Rational manipulation of mRNA folding free energy allows rheostat control of pneumolysin production by Streptococcus pneumoniae

Estimates of differential toxin expression governing heterogeneous intracellular lifespans of Streptococcus pneumoniae

Following invasion of the host cell, pore-forming toxins secreted by pathogens compromise vacuole integrity and expose the microbe to diverse intracellular defence mechanisms. However, the quantitative correlation between toxin expression levels and consequent pore dynamics, fostering the intracellular life of pathogens, remains largely unexplored. In this study, using Streptococcus pneumoniae and its secreted pore-forming toxin pneumolysin (Ply) as a model system, we explored various facets of host-pathogen interactions in the host cytosol. Using time-lapse fluorescence imaging, we monitored pore formation dynamics and lifespans of different pneumococcal subpopulations inside host cells. Based on experimental histograms of various event timescales such as pore formation time, vacuolar death or cytosolic escape time and total degradation time, we developed a mathematical model based on first-passage processes that could correlate the event timescales to intravacuolar toxin accumulation. This allowed us to estimate Ply production rate, burst size and threshold Ply quantities that trigger these outcomes. Collectively, we present a general method that illustrates a correlation between toxin expression levels and pore dynamics, dictating intracellular lifespans of pathogens.

Modeling the ribosomal small subunit dynamic in Saccharomyces cerevisiae based on TCP-seq data

Translation Complex Profile Sequencing (TCP-seq), a protocol that was developed and implemented on Saccharomyces cerevisiae, provides the footprints of the small subunit (SSU) of the ribosome (with additional factors) across the entire transcriptome of the analyzed organism. In this study, based on the TCP-seq data, we developed for the first-time a predictive model of the SSU density and analyzed the effect of transcript features on the dynamics of the SSU scan in the 5′UTR. Among others, our model is based on novel tools for detecting complex statistical relations tailored to TCP-seq. We quantitatively estimated the effect of several important features, including the context of the upstream AUG, the upstream ORF length and the mRNA folding strength. Specifically, we suggest that around 50% of the variance related to the read counts (RC) distribution near a start codon can be attributed to the AUG context score. We provide the first large scale direct quantitative evidence that shows that indeed AUG context affects the small sub-unit movement. In addition, we suggest that strong folding may cause the detachment of the SSU from the mRNA. We also identified a number of novel sequence motifs that can affect the SSU scan; some of these motifs affect transcription factors and RNA binding proteins. The results presented in this study provide a better understanding of the biophysical aspects related to the SSU scan along the 5′UTR and of translation initiation in S. cerevisiae, a fundamental step toward a comprehensive modeling of initiation.

Lower Density and Shorter Duration of Nasopharyngeal Carriage by Pneumococcal Serotype 1 (ST217) May Explain Its Increased Invasiveness over Other Serotypes

Streptococcus pneumoniae is a frequent colonizer of the human nasopharynx and a major cause of life-threating invasive infections such as pneumonia, meningitis and sepsis. Over 1 million people die every year due to invasive pneumococcal disease (IPD), mainly in developing countries. Serotype 1 is a common cause of IPD; however, unlike other serotypes, it is rarely found in the carrier state in the nasopharynx, which is often considered a prerequisite for disease. The aim of this study was to understand this dichotomy. We used murine models of carriage and IPD to characterize the pathogenesis of African serotype 1 (sequence type 217) pneumococcal strains obtained from the Queen Elizabeth Central Hospital in Blantyre, Malawi. We found that ST217 pneumococcal strains were highly virulent in a mouse model of invasive pneumonia, but in contrast to the generally accepted assumption, can also successfully establish nasopharyngeal carriage. Interestingly, we found that cocolonizing serotypes may proliferate in the presence of serotype 1, suggesting that acquisition of serotype 1 carriage could increase the risk of developing IPD by other serotypes. RNA sequencing analysis confirmed that key virulence genes associated with inflammation and tissue invasiveness were upregulated in serotype 1. These data reveal important new insights into serotype 1 pathogenesis, with implications for carriage potential and risk of invasive disease through interactions with other cocolonizing serotypes, an often-overlooked factor in transmission and disease progression.IMPORTANCE The pneumococcus causes serious diseases such as pneumonia, sepsis, and meningitis and is a major cause of morbidity and mortality worldwide. Serotype 1 accounts for the majority of invasive pneumococcal disease cases in sub-Saharan Africa but is rarely found during nasopharyngeal carriage. Understanding the mechanisms leading to nasopharyngeal carriage and invasive disease by this serotype can help reduce its burden on health care systems worldwide. In this study, we also uncovered the potential impact of serotype 1 on disease progression of other coinfecting serotypes, which can have important implications for vaccine efficacy. Understanding the interactions between different serotypes during nasopharyngeal carriage may lead to improved intervention methods and therapies to reduce pneumococcal invasive disease levels.

