative contributions of prokaryotic and eukaryotic expression in production of the vaccine antigen. Gene expression in DNA vaccines is commonly under the control of the eukaryotic cytomegalovirus (CMV) promoter. The aim of this study was to identify and disable putative bacterial promoters within the CMV promoter and evaluate the immunogenicity of the resulting DNA vaccine delivered orally by S. typhimurium.

Methodology/Principal Findings

The results reported here clearly demonstrate the presence of bacterial promoters within the CMV promoter. These promoters have homology to the bacterial consensus sequence and functional activity. To disable prokaryotic expression from the CMV promoter a series of genetic manipulations were performed to remove the two major bacterial promoters and add a bacteria transcription terminator downstream of the CMV promoter. S. typhimurium was used to immunise BALB/c mice orally with a DNA vaccine encoding the C-fragment of tetanus toxin (TT) under control of the original or the modified CMV promoter. Although both promoters functioned equally well in eukaryotic cells, as indicated by equivalent immune responses following intramuscular delivery, only the original CMV promoter was able to induce an anti-TT specific response following oral delivery by S. typhimurium.

Conclusions

These findings suggest that prokaryotic expression of the antigen and co-delivery of this protein by Salmonella are at least partially responsible for the successful oral delivery of C-fragment DNA vaccines containing the CMV promoter by S. typhimurium.

Development of non-antibiotic-resistant, chromosomally based, constitutive and inducible expression systems for aroA-attenuated Salmonella enterica Serovar Typhimurium

Live-vaccine delivery systems expressing two model antigens from Mycoplasma hyopneumoniae, F2(P97) (Adh) and NrdF, were constructed using Salmonella enterica serovar Typhimurium aroA (STM-1), and immunogenicity in mice was evaluated. Recombinant plasmid-based expression (PBE) and chromosomally based expression (CBE) systems were constructed. The PBE system was formed by cloning both antigen genes into pJLA507 to create an operon downstream of temperature-inducible promoters. Constitutive CBE was achieved using a promoter-trapping technique whereby the promoterless operon was stably integrated into the chromosome of STM-1, and the expression of antigens was assessed. The chromosomal position of the operon was mapped in four clones. Inducible CBE was obtained by using the in vivo-induced sspA promoter and recombining the expression construct into aroD. Dual expression of the antigens was detected in all systems, with PBE producing much larger quantities of both antigens. The stability of antigen expression after in vivo passage was 100% for all CBE strains recovered. PBE and CBE strains were selected for comparison in a vaccination trial. The vaccine strains were delivered orally into mice, and significant systemic immunoglobulin M (IgM) and IgG responses against both antigens were detected among all CBE groups. No significant immune response was detected using PBE strains. Expression of recombinant antigens in S. enterica serovar Typhimurium aroA from chromosomally located strong promoters without the use of antibiotic resistance markers is a reliable and effective method of inducing a significant immune response.

Rational design of Salmonella recombinant vaccines

Salmonella enterica is an important pathogen of animals and humans causing a variety of infectious diseases. The large number of cases of typhoid fever due to S. enterica serovar Typhi infections gives rise to the continuous need for improved vaccines against this life-threatening infection. However, S. enterica is also an interesting organism to act as a live attenuated carrier for the presentation of recombinant heterologous antigens. Comprehensive experimental studies have been performed and a detailed knowledge of the molecular mechanisms of important virulence factors is available. This allows the rationale design of improved Salmonella carrier strains and the development of novel strategies for the expression and presentation of recombinant antigens. Here, we review recent advances in generation of live attenuated Salmonella vaccines and discuss criteria for expression strategies of heterologous antigens by Salmonella carrier strains.

Oral vaccination with salmonella simultaneously expressing Yersinia pestis F1 and V antigens protects against bubonic and pneumonic plague

