链球菌突破血脑屏障的作用机制研究进展

血脑屏障(blood-brain barrier,BBB)是中枢神经系统(central nervous system,CNS)的天然结构和功能屏障之一,可有效阻止病原菌的入侵。然而病原菌能通过其自身毒力因子与脑内皮细胞相互作用,诱导宿主免疫应答反应,分泌大量细胞因子、趋化因子等,破坏紧密连接蛋白,最终突破血脑屏障,引起细菌性脑膜炎,产生不可逆的神经系统损伤。链球菌(Streptococcus)作为引起细菌性脑膜炎的重要病原菌,关于其突破血脑屏障分子机制研究已有显著进展。本文针对主要的链球菌,包括肺炎链球菌(Streptococcus pneumoniae)、猪链球菌(Streptococcus suis)、B型链球菌(group B Streptococcus,GBS)、马链球菌等突破血脑屏障的作用机制研究进展进行综述。     

A current review of pathogenicity determinants of Streptococcus sp.

The genus Streptococcus comprises important pathogens, many of them are part of the human or animal microbiota. Advances in molecular genetics, taxonomic approaches and phylogenomic studies have led to the establishment of at least 100 species that have a severe impact on human health and are responsible for substantial economic losses to agriculture. The infectivity of the pathogens is linked to cell-surface components and/or secreted virulence factors. Bacteria have evolved sophisticated and multifaceted adaptation strategies to the host environment, including biofilm formation, survival within professional phagocytes, escape the host immune response, amongst others. This review focuses on virulence mechanism and zoonotic potential of Streptococcus species from pyogenic (S. agalactiae, S. pyogenes) and mitis groups (S. pneumoniae).     

Streptococcal toxins: role in pathogenesis and disease

Group A Streptococcus (Streptococcus pyogenes), group B Streptococcus (Streptococcus agalactiae) and Streptococcus pneumoniae (pneumococcus) are host‐adapted bacterial pathogens among the leading infectious causes of human morbidity and mortality. These microbes and related members of the genus Streptococcus produce an array of toxins that act against human cells or tissues, resulting in impaired immune responses and subversion of host physiological processes to benefit the invading microorganism. This toxin repertoire includes haemolysins, proteases, superantigens and other agents that ultimately enhance colonization and survival within the host and promote dissemination of the pathogen.     

The ins and outs of bacterial iron metabolism

Iron is a critical nutrient for the growth and survival of most bacterial species. Accordingly, much attention has been paid to the mechanisms by which host organisms sequester iron from invading bacteria and how bacteria acquire iron from their environment. However, under oxidative stress conditions such as those encountered within phagocytic cells during the host immune response, iron is released from proteins and can act as a catalyst for Fenton chemistry to produce cytotoxic reactive oxygen species. The transitory efflux of free intracellular iron may be beneficial to bacteria under such conditions. The recent discovery of putative iron efflux transporters in Salmonella enterica serovar Typhimurium is discussed in the context of cellular iron homeostasis.     

Fluoride‐sensitivity of growth and acid production of oral Actinomyces: comparison with oral Streptococcus

Actinomyces are predominant oral bacteria; however, their cariogenic potential in terms of acid production and fluoride sensitivity has not been elucidated in detail and compared with that of other caries‐associated oral bacteria, such as Streptococcus. Therefore, this study aimed to elucidate and compare the acid production and growth of Actinomyces and Streptococcus in the presence of bicarbonate and fluoride to mimic conditions in the oral cavity. Acid production from glucose was measured by pH‐stat at pH 5.5 and 7.0 under anaerobic conditions. Growth rate was assessed by optical density in anaerobic culture. Although Actinomyces produced acid at a lower rate than did Streptococcus, their acid production was more tolerant of fluoride (ID_{acid production} 50 = 110–170 ppm at pH 7.0 and 10–13 ppm at pH 5.5) than that of Streptococcus (ID_{acid production} 50 = 36–53 ppm at pH 7.0 and 6.3–6.5 ppm at pH 5.5). Bicarbonate increased acid production by Actinomyces with prominent succinate production and enhanced their fluoride tolerance (ID_{acid production} 50 = 220–320 ppm at pH 7.0 and 33–52 ppm at pH 5.5). Bicarbonate had no effect on these variables in Streptococcus. In addition, although the growth rate of Actinomyces was lower than that of Streptococcus, Actinomyces growth was more tolerant of fluoride (ID_growth 50 = 130–160 ppm) than was that of Streptococcus (ID_growth 50 = 27–36 ppm). These results indicate that oral Actinomyces are more tolerant of fluoride than oral Streptococcus, and bicarbonate enhances the fluoride tolerance of oral Actinomyces. Because of the limited number of species tested here, further study is needed to generalize these findings to the genus level.     