Structural insights into loss of function of a pore forming toxin and its role in pneumococcal adaptation to an intracellular lifestyle

The opportunistic pathogen Streptococcus pneumoniae has dual lifestyles: one of an asymptomatic colonizer in the human nasopharynx and the other of a deadly pathogen invading sterile host compartments. The latter triggers an overwhelming inflammatory response, partly driven via pore forming activity of the cholesterol dependent cytolysin (CDC), pneumolysin. Although pneumolysin-induced inflammation drives person-to-person transmission from nasopharynx, the primary reservoir for pneumococcus, it also contributes to high mortality rates, creating a bottleneck that hampers widespread bacterial dissemination, thus acting as a double-edged sword. Serotype 1 ST306, a widespread pneumococcal clone, harbours a non-hemolytic variant of pneumolysin (Ply-NH). Performing crystal structure analysis of Ply-NH, we identified Y150H and T172I as key substitutions responsible for loss of its pore forming activity. We uncovered a novel inter-molecular cation-π interaction, governing formation of the transmembrane β-hairpins (TMH) in the pore state of Ply, which can be extended to other CDCs. H150 in Ply-NH disrupts this interaction, while I172 provides structural rigidity to domain-3, through hydrophobic interactions, inhibiting TMH formation. Loss of pore forming activity enabled improved cellular invasion and autophagy evasion, promoting an atypical intracellular lifestyle for pneumococcus, a finding that was corroborated in in vivo infection models. Attenuation of inflammatory responses and tissue damage promoted tolerance of Ply-NH-expressing pneumococcus in the lower respiratory tract. Adoption of this altered lifestyle may be necessary for ST306 due to its limited nasopharyngeal carriage, with Ply-NH, aided partly by loss of its pore forming ability, facilitating a benign association of SPN in an alternative, intracellular host niche.

Heterologous expression of cyclodextrin glycosyltransferase from Paenibacillus macerans in Escherichia coli and its application in 2-O-α-D-glucopyranosyl-L-ascorbic acid production

BACKGROUND

Cyclodextrin glucanotransferase (CGTase) can transform L-ascorbic acid (L-AA, vitamin C) to 2-O-α-D-glucopyranosyl-L-ascorbic acid (AA-2G), which shows diverse applications in food, cosmetic and pharmaceutical industries.

RESULTS

In this study, the cgt gene encoding α-CGTase from Paenibacillus macerans was codon-optimized (opt-cgt) and cloned into pET-28a (+) for intracellular expression in E. coli BL21 (DE3). The Opt-CGT was purified by Ni2+-NTA resin with a 55% recovery, and specific activity was increased significantly from 1.17 to 190.75 U·mg- 1. In addition, the enzyme was adopted to transform L-AA into 9.1 g/L of AA-2G. Finally, more economic substrates, including β-cyclodextrin, soluble starch, corn starch and cassava starch could also be used as glycosyl donors, and 4.9, 3.5, 1.3 and 1.5 g/L of AA-2G were obtained, respectively.

CONCLUSIONS

N-terminal amino acid is critical to the activity of CGTase suggested by its truncation study. Furthermore, when the Opt-CGT was flanked by His6-tags on the C- and N-terminal, the recovery of purification by Ni2+-NTA resin is appreciably enhanced. α-cyclodextrin was the ideal glycosyl donor for AA-2G production. In addition, the selection of low cost glycosyl donors would make the process of AA-2G production more economically competitive.

Heterogeneity in pneumolysin expression governs the fate of Streptococcus pneumoniae during blood-brain barrier trafficking

Outcome of host-pathogen encounter is determined by the complex interplay between protective bacterial and host defense strategies. This complexity further amplifies with the existence of cell-to-cell phenotypic heterogeneity in pathogens which remains largely unexplored. In this study, we illustrated that heterogeneous expression of pneumolysin (Ply), a pore-forming toxin of the meningeal pathogen, S. pneumoniae (SPN) gives rise to stochastically different bacterial subpopulations with variable fate during passage across blood-brain barrier (BBB). We demonstrate that Ply mediated damage to pneumococcus containing vacuolar (PCV) membrane leads to recruitment of cytosolic "eat-me" signals, galectin-8 and ubiquitin, targeting SPN for autophagic clearance. However, a majority of high Ply producing subset extensively damages autophagosomes leading to pneumococcal escape into cytosol and efficient clearance by host ubiquitination machinery. Interestingly, a low Ply producing subset halts autophagosomal maturation and evades all intracellular defense mechanisms, promoting its prolonged survival and successful transcytosis across BBB, both in vitro and in vivo. Ply therefore acts as both, sword and shield implying that its smart regulation ensures optimal disease manifestation. Our elucidation of heterogeneity in Ply expression leading to disparate infection outcomes attempts to resolve the dubious role of Ply in pneumococcal pathogenesis.

The development of fluorescent protein tracing vectors for multicolor imaging of clinically isolated Staphylococcus aureus

Recent advances in fluorescent protein technology provide a wide variety of biological imaging applications; however current tools for bio-imaging in the Gram-positive bacterium Staphylococcus aureus has necessitated further developments for fluorescence intensity and for a multicolor palette of fluorescent proteins. To enhance the expression of multicolor fluorescent proteins in clinical S. aureus strains, we developed new fluorescent protein expression vectors, containing the blaZ/sodp promoter consisting of the β-lactamase gene (blaZ) promoter and the ribosome binding site (RBS) of superoxide dismutase gene (sod). We found S. aureus-adapted GFP (GFP_sa) driven by the blaZ/sodp promoter was highly expressed in the S. aureus laboratory strain RN4220, but not in the clinical strains, MW2 and N315, harboring the endogenous blaI gene, a repressor of the blaZ gene promoter. We therefore constructed a constitutively induced blaZ/sodp promoter (blaZ/sodp(Con)) by introducing substitution mutations into the BlaI binding motif, and this modification allowed enhanced expression of the multicolor GFP variants (GFP_sa, EGFP, mEmerald, Citrine, Cerulean, and BFP) as well as codon-optimized reef coral fluorescent proteins (mCherry and AmCyan) in the S. aureus clinical strains. These new fluorescent probes provide new tools to enhance expression of multicolor fluorescent proteins and facilitate clear visualization of clinical S. aureus strains.