The gut provides a large area for immunization enabling the development of mucosal and systemic Ab responses. To test whether the protective Ags to Yersinia pestis can be orally delivered, the Y. pestis caf1 operon, encoding the F1-Ag and virulence Ag (V-Ag) were cloned into attenuated Salmonella vaccine vectors. F1-Ag expression was controlled under a promoter from the caf1 operon; two different promoters (P), PtetA in pV3, PphoP in pV4, as well as a chimera of the two in pV55 were tested. F1-Ag was amply expressed; the chimera in the pV55 showed the best V-Ag expression. Oral immunization with Salmonella-F1 elicited elevated secretory (S)-IgA and serum IgG titers, and Salmonella-V-Ag(pV55) elicited much greater S-IgA and serum IgG Ab titers than Salmonella-V-Ag(pV3) or Salmonella-V-Ag(pV4). Hence, a new Salmonella vaccine, Salmonella-(F1+V)Ags, made with a single plasmid containing the caf1 operon and the chimeric promoter for V-Ag allowed the simultaneous expression of F1 capsule and V-Ag. Salmonella-(F1+V)Ags elicited elevated Ab titers similar to their monotypic derivatives. For bubonic plague, mice dosed with Salmonella-(F1+V)Ags and Salmonella-F1-Ag showed similar efficacy (>83% survival) against approximately 1000 LD(50) Y. pestis. For pneumonic plague, immunized mice required immunity to both F1- and V-Ags because the mice vaccinated with Salmonella-(F1+V)Ags protected against 100 LD(50) Y. pestis. These results show that a single Salmonella vaccine can deliver both F1- and V-Ags to effect both systemic and mucosal immune protection against Y. pestis.

The integration host factor (IHF) integrates stationary-phase and virulence gene expression in Salmonella enterica serovar Typhimurium

The integration host factor (IHF) is a DNA-binding and -bending protein with roles in local DNA structural organization and transcriptional regulation in Gram-negative bacteria. This heterodimeric protein is composed of the two highly homologous subunits IHFalpha and IHFbeta. DNA microarray analysis was used to define the regulon of genes subject to IHF control in Salmonella enterica serovar Typhimurium (S. Typhimurium). The transcription profile of the wild type was compared with those of mutants deficient in IHFalpha, IHFbeta, or both IHFalpha and IHFbeta. Our data reveal a new connection between IHF and the expression of genes required by the bacterium to undergo the physiological changes associated with the transition from exponential growth to stationary phase. When a mutant lacking IHF entered stationary phase, it displayed downregulated expression of classic stationary-phase genes in the absence of any concomitant change in expression of the RpoS sigma factor. Purified IHF was found to bind to the regulatory regions of stationary-phase genes indicating an auxiliary and direct role for IHF in RpoS-dependent gene activation. Loss of IHF also had a profound influence on expression of the major virulence genes and epithelial cell invasion, indicating a role in co-ordinating regulation of the pathogenic traits with adaptation to stationary phase. Although the three mutants showed considerable overlaps in the genes affected by the ihf lesions, the observed patterns were not identical, showing that S. Typhimurium has not one but three overlapping IHF regulons.

How bacteria and their products provide clues to vaccine and adjuvant development

Evidence has emerged that both vertebrates and invertebrates share innate immune pathways involved in the recognition of and the response to micro-organisms, including bacteria and their products. As a consequence, particular degenerate products of bacteria can stimulate and modulate immune responses and influence acquired immunity and, potentially, protection against disease. New knowledge in this field is beginning to explain how vaccine adjuvants work and will facilitate the future development of novel adjuvants and vaccines.

Hierarchical gene regulators adapt to its host milieus

The facultative intracellular pathogen Salmonella enterica serovar Typhimurium possesses an elaborate set of virulence genes that enables the bacterium successfully to move between and adapt to the environment, different host organisms and various micro-niches within a given host. Expression of virulence attributes is by no means constitutive. Rather, the regulation of virulence determinants is highly coordinated and integrated into normal bacterial physiological responses. By integrating discriminating virulence gene regulators with conserved housekeeping regulatory processes, the bacteria can sense alterations in the repertoire of environmental cues, and translate the sensing events into a pragmatic and coordinated expression of virulence genes. While the description of transmissible genetic elements that import global gene regulatory factors into a cell brings conceptual problems into the established regulatory network, the existence of mobile gene regulators may actually enable the bacteria to further modulate virulence expression.

Salmonella vaccines for use in humans: present and future perspectives

In recent years there has been significant progress in the development of attenuated Salmonella enterica serovar Typhi strains as candidate typhoid fever vaccines. In clinical trials these vaccines have been shown to be well tolerated and immunogenic. For example, the attenuated S. enterica var. Typhi strains CVD 908-htrA (aroC aroD htrA), Ty800 (phoP phoQ) and chi4073 (cya crp cdt) are all promising candidate typhoid vaccines. In addition, clinical trials have demonstrated that S. enterica var. Typhi vaccines expressing heterologous antigens, such as the tetanus toxin fragment C, can induce immunity to the expressed antigens in human volunteers. In many cases, the problems associated with expression of antigens in Salmonella have been successfully addressed and the future of Salmonella vaccine development is very promising.

