Effect of Ca2+ on morphology and division of Yersinia pestis

Role of Tellurite Resistance Operon in Filamentous Growth of Yersinia pestis in Macrophages

BACKGROUND

Yersinia pestis initiates infection by parasitism of host macrophages. In response to macrophage infections, intracellular Y. pestis can assume a filamentous cellular morphology which may mediate resistance to host cell innate immune responses. We previously observed the expression of Y. pestis tellurite resistance proteins TerD and TerE from the terZABCDE operon during macrophage infections. Others have observed a filamentous response associated with expression of tellurite resistance operon in Escherichia coli exposed to tellurite. Therefore, in this study we examine the potential role of Y. pestis tellurite resistance operon in filamentous cellular morphology during macrophage infections.

PRINCIPAL FINDINGS

In vitro treatment of Y. pestis culture with sodium tellurite (Na2TeO3) caused the bacterial cells to assume a filamentous phenotype similar to the filamentous phenotype observed during macrophage infections. A deletion mutant for genes terZAB abolished the filamentous morphologic response to tellurite exposure or intracellular parasitism, but without affecting tellurite resistance. However, a terZABCDE deletion mutant abolished both filamentous morphologic response and tellurite resistance. Complementation of the terZABCDE deletion mutant with terCDE, but not terZAB, partially restored tellurite resistance. When the terZABCDE deletion mutant was complemented with terZAB or terCDE, Y. pestis exhibited filamentous morphology during macrophage infections as well as while these complemented genes were being expressed under an in vitro condition. Further in E. coli, expression of Y. pestis terZAB, but not terCDE, conferred a filamentous phenotype.

CONCLUSIONS

These findings support the role of Y. pestis terZAB mediation of the filamentous response phenotype; whereas, terCDE confers tellurite resistance. Although the beneficial role of filamentous morphological responses by Y. pestis during macrophage infections is yet to be fully defined, it may be a bacterial adaptive strategy to macrophage associated stresses.

The main virulence determinant of Yersinia entomophaga MH96 is a broad-host-range toxin complex active against insects

Through transposon mutagenesis and DNA sequence analysis, the main disease determinant of the entomopathogenic bacterium Yersinia entomophaga MH96 was localized to an ~32-kb pathogenicity island (PAI) designated PAI(Ye₉₆). Residing within PAI(Ye₉₆) are seven open reading frames that encode an insecticidal toxin complex (TC), comprising not only the readily recognized toxin complex A (TCA), TCB, and TCC components but also two chitinase proteins that form a composite TC molecule. The central TC gene-associated region (~19 kb) of PAI(Ye₉₆) was deleted from the Y. entomophaga MH96 genome, and a subsequent bioassay of the ΔTC derivative toward Costelytra zealandica larvae showed it to be innocuous. Virulence of the ΔTC mutant strain could be restored by the introduction of a clone containing the entire PAI(Ye₉₆) TC gene region. As much as 0.5 mg of the TC is released per 100 ml of Luria-Bertani broth at 25°C, while at 30 or 37°C, no TC could be detected in the culture supernatant. Filter-sterilized culture supernatants derived from Y. entomophaga MH96, but not from the ΔTC strain grown at temperatures of 25°C or less, were able to cause mortality. The 50% lethal doses (LD₅₀s) of the TC toward diamondback moth Plutella xylostella and C. zealandica larvae were defined as 30 ng and 50 ng, respectively, at 5 days after ingestion. Histological analysis of the effect of the TC toward P. xylostella larva showed that within 48 h after ingestion of the TC, there was a general dissolution of the larval midgut.

The role of relA and spoT in Yersinia pestis KIM5 pathogenicity

The ppGpp molecule is part of a highly conserved regulatory system for mediating the growth response to various environmental conditions. This mechanism may represent a common strategy whereby pathogens such as Yersinia pestis, the causative agent of plague, regulate the virulence gene programs required for invasion, survival and persistence within host cells to match the capacity for growth. The products of the relA and spoT genes carry out ppGpp synthesis. To investigate the role of ppGpp on growth, protein synthesis, gene expression and virulence, we constructed a ΔrelA ΔspoT Y. pestis mutant. The mutant was no longer able to synthesize ppGpp in response to amino acid or carbon starvation, as expected. We also found that it exhibited several novel phenotypes, including a reduced growth rate and autoaggregation at 26°C. In addition, there was a reduction in the level of secretion of key virulence proteins and the mutant was>1,000-fold less virulent than its wild-type parent strain. Mice vaccinated subcutaneously (s.c.) with 2.5×10⁴ CFU of the ΔrelA ΔspoT mutant developed high anti-Y. pestis serum IgG titers, were completely protected against s.c. challenge with 1.5×10⁵ CFU of virulent Y. pestis and partially protected (60% survival) against pulmonary challenge with 2.0×10⁴ CFU of virulent Y. pestis. Our results indicate that ppGpp represents an important virulence determinant in Y. pestis and the ΔrelA ΔspoT mutant strain is a promising vaccine candidate to provide protection against plague.

