Phase variation in pneumococcal populations during carriage in the human nasopharynx

Streptococcus pneumoniae is one of the world's leading bacterial pathogens, responsible for pneumonia, septicaemia and meningitis. Asymptomatic colonisation of the nasopharynx is considered to be a prerequisite for these severe infections, however little is understood about the biological changes that permit the pneumococcus to switch from asymptomatic coloniser to invasive pathogen. A phase variable type I restriction-modification (R-M) system (SpnIII) has been linked to a change in capsule expression and to the ability to successfully colonise the murine nasopharynx. Using our laboratory data, we have developed a Markov change model that allows prediction of the expected level of phase variation within a population, and as a result measures when populations deviate from those expected at random. Using this model, we have analysed samples from the Experimental Human Pneumococcal Carriage (EHPC) project. Here we show, through mathematical modelling, that the patterns of dominant SpnIII alleles expressed in the human nasopharynx are significantly different than those predicted by stochastic switching alone. Our inter-disciplinary work demonstrates that the expression of alternative methylation patterns should be an important consideration in studies of pneumococcal colonisation.

The role of surfactant protein D in the colonisation of the respiratory tract and onset of bacteraemia during pneumococcal pneumonia

We have shown previously that surfactant protein D (SP-D) binds and agglutinates Streptococcus pneumoniae in vitro. In this study, the role of SP-D in innate immunity against S. pneumoniae was investigated in vivo, by comparing the outcome of intranasal infection in surfactant protein D deficient (SP-D-/-) to wildtype mice (SP-D+/+). Deficiency of SP-D was associated with enhanced colonisation and infection of the upper and lower respiratory tract and earlier onset and longer persistence of bacteraemia. Recruitment of neutrophils to inflammatory sites in the lung was similar in both strains mice in the first 24 hrs post-infection, but different by 48 hrs. T cell influx was greatly enhanced in SP-D-/- mice as compared to SP-D+/+ mice. Our data provides evidence that SP-D has a significant role to play in the clearance of pneumococci during the early stages of infection in both pulmonary sites and blood.

The role of pneumolysin in pneumococcal pneumonia and meningitis

Diseases caused by Streptococcus pneumoniae include pneumonia, septicaemia and meningitis. All these are associated with high morbidity and mortality. The pneumococcus can colonize the nasopharynx, and this can be a prelude to bronchopneumonia and invasion of the vasculature space. Proliferation in the blood can result in a breach of the blood-brain barrier and entry into the cerebrospinal fluid (CSF) where the bacteria cause inflammation of the meningeal membranes resulting in meningitis. The infected host may develop septicaemia and/or meningitis secondary to bronchopneumonia. Also septicaemia is a common precursor of meningitis. The mechanisms surrounding the sequence of infection are unknown, but will be dependent on the properties of both the host and bacterium. Treatment of these diseases with antibiotics leads to clearance of the bacteria from the infected tissues, but the bacteriolytic nature of antibiotics leads to an acute release of bacterial toxins and thus after antibiotic therapy the patients can be left with organ-specific deficits. One of the main toxins released from pneumococci is the membrane pore forming toxin pneumolysin. Here we review the extensive studies on the role of pneumolysin in the pathogenesis of pneumococcal diseases.

Stress response of Listeria monocytogenes isolated from cheese and other foods

The responses to pH and sodium chloride of four strains of Listeria monocytogenes isolated from Portuguese cheese, with a sodium chloride concentration of about 2% (w/v) and a pH value from 5.1 to 6.2, were studied. Two isolates from meat and two clinical isolates related to food-borne listeriosis, in which the implicated food product had about 2-3.5% (w/v) sodium chloride, also were studied. The effect of temperature on pH and sodium chloride sensitivity was also determined. The results show that natural isolates vary in response to these stresses and the data were often at variance with previously published data. Strains varied in sensitivity to low pH and to high sodium chloride concentration but the cheese isolates tended to be more resistant. A lower temperature was associated with a decrease in resistance to low pH and to sodium chloride. All strains showed an acid tolerance response induction when grown at pH 5.5 and although the time required for maximum induction of the response varied between strains, 2 h of acid adaptation, at least, was necessary which is longer than previously reported. Some strains showed an osmotolerance response after incubation in 3.5% (w/v) sodium chloride. Osmoadaptation, in addition to inducing an osmotolerance response, also induced cross-protection against acid shock conditions (pH 3.5). The acid tolerance response also induced a cross-protection against osmotic shock conditions (20% (w/v) sodium chloride). In some cases there was a relationship between the degree of resistance and adaptation, but usually the behaviour of a particular strain was independent of the conditions from which it was isolated.

Listeria monocytogenes adheres to many materials found in food-processing environments

AIMS

To investigate the adhesion of Listeria monocytogenes 10403S to 17 different, food-use approved materials representing metals, rubbers and polymers.

METHODS AND RESULTS

Adhesion assays were conducted by placing 'coupons' of the materials in planktonic cultures at 30 degrees C, and then immediately withdrawing them ('short contact') or leaving them submerged in the cultures for 2 h. Adherent cells were recovered by sonication. In the short contact experiments, the logarithm of the mean viable counts ranged from 3.67 +/- 0.43 to 4.78 +/- 0.38. After 2 h contact time, the numbers of adherent cells had increased significantly for all materials with the exception of polypropylene. The highest count (6.33 +/- 0.31) recorded was for stainless steel 405.

CONCLUSION

Adhesion to a wide range of materials was time-dependent and characterized by reversible and irreversible stages.

SIGNIFICANCE AND IMPACT OF THE STUDY

Adhesion test protocols must account for cell carry-over and cells which are only weakly bound. Material selection may only have a limited role in reducing food contamination by listeria.