Regulated antigen expression in live recombinant Salmonella enterica serovar Typhimurium strongly affects colonization capabilities and specific CD4(+)-T-cell responses

Regulated antigen expression can influence the immunogenicity of live recombinant Salmonella vaccines, but a rational optimization has remained difficult since important aspects of this effect are incompletely understood. Here, attenuated Salmonella enterica serovar Typhimurium SL3261 strains expressing the model antigen GFP_OVA were used to quantify in vivo antigen levels by flow cytometry and to simultaneously follow the crucial early steps of antigen-specific T-cell responses in mice that are transgenic for a T-cell receptor recognizing ovalbumin. Among seven tested promoters, P(pagC) has the highest activity in murine tissues combined with low in vitro expression, whereas P(tac) has a comparable in vivo and a very high in vitro activity. Both SL3261 (pP(pagC)GFP_OVA) and SL3261 (pP(tac)GFP_OVA) cells can induce potent ovalbumin-specific cellular immune responses following oral administration, but doses almost 1,000-fold lower are sufficient for the in vivo-inducible construct SL3261 (pP(pagC)GFP_OVA) compared to SL3261 (pP(tac)GFP_OVA). This efficacy difference is largely explained by impaired early colonization capabilities of SL3261 (pP(tac)GFP_OVA) cells. Based on the findings of this study, appropriate in vivo expression levels for any given antigen can be rationally selected from the increasing set of promoters with defined properties. This will allow the improvement of recombinant Salmonella vaccines against a wide range of pathogens.

Vaccination of mice with live recombinant Salmonella typhimurium aroA against H. pylori: parameters associated with prophylactic and therapeutic vaccine efficacy

Previously we described a recombinant attenuated Salmonella typhimurium aroA strain (SL3261[pYZ97]) with constitutive expression of plasmid encoded Helicobacter pylori urease subunits A and B (UreAB). Single dose oral vaccination effectively induced prophylactic immunity against bacterial challenge in BALB/c mice. Here we successfully extended this approach to several mouse strains with allelic differences in NRAMP-1 and H-2 genes. The respective host determinants are known to influence the immune response against S. typhimurium. A comparative analysis of the vaccine efficacy in C57BL/6 and BALB/c mice showed that the live vaccine confers long lasting immunity in both strains (>18 weeks). In C57BL/6 mice, protection was still observed 54 weeks while not all vaccinated BALB/c were immune when challenged after this time. BALB/c mice also needed higher doses of SL3261[pYZ97] for full protection. We also demonstrate a therapeutic potential of SL3261[pYZ97] in H. pylori infected BALB/c and C57BL/6 mice. Urease- and carrier-specific serum antibody responses as well as the level of colonization by the Salmonella were analyzed in both mouse strains after immunization with low (4 x 10(7)CFU) or high (1 x 10(9)CFU) vaccine doses. The results are discussed in the context of inoculum size and the mode of antigen supply required for effective vaccination with recombinant Salmonella.

Bacterial virulence, proinflammatory cytokines and host immunity: how to choose the appropriate Salmonella vaccine strain?

Salmonella infection in its mammalian host can be dissected into two main components. The co-ordinate expression of bacterial virulence genes which are designed to evade, subvert or circumvent the host response on the one hand, and the host defence mechanisms which are designed to restrict bacterial survival and replication on the other hand. The outcome of infection is determined by the one which succeeds in disturbing this equilibrium more efficiently. This delicate balance between Salmonella virulence and host immunity/inflammation has important implications for vaccine development or therapeutic intervention. Novel Salmonella vaccine candidates and live carriers for heterologous antigens are attenuated strains with defined genetic modifications of metabolic or virulence functions. Although genetic defects of different gene loci can lead to similar degrees of attenuation, effects on the course of infection may vary, thereby altering the quality of the elicited immune response. Studies with gene-deficient animals indicate that Salmonella typhimurium strains with mutations in aroA, phoP/phoQ or ssrA/ssrB invoke different immune responses and that a differential repertoire of pro-inflammatory cytokines is required for clearance. Consequently, Salmonella mutants defective in distinct virulence functions offer the potential to specifically modulate the immune response for defined medical applications.

