A Salmonella locus that controls resistance to microbicidal proteins from phagocytic cells

Defensins

Defensins are a family of small, variably cationic proteins which are highly abundant in the granules of mammalian phagocytes. Three defensins, HNP-1, 2, and 3, comprise 30-50% of total protein in azurophil granules of human neutrophils. Some defensins are broadly antimicrobial, antiviral and cytotoxic, while others are chemotactic, opsonic, or may modulate hormonal responses. The defensin molecule typically consists of 29-34 amino acids with a conserved pattern of disulfide linkage among its 6 cysteines. The three-dimensional fold of defensins forms a highly amphiphilic molecule. Microbicidal and cytotoxic properties of defensins are most likely a consequence of their ability to insert into biological membranes and to generate pores. Defensins are synthesized by phagocytes or their precursors as a 94-95 amino acid charge-neutralized preprodefensin, an arrangement which may avoid cytotoxic injury to the phagocyte. Although defensins were recognized only recently, the existence of homologs in certain invertebrates suggests that they are ancestral components of the host defense system.

Salmonella virulence: new clues to intramacrophage survival

Salmonella are capable of survival in macrophages (cells which have evolved specific mechanisms to kill pathogenic bacteria). One mechanism involves the bactericidal peptides called defensins which insert into phospholipid bilayers to generate transmembrane pores. Synthesis of the Salmonella gene products which determine resistance to defensins has been found to be under the control of a transcriptional regulatory protein termed PhoP.

The opsonizing ligand on Salmonella typhimurium influences incorporation of specific, but not azurophil, granule constituents into neutrophil phagosomes

Phagosomes were purified from human neutrophils ingesting Salmonella typhimurium opsonized with adsorbed normal human serum or with rabbit IgG. Constituents within the phagosome were endogenously labeled by supplying the cells with 125INa during phagocytosis. Lactoferrin and vitamin B12 binding protein (TC1 and TC3), markers for specific granules, were present in the phagosomes from neutrophils ingesting S. typhimurium opsonized with IgG but were 3.5- to 5-fold less prominent in phagosomes from cells phagocytosing Salmonella bearing C3 fragments only. In contrast, iodinated azurophilic granule components, most prominently defensins, were the major constituents in phagosomes prepared under both opsonization conditions. Furthermore, labeled complement (CR1 and CR3) and immunoglobulin (Fc gamma RIII) receptors were incorporated in the phagosome regardless of the ligand mediating phagocytosis. These results suggest that the ligand-receptor interactions mediating phagocytosis influence incorporation of neutrophil-specific granule contents into phagosomes.

Salmonella as an intracellular parasite

Salmonella species are facultative intracellular parasites, capable of penetrating (invading), surviving, and often multiplying within diverse eukaryotic cell types, including epithelial and phagocytic cells. These processes are essential for virulence, and involve both bacterial and host cell products. The use of cultured eukaryotic cells and other model systems has facilitated the study of bacterial-host cell interactions, and has led to a better understanding of the genetic and molecular basis of Salmonella pathogenicity.

Protein phosphorylation and regulation of adaptive responses in bacteria

Bacteria continuously adapt to changes in their environment. Responses are largely controlled by signal transduction systems that contain two central enzymatic components, a protein kinase that uses adenosine triphosphate to phosphorylate itself at a histidine residue and a response regulator that accepts phosphoryl groups from the kinase. This conserved phosphotransfer chemistry is found in a wide range of bacterial species and operates in diverse systems to provide different regulatory outputs. The histidine kinases are frequently membrane receptor proteins that respond to environmental signals and phosphorylate response regulators that control transcription. Four specific regulatory systems are discussed in detail: chemotaxis in response to attractant and repellent stimuli (Che), regulation of gene expression in response to nitrogen deprivation (Ntr), control of the expression of enzymes and transport systems that assimilate phosphorus (Pho), and regulation of outer membrane porin expression in response to osmolarity and other culture conditions (Omp). Several additional systems are also examined, including systems that control complex developmental processes such as sporulation and fruiting-body formation, systems required for virulent infections of plant or animal host tissues, and systems that regulate transport and metabolism. Finally, an attempt is made to understand how cross-talk between parallel phosphotransfer pathways can provide a global regulatory curcuitry.