Growth of calcium-blind mutants of Yersinia pestis at 37 degrees C in permissive Ca2+-deficient environments

Cells of wild-type Yersinia pestis exhibit a low-calcium response (LCR) defined as bacteriostasis with expression of a pCD-encoded type III secretion system (T3SS) during cultivation at 37 degrees C without added Ca(2+) versus vegetative growth with downregulation of the T3SS with Ca(2+) (>or=2.5 mM). Bacteriostasis is known to reflect cumulative toxicity of Na(+), l-glutamic acid and culture pH; control of these variables enables full-scale growth ('rescue') in the absence of Ca(2+). Several T3SS regulatory proteins modulate the LCR, because their absence promotes a Ca(2+)-blind phenotype in which growth at 37 degrees C ceases and the T3SS is constitutive even with added Ca(2+). This study analysed the connection between the LCR and Ca(2+) by determining the response of selected Ca(2+)-blind mutants grown in Ca(2+)-deficient rescue media containing Na(+) plus l-glutamate (pH 5.5), where the T3SS is not expressed, l-glutamate alone (pH 6.5), where l-aspartate is fully catabolized, and Na(+) alone (pH 9.0), where the electrogenic sodium pump NADH : ubiquinone oxidoreductase becomes activated. All three conditions supported essentially full-scale Ca(2+)-independent growth at 37 degrees C of wild-type Y. pestis as well as lcrG and yopN mutants (possessing a complete but dysregulated T3SS), indicating that bacteriostasis reflects a Na(+)-dependent lesion in bioenergetics. In contrast, mutants lacking the negative regulator YopD or the YopD chaperone (LcrH) failed to grow in any rescue medium and are therefore truly temperature-sensitive. The Ca(2+)-blind yopD phenotype was fully suppressed in a Ca(2+)-independent background lacking the injectisome-associated inner-membrane component YscV but not peripheral YscK, suggesting that the core translocon energizes YopD.

Calcium in bacteria: a solution to which problem?

Calcium and calcium-binding proteins including those resembling calmodulin are implicated in numerous diverse processes in bacteria. These processes include chemotaxis, sporulation, virulence, the transport of sugars and proteins, phosphorylation, heat shock, the initiation of DNA replication, septation, nucleoid structure, nuclease activity and recombination, the stability of the envelope, and phospholipid synthesis and configuration. That such varied processes should have a common factor, calcium, suggests major underlying principles of calcium metabolism which have yet to be discovered.

A calcium flux at the termination of replication triggers cell division in Escherichia coli. Hypothesis

Cell division in Escherichia coli is coupled to chromosome replication. Even in the absence of known inducible division inhibitors, perturbations of chromosome replication affect cell division. Early studies suggested that a signal at the termination of replication might trigger subsequent division. Although later studies have suggested that fork encounter during termination is an active process involving specific termination sites and the tus protein, the coupling mechanism between termination and cell division remains to be elucidated. Recently it has been shown that the chromosome of a bacterium, Pseudomonas tabaci, contains a high proportion of calcium. E. coli maintains an intracellular concentration of free calcium identical to that of higher organisms and in dividing cells of E. coli a twenty-fold increase in the level of total calcium in the cytoplasm, a flux, occurs. In this article I propose that during the replication of the chromosome calcium entry balances calcium binding to DNA. At the termination of replication, there is a brief interval between the end of calcium binding to the chromosome and the end of calcium entry or release into the cytoplasm. During this interval the level of free calcium therefore rises. This rise may result in the observed flux by triggering the entry of calcium directly via voltage-gated calcium channels or indirectly via changes in phospholipid configurations. Mechanisms whereby these changes in calcium levels might be coupled to cell division and to a phospholipid control of the cell cycle are discussed.

