Characterization of the CsfC and CsfD proteins involved in the biogenesis of CS5 pili from enterotoxigenic Escherichia coli.

The region required for biosynthesis of CS5 pili consists of six csf genes, with csfA encoding the major subunit. In this study, we describe the characterization of two of the genes constituting the region, csfC and csfD, but also identify the true morphology of the CS5 pilus by high resolution electron microscopy. CsfD was shown to be essential in the initiation of CS5 pilus biogenesis, did not possess any chaperone-like activity for the major subunit, and was an integral minor component of the pilus structure. Studies on CsfD translocation across the outer membrane in Escherichia coli K-12 using a csfA mutant also showed that CsfD is likely to be the first pilin subunit assembled. A specific in-frame deletion in the csfC gene resulted in the complete absence of cell surface CS5 pili and prevented the translocation of CsfA and CsfD pilins across the outer membrane. Specific cell localization studies showed an accumulation of CsfC in the outer membranes of E. coli K-12, while complementation experiments with homologous outer membrane assembly genes from CS1 and CFA/I pili systems were unable to restore assembly of CS5 pili. The CS5 pilus was shown to be a 2 nm flexible fibrillar structure, which adopted a predominantly open helical conformation under the electron microscope.

Comparison of virulence of Legionella longbeachae strains in guinea pigs and U937 macrophage-like cells.

A guinea pig model of experimental legionellosis was established for assessment of virulence of isolates of Legionella longbeachae. The results showed that there were distinct virulence groupings of L. longbeachae serogroup 1 strains based on the severity of disease produced in this model. Statistical analysis of the animal model data suggests that Australian isolates of L. longbeachae may be inherently more virulent than non-Australian strains. Infection studies performed with U937 cells were consistent with the animal model studies and showed that isolates of this species were capable of multiplying within these phagocytic cells. Electron microscopy studies of infected lung tissue were also undertaken to determine the intracellular nature of L. longbeachae serogroup 1 infection. The data showed that phagosomes containing virulent L. longbeachae serogroup 1 appeared bloated, contained cellular debris and had an apparent rim of ribosomes while those containing avirulent L. longbeachae serogroup 1 were compact, clear and smooth.

Isolation and characterization of bacteriophage-resistant mutants of Vibrio cholerae O139.

Vibrio cholerae O139 strains produce a capsule which is associated with complement resistance and is used as a receptor by bacteriophage JA1. Spontaneous JA1-resistant mutants were found to have several phenotypes, with loss of capsule and/or O-antigen from the cell surface. Determination of the residual complement resistance and infant mouse colonization potential of each mutant suggested that production of O-antigen is of much greater significance than the presence of capsular material for both of these properties. Two different in vitro assays of complement resistance were compared and the results of one shown to closely reflect the comparative recoveries of bacteria from the colonization experiments. Preliminary complementation studies implicated two rfb region genes, wzz and wbfP, as being essential for the biosynthesis of capsule but not O-antigen.

Susceptibility of Vibrio cholerae O139 to antibody-dependent, complement-mediated bacteriolysis.

Volunteer studies with Vibrio cholerae O1 have shown that the best correlate of a vaccine's protective efficacy is its propensity to elicit serum bactericidal responses in its recipients. Attempts to detect such responses following infection with V. cholerae O139, however, have met with varying success. Using a tube-based assay which involves viable counting, we now report that strains of serogroup O139 can appear to be sensitive or resistant to a fixed concentration of complement in the presence of antibody, depending on assay conditions. Susceptibility to lysis is critically dependent on the availability of complement, but with O139 indicator strains this is not simply determined by the concentration of serum added to the reaction mix. The nature of the assay diluent and the concentration of indicator bacteria can also dramatically affect bactericidal end points, whereas such variables have minimal significance with O1 indicator bacteria. Although some laboratories use unencapsulated mutant strains to seek evidence of seroconversion following exposure to V. cholerae O139, this is not necessary, and our findings question the significance of capsule expression as a determinant of complement sensitivity when antibody is present. The medium used for growth of the indicator strain and the particular strain used appeared to be unimportant. Each of seven O139 isolates tested was found to be lysed by antibody and complement in our standard assay system, which allowed the detection of significant serum bactericidal responses in 9 of 11 cases of O139 disease.

Pathogenic and vaccine significance of toxin-coregulated pili of Vibrio cholerae E1 Tor.

