The importance of motility and chemotaxis for extra-animal survival of Salmonella enterica serovar Typhimurium and Dublin

AIMS

This study investigated the importance of flagella and motility of Salmonella enterica serovar Typhimurium and Dublin in models of extra-animal survival.

METHODS AND RESULTS

The study was performed using transposon mutants in flagella genes fliC and fljB and in chemotaxis genes cheA, cheB and cheR. Flagella and chemotaxis were found to be of minor importance for attachment to plant leaves, survival in liquid manure and interaction with the nematode C. elegans, while differences were observed between the fliC mutant and the wild-type strain of S. Dublin in interactions with amoebae.

CONCLUSIONS

The study shows that flagella and chemotaxis play a minor role in extra-animal survival of these two serovars of Salmonella under the conditions tested.

SIGNIFICANCE AND IMPACT OF THE STUDY

Extra-animal survival is important in the full infection cycle for zoonotic salmonellae. Such serovars are motile. Even though the current study was only based on the characterization of two serovars, it strongly suggests that motility and chemotaxis are of minor importance during the spread of Salmonella from one animal to the next through the external environment.

Method for Testing Degree of Infectivity of Rhizobium meliloti Strains

The infectiveness of different strains of Rhizobium meliloti was tested with a technique that uses the addition of tetracycline to the root medium. To stop the infection, the antibiotic was added some time after the inoculation of Medicago sativa plants. A coefficient of infectivity for each strain was calculated according to the number of nodules that appeared with and without the addition of the antibiotic. This method seems useful in infectivity studies and is simpler and easier to perform than the test of competence between strains.

Tn10 mutagenesis in Azotobacter vinelandii

The tetracycline-resistant transposon Tn10 and its "high-hopper" derivative Tn10HH104 were introduced into the Azotobacter vinelandii genome using suicide conjugative plasmids derived from pRK2013. Several types of mutants induced by either of these elements are described. Nif- mutants (deficient in nitrogen fixation) were easily isolated, whereas the isolation of other mutant types (auxotrophs, sugar non-users) required special selection conditions. The characterization of the mutations as transposon insertions was often complicated and sometimes required a combination of genetic and physical tests. A common source of complication, the existence of double inserts, was found among the mutants induced by Tn10HH104 but not among those induced by Tn10. Both the high-hopper and the wild-type element proved to undergo secondary transpositions, albeit at different frequencies. Another type of complication, the existence of heterozygotes, occurred because of the high level of redundancy of the A. vinelandii genome.

Salmonella enterica serovar Typhimurium response involved in attenuation of pathogen intracellular proliferation

Salmonella enterica serovar Typhimurium proliferates within cultured epithelial and macrophage cells. Intracellular bacterial proliferation is, however, restricted within normal fibroblast cells. To characterize this phenomenon in detail, we investigated the possibility that the pathogen itself might contribute to attenuating the intracellular growth rate. S. enterica serovar Typhimurium mutants were selected in normal rat kidney fibroblasts displaying an increased intracellular proliferation rate. These mutants harbored loss-of-function mutations in the virulence-related regulatory genes phoQ, rpoS, slyA, and spvR. Lack of a functional PhoP-PhoQ system caused the most dramatic change in the intracellular growth rate. phoP- and phoQ-null mutants exhibited an intracellular growth rate 20- to 30-fold higher than that of the wild-type strain. This result showed that the PhoP-PhoQ system exerts a master regulatory function for preventing bacterial overgrowth within fibroblasts. In addition, an overgrowing clone was isolated harboring a mutation in a previously unknown serovar Typhimurium open reading frame, named igaA for intracellular growth attenuator. Mutations in other serovar Typhimurium virulence genes, such as ompR, dam, crp, cya, mviA, spiR (ssrA), spiA, and rpoE, did not result in pathogen intracellular overgrowth. Nonetheless, lack of either SpiA or the alternate sigma factor RpoE led to a substantial decrease in intracellular bacterial viability. These results prove for the first time that specific serovar Typhimurium virulence regulators are involved in a response designed to attenuate the intracellular growth rate within a nonphagocytic host cell. This growth-attenuating response is accompanied by functions that ensure the viability of intracellular bacteria.

Genetic mapping by duplication segregation in Salmonella enterica

MudP and MudQ elements were used to induce duplications in Salmonella enterica by formation of a triple crossover between two transduced fragments and the host chromosome. The large size (36 kb) of MudP and MudQ is a favorable trait for duplication formation, probably because homology length is a limiting factor for the central crossover. Additional requirements are a multiplicity of infection of 2 or higher in the infecting phage suspensions (which reflects the need of two transduced fragments) and an exponentially growing recipient (which reflects the need of a chromosome replication fork). We describe a set of 11 strains of S. enterica, each carrying a chromosomal duplication with known endpoints. The collection covers all the Salmonella chromosome except the terminus. For mapping, a dominant marker (e.g., a transposon insertion in or near the locus to be mapped) is transduced into the 11-strain set. Several transductants from each cross are grown nonselectively, and haploid segregants are scored for the presence of the marker. If all the segregants contain the transduced marker, it maps outside the duplication interval. If the marker is found only in a fraction of the segregants, it maps within the duplicated region.