Structural analysis of the protein/lipid complexes associated with pore formation by the bacterial toxin pneumolysin

Pneumolysin, a major virulence factor of the human pathogen Streptococcus pneumoniae, is a soluble protein that disrupts cholesterol-containing membranes of cells by forming ring-shaped oligomers. Magic angle spinning and wideline static (31)P NMR have been used in combination with freeze-fracture electron microscopy to investigate the effect of pneumolysin on fully hydrated model membranes containing cholesterol and phosphatidylcholine and dicetyl phosphate (10:10:1 molar ratio). NMR spectra show that the interaction of pneumolysin with cholesterol-containing liposomes results in the formation of a nonbilayer phospholipid phase and vesicle aggregation. The amount of the nonbilayer phase increases with increasing protein concentration. Freeze-fracture electron microscopy indicates the coexistence of aggregated vesicles and free ring-shaped structures in the presence of pneumolysin. On the basis of their size and analysis of the NMR spectra it is concluded that the rings are pneumolysin oligomers (containing 30-50 monomers) complexed with lipid (each with 840-1400 lipids). The lifetime of the phospholipid in either bilayer-associated complexes or free pneumolysin-lipid complexes is > 15 ms. It is further concluded that the effect of pneumolysin on lipid membranes is a complex combination of pore formation within the bilayer, extraction of lipid into free oligomeric complexes, aggregation and fusion of liposomes, and the destabilization of membranes leading to formation of small vesicles.

Role of genetic resistance in invasive pneumococcal infection: identification and study of susceptibility and resistance in inbred mouse strains

From a panel of nine inbred mice strains intranasally infected with Streptococcus pneumoniae type 2 strain, BALB/c mice were resistant and CBA/Ca and SJL mice were susceptible to infection. Further investigation revealed that BALB/c mice were able to prevent proliferation of pneumococci in the lungs and blood, whereas CBA/Ca mice showed no bacterial clearance. Rapidly increasing numbers of bacteria in the blood was a feature of CBA/Ca but not BALB/c mice. In the lungs, BALB/c mice recruited significantly more neutrophils than CBA/Ca mice at 12 and 24 h postinfection. Inflammatory lesions in BALB/c mice were visible much earlier than in CBA/Ca mice, and there was a greater cellular infiltration into the lung tissue of BALB/c mice at the earlier time points. Our data suggest that resistance or susceptibility to intranasal pneumococci may have an association with recruitment and/or function of neutrophils.

Use of green fluorescent protein in visualisation of pneumococcal invasion of broncho-epithelial cells in vivo

The pneumococcus is the principle cause of bacterial pneumonia and also a major cause of bacterial meningitis. The mechanisms and sites of pneumococcal adherence and invasion of the respiratory tract in vivo are not clear however. We have made pneumococci expressing green fluorescent protein (GFP) and used it to trace pneumococcal adherence and invasion in vivo. By using GFP pneumococci we have shown bacterial adherence and invasion of broncho-epithelial cells in vivo by 4 h post-infection, with increases in pneumococcal invasiveness by 24 h. Using confocal image analysis we have shown varying levels of pneumococcal penetration and internalisation into host cells, as well as translocation through epithelial layers. To our knowledge this is the first report of pneumococcal invasion and cellular translocation in vivo.

Structural basis of pore formation by cholesterol-binding toxins

In this paper we describe reconstructions by electron cryo-microscopy of two oligomeric states of the pore-forming toxin pneumolysin. The results are interpreted by the fitting of atomic models of separated domains to the 3-dimensional electron density maps, revealing two steps in the mechanism of pore formation by the family of cholesterol-binding toxins. We briefly describe the observation of the toxin pore in model membranes and contrast the apparent mechanism of pneumolysin with that of other pore-forming toxins.

Rapid methods for testing the efficacy of sterilization-grade filter membranes

The validation of sterilization-grade membranes is integral to ensuring the efficient and safe use of microfiltration systems. Here validation refers to the production of sterile filtrate for sterilizing-grade membranes under challenge test conditions. Current validation methods require 48 h of culture for results to become available, which creates time delays within the manufacturing process and quality control (QC) backlogs. This work compares four methods for the production of filter challenge test data, to the desired test sensitivity, within 24 h using bioluminescent and fluorescent recombinant strains of the test organism Brevundimonas diminuta. These methods should provide a way to implement more rapid QC test regimens for filters.

Role of manganese-containing superoxide dismutase in oxidative stress and virulence of Streptococcus pneumoniae

Streptococcus pneumoniae was shown to contain two types of superoxide dismutase, MnSOD and FeSOD. Levels of MnSOD increased during growth in an aerobic environment. The sodA gene, encoding MnSOD, of virulent S. pneumoniae type 2 strain D39 was inactivated to give mutant D39HY1. Aerobically, D39HY1 had a lower growth rate than the wild type and exhibited susceptibility to the redox-active compound paraquat, but anaerobic growth of D39HY1 was identical to that of the wild type. Virulence studies showed that the median survival time of mice infected intranasally with D39HY1 was significantly longer than that of mice infected with the wild-type pneumococcus. In contrast to the wild type, D39HY1 did not multiply in lungs during the first 24 h but thereafter grew at the same rate as the wild type. Appearance in the bloodstream was also delayed, but growth in the blood was unimpaired by the sodA mutation. The pattern of inflammation in lungs infected with D39HY1 differed from that in wild-type-infected mice. After infection with D39HY1, neutrophils were densely packed around bronchioles, in contrast to the wild-type infection, where neutrophils were more diffusely localized.

Biological properties of pneumolysin

Pneumolysin is a thiol-activated membrane-damaging toxin produced by Streptococcus pneumoniae. The toxin plays a role in virulence of the pneumococcus in animal models of infection. Pneumolysin has a range of biological activity including the ability to lyse eukaryotic cells and to interfere with the function of cells and soluble molecules of the immune system. The use of purified native and mutant toxin and of isogenic mutants of the pneumococcus expressing altered versions of the toxin has allowed the contribution of the various activities of this multifunctional toxin to virulence to be defined.