In vivo visualization of bacterial colonization, antigen expression, and specific T-cell induction following oral administration of live recombinant Salmonella enterica serovar Typhimurium

Live attenuated Salmonella strains that express a foreign antigen are promising oral vaccine candidates. Numerous genetic modifications have been empirically tested, but their effects on immunogenicity are difficult to interpret since important in vivo properties of recombinant Salmonella strains such as antigen expression and localization are incompletely characterized and the crucial early inductive events of an immune response to the foreign antigen are not fully understood. Here, methods were developed to directly localize and quantitate the in situ expression of an ovalbumin model antigen in recombinant Salmonella enterica serovar Typhimurium using two-color flow cytometry and confocal microscopy. In parallel, the in vivo activation, blast formation, and division of ovalbumin-specific CD4(+) T cells were followed using a well-characterized transgenic T-cell receptor mouse model. This combined approach revealed a biphasic induction of ovalbumin-specific T cells in the Peyer's patches that followed the local ovalbumin expression of orally administered recombinant Salmonella cells in a dose- and time-dependent manner. Interestingly, intact Salmonella cells and cognate T cells seemed to remain in separate tissue compartments throughout induction, suggesting a transport of killed Salmonella cells from the colonized subepithelial dome area to the interfollicular inductive sites. The findings of this study will help to rationally optimize recombinant Salmonella strains as efficacious live antigen carriers for oral vaccination.

Enhanced immune responses to viral epitopes by combining macrophage-inducible expression with multimeric display on a Salmonella vector

In this study, the immunogenicity of chimeric 987P fimbriae on a Salmonella vaccine strain was improved by optimizing fimbrial expression. The constitutive tetA promoter and the in vivo activated nirB and pagC promoters were evaluated for their use to express two epitopes of the transmissible gastroenteritis virus (TGEV) spike protein carried by fimbriae which were displayed on a Salmonella vaccine strain. Constructs with the pagC promoter were shown to drive increased expression of chimeric 987P fimbriae in macrophages as well as in Mg(2+)-poor media, mimicking a major environmental signal found in Salmonella-containing endocytic vacuoles of macrophages. Mice immunized orally with a Salmonella vaccine strain which expressed chimeric fimbriae from the pagC promoter elicited significantly higher mucosal and systemic immune responses to both the 987P fimbriae and the TGEV epitopes than mice immunized with the same strain hosting a tetA or nirB promoter-driven expression plasmid. Moreover, only the Salmonella vaccine strains harboring a plasmid with the pagC promoter, with or without an additional tetA promoter in tandem, elicited neutralizing antibodies to TGEV. This indicated that the pagC promoter can be used successfully to improve epitope-display by chimeric fimbriae on Salmonella vaccine strains for the induction of a desired immune response.

The virulence plasmid of Salmonella typhimurium contains an autoregulated gene, rlgA, that codes for a resolvase-like DNA binding protein

The virulence plasmid of Salmonella typhimurium contains a gene, rlgA, that shows strong homology to several reported resolvase-like proteins. This gene maps 5 kb upstream of spv locus, the major virulence determinant on the plasmid. Regulation of rlgA was studied using a lacZ transcriptional reporter fusion. The rlgA gene was found to be repressed at the level of transcription by its own product and to be expressed maximally in the late exponential phase of growth. The transcription start site of the rlgA gene was determined and the RlgA binding site was mapped and found to overlap with the transcription initiation signals. A derivative of the virulence plasmid was constructed with a knockout mutation in rlgA. This mutation did not alter the stability of the virulence plasmid nor did it affect the ability of S. typhimurium to cause systemic disease in mice.

DNA topology and adaptation of Salmonella typhimurium to an intracellular environment

The expression of genes coding for determinants of DNA topology in the facultative intracellular pathogen Salmonella typhimurium was studied during adaptation by the bacteria to the intracellular environment of J774A.1 macrophage-like cells. A reporter plasmid was used to monitor changes in DNA supercoiling during intracellular growth. Induction of the dps and spv genes, previously shown to be induced in the macrophage, was detected, as was expression of genes coding for DNA gyrase, integration host factor and the nucleoid-associated protein H-NS. The topA gene, coding for the DNA relaxing enzyme topoisomerase I, was not induced. Reporter plasmid data showed that bacterial DNA became relaxed following uptake of S. typhimurium cells by the macrophage. These data indicate that DNA topology in S. typhimurium undergoes significant changes during adaptation to the intracellular environment. A model describing how this process may operate is discussed.