Daily time course of whole-shoot gas exchange rates in two drought-exposed Mediterranean shrubs

Effects of drought on water relations, whole-shoot gas-exchange characteristics, and pigment and zeatin concentrations were investigated in the Mediterranean shrubs rosemary (Rosmarinus officinalis L.) and lavender (Lavandula stoechas L.). Two-year-old, greenhouse-grown plants were placed in a whole-shoot gas-exchange measurement system and subjected to 10 days of drought, resulting in severe water stress, and then re-watered for 5 days in order to study their recovery. Water stress resulted in a significant decline in maximum whole-shoot net CO2 assimilation rates (An) for both species that was associated with reductions in leaf area and stomatal conductance. Because shoot dark respiration rate (Rd) was less sensitive to water stress than An, shoot Rd/An ratio increased from about 15 to 95% during water stress. No major changes in chlorophyll and carotenoid concentrations of rosemary leaves were observed during the experiments, but chlorophyll and carotenoid concentrations fell significantly in water-stressed lavender leaves. Zeatin concentrations were higher in rosemary leaves than in lavender leaves during water stress. After re-watering, whole-shoot An and Rd rapidly recovered to their pre-drought rates.

Host adapted serotypes of Salmonella enterica

Salmonella constitutes a genus of zoonotic bacteria of worldwide economic and health importance. The current view of salmonella taxonomy assigns the members of this genus to two species: S. enterica and S. bongori. S. enterica itself is divided into six subspecies, enterica, salamae, arizonae, diarizonae, indica, and houtenae, also known as subspecies I, II, IIIa, IIIb, IV, and VI, respectively [1]. Members of Salmonella enterica subspecies enterica are mainly associated with warm-blooded vertebrates and are usually transmitted by ingestion of food or water contaminated by infected faeces. The pathogenicity of most of the distinct serotypes remains undefined, and even within the most common serotypes, many questions remain to be answered regarding the interactions between the organism and the infected host.

Salmonellosis manifests itself in three major forms: enteritis, septicaemia, and abortion, each of which may be present singly or in combination, depending on both the serotype and the host involved. Although currently over 2300 serovars of Salmonella are recognized, only about 50 serotypes are isolated in any significant numbers as human or animal pathogens [2, 3] and they all belong to subspecies enterica. Of these, most cause acute gastroenteritis characterized by a short incubation period and a severe systemic disease in man or animals, characterized by septicaemia, fever and/or abortion, and such serotypes are often associated with one or few host species [4–6].

It is the intention of this review to present a summary of current knowledge of these host-adapted serotypes of S. enterica. The taxonomic relationships between the serotypes will be discussed together with a comparison of the pathology and pathogenesis of the disease that they cause in their natural host(s). Since much of our knowledge on salmonellosis is based on the results of work on Typhimurium, this serotype will often be used as the baseline in discussion. It is hoped that an appreciation of the differences that exist in the way these serotypes interact with the host will lead to a greater understanding of the complex host–parasite relationship that characterizes salmonella infections.

Regulation of septation: a novel role for SerC/PdxF in Salmonella?

The sfiW locus of Salmonella enterica, previously identified by mutations that suppress the cell division defect of His-constitutive (His(c)) strains, corresponds to serC, the bifunctional gene for phosphoserine-oxoglutarate aminotransferase (SerC) and 2-ketoerythroic acid 4-phosphate transaminase (PdxF). SerC- mutants form small, nearly spherical cells in a wild-type (His+) background, suggesting that the SerC/PdxF product acts as a septation antagonist. Suppression of His(c) filamentation by serC mutations may be explained by loss of the anti-septation activity of SerC/PdxF. The isolation of serC alleles that have lost their biosynthetic activities but are still able to inhibit septum formation suggests that the anti-septation activity of the SerC/PdxF product is unrelated to its known roles in serine and pyridoxine biosynthesis.