Relative roles of pneumolysin and hydrogen peroxide from Streptococcus pneumoniae in inhibition of ependymal ciliary beat frequency

Ciliated ependymal cells line the ventricular system of the brain and the cerebral aqueducts. This study characterizes the relative roles of pneumolysin and hydrogen peroxide (H(2)O(2)) in pneumococcal meningitis, using the in vitro ependymal ciliary beat frequency (CBF) as an indicator of toxicity. We have developed an ex vivo model to examine the ependymal surface of the brain slices cut from the fourth ventricle. The ependymal cells had cilia beating at a frequency of between 38 and 44Hz. D39 (wild-type) and PLN-A (pneumolysin-negative) pneumococci at 10(8) CFU/ml both caused ciliary slowing. Catalase protected against PLN-A-induced ciliary slowing but afforded little protection from D39. Lysed PLN-A did not reduce CBF, whereas lysed D39 caused rapid ciliary stasis. There was no effect of catalase, penicillin, or catalase plus penicillin on the CBF. H(2)O(2) at a concentration as low as 100 microM caused ciliary stasis, and this effect was abolished by coincubation with catalase. An additive inhibition of CBF was demonstrated using a combination of both toxins. A significant inhibition of CBF at between 30 and 120 min was demonstrated with both toxins compared with either H(2)O(2) (10 microM) or pneumolysin (1 HU/ml) alone. D39 released equivalent levels of H(2)O(2) to those released by PLN-A, and these concentrations were sufficient to cause ciliary stasis. The brain slices did not produce H(2)O(2), and in the presence of 10(8) CFU of D39 or PLN-A per ml there was no detectable bacterially induced increase of H(2)O(2) release from the brain slice. Coincubation with catalase converted the H(2)O(2) produced by the pneumococci to H(2)O. Penicillin-induced lysis of bacteria dramatically reduced H(2)O(2) production. The hemolytic activity released from D39 was sufficient to cause rapid ciliary stasis, and there was no detectable release of hemolytic activity from the pneumolysin-negative PLN-A. These data demonstrate that D39 bacteria released pneumolysin, which caused rapid ciliary stasis. D39 also released H(2)O(2), which contributed to the toxicity, but this was masked by the more severe effects of pneumolysin. H(2)O(2) released from intact PLN-A was sufficient to cause rapid ciliary stasis, and catalase protected against H(2)O(2)-induced cell toxicity, indicating a role for H(2)O(2) in the response. There is also a slight additive effect of pneumolysin and H(2)O(2) on ependymal toxicity; however, the precise mechanism of action and the role of these toxins in pathogenesis remain unclear.

Effect of pneumolysin on rat brain ciliary function: comparison of brain slices with cultured ependymal cells

This study compares two models for examining ependymal ciliary function: rat brain slices cut from the fourth ventricle and primary ependymal cells in culture. The cilia from both preparations were very reproducible; each preparation had cilia beating at a constant frequency of between 38 and 44 Hz. With the brain slices, ciliary stasis occurred after 5 d in culture. However, ependymal cells had fully functional cilia for up to 48 d in culture. The pneumococcal toxin, pneumolysin, caused a dose-dependent inhibition of cilia beat frequency within 15 min in both models. There were no significant differences in the mean log 50% inhibitory concentration (pIC50) slice = 0.65 +/- 0.05, equivalent to 4.4 hemolytic units (HU)/mL; cells = 0.57 +/- 0.14, equivalent to 3.7 HU/mL. There were also no significant differences in the mean Hill slope factors for the curves (slice = 1.4 +/- 0.05; cells = 1.6 +/- 0.4). These data demonstrate that both models can be used to examine the acute (15-min) effects of pneumolysin on cilia beat frequency. The main advantage of the primary ependymal culture model is that considerably more cultured ependymal cells (approximately 70%) are available, compared with the number of ependymal cells on the brain slices (approximately 2%), thus reducing the number of animals used. A pure ependymal culture was not achieved (approximately 30% of the cells were not ciliated). The increased survival time of the ependymal cells compared with the brain slices make cultured ependymal cells more useful for examining long-term ciliary function, whereas brain slices may be more useful for examining the interactions between ependymal and other nearby cells.

Host cellular immune response to pneumococcal lung infection in mice

Although there is substantial evidence that pneumolysin is an important virulence factor in pneumococcal pneumonia, relatively little is known about how it influences cellular infiltration into the lungs. We investigated how the inability of mutant pneumococci to produce pneumolysin altered the pattern of inflammation and cellular infiltration into the lungs. The effect on bacterial growth in the lungs also was assessed. There were three phases of growth of wild-type bacteria in the lungs: a decline followed by a rapid increase and then stasis or decline. The absence of pneumolysin was associated with a more rapid early decline and then a much slower increase in numbers. The pattern of inflammatory-cell accumulation also had distinct stages, and the timing of these stages was influenced by the presence of pneumolysin. Neutrophils began to accumulate about 12 to 16 h after infection with wild-type pneumococci. This accumulation occurred after the early decline in pneumococcal numbers but coincided with the period of rapid growth. Following infection with pneumococci unable to make pneumolysin, neutrophil influx was slower and less intense. Coincident with the third stage of pneumococcal growth was an accumulation of T and B lymphocytes at the sites of inflammation, but the accumulation was not associated with an increase in the total number of lymphocytes in the lungs. Lymphocyte accumulation in the absence of pneumolysin occurred but was delayed.