Vibrio cholerae O1 strains are classified into one of two biotypes, classical and E1 Tor, the latter being primarily responsible for cholera cases worldwide since 1961. Recent studies in our laboratory have focused upon the pathogenic and vaccine significance of the toxin-coregulated pili (TCP) produced by strains of E1 Tor biotype. Mutants in which the tcpA gene (encoding the pilin subunit protein) has been inactivated are dramatically attenuated in the infant mouse cholera model, showing markedly reduced colonisation potential in mixed-infection competition experiments. Significantly, in the vaccine context, antibodies to TCP are sufficient to prevent experimental infection, although our data suggest that this protective effect might be limited to strains of homologous biotype. Since we have shown that tcpA sequences are conserved within a biotype but differ between biotypes, this latter observation suggests that the biotype-restricted pilin epitopes might have greater vaccine significance. Similar studies indicate that TCP also play a critical role in colonisation by strains of the recently-recognised O139 serogroup, which is thought to have evolved from an O1 E1 Tor strain. In contrast to the effect of introducing mutations in the tcpA gene, strains carrying inactivated mshA genes (encoding the subunit of the mannose-sensitive haemagglutinin pilus) show unaltered in vivo behaviour. Consistent with this finding is our inability to demonstrate any protective effect associated with antibodies to MSHA. Ongoing approaches to vaccine development are variously aimed at improving the immunogenicity of the current inactivated whole-cell vaccine, or assessing the field efficacy of a promising live attenuated strain. The possible implications of our findings are discussed in relation to both of these options.

CS5 pilus biosynthesis genes from enterotoxigenic Escherichia coli O115:H40.

We have sequenced the entire region of DNA required for the biosynthesis of CS5 pili from enterotoxigenic Escherichia coli O115:H40 downstream of the major subunit gene, designated csfA (for coli surface factor five A). Five more open reading frames (ORFs) (csfB, csfC, csfE, csfF, and csfD) which are transcribed in the same direction as the major subunit and are flanked by a number of insertion sequence regions have been identified. T7 polymerase-mediated overexpression of the cloned csf ORFs confirmed protein sizes based on the DNA sequences that encode them. The expression of only the csf region in E. coli K-12 resulted in the hemagglutination of human erythrocytes and the cell surface expression of CS5 pili, suggesting that the cluster contains all necessary information for CS5 pilus biogenesis and function.

Genetic organization of the regions associated with surface polysaccharide synthesis in Vibrio cholerae O1, O139 and Vibrio anguillarum O1 and O2: a review.

Vibrio cholerae and V. anguillarum are recognized as aquatic-borne human and fish pathogens, respectively. Based upon analyses of several genes and the presence of novel genetic elements it seems that these two species are very closely related. Studies in this laboratory have identified an association of IS1358 with rfb and capsule loci in these two species. The most recent findings suggest that IS1358 is associated with the rfb region in V. cholerae O1 and O139 and in V. anguillarum O1 and O2. In addition, the rfb region in both V. cholerae serogroups and in V. anguillarum O1 is limited at one end by gmhD. These features make it feasible to envisage a mechanism by which the evolution of new rfb genes is taking place involving IS1358 and the region around gmhD. Furthermore, it is possible to envisage that there is or has been an exchange of genetic material between these species leading to new rfb/capsule regions. This review examines the genetics and biosynthesis of the O-antigen and capsule of V. cholerae O1 and O139, as well as the V. anguillarum serogroup O1 and the role of IS1358. Throughout this review we have used the new nomenclature for rfb genes proposed by.

Translocated intimin receptors (Tir) of Shiga-toxigenic Escherichia coli isolates belonging to serogroups O26, O111, and O157 react with sera from patients with hemolytic-uremic syndrome and exhibit marked sequence heterogeneity.

