Length of the flagellar hook and the capacity of the type III export apparatus

Length determination in biology generally uses molecular rulers. The hook, a part of the flagellum of motile bacteria, has an invariant length. Here, we examined hook length and found that it was determined not by molecular rulers but probably by the amount of subunit protein secreted by the flagellar export apparatus. The export apparatus shares common features with the type III virulence-factor secretion machinery and thus may be used more widely in length determination of structures other than flagella.

Recombinant attenuated bacteria for the delivery of subunit vaccines

Using attenuated intracellular bacteria as carriers, we have developed two different approaches for the delivery of subunit vaccines encoding heterologous antigens. The first system is based on the direct secretion of the heterologous antigens in Gram-negative bacteria via the hemolysin secretion system of Escherichia coli into either phagosome or cytosol of infected cells. The second approach is based on the transport of eukaryotic antigen expression vectors by intracellular bacteria like Listeria and Salmonella into the host cell and here, preferably, into the cytosolic compartment. After release of the plasmid DNA from the bacteria, the plasmid-encoded antigens can be expressed directly by the host cell. Finally, we combined both types of subunit vaccines in one live vector - we equipped Salmonella strains with a phagosomal escape function by utilization of the hemolysin secretion system and used this recombinant vaccine strain for the delivery of a eukaryotic antigen expression vector into the cytosol of macrophages.

Genetic analysis of assembly of the Salmonella enterica serovar Typhimurium type III secretion-associated needle complex

Several pathogenic bacteria have evolved a specialized protein secretion system termed type III to secrete and deliver effector proteins into eukaryotic host cells. Salmonella enterica serovar Typhimurium uses one such system to mediate entry into nonphagocytic cells. This system is composed of more than 20 proteins which are encoded within a pathogenicity island (SPI-1) located at centisome 63 of its chromosome. A subset of these components form a supramolecular structure, termed the needle complex, that resembles the flagellar hook-basal body complex. The needle complex is composed of a multiple-ring cylindrical base that spans the bacterial envelope and a needle-like extension that protrudes from the bacterial outer surface. Although the components of this structure have been identified, little is known about its assembly. In this study we examined the effect of loss-of-function mutations in each of the type III secretion-associated genes encoded within SPI-1 on the assembly of the needle complex. This analysis indicates that the assembly of this organelle occurs in discrete, genetically separable steps. A model for the assembly pathway of this important organelle is proposed that involves a sec-dependent step leading to the assembly of the base substructure followed by a sec-independent process resulting in the assembly of the needle portion.

Contribution of Salmonella typhimurium type III secretion components to needle complex formation

The prgHIJK operon encodes components of the Salmonella typhimurium pathogenicity island 1 type III secretion system (TTSS). Previously, prgH and prgK were shown to be required for formation of the supramolecular type III secretion needle complex (NC) [Kubori, T., et al. (1998) Science 280, 602-605]. This work indicates that all prg operon genes are required for NC formation. PrgH multimerizes into a distinct tetrameric-shaped structure that may be an early intermediate of NC assembly and may provide the structural foundation required for PrgK oligomerization. PrgH and PrgK, in the absence of other TTSS components, oligomerize into ring-shaped structures identical in appearance and size to the base of the NC, indicating that they are likely the major inner membrane structural components required for secretion. PrgI and PrgJ cofractionate with the NC and are secreted into the culture supernatant. NC from prgI and prgJ mutants have an identical morphology to the envelope-spanning (basal body) NC components, but are missing the external needle, indicating that PrgI and PrgJ are required for full NC assembly and are likely components of the external needle. Therefore, PrgI and PrgJ are secreted through the NC basal body, composed in part of PrgH/K and InvG/H rings, to participate in assembly of the more distal components of the NC.