Lack of hotspot targets: a constraint for IS30 transposition in Salmonella

IS30 is an insertion element common in E. coli strains but rare or absent in Salmonella. Transfer of the IS30-flanked transposon Tn2700 to Salmonella typhimurium was assayed using standard delivery procedures of bacterial genetics (conjugation and transduction). Tn2700 'hops' were rare and required transposase overproduction, suggesting the existence of host constraints for IS30 activity. Sequencing of three Tn2700 insertions in the genome of S. typhimurium revealed that the transposon had been inserted into sites with a low homology to the IS30 consensus target, suggesting that inefficient Tn2700 transposition to the Salmonella genome might be caused by a lack of hotspot targets. This view was confirmed by the introduction of an IS30 'hot target sequence', whose sole presence permitted Tn2700 transposition without transposase overproduction. Detection of IS30-induced DNA rearrangements in S. typhimurium provided further evidence that the element undergoes similar activities in E. coli and S. typhimurium. Thus, hotspot absence may be the main (if not the only) limitation for IS30 activity in the latter species. If these observations faithfully reproduce the scenario of natural populations, establishment of IS30 in the Salmonella genome may have been prevented by a lack of DNA sequences closely related to the unusually long (24 bp) IS30 consensus target.

DNA adenine methylase mutants of Salmonella typhimurium show defects in protein secretion, cell invasion, and M cell cytotoxicity

Mutants of Salmonella typhimurium lacking DNA adenine methylase are attenuated for virulence in BALB/c mice. LD(50) values of a DNA adenine methylation (Dam)(-) mutant are at least 10(3)- to 10(4)-fold higher than those of the parental strain when administrated by oral or intraperitoneal routes. Dam(-) mutants are unable to proliferate in target organs but persist in low numbers in these locations. Efficient protection to challenge with the virulent parental strain is observed in mice infected with a Dam(-) mutant. Use of the ileal loop assay shows that Dam(-) mutants are less cytotoxic to M cells and fail to invade enterocytes. In the tissue culture model, lack of DNA adenine methylation causes reduced ability to invade nonphagocytic cells. In contrast, no effect is observed either in intracellular proliferation within nonphagocytic cells or in survival within macrophages. The invasion defect of Dam(-) mutants is correlated with a distinct pattern of secreted proteins, which is observed in both PhoP(+) and PhoP(-) backgrounds. Altogether, our observations suggest a multifactorial role of Dam methylation in Salmonella virulence.

Repression of IS200 transposase synthesis by RNA secondary structures

The IS 200 transposase, a 16 kDa polypeptide encoded by the single open reading frame (ORF) of the insertion element, has been identified using an expression system based on T7 RNA polymerase. In wild-type IS 200, two sets of internal inverted repeats that generate RNA secondary structures provide two independent mechanisms for repression of transposase synthesis. The inverted repeat located near the left end of IS 200 is a transcriptional terminator that terminates read-through transcripts before they reach the IS 200 ORF. The terminator is functional in both directions and may terminate >80% of transcripts. Another control operates at the translational level: transposase synthesis is inhibited by occlusion of the ribosome-binding site (RBS) of the IS 200 ORF. The RBS (5'-AGGGG-3') is occluded by formation of a mRNA stem-loop structure whose 3' end is located only 3 nt upstream of the start codon. This mechanism reduces transposase synthesis approximately 10-fold. Primer extension experiments with AMV reverse transcriptase have provided evidence that this stem-loop RNA structure is actually formed. Tight repression of transposase synthesis, achieved through synergistic mechanisms of negative control, may explain the unusually low transposition frequency of IS 200.

The virulence plasmids of Salmonella

Certain Salmonella serovars belonging to subspecies I carry a large, low-copy-number plasmid that contains virulence genes. Virulence plasmids are required to trigger systemic disease; their involvement in the enteric stage of the infection is unclear. Salmonella virulence plasmids are heterogeneous in size (50-90 kb), but all share a 7.8 kb region, spv, required for bacterial multiplication in the reticuloendothelial system. Other loci of the plasmid, such as the fimbrial operon pef, the conjugal transfer gene traT and the enigmatic rck and rsk loci, may play a role in other stages of the infection process. The virulence plasmid of Salmonella typhimurium LT2 is self-transmissible; virulence plasmids from other serovars, such as Salmonella enteritidis and Salmonella choleraesuis, carry incomplete tra operons. The presence of virulence plasmids in host-adapted serovars suggests that virulence plasmid acquisition may have expanded the host range of Salmonella.

Synthesis of FinP RNA by plasmids F and pSLT is regulated by DNA adenine methylation

DNA adenine methylase mutants of Salmonella typhimurium contain reduced amounts of FinP, an antisense RNA encoded by the virulence plasmid pSLT. Lowered FinP levels are detected in both Dam- FinO+ and Dam- FinO- backgrounds, suggesting that Dam methylation regulates FinP production rather than FinP half-life. Reduced amounts of F-encoded FinP RNA are likewise found in Dam- mutants of Escherichia coli. A consequence of FinP RNA scarcity in the absence of DNA adenine methylation is that Dam- mutants of both S. typhimurium and E. coli show elevated levels of F plasmid transfer. Inhibition of F fertility by the S. typhimurium virulence plasmid is also impaired in a Dam- background.