Studies on the structure and mechanism of a bacterial protein toxin by analytical ultracentrifugation and small-angle neutron scattering

Pneumolysin, an important virulence factor of the human pathogen Streptococcus pneumoniae, is a pore-forming toxin which also possesses the ability to activate the complement system directly. Pneumolysin binds to cholesterol in cell membrane surfaces as a prelude to pore formation, which involves the oligomerization of the protein. Two important aspects of the pore-forming activity of pneumolysin are therefore the effect of the toxin on bilayer membrane structure and the nature of the self-association into oligomers undergone by it. We have used analytical ultracentrifugation (AUC) to investigate oligomerization and small-angle neutron scattering (SANS) to investigate the changes in membrane structure accompanying pore formation. Pneumolysin self-associates in solution to form oligomeric structures apparently similar to those which appear on the membrane coincident with pore formation. It has previously been demonstrated by us using site-specific chemical derivatization of the protein that the self-interaction preceding oligomerization involves its C-terminal domain. The AUC experiments described here involved pneumolysin toxoids harbouring mutations in different domains, and support our previous conclusions that self-interaction via the C-terminal domain leads to oligomerization and that this may be related to the mechanism by which pneumolysin activates the complement system.SANS data at a variety of neutron contrasts were obtained from liposomes used as model cell membranes in the absence of pneumolysin, and following the addition of toxin at a number of concentrations. These experiments were designed to allow visualization of the effect that pneumolysin has on bilayer membrane structure resulting from oligomerization into a pore-forming complex. The structure of the liposomal membrane alone and following addition of pneumolysin was calculated by the fitting of scattering equations directly to the scattering curves. The fitting equations describe scattering from simple three-dimensional scattering volume models for the structures present in the sample, whose dimensions were varied iteratively within the fitting program. The overall trend was a thinning of the liposome surface on toxin attack, which was countered by the formation of localized structures thicker than the liposome bilayer itself, in a manner dependent on pneumolysin concentration. At the neutron contrast match point of the liposomes, pneumolysin oligomers were observed. Inactive toxin appeared to bind to the liposome but not to cause membrane alteration; subsequent activation of pneumolysin in situ brought about changes in liposome structure similar to those seen in the presence of active toxin. We propose that the changes in membrane structure on toxin attack which we have observed are related to the mechanism by which pneumolysin forms pores and provide an important perspective on protein/membrane interactions in general. We discuss these results in the light of published data concerning the interaction of gramicidin with bilayers and the hydrophobic mismatch effect.

The effect of the pneumococcal toxin, pneumolysin on brain ependymal cilia

Densely ciliated ependymal cells cover the ventricular surface of the brain and cerebral aqueducts separating cerebrospinal fluid, which is infected in meningitis, from neuronal tissue. We have established an ex vivo model that allows measurement of ependymal ciliary beat frequency, using high-speed video analysis, during incubation with bacterial toxins. Ciliated ependyma, from Wistar rats, was exposed to the pneumococcal toxin, pneumolysin, and a mutant form with markedly reduced cytotoxic activity (;0.1%). Wild-type pneumolysin (1500 HU/ml and 150 HU/ml: 10 and 1 microg/ml) caused rapid ciliary stasis (30-150 s), sloughing of cilia and cytoplasmic extrusion. Ciliary slowing before stasis was seen at 15 HU/ml (0.1 microg/ml); however, no effect on ciliary beat frequency was seen at lower concentrations (1.5 HU/ml and 0.15 HU/ml: 0.01 and 0.001 microg/ml). Mutant pneumolysin, 99.9% deficient in haemolytic activity, caused rapid ciliary stasis at 10 microg/ml but no effect was seen at lower concentrations (1-0.1 microg/ml). Pneumolysin, at levels which may be produced during severe pneumococcal meningitis, may cause rapid ependymal ciliary stasis.

Two structural transitions in membrane pore formation by pneumolysin, the pore-forming toxin of Streptococcus pneumoniae

The human pathogen Streptococcus pneumoniae produces soluble pneumolysin monomers that bind host cell membranes to form ring-shaped, oligomeric pores. We have determined three-dimensional structures of a helical oligomer of pneumolysin and of a membrane-bound ring form by cryo-electron microscopy. Fitting the four domains from the crystal structure of the closely related perfringolysin reveals major domain rotations during pore assembly. Oligomerization results in the expulsion of domain 3 from its original position in the monomer. However, domain 3 reassociates with the other domains in the membrane pore form. The base of domain 4 contacts the bilayer, possibly along with an extension of domain 3. These results reveal a two-stage mechanism for pore formation by the cholesterol-binding toxins.

Self-interaction of pneumolysin, the pore-forming protein toxin of Streptococcus pneumoniae

The pathogenically important cholesterol-binding pore-forming bacterial "thiol-activated" toxins (TATs) are commonly believed to be monomeric in solution and to undergo a transition on membrane binding mediated by cholesterol to an oligomeric pore. We present evidence, gained through the application of a number of biochemical and biophysical techniques with associated modelling, that the TAT from Streptococcus pneumoniae, pneumolysin, is in fact able to self-associate in solution to form the same oligomeric structures. The weak interaction leading to solution oligomerization is manifested at low concentrations in a dimeric toxin form. The inhibition of toxin self-interaction by derivatization of the single cysteine residue in pneumolysin with the thiol-active agent dithio (bis)nitrobenzoic acid indicates that self-interaction is mediated by the fourth domain of the protein, which has a fold similar to other proteins known to self-associate. This interaction is thought to have implications for the understanding of mechanisms of pore formation and complement activation by pneumolysin.

The molecular mechanism of pneumolysin, a virulence factor from Streptococcus pneumoniae

Pneumolysin, a member of the thiol-activated cytolysin family of toxins, is a virulence factor from the Gram-positive bacterium Streptococcus pneumoniae. The toxin forms large oligomeric pores in cholesterol-containing membranes of eukaryotic cells. A plethora of biochemical and mutagenesis data have been published on pneumolysin, since its initial characterization in the 1930s. Here we present an homology model of the monomeric and oligomeric forms of pneumolysin based on the recently determined crystal structure of perfringolysin O and electron microscopy data. A feature of the model is a striking electronegative surface on parts of pneumolysin that may reflect its cytosolic location in the bacterial cell. The models provide a molecular basis for understanding the effects of published mutagenesis and biochemical modifications on the toxic activity of pneumolysin. In addition, spectroscopic data are presented that shed new light on pneumolysin activity and have guided us to hypothesise a detailed model of membrane insertion. These data show that the environment of some tryptophan residues changes on insertion and/or pore formation. In particular, spectroscopic analysis of a tryptophan mutant, W433F, suggests it is the residue mainly responsible for the observed effects. Furthermore, there is no change in the secondary structure content when the toxin inserts into membranes. Finally, the basis of the very low activity shown by a pneumolysin molecule from another strain of S. pneumoniae may be due to the movements of a key domain-domain interface. The molecular basis of pneumolysin-induced complement activation may be related to the structural similarity of one of the domains of pneumolysin to Fc, rather than the presumed homology of the toxin to C-reactive protein as previously suggested.