Molecular characterization and assembly of the needle complex of the Salmonella typhimurium type III protein secretion system

Many bacterial pathogens of plants and animals have evolved a specialized protein-secretion system termed type III to deliver bacterial proteins into host cells. These proteins stimulate or interfere with host cellular functions for the pathogen's benefit. The Salmonella typhimurium pathogenicity island 1 encodes one of these systems that mediates this bacterium's ability to enter nonphagocytic cells. Several components of this type III secretion system are organized in a supramolecular structure termed the needle complex. This structure is made of discrete substructures including a base that spans both membranes and a needle-like projection that extends outward from the bacterial surface. We demonstrate here that the type III secretion export apparatus is required for the assembly of the needle substructure but is dispensable for the assembly of the base. We show that the length of the needle segment is determined by the type III secretion associated protein InvJ. We report that InvG, PrgH, and PrgK constitute the base and that PrgI is the main component of the needle of the type III secretion complex. PrgI homologs are present in type III secretion systems from bacteria pathogenic for animals but are absent from bacteria pathogenic for plants. We hypothesize that the needle component may establish the specificity of type III secretion systems in delivering proteins into either plant or animal cells.

Salmonella maintains the integrity of its intracellular vacuole through the action of SifA

A method based on the Competitive Index was used to identify Salmonella typhimurium virulence gene interactions during systemic infections of mice. Analysis of mixed infections involving single and double mutant strains showed that OmpR, the type III secretion system of Salmonella pathogenicity island 2 (SPI-2) and SifA [required for the formation in epithelial cells of lysosomal glycoprotein (lgp)-containing structures, termed Sifs] are all involved in the same virulence function. sifA gene expression was induced after Salmonella entry into host cells and was dependent on the SPI-2 regulator ssrA. A sifA(-) mutant strain had a replication defect in macrophages, similar to that of SPI-2 and ompR(-) mutant strains. Whereas wild-type and SPI-2 mutant strains reside in vacuoles that progressively acquire lgps and the vacuolar ATPase, the majority of sifA(-) bacteria lost their vacuolar membrane and were released into the host cell cytosol. We propose that the wild-type strain, through the action of SPI-2 effectors (including SpiC), diverts the Salmonella-containing vacuole from the endocytic pathway, and subsequent recruitment and maintenance of vacuolar ATPase/lgp-containing membranes that enclose replicating bacteria is mediated by translocation of SifA.

Use of confocal microscopy to detect Salmonella typhimurium within host cells associated with Spv-mediated intracellular proliferation

The major limitation in histological examination of orally inoculated mice of Salmonella typhimurium has been the difficulty of attaining high enough levels for immunochemical detection. This problem has been solved by the use of confocal laser scanning microscopy (CLSM) analysis, which allows detection of bacteria in the immunostained sections of mouse spleens at a minimum rate of approximately 1000 colony-forming units (cfu)/spleen. Here, we demonstrate that over 80% of salmonellae of the wild type of S. typhimurium were detected intracellularly within Mac-1 positive cells by the CLSM analysis of immunostained sections from spleens in orally or subcutaneously inoculated mice. Only 40% of salmonellae of the spv -deleted strain were detected inside Mac-1 positive cells. These data suggest that the spv genes play a key role in intracellular proliferation within phagocytes in the mouse spleen.

Comparative study of the invasiveness of Salmonella serotypes Typhimurium, Choleraesuis and Dublin for Caco-2 cells, HEp-2 cells and rabbit ileal epithelia

Patterns of invasiveness of Salmonella serotypes Typhimurium, Choleraesuis and Dublin in Caco-2 cells (without centrifugation) were compared with previously published studies of the rabbit ileal invasion assay (RIIA) and (where relevant) a HEp-2 cell invasion assay. Optimal conditions for the use of Caco-2 cell monolayers in bacterial invasion assays were defined. Centrifuge-assisted attachment of bacteria to cells was not used routinely as this increased the invasiveness of known hypo-invasive strains and detachment of Caco-2 cells. Inocula with too high bacterial numbers resulted in rapid acidification of media and detachment of the monolayers. The invasiveness of Typhimurium strains TML, WAKE, WII8, LT7, SL1027 and M206 in Caco-2 cells reflected that seen in the RIIA. The invasiveness of Choleraesuis strain A50 was similar to that in the RIIA except that bacteria grown at 37 degrees C and used without storage at 4 degrees C were slightly more invasive than those grown at 37 degrees C and stored at 4 degrees C before use. Dublin strain 3246 showed no apparent temperature-regulated invasiveness in Caco-2 cells, in contrast to the results observed in the RIIA. Dublin strain 3246 did not cleave tight junctions in the Caco-2 cell monolayer as it did in rabbit ileal epithelia both in vitro and in vivo. Three TnphoA insertion LPS mutants of Typhimurium TML were uniformly hypo-invasive in both Caco-2 cells and the RIIA; in contrast, they were differentially invasive in HEp-2 cells. Three smooth TnphoA insertion mutants of Typhimurium TML (invH, invG and pagC) were hypo-invasive in both the Caco-2 and HEp-2 cell invasion assays but not in the RIIA.