Phylogenomic analysis of natural selection pressure in <Emphasis Type="Italic">Streptococcus</Emphasis> genomes

Background  

In comparative analyses of bacterial pathogens, it has been common practice to discriminate between two types of genes: (i) those shared by pathogens and their non-pathogenic relatives (core genes), and (ii) those found exclusively in pathogens (pathogen-specific accessory genes). Rather than attempting to a priori delineate genes into sets more or less relevant to pathogenicity, we took a broad approach to the analysis of Streptococcus species by investigating the strength of natural selection in all clusters of homologous genes. The genus Streptococcus is comprised of a wide variety of both pathogenic and commensal lineages, and we relate our findings to the pre-existing knowledge of Streptococcus virulence factors.     

Nitrogenous compounds stimulate glucose‐derived acid production by oral Streptococcus and Actinomyces

Both Streptococcus and Actinomyces can produce acids from dietary sugars and are frequently found in caries lesions. In the oral cavity, nitrogenous compounds, such as peptides and amino acids, are provided continuously by saliva and crevicular gingival fluid. Given that these bacteria can also utilize nitrogen compounds for their growth, it was hypothesized that nitrogenous compounds may influence their acid production; however, no previous studies have examined this topic. Therefore, the present study aimed to assess the effects of nitrogenous compounds (tryptone and glutamate) on glucose‐derived acid production by Streptococcus and Actinomyces. Acid production was evaluated using a pH‐stat method under anaerobic conditions, whereas the amounts of metabolic end‐products were quantified using high performance liquid chromatography. Tryptone enhanced glucose‐derived acid production by up to 2.68‐fold, whereas glutamate enhanced Streptococcus species only. However, neither tryptone nor glutamate altered the end‐product profiles, indicating that the nitrogenous compounds stimulate the whole metabolic pathways involving in acid production from glucose, but are not actively metabolized, nor do they alter metabolic pathways. These results suggest that nitrogenous compounds in the oral cavity promote acid production by Streptococcus and Actinomyces in vivo.     

Iron and Pathogenesis of Shigella: Iron Acquisition in the Intracellular Environment

Shigella species are able to grow in a variety of environments, including intracellularly in host epithelial cells. Shigella have a number of different iron transport systems that contribute to their ability to grow in these diverse environments. Siderophore iron uptake systems, heme transporters, and ferric and ferrous iron transport systems are present in these bacteria, and the genes encoding some of these systems appear to have spread among the Shigella species by horizontal transmission. Iron is not only essential for growth of Shigella but also plays an important role in regulation of metabolic processes and virulence determinants in Shigella. This regulation is mediated by the repressor protein Fur and the small RNA RyhB.     

ABC transporter FtsABCD of <Emphasis Type="Italic">Streptococcus pyogenes</Emphasis> mediates uptake of ferric ferrichrome

Background  

The Streptococcus pyogenes or Group A Streptococcus (GAS) genome encodes three ABC transporters, namely, FtsABCD, MtsABC, and HtsABC, which share homology with iron transporters. MtsABC and HtsABC are believed to take up ferric (Fe³⁺) and manganese ions and heme, respectively, while the specificity of FtsABCD is unknown.     

Iron Acquisition and Transport in Staphylococcus aureus

Pathogenic Gram-positive bacteria encounter many obstacles in route to successful invasion and subversion of a mammalian host. As such, bacterial species have evolved clever ways to prevent the host from clearing an infection, including the production of specialized virulence systems aimed at counteracting host defenses or providing protection from host immune mechanisms. Positioned at the interface of bacteria/host interactions is the bacterial cell wall, a dynamic surface organelle that serves a multitude of functions, ranging from physiologic processes such as structural scaffold and barrier to osmotic lysis to pathogenic properties, for example the deposition of surface molecules and the secretion of cytotoxins. In order to succeed in a battle with host defenses, invading bacteria need to acquire the nutrient iron, which is sequestered within host tissues. A cell-wall based iron acquisition and import pathway was uncovered in Staphylococcus aureus. This pathway, termed the isd or iron-responsive surface determinant locus, consists of a membrane transporter, cell wall anchored heme-binding proteins, heme/haptoglobin receptors, two heme oxygenases, and sortase B, a transpeptidase that anchors substrate proteins to the cell wall. Identification of the isd pathway provides an additional function to the already bountiful roles the cell wall plays in bacterial pathogenesis and provides new avenues for therapeutics to combat the rise of antimicrobial resistance in S. aureus. This review focuses on the molecular attributes of this locus, with emphasis placed on the mechanism of iron transport and the role of such a system during infection.     