Amino acid changes affecting the activity of pneumolysin alter the behaviour of pneumococci in pneumonia

Pneumolysin is a multi-functional toxin produced by Streptococcus pneumoniae. The toxin has distinct cytotoxic activity and complement-activating activity mediated by different parts of the toxin molecule. Mice challenged intranasally with a type 2 pneumococcal strain contract bronchopneumonia and bacteremia [1]. Mice were infected intranasally with isogenic mutants of this strain in which the chromosomal pneumolysin gene carried point mutations affecting either or both properties of pneumolysin. Reduction in either cytotoxic activity or complement activation by pneumolysin decreased the virulence of the mutant pneumococci. However, it was the ability to activate complement that most affected the behaviour of pneumococci in the lungs and associated bacteremia in the first 24 h following infection.

A conserved tryptophan in pneumolysin is a determinant of the characteristics of channels formed by pneumolysin in cells and planar lipid bilayers

Pneumolysin is one of the family of thiol-activatable, cytolytic toxins. Within these toxins the amino acid sequence Trp-Glu-Trp-Trp is conserved. Mutations made in this region of pneumolysin, residues 433-436 inclusive, did not affect cell binding or the formation of toxin oligomers in the target cell membrane. However, the mutations did affect haemolysis, leakage of low-molecular-mass metabolites from Lettre cells and the induction of conductance channels across planar lipid bilayers. Of eight modified pneumolysins examined, Trp-433-->Phe showed the smallest amount of haemolysis or leakage (less than 5% of wild type). Pneumolysin-induced leakage from Lettre cells was sensitive to inhibition by bivalent cations but the extent of inhibition varied depending on the modification. Leakage by the mutant Trp-433-->Phe was least sensitive to cation inhibition. The ion-conducting channels formed across planar lipid bilayers exhibit small (less than 30 pS), medium (30 pS-1 nS) and large (more than 1 nS) conductance steps. Small- and medium-sized channels were preferentially closed by bivalent cations. In contrast with wild-type toxin, which formed predominantly small channels, the modified toxin Trp-433-->Phe formed large channels that were insensitive to cation-induced closure. Polysaccharides of molecular mass more than 15 kDa inhibited haemolysis by wild-type toxin, but polysaccharide of up to 40 kDa did not prevent haemolysis by Trp-433-->Phe. Electron microscopy revealed that Trp-433-->Phe formed oligomeric arc and ring structures with dimensions identical with those of wild-type toxin, and that the ratio of arcs to rings formed was the same for wild-type toxin and the Trp-433-->Phe variant. We conclude that the change Trp-433-->Phe affects channel formation at a point subsequent to binding to the cell membrane and the formation of oligomers, and that the size of arc and ring structures revealed by electron microscopy does not reflect the functional state of the channels.

A role for pneumolysin but not neuraminidase in the hearing loss and cochlear damage induced by experimental pneumococcal meningitis in guinea pigs

We investigated the roles of pneumolysin and neuraminidase in the pathogenesis of deafness and cochlear damage during experimental pneumococcal meningitis. Anesthetized guinea pigs were inoculated intracranially with 7.5 log10 CFU of either (i) wild-type Streptococcus pneumoniae D39 (n = 8), (ii) PLN-A, a defined isogenic derivative of D39 deficient in pneumolysin (n = 5), or (iii) deltaNA1, a new derivative of D39 deficient in neuraminidase constructed by insertion-duplication mutagenesis of the nanA gene (n = 5). To quantify hearing loss, the auditory nerve compound action potential evoked by a tone pulse was recorded from the round window membrane of the cochlea every 3 h for 12 h. The organ of Corti was intravitally fixed for subsequent examination by high-resolution scanning and transmission electron microscopy. All animals sustained similar meningeal inflammatory responses. PLN-A induced significantly less hearing loss than D39 over the frequency range of 3 to 10 kHz. Levels of mean hearing loss at 10 kHz 12 h postinoculation were as follows: D39, 50 dB; deltaNA1, 52 dB (P = 0.76 versus D39), and PLN-A, 12 dB (P < 0.0001 versus D39). The mean rates of hearing loss at 10 kHz were 4.4 dB/h for D39, 4.3 dB/h for deltaNA1, and just 1.0 dB/h for PLN-A (P < 0.0001 versus D39). Suppurative labyrinthitis was universal. PLN-A induced the accumulation of less protein in the cerebrospinal fluid (P = 0.04 versus D39). Infection with D39 and deltaNA1 induced significant damage to the reticular lamina, the sensory hair cells, and supporting cells of the organ of Corti. By contrast, after infection with PLN-A, the organ of Corti appeared virtually intact. Pneumolysin seems to be the principal cause of cochlear damage in this model of meningogenic deafness. No clear pathogenic role was demonstrated for neuraminidase.

Structural and functional characterisation of two proteolytic fragments of the bacterial protein toxin, pneumolysin

Proteolytic cleavage of the bacterial protein toxin pneumolysin with protease K creates two fragments of 37 and 15 kDa. This paper describes the purification of these two fragments and their subsequent physical and biological characterisation. The larger fragment is directly involved in the cytolytic mechanism of this pore-forming protein, via membrane binding and self-association. The smaller fragment lacks ordered structure or discernible activity.

Physiological and biochemical studies on psychrotolerance in Listeria monocytogenes

Studies were undertaken to explain the ability of Listeria monocytogenes to grow at low temperatures in a chemostat. It was found that when grown in continuous culture at a dilution rate of 0.02 h-1 L. monocytogenes had a lower proportion of anteiso-17:0, and a higher proportion of anteiso-15:0, and smaller chain fatty acids when grown at 10 degrees C compared to 30 degrees C. A previously unreported glycolipid was only seen after growth at low temperature. Growth temperature had no effect on the rate of glucose uptake.