Characterization of FimY as a coactivator of type 1 fimbrial expression in Salmonella enterica serovar Typhimurium

Type 1 fimbriae of Salmonella enterica serovar Typhimurium are surface appendages that carry adhesins specific for mannosylated host glycoconjugates. Regulation of the major fimbrial subunit is thought to be controlled by a number of ancillary fim genes, including fimZ, fimY, fimW, and fimU. Previous studies using a FimZ mutant have indicated that this protein is necessary for fimA expression, and in vitro DNA binding assays determined that FimZ is a transcriptional activator that binds directly to the fimA promoter. To determine the role of FimY as a potential regulator of fimbrial expression, a fimY mutant of serovar Typhimurium was generated by allelic exchange. This mutant was found to be phenotypically nonfimbriate. No transcription from the fimA promoter was detected in a fimY mutant containing a fimA-lacZ reporter construct located on the chromosome. In addition, transcription from the cloned fimY promoter was not detected in Escherichia coli unless both FimZ and FimY were present, indicating that these proteins also act as coactivators of fimY expression. Consistent with these results, there is no transcription from a fimY-lacZ reporter construct within a serovar Typhimurium fimY or fimZ mutant. Studies using the fimY-lacZ construct reveal that expression of this gene varies with environmental conditions in a manner similar to fimA expression. Extensive in vitro DNA binding assays using extracts from E. coli that overexpress FimY, as well as partially purified FimY, were unable to identify a specific interaction between FimY and the fimA or fimY promoter. The results indicate that FimY is a positive regulator of fimbrial expression and that this protein acts in cooperation with FimZ to regulate the expression of Salmonella type 1 fimbrial appendages.

Survival and filamentation of Salmonella enterica serovar enteritidis PT4 and Salmonella enterica serovar typhimurium DT104 at low water activity

In this study we investigated the long-term survival of and morphological changes in Salmonella strains at low water activity (a(w)). Salmonella enterica serovar Enteritidis PT4 and Salmonella enterica serovar Typhimurium DT104 survived at low a(w) for long periods, but minimum humectant concentrations of 8% NaCl (a(w), 0. 95), 96% sucrose (a(w), 0.94), and 32% glycerol (a(w), 0.92) were bactericidal under most conditions. Salmonella rpoS mutants were usually more sensitive to bactericidal levels of NaCl, sucrose, and glycerol. At a lethal a(w), incubation at 37 degrees C resulted in more rapid loss of viability than incubation at 21 degrees C. At a(w) values of 0.93 to 0.98, strains of S. enterica serovar Enteritidis and S. enterica serovar Typhimurium formed filaments, some of which were at least 200 microm long. Filamentation was independent of rpoS expression. When the preparations were returned to high-a(w) conditions, the filaments formed septa, and division was complete within approximately 2 to 3 h. The variable survival of Salmonella strains at low a(w) highlights the importance of strain choice when researchers produce modelling data to simulate worst-case scenarios or conduct risk assessments based on laboratory data. The continued increase in Salmonella biomass at low a(w) (without a concomitant increase in microbial count) would not have been detected by traditional microbiological enumeration tests if the tests had been performed immediately after low-a(w) storage. If Salmonella strains form filaments in food products that have low a(w) values (0.92 to 0.98), there are significant implications for public health and for designing methods for microbiological monitoring.