The capacity to form attaching and effacing (A/E) lesions on the surfaces of enterocytes is an important virulence trait of several enteric pathogens, including enteropathogenic Escherichia coli (EPEC) and Shiga-toxigenic E. coli (STEC). Formation of such lesions depends upon an interaction between a bacterial outer membrane protein (intimin) and a bacterially encoded receptor protein (Tir) which is exported from the bacterium and translocated into the host cell membrane. Intimin, Tir, and several other proteins necessary for generation of A/E lesions are encoded on a chromosomal pathogenicity island termed the locus for enterocyte effacement (LEE). Reports of sequence heterogeneity and antigenic variation in the region of intimin believed to be responsible for receptor binding raise the possibility that the receptor itself is also heterogeneous. We have examined this by cloning and sequencing tir genes from three different STEC strains belonging to serogroups O26, O111, and O157. The deduced amino acid sequences for the Tir homologues from these strains varied markedly, exhibiting only 65.4, 80.2, and 56.7% identity, respectively, to that recently reported for EPEC Tir. STEC Tir is also highly immunogenic in humans. Western blots of E. coli DH5alpha expressing the various STEC tir genes cloned in pBluescript [but not E. coli DH5alpha(pBluescript)] reacted strongly with convalescent sera from patients with hemolytic-uremic syndrome (HUS) caused by known LEE-positive STEC. Moreover, no reaction was seen when the various clone lysates were probed with serum from a patient with HUS caused by a LEE-negative STEC or with serum from a healthy individual. Covariation of exposed epitopes on both intimin and Tir may be a means whereby STEC avoid host immune responses without compromising adhesin-receptor interaction.

Sequence-based classification scheme for the genus Legionella targeting the mip gene.

The identification and speciation of strains of Legionella is often difficult, and even the more successful chromatographic classification techniques have struggled to discriminate newly described species. A sequence-based genotypic classification scheme is reported, targeting approximately 700 nucleotide bases of the mip gene and utilizing gene amplification and direct amplicon sequencing. With the exception of Legionella geestiana, for which an amplicon was not produced, the scheme clearly and unambiguously discriminated among the remaining 39 Legionella species and correctly grouped 26 additional serogroup and reference strains within those species. Additionally, the genotypic classification of approximately 150 wild strains from several continents was consistent with their phenotypic classification, with the exception of a few strains where serological cross-reactivity was complex, potentially confusing the latter classification. Strains thought to represent currently uncharacterized species were also found to be genotypically unique. The scheme is technically simple for a laboratory with even basic molecular capabilities and equipment, if access to a sequencing laboratory is available.

Molecular analysis of Shiga toxigenic Escherichia coli O111:H- proteins which react with sera from patients with hemolytic-uremic syndrome.

Western blot analysis was used to assess the reactivity of convalescent-phase sera from patients who were associated with an outbreak of hemolytic-uremic syndrome (HUS) caused by fermented sausage contaminated with Shiga toxin-producing Escherichia coli (STEC). The predominant STEC isolated from HUS patients belonged to serotype O111:H-, and reactivity to O111:H- whole-cell lysates, treated or untreated with proteinase K, was examined. As expected, all five serum samples demonstrated a marked anti-lipopolysaccharide response, but several protein bands were also immunoreactive, particularly one with an apparent size of 94 kDa. One convalescent-phase serum sample was subsequently used to screen an O111:H- cosmid bank and 2 of 900 cosmid clones were found to be positive, both of which contained a similar DNA insert. Western blot analysis of one of these clones identified three major immunoreactive protein bands of approximately 94, 70, and 50 kDa. An immune response to the three proteins was detectable with all five convalescent-phase serum samples but not with healthy human serum. Immunoreactive 94- and 50-kDa species were produced by a deletion derivative of the cosmid containing a 7-kb STEC DNA insert. Sequence analysis of this region indicated that it is part of the locus for enterocyte effacement, including the eaeA gene which encodes intimin. The deduced amino acid sequence of the O111:H- intimin was 88.6% identical to intimin from O157:H7 STEC, and the most divergent region was the 200 residues at the carboxyl terminus, which were only 75% identical. Such variation may be antigenically significant as serum from a HUS patient infected only with the O111:H- STEC reacted with intimin from an enteropathogenic E. coli O111 strain, as well as several other eaeA-positive STEC isolates, but not with an eaeA-positive STEC belonging to serotype O157:H-. Sera from two of the other HUS patients also failed to react with intimin from this latter strain. However, intimin from O157:H- STEC did react with serum from a patient infected with both O111:H- and O157:H- STEC.

Sequence analysis of the mip gene of the soilborne pathogen Legionella longbeachae.