Cell division inhibition in Salmonella typhimurium histidine-constitutive strains: an ftsI-like defect in the presence of wild-type penicillin-binding protein 3 levels

Histidine-constitutive (Hisc) strains of Salmonella typhimurium undergo cell division inhibition in the presence of high concentrations of a metabolizable carbon source. Filaments formed by Hisc strains show constrictions and contain evenly spaced nucleoids, suggesting a defect in septum formation. Inhibitors of penicillin-binding protein 3 (PBP3) induce a filamentation pattern identical to that of Hisc strains. However, the Hisc septation defect is caused neither by reduced PBP3 synthesis nor by reduced PBP3 activity. Gross modifications of peptidoglycan composition are also ruled out. D-Cycloserine, an inhibitor of the soluble pathway producing peptidoglycan precursors, causes phenotypic suppression of filamentation, suggesting that the septation defect of Hisc strains may be caused by scarcity of PBP3 substrate.

The P22 Erf protein and host RecA provide alternative functions for transductional segregation of plasmid-borne duplications

A tandem DNA duplication carried on a ColE1-derived plasmid segregates at high frequency upon generalized transduction by phage P22 HT. Transductional segregation of the plasmid-borne duplication can be promoted either by RecA or by the Erf function of P22, indicating that transductional segregation is a consequence of the recombination events that re-circularize the plasmid in the recipient cell. RecA-mediated and Erf-mediated transduction give similar frequencies of duplication segregation and yield the same types of segregation products, indicating that two distinct recombination machineries (RecA + RecBCD and Erf + RecBCD) perform similar or identical recombination reactions on plasmid DNA substrates transduced by bacteriophage P22 HT.

The sfiX, rfe and metN genes of Salmonella typhimurium and their involvement in the His(c) pleiotropic response

Two loci involved in the pleiotropic response of His(c) strains of Salmonella typhimurium (sfiX and sfiY) have been characterized at the molecular level. The sfiX gene (CS 44) has been identified as a homolog of the E. coli gene sanA, located downstream of the cytidine deaminase gene (cdd). The cdd-sanA (or cdd-sfiX) operon shows a highly conserved structure in E. coli and Salmonella. Like its E. coli homolog, the sfiX gene of S. typhimurium is required for vancomycin resistance at high temperature. The dual effect of sfiX mutations (induction of vancomycin sensitivity and suppression of cell division inhibition) suggests a link between SfiX function and murein synthesis. The sfiY locus (CS 85), contains two genes arranged in a single transcriptional unit. The upstream gene is a homolog of the E. coli gene rfe; mutations in this gene suppress the cell division defect of His(c) strains. The suppressor effect of rfe mutations can be reproduced by tunicamycin, suggesting that suppression of filamentation results from an increase in the intracellular concentration of UDP-N-acetyl-D-glucosamine. The gene located downstream of rfe is also found in E. coli but its function is unknown. Insertions in rfe suppress the methionine requirement of His(c) strains of S. typhimurium by a polar effect on the downstream gene, tentatively designated metN. Complementation with a rfe+ clone indicates that the rfe gene is not involved in the methionine requirement of His(c) strains. Thus metN expression appears to cause methionine auxotrophy in a His(c) background.

Underground metabolism

All enzymes are able to use alternative substrates. When these are naturally occurring metabolites, an 'underground reaction' takes place. Examples are presented in which underground metabolism of this sort produces an observable phenotype. Although biological processes can be remarkably accurate, evolution has selected error rates far from perfect. It is suggested here that a certain level of metabolic inaccuracy, in addition to saving energy, may also confer an evolutionary advantage, for example by providing metabolic plasticity. Since underground reactions are unpredictable from DNA sequence data, caution is in order when interpreting correlations between genetic disorders and pathological syndromes.

Cloning with Mud-P22 hybrid prophages: mapping of IS200 elements on the chromosome of Salmonella typhimurium LT2

Specific DNA fragments from the chromosome of Salmonella typhimurium LT2 were packaged in P22 capsids by induction of "locked-in" Mud-P22 hybrid prophages. High yields of the packaged DNA were obtained upon capsid disruption. DNA hybridization using a fragment of insertion sequence IS200 as probe permitted physical mapping of IS200 elements on the chromosome of S. typhimurium LT2 within +/- 1 centisome (CS). IS200 copies were found at the following locations: CS 24 (copy VI), CS 53 (copy V), CS 63 (copy I), CS 80 (copy II) and CS 93 (copy III). Copy IV, previously mapped near fliA (CS 42), was not included in our study.