AgfD, the checkpoint of multicellular and aggregative behaviour in Salmonella typhimurium regulates at least two independent pathways

The regulatory programme of multicellular behaviour in Salmonella typhimurium is determined by mutations in the agfD promoter. AgfD has already been identified to regulate the extracellular matrix associated with the multicellular morphotype composed of thin aggregative fimbriae (agf). To detect additional components contributing to the multicellular morphotype in S. typhimurium, we constructed a mutant in agfD, the positive transcriptional regulator of the agfBA(C) operon encoding for fimbrial subunit proteins. The agfD mutant lacked any form of multicellular behaviour as shown by analysis at the macroscopic and microscopic level. In contrast, the agfBA mutant unable to form thin aggregative fimbriae still maintained long-range intercellular adhesion. Promoter and expression analysis revealed that the genes downstream of agfD agfEFG most likely did not contribute to the remaining aggregative behaviour. Screening of transcriptional fusions for agfD dependency uncovered adrA, a homologue of yaiC in Escherichia coli. Environmental factors regulating adrA correspond to the regulation of thin aggregative fimbriae. AdrA is a putative transmembrane protein with a C-terminal GGDEF domain of unknown function although it is present in over 50 bacterial proteins. AdrA mutant cells, which still formed thin aggregative fimbriae with all binding characteristics, exhibited community behaviour but, unlike the wild type, lacked long-range intercellular adhesion. An agfBA adrA double mutant behaved like the agfD mutant. Therefore, it was concluded that agfD regulates at least two independent pathways contributing to the multicellular morphotype in S. typhimurium.

The effects of chlorpromazine on the outer cell wall of Salmonella typhimurium in ensuring resistance to the drug

Chlorpromazine (CPZ), a compound employed for the management of psychosis, has a wide ranging antibacterial activity. The growth of Salmonella typhimurium100 mg/l), was initially inhibited during the first 8-16 h of exposure to concentrations of CPZ below the MIC. During this period of transient susceptibility, the distribution of ribosomes was markedly altered in a concentration dependent manner; the rough cell wall was transformed into a smooth form. The protein composition of the outer cell wall of 55 kDa was markedly decreased, whilst there was an increased number of high molecular weight proteins. After 16 h of exposure to sub-MIC levels of CPZ, the inhibitory effect of the drug was no longer apparent whereas the effects noted on the cell wall were retained. These Salmonella were, as the control, agglutinated by O antigen specific antibody. Whereas agglutination of the control Salmonella was blocked by the presence of CPZ at concentrations that induced the cell-wall effects, agglutination of CPZ exposed-Salmonella for periods in excess of 16 h was not blocked by any concentration of CPZ. These results suggested that eventual resistance to CPZ was dependent upon changes induced by CPZ at the cell wall level. The results also suggested that the CPZ binds to the 55 kDa protein and that such binding interfered with the recognition of the O antigen by antibody.

Modulation of host immune responses stimulated by Salmonella vaccine carrier strains by using different promoters to drive the expression of the recombinant antigen

We evaluated whether immune responses stimulated by Salmonella vaccine carriers can be modulated by using different promoters to drive antigen expression. Mice were orally immunized with strains transfected with plasmids carrying beta-galactosidase (beta-gal) under the control of either a constitutive or an in vivo-activated promoter. While alpha-gal-reactive IgG1, IgG2a, IgG2b and IgG3 were detected in sera of mice immunized with Salmonella expressing constitutively beta-gal, higher titers dominated by IgG2a and IgG2b were detected in sera when the in vivo-activated promoter was used. beta-gal-specific proliferative responses of spleen-derived CD4+ T lymphocytes were similar in both groups. However, CD4+ T lymphocytes from mice immunized with the constitutive promoter secreted IL-4, IL-5, IL-6, IL-10 and IFN-gamma (Th1/Th2 pattern), whereas CD4+ cells mainly secreted IFN-gamma (Th1 pattern) when the second construct was used. The spleens of all immunized mice contained beta-gal-reactive CD8+ CTL precursors. The vaccine prototypes were tested for their capacity to control seeding and/or development within the lung of an intravenously delivered aggressive fibrosarcoma transfected with beta-gal. Reduced metastasis and significantly increased mean survival times were observed in all vaccinated mice. However, protection was improved when the carrier expressed beta-gal upon infection (80 % versus 50% survival, p < 0.05).