Identification of a siderophore utilization locus in nontypeable <Emphasis Type="Italic">Haemophilus influenzae</Emphasis>

Background  

Haemophilus influenzae has an absolute aerobic growth requirement for either heme, or iron in the presence of protoporphyrin IX. Both iron and heme in the mammalian host are strictly limited in their availability to invading microorganisms. Many bacterial species overcome iron limitation in their environment by the synthesis and secretion of small iron binding molecules termed siderophores, which bind iron and deliver it into the bacterial cell via specific siderophore receptor proteins on the bacterial cell surface. There are currently no reports of siderophore production or utilization by H. influenzae.     

链球菌-韦荣菌共生对系统性疾病的诊断价值研究

口腔是人体生理功能的窗口,也是种类和数量繁多的微生物库.口腔微生态变化能够反映宿主与环境因素的相互作用,进而影响机体健康和疾病的进展.其中,链球菌属(Streptococcus)和韦荣菌属(Veillonella)是口腔最早的定殖菌和典型共生菌,共同参与口腔早期生物膜形成.大量研究显示,链球菌和韦荣菌共生失调不仅与龋病...     

Differentially regulated high‐affinity iron assimilation systems support growth of the various cell types in the dimorphic pathogen Talaromyces marneffei

Iron is a key trace element important for many biochemical processes and its availability varies with the environment. For human pathogenic fungi iron acquisition can be particularly problematical because host cells sequester free iron as part of the acute‐phase response to infection. Fungi rely on high‐affinity iron uptake systems, such as reductive iron assimilation (RIA) and siderophore‐mediated iron uptake (non‐RIA). These have been extensively studied in pathogenic fungi that exist outside of host cells, but much less is known for intracellular fungal pathogens. Talaromyces marneffei is a dimorphic fungal pathogen endemic to Southeast Asia. In the host T. marneffei resides within macrophages where it grows as a fission yeast. T. marneffei has genes of both iron assimilation systems as well as a paralogue of the siderophore biosynthetic gene sidA, designated sidX. Unlike other fungi, deletion of sidA or sidX resulted in cell type‐specific effects. Mutant analysis showed that T. marneffei yeast cells also employ RIA for iron acquisition, providing an additional system in this cell type that differs substantially from hyphal cells. These data illustrate the specialized iron acquisition systems used by the different cell types of a dimorphic fungal pathogen and highlight the complexity in siderophore‐biosynthetic pathways amongst fungi.     

Determination of the mimic epitope of the M-like protein adhesin in swine <Emphasis Type="Italic">Streptococcus equi</Emphasis> subsp. <Emphasis Type="Italic">zooepidemicus</Emphasis>

Background  

The M-like protein, also known as SzP, is expressed on the surface of Streptococcus equi subsp. zooepidemicus (S. zooepidemicus). Previous studies demonstrated that SzP is similar to M protein of group A Streptococcus in the structure and characteristics of antiphagocytosis. The M protein is an adhesin that can bind to the host cells, however it is not known whether the SzP of S. zooepidemicus also functions as an adhesin. We conducted an investigation to determine SzP as an adhesin, and one SzP epitope was identified to be responsible for mediating binding to HEp-2 cells.     

A novel iron‐ and copper‐binding protein in the Lyme disease spirochaete

Iron and copper are transition metals that can be toxic to cells due to their abilities to react with peroxide to generate hydroxyl radical. Ferritins and metallothioneins are known to sequester intracellular iron and copper respectively. The Lyme disease pathogen Borrelia burgdorferi does not require iron, but its genome encodes a ferritin‐like Dps (D NA‐binding p rotein from s tarved bacteria) molecule, which has been shown to be important for the spirochaete's persistence in the tick and subsequent transmission to a new host. Here, we show that the c arboxyl‐terminal c ysteine‐r ich (CCR) domain of this protein functions as a copper‐binding metallothionein. This novel fusion between Dps and metallothionein is unique to and conserved in all Borrelia species. We term this molecule BicA for B orrelia i ron‐ and c opper‐binding protein A . An isogenic mutant lacking BicA had significantly reduced levels of iron and copper and was more sensitive to iron and copper toxicity than its parental strain. Supplementation of the medium with iron or copper rendered the spirochaete more susceptible to peroxide killing. These data suggest that an important function of BicA is to detoxify excess iron and copper the spirochaete may encounter during its natural life cycle through a tick vector and a vertebrate host.     