Effects of growth temperature, 47-megadalton plasmid, and calcium deficiency on the outer membrane protein porin and lipopolysaccharide composition of Yersinia pestis EV76

The expression of several virulence determinants of Yersinia pestis is known to be dependent on the in vitro growth temperature. One of these, calcium dependence, is associated with the presence of a 47-megadalton plasmid. We have examined the effects of incubation temperature, calcium in the growth medium, the presence of the 47-megadalton plasmid on the outer membrane protein, and the lipopolysaccharide composition of Y. pestis EV76. When cells were grown at 37 degrees C as opposed to 26 degrees C, a change in lipopolysaccharide composition and a decrease in the amount of an outer membrane protein (protein E) were observed. The lipopolysaccharide obtained from cells incubated at 37 degrees C had a lower proportion of 2 keto-3-deoxyoctanate, a lower phosphate to 2-keto-3-deoxyoctanate ratio, and an increased gel mobility upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis when compared with lipopolysaccharide obtained from cells grown at 26 degrees C. Because of its growth temperature-related abundance, we investigated the nature of protein E. This protein had physical properties similar to those of other enterobacterial porins, including apparent formation of an oligomer on sodium dodecyl sulfate-polyacrylamide gels when solubilized at low temperature, acidic isoelectric point, and strong noncovalent association with the peptidoglycan. Protein E was purified and shown to form an aqueous channel in planar lipid membranes with a conductance of 1.1 nS in 1 M KCl. In addition to growth temperature-related alterations in the lipopolysaccharide and porin components of the outer membrane, the amount of three spots in two-dimensional polyacrylamide gels was shown to be related to the temperature or the presence of calcium during growth. One of these spots was shown to contain residual unmodified portions of two major heat-modifiable proteins which failed to shift to their heat-modified positions on gels, despite solubilization at 100 degrees C for 10 min before electrophoresis. The other two spots were the heat-modified and unmodified forms of another outer membrane protein (J) which did not appear in the isoelectric focusing gel of cells grown at 37 degrees C. It is proposed that the appearance of these spots in two-dimensional analyses is related to the lipopolysaccharide composition of the cells from which the outer membrane is derived and reflects lipopolysaccharide-protein interactions or calcium-protein interactions.

Reactivation of Rickettsia rickettsii in Dermacentor andersoni ticks: an ultrastructural analysis

Virulent Rickettsia in Dermacentor andersoni lose their pathogenicity and virulence for guinea pigs when subjected to physiological stresses, such as starvation (overwintering), of its tick vector. However, incubation of infected ticks at an elevated temperature (37 degrees C) for 24 to 48 h or feeding for a time (usually greater than 10 h) induces R. rickettsii to revert to a virulent state, a phenomenon defined as "reactivation." Electron microscopy reveals that the microcapsular and slime layers of R. rickettsii undergo changes dependent upon the physiological conditions within the tick vector. In engorged ticks, the microcapsular layer is readily identified as a discrete layer, approximately 16 nm thick, composed of globular subunits that have a periodicity of approximately 10 nm. The slime layer external to the microcapsular layer forms a discrete electron-lucent zone around the rickettsia. In starved ticks, neither the microcapsular layer nor slime layer remains a discrete entity. Instead, they are shed and form stringy, shredded, and somewhat flocculent strands of low electron density without periodicity. Incubation at 37 degrees C or feeding of starved infected ticks results in the restoration of a discrete microcapsular and slime layer. These reversible structural modifications are linked to physiological changes in the tick host and correlate with reactivation, i.e., restoration of pathogenicity and virulence of R. rickettsii.

RNA synthesis in Yersinia pestis during growth restriction in calcium-deficient medium

