Academic literature on the topic 'Opacity phase variation'

ABSTRACTAcinetobacter baumanniistrain AB5075 produces colonies with two opacity phenotypes, designated opaque and translucent. These phenotypes were unstable and opaque and translucent colony variants were observed to interconvert at high frequency, suggesting that a phase-variable mechanism was responsible. The frequency of phase variation both within colonies and in broth cultures increased in a cell density-dependent manner and was mediated by the accumulation of an extracellular factor. This factor was distinct from the knownA. baumanniisignaling molecule 3-OH C12-homoserine lactone. Opaque and translucent colony variants exhibited a number of phenotypic differences, including cell morphology, surface motility, biofilm formation, antibiotic resistance, and virulence in aGalleria mellonellamodel. Additional clinical isolates exhibited a similar phase-variable control of colony opacity, suggesting that this may be a common feature ofA. baumannii.IMPORTANCEA novel phase-variable mechanism has been identified inAcinetobacter baumanniithat results in an interconversion between opaque and translucent colony phenotypes. This phase variation also coordinately regulates motility, cell shape, biofilm formation, antibiotic resistance, and virulence. The frequency of phase variation is increased at high cell density via a diffusible extracellular signal. To our knowledge, this report presents the first example of phase variation inA. baumanniiand also the first example of quorum sensing-mediated control of phase variation in a bacterium. The findings are important, as this phase-variable mechanism can be identified only via changes in colony opacity using oblique light; therefore, many researchers studyingA. baumanniimay unknowingly be working with different colony variants.

Simulation studies have proposed a significant thermal effect of water ice clouds on the Martian atmosphere and climate. However, previous studies focused more on seasonal variations but less on short-term changes. In this work, we used the MCS multi-local time data to investigate the water ice diurnal variations on Mars. We quantified its diurnal variations with amplitude and phase by applying the tidal fitting method to the water ice abundance. In addition, we found a close correlation (antiphase relation) between the thermal tide and water ice diurnal variations during the aphelion seasons that was not sensitive to both the background water ice and dust opacity but increased with the tidal amplitude. In the perihelion seasons, the antiphase relation was sensitive to the water ice and dust opacity, both affected by the dust storm activity. Finally, the statistic results suggested an unexpected low threshold of diurnal tide amplitude (2 to 3 K) for generating a relevant water ice diurnal variation, accounting for the ubiquitous water ice diurnal variations in the Martian atmosphere. These new observational results can help further understand the phase transition process between ice and vapor in the Martian atmosphere and better constrain the Martian global climate model in the future.

ABSTRACT The neisserial opacity (Opa) proteins are a family of antigenically distinct outer membrane proteins that undergo phase-variable expression. Opa+ variants of Neisseria gonorrhoeae strain FA1090 are selected in a cyclical pattern from the lower genital tract of estradiol-treated mice. Here we show that cyclical recovery of Opa+ gonococci does not occur in ovariectomized mice; therefore, the reproductive cycle plays a role in the selection kinetics in vivo. As predicted by the selection pattern shown by wild-type gonococci, we demonstrated that a constitutive Opa-expressing strain was more fit than an Opa-deficient mutant in the early and late phases of infection. We found no evidence that Opa-mediated colonization selects for Opa+ variants during murine infection based on adherence assays with cultured murine epithelial cells. We also tested the hypothesis that complement selects for Opa protein expression during infection. Although some Opa+ variants of a serum-sensitive derivative of strain FA1090 were more resistant to the bactericidal activity of normal human serum, selection for Opa expression was not abrogated in C3-depleted mice. Finally, as previously reported, Opa+ gonococci were more sensitive to serine proteases. Thus, proteases or protease inhibitors may contribute to the observed in vivo selection pattern. We concluded that Opa proteins promote persistence of N. gonorrhoeae in the female genital tract and that opa gene phase variation allows gonococci to evade or capitalize upon unidentified host factors of the mammalian reproductive cycle. This work revealed an intimate interaction between pathogen and host and provides evidence that hormonally related factors shape bacterial adaptation.