Pathogenic Streptococcus strains employ novel escape strategy to inhibit bacteriostatic effect mediated by mammalian peptidoglycan recognition protein

Pathogenic streptococcal species are responsible for some of the most lethal and prevalent animal and human infections. Previous reports have identified a candidate pathogenicity island (PAI) in two highly virulent clinical isolates of Streptococcus suis type 2, a causative agent of high‐mortality streptococcal toxic shock syndrome. This PAI contains a type‐IVC secretion system C subgroup (type‐IVC secretion system) that is involved in the secretion of unknown pathogenic effectors that are responsible for streptococcal toxic shock syndrome caused by highly virulent strains of S. suis. Both virulence protein B4 and virulence protein D4 were demonstrated to be key components of this type‐IVC secretion system. In this study, we identify a new PAI family across 3 streptococcal species; Streptococcus genomic island contains type‐IV secretion system, which contains a genomic island type‐IVC secretion system and a novel PPIase molecule, SP1. SP1 is shown to interact with a component of innate immunity, peptidoglycan recognition protein (PGLYRP‐1) and to perturb the PGLYRP‐1‐mediated bacteriostatic effect by interacting with protein PGLYRP‐1. Our study elucidates a novel mechanism by which bacteria escape by components of the innate immune system by secretion of the SP1 protein in pathogenic Streptococci, which then interacts with PGLYRP‐1 from the host. Our results provide potential targets for the development of new antimicrobial drugs against bacteria with resistance to innate host immunity.     

Apoptotic cell death of macrophages by iron-stressed <Emphasis Type="Italic">Salmonella enterica </Emphasis>serovar Typhimurium

Salmonella spp. have been shown to cause apoptosis of various host cell types as a part of their infection process. However, the induction of apoptosis remains to be looked into under the different host environments experienced by the pathogens. One of these is iron limitation, due to binding of iron in the host with proteins like lactoferrin, transferrin, haptoglobulin and hemoglobin etc. making non-availability of free iron to the pathogen for its growth and metabolism. In order to simulate the iron-limited in vivo situation, we studied the potential of Salmonella enterica serovar Typhimurium and its proteins under in vitro-created iron-stressed conditions, to cause apoptosis of macrophages (the first line of defence system). The apoptotic potential was evaluated qualitatively and quantitatively by various methods like assessment of nucleosomal DNA ladder (hallmark of apoptosis) and morphological evaluation by DNA intercalating dyes like acridine orange staining and Hoechst 33342-propidium iodide co-staining. It was observed that iron limitation could cause apoptotic cell death in a higher number of cells with the overexpression of proteins with subunit molecular weights of approximately 89, 54, 32 and 20 kDa. Salmonella may initiate apoptosis as a virulence strategy, but the death of host cells by the process of apoptosis rather than necrosis after getting a suicidal signal might be helpful for the host in order to save the surrounding cells, as well as to the parasite to enable it to spread systemically without inducing an inflammatory response.     

The role of ecological availability and host plant characteristics in determining host use by the bogus yucca moth Prodoxus decipiens

1. Specialisation in host plant use is strongly correlated with speciation in many plant‐feeding insects. Specialised taxa, however, could be restricted in host range due to limits in ecological availability of host plant species rather than trade‐offs in using alternate host species. 2. Moths in the genus Prodoxus are extreme specialists on Yucca and speciation is closely tied to host plant shifts. However, many Yucca ranges are allopatric. This study examined whether the bogus yucca moth Prodoxus decipiens is limited in host range because of biogeographic factors or due to differences in the characteristics of host plant species. 3. In a common garden, local P. decipiens moths that use Yucca filamentosa were exposed to individuals of five Yucca species, two that are known hosts of P. decipiens in other parts of its range and three that are used by its sister species, Prodoxus quinquepunctellus. 4. Local moths were attracted to flowers of all Yucca species and females attempted oviposition in the flowering stalks of all species. However, larvae successfully completed development to diapause in only one of the five host plant species. Larval development on non‐natal Yucca species was significantly reduced compared with the local host. 5. The results suggest that differences in host plant characteristics among Yucca species would result in strong natural selection during a host shift. Thus, specialisation in host plant use is probably due to trade‐offs involved with using novel host plant species as well as ecological availability.