Structural and functional analogy between pneumolysin and proaerolysin

Pneumolysin and proaerolysin are bacterial toxins that form pores in host cells by oligomerization. We propose that they may have similar structures despite a poor sequence identity. The crystal structure of proaerolysin reveals a protein composed of four domains, arranged in the shape of an elongated comma. Electron microscopy of the pneumolysin monomer shows a similar arrangement of domains. The sequence of pneumolysin recognizes the template of proaerolysin from a library of protein folds. A three-dimensional model of pneumolysin has been constructed by the comparative approach using the structure of proaerolysin. This model, together with results on the activity of site-specific mutants and the positions of antigenic sites, has been used to propose functional roles of individual domains.

Streptococcus pneumoniae produces a second haemolysin that is distinct from pneumolysin

Pneumococci are described as alpha-haemolytic but under certain circumstances they produce zones of beta-haemolysis on blood-containing medium. This observation was investigated using wild type strains and a genetically-modified strain unable to produce the haemolytic toxin, pneumolysin. beta-haemolysis was produced by all pneumococci tested. It was not inhibited by anti-pneumolysin antibody but could be inactivated by cholesterol. These data confirm that pneumococci elaborate a second haemolysin, distinct from pneumolysin.

Response of Mycobacterium smegmatis to acid stress

Evidence was sought for the existence of an inducible acid tolerance response in Mycobacterium smegmatis. Exposure of M. smegmatis to a sub-lethal, adaptive acidic pH was found to confer a significant level of protection against subsequent exposure to a lethal pH, compared to unadapted cells. Adaptation was dependent on de novo protein synthesis.

Distinct roles for pneumolysin's cytotoxic and complement activities in the pathogenesis of pneumococcal pneumonia

Pneumolysin, the major Streptococcus pneumoniae cytotoxin, contributes to the early pathogenesis of invasive pneumococcal pneumonia by facilitating intrapulmonary bacterial growth and invasion into the blood. Pneumolysin is a multifunctional toxin, with distinct cytolytic ("hemolytic") and complement-activation ("complement") activities that have been mapped to several regions of the molecule. To characterize the specific contributions of pneumolysin's hemolytic and complement properties to the pathogenesis of pneumococcal pneumonia, we compared the in vivo effects of type 2 S. pneumoniae mutant strains, which produce pneumolysins deficient in these activities. The absence of either pneumolysin's hemolytic or complement activities rendered mutant strains less virulent than the wild-type strain during pulmonary infection. Pneumolysin's hemolytic activity correlated with acute lung injury and bacterial growth at 3 and 6 h after endotracheal instillation. In contrast, pneumolysin's complement activity correlated with bacterial growth and bacteremia at 24 h after pulmonary infection. Pneumolysin's complement activity was not associated with the degree of alveolar-capillary injury or recruitment of leukocytes during initial pulmonary infection. However, pneumolysin's complement activity inhibited killing of mutant bacteria in an in vitro complement-dependent neutrophil killing assay. Thus, both pneumolysin's hemolytic and complement activities made specific contributions to the early pathogenesis of pneumococcal pneumonia at different stages of infection and by different mechanisms.

Functional analysis of pneumolysin by use of monoclonal antibodies

We have produced a panel of monoclonal antibodies to pneumolysin, the membrane-damaging toxin from Streptococcus pneumoniae. We have used these antibodies to identify three regions of the toxin sequence that are involved in the lytic mechanism of this toxin. Two of these sites probably form the cell binding site of this toxin. Antibodies to the third site inhibit the lytic action of this toxin but not the binding of this toxin to cells. This site is engaged in the oligomerization process involved in the formation of pores in cell membranes. Two of these epitopes are also present in the related toxin perfringolysin O.

Subunit organisation and symmetry of pore-forming, oligomeric pneumolysin

We present a detailed analysis of the oligomeric subunit organisation of pneumolysin by the use of negative stain electron microscopy and image processing to produce a projection density map. Analysis of the rotational symmetry has revealed a large and variable subunit number, between 40-50. The projected subunit density by rotational averaging shows at least two distinct subunit domains at different radial positions. Side views of the rings reveal further details concerning the dimensions of the oligomer in the membrane. On the basis of these observations and our previous knowledge of the monomer domain structure we propose that the 4-domain subunits are packed in a square planar arrangement to form the pneumolysin oligomer.

The role of pneumolysin and autolysin in the pathology of pneumonia and septicemia in mice infected with a type 2 pneumococcus

Mice were infected intranasally with a serotype 2 pneumococcus, a pneumolysin-negative derivative (PLN-A), or an autolysin-negative derivative (AL-2). Numbers of wild type pneumococci were seen in the lung from approximately 12 h after infection and were first detected in the blood around this time. Immunofluorescent staining of lung sections showed that pneumolysin was produced in vivo. Pneumococcal infection resulted in alteration of the composition of the blood but not the bone marrow. Some of the hematologic changes did not occur after PLN-A. PLN-A had a slower growth rate in the lung and bacteremia was delayed. AL-2 was rapidly cleared from the lungs and was not detected in the blood. These events paralleled the pattern of histology in the lung, with the severity of inflammation reduced with PLN-A and no inflammation or hematologic changes with AL-2.