Molecular organization of genes constituting the virulence determinant on the Salmonella typhimurium 96 kilobase pair plasmid

The ability of intracellular growth is plasmid-dependent in Salmonella typhimurium. Only a small portion of this 96 kilobase pair plasmid appears essential for intracellular growth. The genetic organization of this region (the essential virulence determinant) was resolved. Fragments of the virulence determinant were cloned from the 96-kb plasmid pEX102 and transformed into minicell-producing E. coli. Plasmid-directed protein synthesis was investigated in metabolically labeled minicells. This analysis indicated the presence of at least four genes, mkaA, mkaB, mkaC and mkaD, within the virulence determinant encoding proteins of 70, 31, 30 and 29 kDa, respectively. The genes were positioned on the restriction map of the 96-kb virulence plasmid and the map locations confirmed by nucleotide sequence analysis of two new virulence genes (mkaB and mkaC).

A Salmonella typhimurium virulence gene linked to flg

Isogenic pairs of strains of Salmonella typhimurium which differed only in whether or not they were flagellate were found to be equally virulent in C57BL/6J mice infected orally, intravenously, or intraperitoneally. Therefore, we investigated the genetic basis for our previous observation that in this mouse model, nonflagellate delta flagABCDE25 strains were reduced in virulence compared with isogenic wild-type flagellate strains. The recombinant plasmid pMH6, which contains several flg+ genes and a segment of the S. typhimurium chromosome adjacent to the flg genes, was introduced into a delta flgABCDE25 mutant. This restored virulence in mice challenged intraperitoneally, which suggested that a virulence gene occurs adjacent to the flg genes. When plasmid pMH64, which lacks the chromosomal segment adjacent to the flg genes, was introduced into the same delta flgABCDE25 mutant, virulence was not restored. In contrast, the introduction of pMH71, a plasmid which retains the chromosomal segment adjacent to the flg genes, restored virulence. We concluded that a hitherto unknown virulence gene, which we have named mviS, occurs adjacent to the flg genes and that its absence in delta flgABCDE25 mutants, rather than the nonflagellate phenotype of the delta flgABCDE25 mutants, caused the previously reported attenuation of such mutants.

Families of bacterial signal-transducing proteins

Bacteria can respond to a variety of environmental stimuli by means of systems generally composed of two proteins. The first protein (sensor or transmitter) is usually a transmembrane protein with cytoplasmic and extracytoplasmic domains. The extracytoplasmic domain (sensor) senses the environment and transfers the signal through the transmembrane domain to the cytoplasmic domain (transmitter), which has kinase activity. The second protein is located in the cytoplasm and contains an amino-terminal domain (receiver), which can be phosphorylated by the transmitter, and a carboxy-terminal region (regulator), which regulates gene expression by binding to DNA. The transmitter and receiver modules (the kinase and its target) are conserved in all signal-transducing systems and are the 'core structure' of this two-component system. The sensors and the regulators vary according to the stimuli they respond to and the DNA structure they interact with. On the basis of their sequence homology, the proteins belonging to such two-component systems can be classified into different families, which are summarized in this review.

Functional homology of virulence plasmids in Salmonella gallinarum, S. pullorum, and S. typhimurium

The virulence-associated plasmids of strains of Salmonella gallinarum and S. pullorum were transferred separately by mobilization with the F plasmid into virulence plasmid-cured derivatives of S. gallinarum, S. pullorum, and S. typhimurium and into a prototrophic Escherichia coli K-12 strain. The transconjugants were tested for virulence in chickens of different ages and in mice. The S. gallinarum and S. pullorum plasmids were able to restore full virulence in the three Salmonella strains, thus demonstrating functional homology in virulence plasmids from these Salmonella serotypes and biotypes. The virulence phenotypes of the transconjugants remained the same as that of the parent strain of the recipient. This, together with the fact that E. coli K-12 containing either of the virulence plasmids was avirulent for chickens, suggested that in addition to plasmid genes, chromosomal genes are important in determining virulence, particularly in determining the ability to survive and multiply in the cells of the reticuloendothelial system. The virulence plasmids were not self-transmissible and could not be transduced by temperate bacteriophages lysogenizing field strains of S. gallinarum. They were not in the same incompatibility group as F but were fi+.

Salmonella typhimurium phoP virulence gene is a transcriptional regulator

Salmonella typhimurium is a facultative intracellular pathogen capable of surviving within host phagocytic cells. Salmonella strains carrying phoP mutations are avirulent, unable to survive in macrophages, and extremely sensitive to peptides having antimicrobial activity such as the host-derived defensins. We present here the DNA sequence of the phoP gene and show that the deduced amino acid sequence of phoP has extensive homology with the Escherichia coli transcriptional regulators PhoB and OmpR, which control the expression of loci in response to different environmental stimuli. The psiD locus, which is regulated by phosphate availability, was found to be under the control of the phoP gene product. Sequences homologous to phoP were found in several Gram-negative species and in the yeast Saccharomyces cerevisiae.