Identification of Salmonella abortusovis by PCR amplification of a serovar-specific IS200 element

Field and collection isolates of Salmonella abortusovis carry one IS200 element in a distinct chromosome location. IS200 is not found in the corresponding region of the chromosome of other Salmonella serovars. Sequencing of the boundaries of the S. abortusovis-specific IS200 insertion permitted the design of primers for the amplification of this IS200 element by PCR. Isolates of S. abortusovis are identified by the amplification of a DNA fragment of about 900 bp or larger. PCR amplification of DNA from salmonellae other than S. abortusovis yields either a fragment of about 200 bp or no product. The high specificity of the assay is confirmed by the absence of cross-reactivity with the following templates: (i) sheep DNA, (ii) DNAs from abortion-causing agents other than S. abortusovis, and (iii) DNAs from microorganisms that do not cause abortion but are common in flocks.

Conserved structure of IS200 elements in Salmonella

Sequence analysis of three IS200 elements (two from Salmonella typhimurium, one from Salmonella abortusovis) reveals a highly conserved structure, with a length of 707-708 bp and absence of terminal repeats. IS200 contains an open-reading-frame (ORF) which potentially encodes a peptide of 151 amino acids, with a putative ribosome-binding-site properly placed upstream of the ORF. A potential RNA stem-loop structure that might occlude the ribosome-binding-site of the ORF is also found. Another conserved trait is a potential RNA hairpin which resembles a Rho-independent transcription terminator, located near one end of IS200. The junctions between IS200 and host DNA sequences are A+T-rich. Upon insertion, IS200 duplicates 1-2 bp of host DNA sequences. The observation that IS200 elements characterized as 'hops' are roughly identical to those residing in the Salmonella genome suggests that IS200 transposition is unlikely to generate inactive copies. If such is the case and many or all IS200 elements are active, the extremely low frequency of IS200 transposition may reflect the normal behavior of the element.

DNA adenine methylase mutants of Salmonella typhimurium and a novel dam-regulated locus

Mutants of Salmonella typhimurium lacking DNA adenine methylase were isolated; they include insertion and deletion alleles. The dam locus maps at 75 min between cysG and aroB, similar to the Escherichia coli dam gene. Dam- mutants of S. typhimurium resemble those of E coli in the following phenotypes: (1) increased spontaneous mutations, (2) moderate SOS induction, (3) enhancement of duplication segregation, (4) inviability of dam recA and dam recB mutants, and (5) suppression of the inviability of the dam recA and dam recB combinations by mutations that eliminate mismatch repair. However, differences between S. typhimurium and E. coli dam mutants are also found: (1) S. typhimurium dam mutants do not show increased UV sensitivity, suggesting that methyl-directed mismatch repair does not participate in the repair of UV-induced DNA damage in Salmonella. (2) S. typhimurium dam recJ mutants are viable, suggesting that the Salmonella RecJ function does not participate in the repair of DNA strand breaks formed in the absence of Dam methylation. We also describe a genetic screen for detecting novel genes regulated by Dam methylation and a locus repressed by Dam methylation in the S. typhimurium virulence (or "cryptic") plasmid.

Strain typing with IS200 fingerprints in Salmonella abortusovis

A collection of Salmonella abortusovis isolates was examined for the presence of insertion element IS200. All proved to contain three or four copies of the element. One IS200 hybridization band of approximately 9 kb was found in all isolates, indicating that all S. abortusovis strains carry an IS200 element in similar or identical locations; this band can be potentially useful for serovar identification. S. abortusovis collection isolates from distinct geographic areas were highly polymorphic, suggesting that IS200 fingerprints might provide information on the geographic origin of S. abortusovis strains. Isolates obtained from the same geographic area (the island of Sardinia, Italy) were less polymorphic: all shared three constant IS200 hybridization bands, indicating that they derive from a single ancestor. Most strains analyzed contained an additional copy of IS200 in the variable region of the virulence plasmid. Certain Sardinian flocks proved to be infected by only one S. abortusovis strain, while others harbored two strains. Strain typing with IS200 fingerprints proved to be more reliable than plasmid analysis, because the latter yielded a high degree of polymorphism, even among isolates from the same flock.

recB recJ mutants of Salmonella typhimurium are deficient in transductional recombination, DNA repair and plasmid maintenance

recB recJ mutants of Salmonella typhimurium are deficient in transduction of chromosomal markers and ColE1-derived plasmids, and also in the maintenance of ColE1 and F plasmids. Plasmid instability is less severe in recD recJ strains; ColE1 plasmid DNA preparations from these strains show an increased yield of high molecular weight (HMW) linear multimers and a concomitant reduction in plasmid monomers compared to the wild type. Plasmids remain unstable in recA recD recJ mutants; since these do not produce HMW linear concatemers, we propose that a decrease in monomer production leads to plasmid instability. recB recJ strains also display decreased viability, a component of which may be related to their deficiency in DNA repair. In contrast to their severe defects in recombination, DNA repair and plasmid maintenance, recB recJ mutants of S. typhimurium behave similarly to the wild type in the segregation of chromosome duplications. The latter observation suggests that neither RecBCD nor RecJ functions are required for chromosomal recombination events that do not involve the use of free ends as recombination substrates.