Effect of defined point mutations in the pneumolysin gene on the virulence of Streptococcus pneumoniae

The thiol-activated toxin pneumolysin is a known pneumococcal virulence factor, with both cytotoxic (hemolytic) and complement activation properties. Copies of the pneumolysin gene carrying defined point mutations affecting either or both of these properties were introduced into the chromosome of Streptococcus pneumoniae D39 by insertion-duplication mutagenesis. The virulences of these otherwise isogenic strains were then compared. There was no significant difference in either the median survival time or overall survival rate between mice challenged with D39 derivatives producing the wild-type toxin and those expressing a pneumolysin gene with an Asp-385-->Asn mutation, which abolishes the complement activation property. However, mice challenged with strains carrying either His-367-->Arg or Trp-433-->Phe plus Cys-428-->Gly mutations, which reduce hemolytic activity to approximately 0.02 and 0.0001% of the wild-type level, respectively, had significantly greater median survival times and overall survival rates than mice challenged with D39 derivatives expressing a wild-type pneumolysin gene. No additional reduction in virulence was observed when mice were challenged with a D39 derivative carrying Trp-433-->Phe, Cys-428-->Gly, and Asp-385-->Asn, rather than Trp-433-->Phe and Cys-428-->Gly, mutations in the pneumolysin gene. Thus, it appears that in the intraperitoneal challenge model, the contribution of pneumolysin to virulence is largely attributable to its hemolytic (cytotoxic) properties rather than to its capacity to activate complement. Interestingly, however, the amount of pneumolysin required for full virulence may be very small, as D39 derivatives carrying the Trp-433-->Phe mutation (which reduces hemolytic activity to 0.1% of the wild-type level) had intermediate virulence.

Growth and virulence of a complement-activation-negative mutant of Streptococcus pneumoniae in the rabbit cornea

Our previous work has demonstrated the importance of pneumolysin in the virulence of S. pneumoniae in a rabbit intracorneal model. This was accomplished by showing that deletion of the gene encoding pneumolysin resulted in reduced virulence, whereas restoration of the wild-type gene resulted in restoration of the virulent phenotype. To assess the importance of a particular domain in the pneumolysin molecule, we have now constructed a strain which produces a pneumolysin molecule which is hemolytic but which bears a site-specific mutation in the domain known to be associated with the complement-activating properties of this molecule. Comparison of the virulence of this strain with that of a strain bearing the wild-type gene showed statistically significantly lower total slit lamp examination (SLE) scores at 12, 18, 24, and 36 h (particularly with respect to fibrin formation), but no difference at 48 h. Determination of colony forming units (CFU) in eyes infected with the two strains showed approximately 10(6) bacteria per cornea until 36 h. Between 36 and 48 h, the bacteria were almost completely cleared with very few bacteria recoverable at the later time point. The loss of virulence observed with this mutation in the complement-activation domain of pneumolysin, though less than that observed with the gene deletion mutant, suggests that complement activation by pneumolysin has a significant role in the pathology observed in this model of corneal infection.

Interaction of pneumolysin-sufficient and -deficient isogenic variants of Streptococcus pneumoniae with human respiratory mucosa

Streptococcus pneumoniae is the most common cause of community-acquired pneumonia, and pneumolysin, a hemolytic toxin, is thought to be an important virulence factor. We have studied the interaction of a pneumolysin-sufficient type II S. pneumoniae strain (PL+) and an otherwise identical pneumolysin-deficient derivative (PL-) with human respiratory mucosa in an organ culture with an air interface for up to 48 h. Ciliary beat frequency (CBF) was measured by a photometric technique, and adherence to and invasion of the epithelium were assessed by scanning and transmission electron microscopy. PL+ and PL- caused a progressive fall in CBF compared with the control which became significant (P < 0.01) at 24 h for PL+ and at 48 h for PL-. At 24 h, there was a significant increase in the percentage of the mucosa of the organ culture that was damaged for PL+ compared with the control (P < 0.01) and PL- (P < 0.02). At 48 h, there was a significant increase in mucosal damage for both PL+ (P < 0.005) and PL- (P < 0.05) compared with the control. At 24 and 48 h, PL+ and PL- adhered predominantly to mucus and damaged cells. PL+ infection alone caused separation of tight junctions between epithelial cells, and at 48 h PL+ cells were adherent to the separated edges of otherwise healthy unciliated cells. PL+ and PL- both caused damage to the epithelial cell ultrastructure. S. pneumoniae infection caused patchy damage to the respiratory mucosa and a lowered CBF. These changes were more severe and occurred earlier with the pneumolysin-sufficient variant.

Comparative study of the growth of Listeria monocytogenes in defined media and demonstration of growth in continuous culture

A basic requirement for physiological studies with Listeria monocytogenes is a chemically defined medium that supports growth of the bacterium in batch and continuous culture. A number of such media have been devised but comparative studies of their efficiency are few and none has been used in continuous culture. Six of the media were compared for their ability to sustain sequential growth of L. monocytogenes in static and aerated batch culture with glucose as sole carbon source. The most suitable, judged on the basis of ease of preparation, growth rate and yield, was that of Trivett and Meyer (1971). This medium was shown to support growth of L. monocytogenes NCTC 7973 in continuous culture in a chemostat. A lytic phenomenon, noted with the same strain under anaerobic conditions and in batch culture in the chemostat, is discussed.

Dual function of pneumolysin in the early pathogenesis of murine pneumococcal pneumonia

Streptococcus pneumoniae is one of the most common etiologic agents of community-acquired pneumonia, particularly bacteremic pneumonia. Pneumolysin, a multifunctional cytotoxin, is a putative virulence factor for S. pneumoniae; however, a direct role for pneumolysin in the early pathogenesis of pneumococcal pneumonia has not been confirmed in vivo. We compared the growth of a pneumolysin-deficient (PLY[-]) type 2 S. pneumoniae strain with its isogenic wild-type strain (PLY[+]) after direct endotracheal instillation of bacteria into murine lungs. Compared with PLY(-) bacteria, infection with PLY(+) bacteria produced greater injury to the alveolar-capillary barrier, as assayed by albumin concentrations in alveolar lavage, and substantially greater numbers of PLY(+) bacteria were recovered in alveolar lavages and lung homogenates at 3 and 6 h after infection. The presence of pneumolysin also contributed to the development of bacteremia, which was detected at 3 h after intratracheal instillation of PLY(+) bacteria. The direct effects of pneumolysin on lung injury and on the ability of pneumococci to evade local lung defenses was confirmed by addition of purified recombinant pneumolysin to inocula of PLY(-) pneumococci, which promoted growth of PLY(-) bacteria in the lung to levels comparable to those seen with the PLY(+) strain. We further demonstrated the contributions of both the cytolytic and the complement-activating properties of pneumolysin on enhanced bacterial growth in murine lungs using genetically modified pneumolysin congeners and genetically complement-deficient mice. Thus, pneumolysin facilitates intraalveolar replication of pneumococci, penetration of bacteria from alveoli into the interstitium of the lung, and dissemination of pneumococci into the bloodstream during experimental pneumonia. Moreover, both the cytotoxic and the complement-activating activities of pneumolysin may contribute independently to the acute pulmonary injury and the high rates of bacteremia which characterize pneumococcal pneumonia.