To understand the basis of pathogenesis by Legionella longbeachae serogroup 1, the importance of the Mip protein in this species was examined. Amino-terminal analysis of the purified, cloned L. longbeachae serogroup 1 ATCC 33462 Mip protein confirmed that the cloned gene protein was expressed and processed in an Escherichia coli background. DNA sequence analysis of plasmid pIMVS27, containing the entire L. longbeachae serogroup 1 mip gene, revealed a high degree of homology to the mip gene of Legionella pneumophila serogroup 1, 76% homology at the DNA level and 87% identity at the amino acid level. Primer extension analysis determined that the start site of transcription was the same for both species, with some differences observed for the -10 and -35 promoter regions. Primers designed from the mip gene sequence obtained for L. longbeachae serogroup 1 ATCC 33462 were used to amplify the mip genes from L. longbeachae serogroup 2 ATCC 33484 and an Australian clinical isolate of L. longbeachae serogroup 1 A5H5. The mip gene from A5H5 was 100% identical to the type strain sequence. The serogroup 2 strain of L. longbeachae differed by 2 base pairs in third-codon positions. Allelic exchange mutagenesis was used to generate an isogenic mip mutant in ATCC 33462 and strain A5H5. The ATCC mip mutant was unable to infect a strain of Acanthamoebae sp. both in liquid and in a potting mix coculture system, while the A5H5 mip mutant behaved in a manner siilar to that of L. pneumophila serogroup 1, i.e., it displayed a reduced capacity to infect and multiply within Acanthamoebae. To determine if this mutation resulted in reduced virulence in the guinea pig animal model, the A5H5 mip mutant and its parent strain were assessed for their abilities to establish an infection after aerosol exposure. Unlike the virulent parent strain, the mutant strain did not kill any animals under two different dose regimes. The data indicate that the Mip protein plays an important role in the intracellular life cycle of L. longbeachae serogroup 1 species and is required for full virulence.

Antibodies to lipopolysaccharide block adherence of Shiga toxin-producing Escherichia coli to human intestinal epithelial (Henle 407) cells.

Shiga toxin-producing Escherichia coli (STEC) are a diverse group of organisms known to cause diarrhoea, haemorrhagic colitis and haemolytic uraemic syndrome (HUS) in humans. During the early stage of infection, numbers of STEC in the gut may be very high (of the order of 10(9)/g faeces), but as disease progresses, the numbers may drop rapidly such that STEC are undetectable within a week. Convalescent sera from patients recovering from HUS frequently contain high levels of antibody to E. coli lipopolysaccharide (LPS) of the infecting serotype, and it is possible that a local immune response to LPS contributes to elimination of the organism from the gut. We have recently demonstrated that STEC strains isolated from HUS cases have enhanced adherence to a human intestinal epithelial cell line (Henle 407) compared with STEC strains from non-human sources. In this study, we examined the capacity of STEC strains belonging to O-antigen types O111 and O157 to adhere to human intestinal epithelial (Henle 407) cells in the presence or absence of anti-LPS. Adherence was inhibited by up to 95% by anti-LPS of the homologous, but not heterologous serotype. This effect was not an artefact of serum bactericidal or agglutinating activity. Preincubation with purified homologous or heterologous LPS did not prevent adherence, suggesting that LPS was not acting as an adhesin per se. Nevertheless, these findings raise the possibility that oral administration of preparations containing anti-LPS may interfere with colonization of the human gut by STEC, and therefore could be of potential therapeutic value if administered early in the course of infection.

Lipopolysaccharide O-antigen expression and the effect of its absence on virulence in rfb mutants of Vibrio cholerae O1.

Using defined rfb mutants, defective in the biosynthesis of the O-antigen of the lipopolysaccharide (LPS), and monoclonal antibodies (MAbs) to the A, B and C LPS antigens, we have examined the distribution of the antigens and the effects of their loss. By immunogold electron microscopy, it has been possible to determine the relative amounts of the A, B and C antigens on Inaba and Ogawa cells, confirming previous studies based upon bacterial agglutination and hemagglutination inhibitions. These antigens are absent from rfb::Tn mutants selected as resistant to phages which have been shown to use the O-antigen as their receptor. These mutants were severely attenuated as measured by both LD50 and their ability to compete with the wild-type parents when analyzed in the infant mouse cholera model. These mutants were unchanged in the export of cholera toxin or other secreted proteins but revealed an altered outer membrane protein profile. The competition defect suggested an effect on TCP (toxin-coregulated pilus). An analysis of the rfb::Tn mutants revealed that they were unable to assemble TCP on their surface, but the major subunit, TcpA, could be found as an intracellular pool. These mutants could be complemented back to wild-type using the cloned rfb region, implying that functional TCP assembly is dependent upon an intact LPS.