ABSTRACT Phase variation in the colonial opacity of Streptococcus pneumoniae has been implicated as a factor in bacterial adherence, colonization, and invasion in the pathogenesis of pneumococcal disease. Additionally, the synergistic effects of influenza A virus and S. pneumoniae in the development of otitis media (OM) have been reported. This study examined the ability of opaque or transparent S. pneumoniae from the same strain in combination with an antecedent influenza A virus infection to colonize the nasopharynx and invade the middle ear in the chinchilla model. Our data indicated that there was no significant difference in the level of nasopharyngeal colonization and induction of OM between the opaque and transparent variants unless there was a prior challenge with influenza A virus. Subsequent to influenza A virus infection, there was a significant difference between the variants in the ability to colonize and persist in the nasopharynx and middle ear. The concentrations of the opaque variant in nasopharyngeal-lavage samples and middle-ear fluid remained consistently higher than those of the transparent variant for 10 days postinoculation. Data from this study indicate that the effects of influenza A virus on the pathogenesis of experimental S. pneumoniae-induced OM differ depending on the opacity phenotype involved.

Streptococcus pneumoniae (pneumococcus) is a respiratory commensal pathogen that causes a range of infections, particularly in young children and the elderly. Pneumococci undergo spontaneous phase variation in colony opacity phenotype, in which DNA rearrangements within the Type I restriction-modification (R-M) system specificity gene hsdS can potentially generate up to six different hsdS alleles with differential DNA methylation activity, resulting in changes in gene expression. To gain a broader perspective of this system, we performed bioinformatic analyses of Type I R-M loci from 18 published pneumococcal genomes, and one R-M locus sequenced for this study, to compare genetic content, organization, and homology. All 19 loci encoded the genes hsdR, hsdM, hsdS, and at least one hsdS pseudogene, but differed in gene order, gene orientation, and hsdS target recognition domain (TRD) content. We determined the coding sequences of 87 hsdS TRDs and excluded seven from further analysis due to the presence of premature stop codons. Comparative analyses revealed that the TRD 1.1, 1.2, and 2.1 protein sequences had single amino acid substitutions, and TRD 2.2 and 2.3 each had seven differences. The results of this study indicate that variability exists among the gene content and arrangements within Type I R-M loci may provide an additional level of divergence between pneumococcal strains, such that phase variation-mediated control of virulence factors may vary significantly between individual strains. These findings are consistent with presently available transcript profile data.

Neisseria meningitidis causes 500,000 cases of meningitis and septicaemia annually worldwide, with a mortality rate of approximately 10%. Most disease in developed countries is caused by serogroup B infection, against which there is no universal vaccine. Opa proteins are major meningococcal outer membrane proteins, and a limited number of Opa variants have been associated with hyperinvasive serogroup B meningococci, suggesting their use as a potential novel vaccine. Immunisation of mice with recombinant Opa elicited high levels of meningococcal-specific serum bactericidal antibody (SBA), demonstrating proof in principle of this approach. Opa proteins mediate bacterial adherence to host cells and modulate human cellular immunity, and there are conflicting data regarding their effects on CD4⁺ T cells. opa genes from N. meningitidis strain H44/76 were cloned into the plasmid vector pBluescript, disrupted using antibiotic resistance cassettes and transformed into H44/76 to sequentially disrupt the four opa genes. This produced a unique panel of 15 isogenic Opa-deficient strains, including an Opa-negative strain, which enabled investigation of the immunomodulatory role of surface-expressed Opa proteins. There was no consistent effect of Opa expressed on the surface of OMVs and inactivated bacteria on CD4⁺ T cells, with significant heterogeneity of responses between individuals. The rate of Opa phase variation was between 10^-3 and 10^-4, and increased 180-fold following transformation of bacteria with unrelated DNA. These data support further investigation of Opa as a potential meningococcal vaccine component, and highlight the importance of host and bacterial factors in the development of OMV vaccines.