Physiological effects of high hydrostatic pressure treatments on Listeria monocytogenes and Salmonella typhimurium

The effect of a high hydrostatic pressure treatment on the Gram-positive Listeria monocytogenes strain Scott A and the Gram-negative Salmonella typhimurium strain Mutton (ATCC13 311) has been determined in stationary phase cell suspensions. Pressure treatments were done at room temperature for 10 min in sodium citrate (pH 5.6) and sodium phosphate (pH 7.0) suspension buffers. Increasing pressure treatments resulted in an exponential decrease of cell counts. Salmonella typhimurium suspended at low pH was more sensitive to pressure treatments. Progressive morphological changes were evident with the pressure increase. Cell lysis only appeared with the highest pressure treatments. Cell volume was not affected by pressure treatment. A progressive decrease of deltapH (pHin - pHout), intracellular potassium and ATP contents was demonstrated with the pressure increase. A parallel lowering of membrane potentials was measured.

A recombinant Salmonella typhimurium vaccine strain is taken up and survives within murine Peyer's patch dendritic cells

The attenuated Salmonella typhimurium PhoPc strain is avirulent but immunogenic via the oral route in mice and is attenuated in survival in macrophage cell lines. In this study, the fate of PhoPc bacteria expressing green fluorescent protein was investigated in murine Peyer's patches. The survival of PhoPc was monitored after orogastric inoculation of BALB/c mice. Bacteria persisted for several weeks in the Peyer's patches and were also recovered from the mesenteric lymph nodes and spleen. Confocal microscopy analysis identified dendritic cells as the Peyer's patch cell type that internalized PhoPc expressing green fluorescent protein at early time points. In addition, live PhoPc were found in Peyer's patch dendritic cells and not in B cells 3 days after orogastric inoculation. Taken together, these results provide strong evidence that PhoPc is internalized and survives within Peyer's patch dendritic cells. As these cells are potent antigen-presenting cells, these data could explain the immunogenicity of S. typhimurium vaccine strains in vivo.

[Ultrastructural organization of Salmonella typhimurium cells during long-term starvation and transfer to an unculturable state]

Electron microscopic and immunocytochemical studies of Salmonella typhimurium culture were carried out under conditions of cell transfer into an unculturable state induced by carbon, phosphorus, and nitrogen starvation. Morphological variants of bacterial cells were detected in the course of cell culturing under conditions of starvation. Electron microscopy showed that O-antigen was retained in salmonella after long starvation and transfer into an unculturable state.

Crystallization of lipopolysaccharide from a Salmonella typhimurium semi-rough (SR) mutant

Salmonella typhimurium SR-form lipopolysaccharide (LPS), consisting of a single repeating unit of the O-antigenic polysaccharide, linked to the R-core consisting of oligosaccharide that is, in turn, linked to lipid A, formed crystals whose shapes were hexagonal plates, discoids, and solid columns when precipitated by the addition of 2 volumes of 95% ethanol containing 375 mM MgCl2 and kept in 70% ethanol containing 250 mm MgCl2 at 4 C for 10 days. Among these crystals, the basic form is considered to be the hexagonal plates. Analyses of hexagonal plate crystals showed that they consist of hexagonal lattices with a lattice constant (a axis) of 4.62 A and longitudinal axis (c axis) of approximately 100 A. In X-ray diffraction patterns in the low-angle region, crystals of S. typhimurium SR-form LPS exhibited much less distinct reflections when compared with crystals of synthetic Escherichia coli-type lipid A. In contrast to the previous finding that S. minnesota S-form LPS possessing the O-antigenic polysaccharide does not crystallize under the same experimental conditions as used in the present study, the presence of a single repeating unit of the O-antigenic polysaccharide does not inhibit crystallization.

Substrate specificity switching of the flagellum-specific export apparatus during flagellar morphogenesis in Salmonella typhimurium

During flagellar morphogenesis in Salmonella typhimurium, the flagellum-specific anti-sigma factor FlgM is exported out of the cells only after completion of hook assembly. In this study, we examined the export of the flagellar proteins, FlgD (hook capping protein), FlgE (hook protein), FlgK and FlgL (hook-filament junction proteins), FliD (filament capping protein), and FliC (flagellin), before and after completion of hook assembly. Like the FlgM protein, the FlgK, FlgL, FliD, and FliC proteins are exported efficiently only after completion of hook assembly. On the other hand, the FlgD and FlgE proteins are exported efficiently before, but poorly after, hook completion. These results indicate that the export properties are different between these two groups and that their export order exactly parallels the assembly order of the hook-filament structure. We propose that the substrate specificity switching occurs in the flagellum-specific export apparatus upon completion of hook assembly.