A functional homolog of Escherichia coli NhaR in Vibrio cholerae.

Escherichia coli NhaR controls expression of a sodium/proton (Na+/H+) antiporter, NhaA. The Vibrio cholerae NhaR protein shows over 60% identity to those of Escherichia coli and Salmonella enteritidis. V. cholerae NhaR complements an E. coli nhaR mutant for growth in 100 mM LiCl-33 mM NaCl, pH 7.6, and enhances the Na+-dependent induction of an E. coli chromosomal nhaA::lacZ fusion. These findings indicate functional homology to E. coli NhaR. Two V. cholerae nhaR mutants were constructed by using kanamycin resistance cartridge insertion at different sites to disrupt the gene. Both mutants showed sensitivity to growth in 120 mM LiCl, pH 9.2, compared with the wild-type strain and could be complemented by the introduction of V. cholerae nhaR on a low-copy-number plasmid. An nhaR mutation had no detectable effect on the virulence of the V. cholerae strain in the infant mouse model, suggesting that the antiporter system involved is not required in vivo, at least in this animal model.

Characterization of the Vibrio cholerae El Tor lipase operon lipAB and a protease gene downstream of the hly region.

We have cloned and sequenced a region encoding a lipase operon and a putative, previously uncharacterized metalloprotease of Vibrio cholerae O1. These lie downstream of hlyA and hlyB, which encode the El Tor hemolysin and methyl-accepting chemotactic factor, respectively. Previous reports identified the hlyC gene downstream of hlyAB, encoding an 18.3-kDa protein. However, we now show that this open reading frame (ORF) encodes a 33-kDa protein, and since the amino acid sequence is highly homologous to the triacylglyceride-specific lipase of Pseudomonas spp., hlyC has been renamed lipA. LipA contains the highly conserved pentapeptide and catalytic triad amino acid regions of the catalytic sites of other lipases. The region downstream of lipA has been sequenced and has revealed ORFs lipB and prtV. The amino acid sequence of lipB is homologous to those of the accessory lipase proteins (lipase-specific foldase) required by Pseudomonas and various other bacterial species for the production of mature active lipase, and in agreement with this, we show that both lipA and lipB are required to restore a lipase-deficient lipA null mutant of V. cholerae. The intergenic stop codon for lipA overlaps the ribosome-binding site for lipB, and a stem-loop resembling a rho-independent terminator is present immediately downstream from lipB, suggesting that lipA and lipB form a lipase operon in V. cholerae. prtV lies downstream of lipAB but is transcribed in the opposite direction and is predicted to share the same putative transcriptional terminator with lipAB. The zinc-binding and catalytic domains conserved among many metalloproteases are present in PrtV, which is highly homologous to the immune inhibitor A (InA) metalloprotease of Bacillus thuringiensis. PrtV was visualized as approximately 102 kDa, which is consistent with the coding capacity of the gene. The genetic organization of this region suggests that it is possibly part of a pathogenicity island, encoding products capable of damaging host cells and/or involved in nutrient acquisition by V. cholerae. However, neither lipA nor prtV null mutants were attenuated in the infant mouse model, nor did they exhibit reduced colonization potential compared with wild type in competition experiments.

Interspecies sequence differences in the Mip protein from the genus Legionella: implications for function and evolutionary relatedness.

The nucleotide sequence of the mip genes and their inferred amino acid sequences were determined from 35 Legionella species and compared with the published sequences for L. pneumophila, L. micdadei and L. longbeachae. The sequences were 69-97% conserved at the nucleotide level and 82-99% at the amino acid level, with total conservation of amino acids determined to be associated with sites known to be involved in peptidyl prolyl cis-trans isomerase activity. No apparent difference could be determined in the arrangement of amino acids that would predict a functional difference in Mip from species associated with disease and Mip from species isolated only from the environment. Additionally, a phylogenetic comparison of the sequences with published 16S RNA sequences, using both genetic distance and maximum parsimony methods, was performed. Few relationships were apparent that were well supported by both data sets, the most robust being a clade comprising ([(cincinnatiensis, longbeachae, sainthelensi, santicrucis) gratiana] (moravica, quateirensis, shakespearei, worsleiensis) anisa, bozemanii, cherrii, dumoffii, gormanii, jordanis, parisiensis, pneumophila, steigerwaltii, tucsonensis, and wadsworthii).