Yersinia pestis requires 2.5 mM Ca(2+) for growth at 37 degrees C but not at 26 degrees C. After a shift from 26 to 37 degrees C in a Ca(2+)-deficient medium, an ordered series of metabolic alterations occur which result in transition from a growing cell to a viable but non-proliferating cell. The earliest known alteration in normal metabolism associated with this transition is a termination of net RNA synthesis. Competitive RNA/DNA hybridizations with uniformly labeled RNA and stable RNA competitor indicated identical mRNA to stable RNA ratios in growing cells and non-proliferating Ca(2+)-deprived cells. Similar hybridizations with pulse-labeled RNA demonstrated that growing cells synthesized 57% mRNA, 37% rRNA, and 5% tRNA, whereas Ca(2+)-deprived cells synthesized 95% mRNA, 4.7% rRNA, and 0.7% tRNA. After addition of radioactive uracil and rifampin to growing and Ca(2+)-deprived cells, decay of approximately 40 and 90% of the newly synthesized RNA was found for growing and Ca(2+)-deprived cells, respectively. The half-life of the mRNA was found to be 1.5 min for growing cells and 4.5 min for Ca(2+)-deprived cells. Y. pestis elicited increases in the levels of guanosine tetraphosphate and guanosine pentaphosphate in response to amino acid deprivation and yielded transient increases in the levels of these phosphorylated nucleotides after a shift from 26 to 37 degrees C. These increases were independent of Ca(2+) availability and preceded the alteration in RNA synthesis by more than 1 h. The levels of these phosphorylated nucleotides then stabilized at about 80 and 40 pmol for Ca(2+)-deprived and Ca(2+)-supplemented cultures, respectively, and did not increase further in the Ca(2+)-deprived culture at the time corresponding to the reduction in stable RNA synthesis. These findings indicate that the early lesion in RNA synthesis associated with the growth restriction of Ca(2+)-deprived Y. pestis reflects a block in stable RNA synthesis and that this effect is not mediated by guanosine tetraphosphate or guanosine pentaphosphate.

Characterization of plasmids and plasmid-associated determinants of Yersinia enterocolitica pathogenesis

Yersinia enterocolitica isolates harboring a particular species of plasmid deoxyribonucleic acid showed a high degree of lethality for gerbils and caused the detachment of HEp-2 tissue cell monolayers. Strains cured of their plasmid deoxyribonucleic acid showed loss of these properties. However, invasiveness of HEp-2 cells was shown not to be a plasmid-mediated property. The expression of plasmid-associated properties, including at least three major outer membrane polypeptides, occurred during growth at 37 but not at 25 degrees C and was related to the concentration of calcium in the growth medium. The plasmid species associated with these properties ranged in molecular mass from 40 x 10(6) to 48 x 10(6) daltons and comprised a family of related plasmids.

Consequences of Ca2+ deficiency on macromolecular synthesis and adenylate energy charge in Yersinia pestis

A 37 but not 26 degrees C virulent Yersinia pestis is known to require at least 2.5 mM Ca2+ for growth; this requirement is potentiated by Mg2+. After shift of log-phase cells (doubling time of 2 h) from 26 to 37 degrees C in Ca2+-deficient medium, shutoff of net ribonucleic acid synthesis preceded that of protein and cell mass. With 2.5 mM Mg2+, about two doublings in cell mass and number occurred before restriction with synthesis of sufficient deoxyribonucleic acid to account for initiation and termination of two postshift rounds of chromosome replication. Temperature shift with 20 mMMg2+ resulted in a single doubling of cell mass and number with one round of chromosome replication. Subsequent to shutoff of ribonucleic acid accumulation, ribonucleoside but not deoxyribonucleoside triphosphate pools became reduced to about 50% of normal values and the adenylate energy change fell from about 0.8, typical of growing cells, to about 0.6. Excretion of significant concentrations of adenine nucleotides under both permissive and restrictive conditions was observed. Only trace levels (less than 0.01 microM ol/g [dry weight]) of guaninosine 5'-diphosphate 3'-diphosphate accumulated under restrictive or permissive conditions; guanosine 5'-triphosphate 3'-diphosphate was not detected. Return of fully restricted cells from 37 to 26 degrees C with Ca2+ resulted in prompt growth, whereas addition of Ca2+ at 37 degrees C was ineffective. This finding indicates that the observed temperature-sensitive lesion in ribonucleic acid synthesis that results in restriction can be prevented but not reversed by cultivation with Ca2+.

Yersinia pestis: correlation of ultrastructures and immunological status

Ultrastructural identification and localization of the fraction 1 "envelope" antigen in the plague bacillus Yersinia pestis were the primary objectives of this brief study. The antigenicity of extra-cellular material between the bacilli in undisturbed cultured colonies and that of the pathogen per se were measured and correlated by means of the semi quantitative complement fixation method after incubation for 72 h at 37 C. When the amount of extracellular substance in wild-type T1 (virulent) bacteria was compared by electron microscopy with that in avirulent strains of Y. pestis, with and without passage through guinea pigs, we found that the material of interest was greatly attenuated or even absent in colonies that had not been passed through animals, whereas passage markedly augmented production of the material. We also explored the requirement for larger quantities of Ca(2+) and Mg(2+) in the culture media and discovered that without these ions production of the extracellular material was limited. These observations support the hypothesis that this extracellular substance between cultured Y. pestis bacilli of various strains represents the source of the fraction 1 envelope antigen.