[The effect of Aerococcus viridans on the properties of salmonellae and staphylococci in vitro]

According to electron microscopy the repeated contacts of Aerococcus viridans 167 (industrial strain for the probiotic Aerobact production) with Salmonella typhimurium and Staphylococcus aureus caused the deep changes of cellular structure, down to bacteriolysis. Remaining viable pathogenic cells gave posterity with changed biochemical properties and virulence. S. typhimurium completely lost typical biochemical properties, its mobility and ability to be agglutinated by O-specific Salmonella serum reduced after the fifth joint passage. Sharp decrease of the leucotoxic and loss of the lethal and necrotic activity of the S. aureus toxin were observed. The latter was characterized by the smaller number of protein fractions in comparison with control series on electrophoregrams and densitograms. Pathogenic bacteria, playing a role in formation of the pathological intestinal microbiocenosis (S. typhimurium) and being hospital infections (S. aureus) acquired increased sensitivity to antibiotics. The carried out researches open some mechanisms of bactericidal and bacteriostatic action of A. viridans (group 12, Bergey, 1994) and their metabolic products, causing antagonistic effect during direct action on infectious microorganisms.

Role of the outermost subdomain of Salmonella flagellin in the filament structure revealed by electron cryomicroscopy

A mutant strain of Salmonella typhimurium, SJW46, has flagellar filaments supercoiled in the same form as the wild-type strain, SJW1103, and swims normally. However, its flagellar filaments are mechanically unstable and show anomalous behaviors of polymorphism. Flagellin from SJW46 has a large central deletion from Ala204 to Lys292 of SJW1103 flagellin, which has been thought to be located in the outer surface of the filament. Since the filament structure is determined by intersubunit interactions of the terminal regions in the densely packed core of the filament, no serious involvement of the deleted portion was expected in the filament stability and polymorphism. In order to locate the deleted portion and to understand the underlying mechanism of these anomalous characteristics, we carried out structure analysis of the L-type straight filament reconstituted from a mutant flagellin of SJW46 (SJW46S) and compared the structure with that of the SJW1660 filament, which is also the L-type but composed of flagellin with no deletion. The deleted portion was identified as the outermost subdomain, and the structure in the core region showed no appreciable differences. The structure revealed the previously identified folding of flagellin in further detail, and the significance of intersubunit interactions between outer domains, which are present in the SJW1660 filament but absent in the SJW46 filament. This suggests that these contacts have a significant contribution to the filament stability and polymorphic behavior, despite the fact that the contacting surface area occupies only a minor portion of the whole intersubunit interactions.

Architectural features of the Salmonella typhimurium flagellar motor switch revealed by disrupted C-rings

The three-dimensional surface topology of rapid-frozen Salmonella typhimurium flagellar hook basal body complexes was studied by stereo-examination of thin-film metal replicas. The complexes contained the extended cytoplasmic structure, composed of the switch complex proteins; FliG, FliM, and FliN. Distinct nanometer-scale element arrays, separated by grooves, defined the outer surface of the cytoplasmic (C-) ring. The number of array elements was comparable to previously determined FliG and FliM copy numbers in the basal body. In addition to basal body complexes lacking C-rings, complexes containing incomplete C-rings were identified. The incomplete C-rings had lost segments of the proximal array. Basal bodies with the distal C-ring array alone were not found. These findings are compatible with the spatial organization of the flagellar switch suggested by previous biochemical data.