Suppression of the pleiotropic effects of HisH and HisF overproduction identifies four novel loci on the Salmonella typhimurium chromosome: osmH, sfiW, sfiX, and sfiY

Insertion mutations that suppress some or all the pleiotropic effects of HisH and HisF overproduction were obtained by using transposons Tn10dTet and Tn10dCam. All suppressor mutations proved to be recessive, indicating that their effects were caused by loss of function; thus, the suppressors identify genes that are necessary to trigger the pleiotropic response when HisH and HisF are overproduced. Genetic mapping of the suppressor mutations identifies four novel loci on the Salmonella typhimurium genetic map. Mutations in osmH (min 49) behave as general suppressors that abolish all manifestations of the pleiotropic response. Mutations in sfiY (min 83) suppress cell division inhibition and thermosensitivity but not osmosensitivity. Mutations that suppress only cell division inhibition define another locus, sfiX (min 44). A fourth novel locus, sfiW (min 19), is also involved in cell division inhibition. The phenotype of sfiW mutations is in turn pleiotropic: they suppress cell division inhibition, make S. typhimurium unable to grow in minimal media, and cause slow growth and abnormal colony and cell shape. The inability of sfiW mutants to grow in minimal medium cannot be relieved by any known nutritional requirement or by the use of carbon sources other than glucose. The hierarchy of suppressor phenotypes and the existence of epistatic effects among suppressor mutations suggest a pathway-like model for the Hisc pleiotropic response.

Role of Erf recombinase in P22-mediated plasmid transduction

In the absence of host RecA function, plasmid transduction by bacteriophage P22 can be mediated by Erf recombinase. Erf is not carried on the infecting particle but synthesized upon infection. In the recipient cell, Erf can promote both generalized plasmid transduction (which requires the circularization of plasmids transduced as linear multimers) and specialized plasmid transduction (which requires the release of plasmid DNA from linear plasmid-phage cointegrates). Both processes of Erf-mediated plasmid transduction require host RecBCD function. In contrast, RecBCD is not required for Erf-mediated circularization of P22 DNA.

New methods in Salmonella genetics

This review summarizes several recent developments in Salmonella genetics; some of the procedures described can be easily adapted to Escherichia coli and have also potential applications in non-enteric bacteria. The novel methods outlined include genetic mapping procedures, ancillary tools for cloning, a strategy for analyzing DNA-protein interactions in vivo, a method for plasmid curing and a procedure for the detection of bacterial virulence genes.

Compositional heterogeneity of the Escherichia coli genome: a role for VSP repair?

E. coli genes that contain a high frequency of the tetranucleotide CTAG are also rich in the tetramers CTTG, CCTA, CCAA, TTGG, TAGG, and CAAG (group-I tetramers). Conversely, E. coli genes lacking CTAG are rich in the tetranucleotides CCTG, CCAG, CTGG, and CAGG (group-II tetramers). These two gene samples differ also in codon usage, amino acid composition, frequency of Dcm sites, and contrast vocabularies. Group-I tetramers have in common that they are depleted by very-short-patch repair (VSP), while group-II tetramers are favored by VSP activity. The VSP system repairs G:T mismatches to G:C, thereby increasing the overall G+C content of the genome; for this reason the CTAG-rich sample has a lower G+C content than the CTAG-poor sample. This compositional heterogeneity can be tentatively explained by a low level of VSP activity on the CTAG-rich sample. A negative correlation is found between the frequency of group-I tetramers and the level of gene expression, as measured by the Codon Adaptation Index (CAI). A possible link between the rate of VSP activity and the level of gene expression is considered.