Immunization of mice with pneumolysin toxoid confers a significant degree of protection against at least nine serotypes of Streptococcus pneumoniae

Pneumolysin is the thiol-activated cytolysin produced by Streptococcus pneumoniae. Mice were immunized with a genetically engineered toxoid version of pneumolysin, which was derived from a serotype 2 pneumococcus. The toxoid carried the mutation Trp-433-->Phe. Alum was used as the adjuvant. Immunized mice had significantly increased levels of anti-pneumolysin antibodies, principally immunoglobulin G1. Mice were challenged intraperitoneally or intranasally with 12 strains covering capsular serotypes 1 to 6, 7F, 8, and 18C. Following challenge, the survival rate and/or the time of death of nonsurvivors (survival time) was significantly greater than that of sham-immunized mice for all nine serotypes. However, differences in the degree of protection were noted between different strains. The route of challenge also appeared to influence the degree of protection. Nevertheless, the significant, albeit in some cases partial, protection provided against all nine pneumococcal serotypes supports the conclusion that pneumolysin toxoids warrant consideration for inclusion in a human vaccine.

The application of luciferase as a reporter of environmental regulation of gene expression in mycobacteria

This paper describes the construction of pSG10, the first mycobacterial promoter probe shuttle vector to use the structural gene of a bacterial luciferase as a reporter gene. To examine the utility of using bacterial luciferase to measure gene expression in mycobacteria, the authors have used this vector to monitor the induction of the acetamidase gene promoter of Mycobacterium smegmatis. Luciferase proved to be a rapid, sensitive and easily assayable reporter of changes in gene activity in response to environment in mycobacteria.

Strains of Mycobacterium tuberculosis differ in susceptibility to reactive nitrogen intermediates in vitro

The effects on the viability of Mycobacterium tuberculosis strains and one Mycobacterium bovis strain from exposure to sodium nitrite for 24 h, in both neutral and acidic media, were tested. The in vitro resistance of mycobacteria to reactive nitrogen intermediates, generated at an acidic pH, was found to have a significant (P < 0.05) positive correlation to the virulence of strains in guinea pigs.

Modeling the bacterial protein toxin, pneumolysin, in its monomeric and oligomeric form

Pneumolysin is a member of the family of related bacterial thiol-activated toxins, which share structural similarities and a proposed common cytolytic mechanism. Currently the molecular mechanism of membrane damage caused by these toxins remains a matter of controversy. A prerequisite for defining this mechanism is a detailed knowledge of the monomeric and oligomeric pneumolysin structures. We present for the first time details of the monomeric structure of a thiol-activated toxin, pneumolysin. Electron microscope images of metal-shadowed pneumolysin monomers show an asymmetric molecule composed of four domains. We have studied the conformation of pneumolysin monomer by low resolution hydrodynamic bead modeling procedures. The bead model dimensions and shape are derived solely from the electron micrographs. The bead model has been evaluated in terms of the predicted solution properties, which in turn have been compared to the experimental values of the sedimentation coefficient, s(20,w)0, obtained by analytical ultracentrifugation and the intrinsic viscosity, [eta]. Pneumolysin oligomers, observed as ring- and arc-shaped structures, were also examined by electron microscopy. Metal shadowing and negative staining methods were used to establish the overall dimensions of the oligomer and were used to produce a morphological model for the oligomer, incorporating monomer subunits based on the hydrodynamic bead model.

Pneumolysin activates phospholipase A in pulmonary artery endothelial cells

Pneumolysin has been identified as a virulence factor in Streptococcus pneumoniae disease. In addition to producing tissue injury through its cytolytic effect, pneumolysin might injure tissues indirectly by eliciting an inflammatory response. We demonstrate for the first time that pneumolysin is a rapid and potent activator of cellular phospholipase A in bovine pulmonary artery endothelial cells. In contrast to other toxin-activated phospholipases, pneumolysin-stimulated phospholipase A showed no substrate specificity among major cellular membrane phospholipids. Phospholipase A activation required the formation of functional transmembrane pores by pneumolysin rather than membrane lipid perturbation. Pneumolysin stimulation of phospholipase A was calcium dependent; however, pneumolysin did not appear to function simply as a calcium ionophore. Pneumolysin was capable of stimulating purified bee and snake venom phospholipase A2s against a phospholipid substrate isolated from endothelial cells. Thus, pneumolysin stimulates cellular phospholipase A and the resulting products might further injure tissues by direct cytolytic effect or by evoking inflammatory responses.

A neuraminidase from Streptococcus pneumoniae has the features of a surface protein

A gene from Streptococcus pneumoniae (nanA), with features entirely consistent with a neuraminidase gene, has been sequenced. High levels of neuraminidase activity were obtained after cloning of this gene, without flanking sequences, into a high-expression vector. RNA hybridization studies have shown that the gene is transcribed by a virulent pneumococcus strain. The predicted molecular weight of the protein and certain amino acid sequences are typical of other neuraminidases. NanA contains the four copies of the sequence SXDXGXTW that is present in all the bacterial neuraminidases previously described. Kyte and Doolittle analysis showed that NanA is a hydrophilic protein with hydrophobic domains at the N terminus and the C terminus. A putative signal peptide was found in the N terminus of this protein, indicating that the protein is exported from the pneumococcus. The C terminus has the features of the anchor motif found in other surface proteins from gram-positive bacteria. Electron microscopy studies showed the presence of neuraminidase associated with the cell surface of the pneumococcus.