A two-component regulatory system (phoP phoQ) controls Salmonella typhimurium virulence

We have determined that Salmonella typhimurium strains with mutations in the positive regulatory locus phoP are markedly attenuated in virulence for BALB/c mice. The DNA sequence for the phoP locus indicates that it is composed of two genes present in an operon, termed phoP and phoQ. The deduced amino acid sequence of the phoP and phoQ gene products are highly similar to other members of bacterial two-component transcriptional regulators that respond to environmental stimuli. S. typhimurium strains with transposon insertions that create transcriptional and translational gene fusions that require phoP and phoQ for expression have been isolated and have different chromosomal locations, indicating that this system is a regulon. One of these fusion strains, containing a mutation in a gene termed pagC, has a virulence defect. Other strains, including those containing mutations in the phoN gene, encoding an acid phosphatase, have wild-type virulence. Strains with pagC, phoP, or phoQ mutations have decreased survival in cultured mouse macrophages. When used as live vaccines in mice, strains with phoP or phoQ mutations afford partial protection to subsequent challenge by wild-type S. typhimurium.

Virulence and vaccine potential of phoP mutants of Salmonella typhimurium

We have constructed Salmonella typhimurium phoP mutants and found them to be avirulent and able to induce a protective immune response. BALB/c mice survived challenge with phoP derivatives of the highly virulent S. typhimurium strains SR-11 and SL1344 when inoculated intraperitoneally and per oral with doses equivalent to 10(4) 50% lethal doses (LD50) of the parent virulent strains. The avirulent mutants were able to establish an infection of the Peyer's patches of orally infected animals for up to 10 days after inoculation but were very inefficient at reaching the spleens. Despite the low level of infectivity of these mutants, immunized animals developed a delayed-type hypersensitivity (DTH) response to Salmonella antigens and resisted challenge with up to 10(4) LD50 of the virulent parent strain 30 days after immunization.

Common themes in microbial pathogenicity

A bacterial pathogen is a highly adapted microorganism which has the capacity to cause disease. The mechanisms used by pathogenic bacteria to cause infection and disease usually include an interactive group of virulence determinants, sometimes coregulated, which are suited for the interaction of a particular microorganism with a specific host. Because pathogens must overcome similar host barriers, common themes in microbial pathogenesis have evolved. However, these mechanisms are diverse between species and not necessarily conserved; instead, convergent evolution has developed several different mechanisms to overcome host barriers. The success of a bacterial pathogen can be measured by the degree with which it replicates after entering the host and reaching its specific niche. Successful microbial infection reflects persistence within a host and avoidance or neutralization of the specific and nonspecific defense mechanisms of the host. The degree of success of a pathogen is dependent upon the status of the host. As pathogens pass through a host, they are exposed to new environments. Highly adapted pathogenic organisms have developed biochemical sensors exquisitely designed to measure and respond to such environmental stimuli and accordingly to regulate a cascade of virulence determinants essential for life within the host. The pathogenic state is the product of dynamic selective pressures on microbial populations.

High activity of acid phosphatase of Pseudomonas pseudomallei as a possible attribute relating to its pathogenicity

Phosphatase activities were compared quantitatively among selected species of pseudomonads. P. pseudomallei showed the highest activity of a bell-shaped pH pattern with a peak at around pH 5.0. P. cepacia had a similar pattern of milder intensity. In contrast, P. aeruginosa revealed an alkaline phosphatase activity with a pH optimum higher than 8.0, but the level of activity was much lower than those of the above two species. The enzymatic reactions of other species were slight or negligible at their optimum pH in the same test system. These data were discussed in reference to their growth behavior in different pH environments and also in connection with such recent information that the high activity of microbial acid phosphatase may be a favorable attribute to their intracellular parasitism.

Coordinate regulation and sensory transduction in the control of bacterial virulence

Genes and operons that encode bacterial virulence factors are often subject to coordinate regulation. These regulatory systems are capable of responding to various environmental signals that may be encountered during the infectious cycle. For some pathogens, proteins that mediate sensory transduction and virulence control are similar to components of other bacterial information processing systems. Understanding the molecular mechanisms governing global regulation of pathogenicity is essential for understanding bacterial infectious diseases.