Changes of ploidy during the Azotobacter vinelandii growth cycle

The size of the Azotobacter vinelandii chromosome is approximately 4,700 kb, as calculated by pulsed-field electrophoretic separation of fragments digested with the rarely cutting endonucleases SpeI and SwaI. Surveys of DNA content per cell by flow cytometry indicated the existence of ploidy changes during the A. vinelandii growth cycle in rich medium. Early-exponential-phase cells have a ploidy level similar to that of Escherichia coli or Salmonella typhimurium (probably ca. four chromosomes per cell), but a continuous increase of DNA content per cell is observed during growth. Late-exponential-phase cells may contain > 40 chromosomes per cell, while cells in the early stationary stage may contain > 80 chromosomes per cell. In late-stationary-phase cultures, the DNA content per cell is even higher, probably over 100 chromosome equivalents per cell. A dramatic change is observed in old stationary-phase cultures, when the population of highly polyploid bacteria segregates cells with low ploidy. The DNA content of the latter cells resembles that of cysts, suggesting that the process may reflect the onset of cyst differentiation. Cells with low ploidy are also formed when old stationary-phase cultures are diluted into fresh medium. Addition of rifampin to exponential-phase cultures causes a rapid increase in DNA content, indicating that A. vinelandii initiates multiple rounds of chromosome replication per cell division. Growth in minimal medium does not result in the spectacular changes of ploidy observed during rapid growth; this observation suggests that the polyploidy of A. vinelandii may not exist outside the laboratory.

The pleiotropic effects of his overexpression in Salmonella typhimurium do not involve AICAR-induced mutagenesis

Inhibition of cell division associated with overexpression of hisH and hisF in Salmonella typhimurium is strongly reminiscent of a cellular response to DNA damage. On these grounds, we investigated the involvement of a metabolite which appeared to represent a possible candidate for an endogenous mutagen: the base analog 5-amino-4-carboxamide imidazole riboside 5'-phosphate (AICAR), a by-product of HisH and HisF activity. However, we showed that AICAR is not an endogenous mutagen in S. typhimurium. Other types of DNA damage induced by his overexpression seem also unlikely, since similar mutation rates are found in hisO+ and hisOc strains. We also show that AICAR production is not involved in the pleiotropic effects of his overexpression, since these are still observed in strains devoid of AICAR. Thus inhibition of cell division resulting from HisH and HisF overexpression must operate through a mechanism unrelated to the role of these proteins in histidine biosynthesis.

Host RecJ is required for growth of P22 erf bacteriophage

Growth of bacteriophage P22 erf is known to require host RecA recombination function. We show that the RecA function is necessary but not sufficient to restore the plaque-forming ability of phage P22 erf; such mutant phage also requires host RecJ function. The residual efficiency of plaquing of P22 erf in a recJ background (0.03%) is completely abolished in recJ recB hosts (< 0.001%), suggesting that the RecBCD nuclease can provide an alternative function allowing phage growth. One tentative explanation is that circularization of P22 erf DNA mostly proceeds through the RecF pathway of recombination; however, less efficient circularization via the RecBCD pathway may also occur. In a recJ background, lysates obtained upon induction of an erf prophage show reduced yield (10%), suggesting that growth of P22 erf may require host RecJ in a step(s) other than circularization of phage DNA.

Gene dosage analysis in Azotobacter vinelandii

For more than a decade, Azotobacter vinelandii has been considered a polyploid bacterium on the basis of physical studies of chromosome size and DNA content per cell. However, as described in the present work, many genetic operations can be performed in A. vinelandii without the constraints expected in a polyploid bacterium: (i) reversion of transposon-induced mutations is usually associated with loss of the transposable element; (ii) revertants retaining the transposon always carry secondary transpositions; (iii) heterozygotic transconjugants and transformants are unstable and segregate homozygotic colonies even in the absence of selection. Physical monitoring of segregation, achieved by colony hybridization, indicates that phenotypic expression of an allele is always correlated with its physical presence, thus ruling out the existence of either threshold dosage requirements or transcriptionally inactive DNA. Chromosomal lac fusions constructed by double crossover with a linearized plasmid show a segregation pattern consistent with the inheritance of one or several chromosomes per daughter cell. Analysis of the delay required for the expression of recessive chromosomal mutations such as rif, nal and str provides further evidence that A. vinelandii is not a polyploid bacterium.

Isolation and characterization of Rhizobium meliloti mutants affected in exopolysaccharide production

Rhizobium meliloti mutants affected in the production of exopolysaccharide (EPS) were isolated after N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis. The mutants were classified into three phenotypic classes: (I) Exo-, rough mutants lacking exopolysaccharide; (II) Exos (for "small") which form tiny, compact colonies and synthesize reduced amounts of EPS; and (III) Exoc (for "constitutive"), hypermucoid mutants which overproduce EPS. Hypermucoid strains showed increased resistance to desiccation. All the mutants were able to nodulate, although a significant decrease in infectivity degree and/or competitiveness was found in rough and compact strains. Two mutants proved to be deficient in nitrogen fixation. Complementation analysis with cloned R. meliloti exo genes could not be applied to the study of these Fix- mutants because introduction of plasmids derived from cosmid vector pLAFR1 caused loss of nodulating ability. However, complementation of calcofluor staining and EPS production was observed. Complementation with certain exo genes also caused a marked increase in motility.