Streptococcus pneumoniae (the pneumococcus) is a common nasopharyngeal commensal in humans that can invade deeper tissues causing a range of diseases, resulting in significant global morbidity and mortality. Any given strain of S. pneumoniae can undergo phase variation between two different colony phenotypes, termed opaque (O) and transparent (T), as determined by their morphology on clear, solid media. In animal models, the O form is more virulent and commonly isolated from normally sterile sites, such as the blood, lungs and brain, compared to its T counterpart, which is more likely to reside in the nasopharynx. To date, the mechanism involved in the switch between opacity phenotypes is not known and there is discordance in the literature in terms of the precise molecular differences between them. This project set out to comprehensively characterise phenomic (2D-DIGE), transcriptomic (DNA microarray) and genomic (IonTorrent sequencing) differences between opacity variants in three unrelated pneumococcal strains (D39, serotype 2; WCH16, serotype 6A; and WCH43, serotype 4). Proteomic, transcriptomic and genomic analyses revealed strain-specific differences associated with phase variation, but no single difference was consistent across the three strains tested. Nevertheless, there were examples of proteins and genes belonging to the same putative functional groups that were regulated similarly in both the O and T phase between strains. One example was the upregulation of proteins and genes related to genetic competence in the O variants. However, mutagenesis of one such gene, encoding the competence stimulating peptide receptor (comD), did not alter opacity phenotype. There were also inconsistencies between genes identified as differentially expressed at an mRNA and protein level, such as genes involved in cell division, and amino acid biosynthesis and acquisition. These genes were upregulated at an mRNA level in the T variants, but no such upregulation in protein expression was identified by proteomic analysis. At a genomic level, all single nucleotide polymorphisms identified by IonTorent sequencing were independently verified. However, the insertion/deletions, particularly those associated with homopolymeric tracts were all found to be false call-outs, which is a limitation of this sequencing technology. The role of epigenetic changes mediated by genetic rearrangements in a Type I restriction-modification (RM) system on opacity phenotype was also investigated. These rearrangements result in switching between six alternative DNA methylation site specificities impacting on genomic methylation patterns. Examination of six “locked” mutants (SpnD39IIIA-F) with monospecific DNA methylation patterns, indicated that there was an epigenetic impact on colony opacity phenotype and virulence. Importantly, these genetic rearrangements at the Type I RM locus also occurred during experimental infection. However, there were inconsistencies between the opacity phenotypes of locked mutants and the RM allele distribution in wild-type D39O and D39T. Thus, RM allele switching cannot fully account for colony opacity phase variation in pneumococci. This study identified that pneumococcal phase variation is a complex, multifactorial phenomenon and different strains employ alternative mechanisms to attain opacity phenotypes. Furthermore, epigenetic changes impact pneumococcal opacity morphology and pathogenicity. Hence, the role of epigenetic factors in phase variation and pathogenesis should be investigated in future studies.
Thesis (Ph.D.) -- University of Adelaide, School of Biological Sciences, 2016.

The solid particle receiver (SPR) is a direct absorption central receiver that can provide a solar interface with thermal storage for thermochemical hydrogen production processes requiring heat input at temperatures up to 1000 C. In operation, a curtain consisting of ∼690 μm ceramic particles is dropped within the receiver cavity and directly illuminated by concentrated solar energy. The heated particles exit the receiver and may either be stored or sent through a heat exchanger to provide process heat input. The performance of the receiver is dependent on the characteristics of the particle flow including velocity and opacity (optical density). In addition, because the SPR will have an open aperture there is also a possibility that the flow may be disturbed by high ambient winds. Computational models have been and are currently being used to simulate receiver performance at power levels up to several MWt. However, due to the complex two-phase nature of the solid particle flow, such models rely on experimental data both to provide physical input, such as boundary conditions, as well as to provide a point of comparison for model validation. In this paper, we present experimental results from tests performed using a small scale unheated solid particle curtain. These tests focus on the measurement of the flow characteristics of the solid particle curtain as it falls from a near-zero velocity discharge slot to a collection point three meters below. The results include measured values for the variation of velocity, solids volume fraction, curtain width, and curtain opacity along the length of the curtain.