Shiga toxin-producing Escherichia coli isolates from cases of human disease show enhanced adherence to intestinal epithelial (Henle 407) cells.

Shiga toxin-producing Escherichia coli (STEC) strains are a diverse group of organisms which are known to cause diarrhea and hemorrhagic colitis in humans. We have recently described a large food-borne outbreak of STEC disease caused by contaminated semidry fermented sausage (A. W. Paton, R. Ratcliff, R. M. Doyle, J. Seymour-Murray, D. Davos, J. A. Lanser, and J. C. Paton, J. Clin. Microbiol. 34:1622-1627, 1996). STEC strains belonging to several O serotypes were isolated from the contaminated food source, but of these, only a subset were isolated from patients with diarrhea or hemolytic-uremic syndrome (HUS). In the present study, we characterized these STEC isolates with respect to the presence of putative virulence-associated genes and the capacity to adhere to a human intestinal epithelial cell line (Henle 407). The O111:H- STEC strain 95NR1 (isolated from one of the outbreak HUS patients) was shown to adhere to Henle 407 cells in a dose-dependent, mannose-resistant fashion. Microscopic examination revealed a diffuse pattern of adherence for this as well as several other STEC strains. Interestingly, the adherence of STEC strains from HUS cases (both outbreak related and sporadic) was significantly greater than that of STEC strains found in the contaminated food source but not found in any patients. These studies support the hypothesis that an enhanced capacity to adhere to intestinal cells is one of the factors which distinguishes human-virulent STEC strains from those of lesser clinical significance.

The tcp gene cluster of Vibrio cholerae.

The toxin co-regulated pilus (TCP) has been identified as a critical colonization factor in both animal models and humans for Vibrio cholerae O1. The major pilin subunit, TcpA (and also TcpB), is similar to type-4 pilins but TCP probably more appropriately belongs to a sub-class which includes the bundle-forming pilus of enteropathogenic Escherichia coli. The genes for TCP biosynthesis and assembly are clustered with the exception of housekeeping functions such as TcpG (=DsbA, a periplasmic disulfide bond epimerase). The nt sequences from El Tor and classical strains show only minor differences corresponding to the major regulatory regions and in TcpA itself. These differences are thought to account for the alternate conditions required for expression of TCP by the two biotypes and the antigenic variation and lack of cross-protection. Aside from the TcpA only a few of the proteins have had their roles in TCP biogenesis defined. Regulation of TCP is controlled by the ToxR regulon via ToxT with a possible involvement of TcpP and the cAMP-CRP system. Experiments using the infant mouse cholera model have now shown that TCP is a colonization factor and protective antigen for both classical and El Tor O1 strains and in the O139 Bengal serotype and that the mannose-sensitive haemagglutinin pilus does not appear to play a comparable role.

Translocation failure in a type-4 pilin operon: rfb and tcpT mutants in Vibrio cholerae.

Defined chromosomal mutations that lead to assembly failure of the toxin coregulated pilus (TCP) of Vibrio cholerae provide useful insights into the biogenesis of a type-4 pilus. Mutants in rfb affecting LPS O-antigen biosynthesis, and strains depleted of the cytoplasmic membrane-associated ATP-binding protein TcpT, provide contrasting TCP export-defective phenotypes acting at different locations. Mutants in the perosamine biosynthesis pathway of V. cholerae 569B result in an rfb phenotype with an LPS consisting only of core oligosaccharide and lipid A. Such strains are unable to assemble TCP, and TcpA subunits are found in the periplasm and membrane fractions. In both rfb and tcpT mutants, the export defect is specific and complete. TcpT is a member of a large family of cytoplasmic membrane-associated ATP-binding proteins which are essential in type-4 pilin systems and in many non-pilin outer membrane transporters in Gram-negative bacteria. The behaviour of translocation-arrested TcpA in rfb and tcpT mutants is indistinguishable from that within assembled pilus under a range of conditions including flotation in density gradients, chemical cross-linking, and detergent extraction experiments. From the data presently available, it would appear that TcpA requires TcpT-mediated translocation from the cytoplasmic membrane and that TcpT stabilizes the subunit at or immediately beyond this stage, before crossing the outer membrane.