Distribution of insertion sequence IS200 in Salmonella and Shigella

Two DNA probes for the detection of insertion sequence IS200 by either Southern blotting or colony hybridization were constructed. One of the probes is a 300 bp EcoRI-HindIII fragment of IS200 cloned onto pBluescript KS(+); the other is a tail-to-tail dimer of the same fragment cloned onto pUC19. A survey of the presence of IS200 among enteric bacteria revealed that more than 90% of the pathogenic or food-poisoning isolates of Salmonella spp. examined contained one or more copies of insertion sequence IS200, with the exception of the subgenus I serovar S. agona in which IS200 is not found. Although insertion sequence IS200 was first considered a Salmonella-specific element, it also exists in many isolates of Shigella sonnei and Shigella flexneri, but not in Shigella dysenteriae.

Genomic imprinting in microorganisms

Genomic imprinting is an epigenetic mark introduced on a DNA molecule without alteration of the base sequence. Upon replication, the primary mark is propagated to the daughter DNA molecules. Epigenetic DNA modification often serves as a regulatory signal and may play a crucial role in many developmental processes. Although this mode of gene regulation was first discovered in multicellular eukaryotes, cases of imprinting have been recently found in lower eukaryotes, bacteria and phage. Thus it may be reasonable to list DNA modification among the major mechanisms that regulate gene expression.

sulA-independent division inhibition in his-constitutive strains of Salmonella typhimurium

Constitutive expression of the S. typhimurium histidine operon causes multiple phenotypic changes including strong filamentation. However, the SOS regulatory network is not involved in the inhibition of cell division. The possibility of SOS-independent activation of sulA gene transcription has been ruled out using sulA-lacZ fusions. These results suggest the existence of a pathway of division inhibition unrelated to sulA and not regulated by the SOS system.

Gibberella fujikuroi mutants obtained with UV radiation and N-methyl-N'-nitro-N-nitrosoguanidine

N-methyl-N'-nitro-N-nitrosoguanidine (nitrosoguanidine) and to a lesser extent UV radiation are very mutagenic for Gibberella microconidia. The recommended nitrosoguanidine doses lead to much higher frequencies of mutants than are found in other microorganisms. The frequency of mutants among the survivors increases linearly with the nitrosoguanidine dose (molar concentration X time); the absolute number of viable mutants in a given population reaches a maximum for a dose of ca. 0.7 M X s. The microconidia are uninucleate. The onset of germination brings about increased lethality of nitrosoguanidine, but it does not modify the action of UV radiation. Mycelia are more resistant than spores to both agents. Visible illumination effectively prevents lethality when given immediately after UV radiation. Auxotrophs and color mutants are very easily obtained. Pink adenine auxotrophs and several classes of color mutants are affected in the biosynthesis of the carotenoid pigment, neurosporaxanthin.

Transposition of Tn 1 to the Rhizobium meliloti genome

A derivative of the IncP1 plasmid RP4, carrying the thermoinducible prophage Mucts62, was obtained in Escherichia coli K12 J53 (RP4). It was impossible to maintain the hybrid plasmid RP4::Mucts62 in Rhizobium meliloti GR4. Thus, it was used as a vehicle for introducing the ampicillin-resistant transposon Tn1 into the R. meliloti genome. Transposition of Tn1 did not generate auxotrophic strains, suggesting that the insertion of Tn1 into the R. meliloti genome was relatively specific. Two chromosomal hot spots for Tn1 insertion were identified by cotransductional analysis, after general transduction by phage DF2. Plasmid-curing experiments, carried out by heat treatment, revealed that symbiotic plasmid(s) also contain at least one site for Tn1 insertion.

Rough and fine linkage mapping of the Rhizobium meliloti chromosome

A circular linkage map of the Rhizobium meliloti chromosome, obtained from R68.45-mediated crosses, has been revised by cotransductional analysis, after general transduction by DF2 phage. Three short chromosomal regions have been mapped by cotransduction. Comparison between conjugal and cotransductional data suggests that R68.45-mediated linkage measures are indeed rough. Cotransduction seems to be a useful tool for improving the linkage map of R. meliloti.

General transduction in Rhizobium meliloti by a thermosensitive mutant of bacteriophage DF2

A simiplified version of bacteriophage DF2 transduction, using the thermosensitive mutant DF2tsC, is described. The transduction frequencies of Rhizobium meliloti markers by DF1tsC are of about 10(-6) or higher. The development of transductants at 34 degrees C does not require the addition of antiserum for phage neutralization.