Flk couples flgM translation to flagellar ring assembly in Salmonella typhimurium

The hook-basal body (HBB) is a key intermediate structure in the flagellar assembly pathway in Salmonella typhimurium. The FlgM protein inhibits the flagellum-specific transcription factor sigma28 in the absence of the intact HBB structure and is secreted out of the cell following HBB completion. The flk gene encodes a positive regulator of the activity of FlgM at an assembly step just prior to HBB completion: at the point of assembly of the P- and L-rings. FlgM inhibition of sigma28-dependent class 3 flagellar gene transcription was relieved in P- and L-ring assembly mutants (flgA, flgH, and flgI) by introduction of a null mutation in the flk gene (J. E. Karlinsey et al., J. Bacteriol. 179:2389-2400, 1997). In P- and L-ring mutant strains, recessive mutations in flk resulted in a reduction in intracellular FlgM levels to those seen in wild-type (Fla+) strains. The reduction in intracellular FlgM levels by mutations in the flk gene was concomitant with a 10-fold increase in transcription of the flgMN operon compared to that of the isogenic flk+ strain, while transcription of the flgAMN operon was unaffected. This was true for both direct measurement of the flgAMN and flgMN mRNA transcripts by RNase T2 protection assays and for lac operon fusions to either the flgAMN or flgMN promoter. Loss of Flk did not allow secretion of FlgM through basal-body structures lacking the P- and L-rings. Intracellular FlgM was stable to proteolysis, and turnover occurred primarily after export out of the cell. Loss of Flk did not result in increased FlgM turnover in either P- or L-ring mutant strains. With lacZ translational fusions to flgM, a null mutation in flk resulted in a significant reduction of flgM-lacZ mRNA translation, expressed from the class 3 flgMN promoter, in P- and L-ring mutant strains. No reduction in either flgAMN or flgMN mRNA stability was measured in the absence of Flk in Fla+, ring mutant, or HBB deletion strains. We conclude that the reduction in the intracellular FlgM levels by mutation in the flk gene is only at the level of flgM mRNA translation.

[Electron microscopic study of pathogenic bacteria on environmental objects]

The morphological picture of different bacteria (Salmonella typhimurium, Proteus vulgaris, Klebsiella pneumoniae, Pseudomonas aeruginosa, Yersinia enterocolitica O3, Y.pseudotuberculosis 1, Y.frederiksenii, Y.intermedia, Y.kristensenii) on environmental objects was studied with the use of scanning electron microscopy (SEM). Bacteria adhered to the surface of pieces of fodder, egg shell, cabbage leaves and form microcolonies, whose morphology was similar to colonies, grown on nutrient media. The cells produced extracellular substances, seen in SEM as integuments. These integuments were gourd to protect the population from the action of unfavorable factors.

Multiple fimbrial adhesins are required for full virulence of Salmonella typhimurium in mice

Adhesion is an important initial step during bacterial colonization of the intestinal mucosa. However, mutations in the Salmonella typhimurium fimbrial operons lpf, pef, or fim only moderately alter mouse virulence. The respective adhesins may thus play only a minor role during infection or S. typhimurium may encode alternative virulence factors that can functionally compensate for their loss. To address this question, we constructed mutations in all four known fimbrial operons of S. typhimurium: fim, lpf, pef, and agf. A mutation in the agfB gene resulted in a threefold increase in the oral 50% lethal dose (LD50) of S. typhimurium for mice. In contrast, an S. typhimurium strain carrying mutations in all four fimbrial operons (quadruple mutant) had a 26-fold increased oral LD50. The quadruple mutant, but not the agfB mutant, was recovered in reduced numbers from murine fecal pellets, suggesting that a reduced ability to colonize the intestinal lumen contributed to its attenuation. These data are evidence for a synergistic action of fimbrial operons during colonization of the mouse intestine and the development of murine typhoid fever.

Surface characteristics of gram-negative and gram-positive bacteria in an atomic force microscope image

Bacterial images can be obtained rather easily with an atomic-force microscope (AFM) in the magnification range of 5,000 to 30,000 times without any pretreatment of the specimens for such observations as chemical fixation, dehydration or staining. The bacterial shapes or the presence of flagella can be clearly recognized in these magnification ranges. In addition, we were also able to distinguish between gram-negative and gram-positive bacteria based on the specific wavy surface appearance of the former. AFM could thus be a useful tool for the identification of bacteria in the resolution range between electron and light microscopy.