INTRODUCTION — Streptococcus pneumoniae occupies an important position in the history of microbiology:
●The organism was first identified in 1881.
●Its role in causing lobar pneumonia was appreciated by the late 1880s.
●The central role of antibody in host defense against extracellular organisms was first described for the pneumococcus.
●The first recognition that antibody directed to the capsular polysaccharide of a bacteria could be protective was shown for the pneumococcus; this observation forms the basis for many current bacterial vaccines.
●The discovery of DNA as the material for genetic exchange was made from the pneumococcus.
Despite the extensive study of this pathogen and the availability of a vaccine covering 23 different serotypes, S. pneumoniae remains a major cause of morbidity and mortality in children and older adults worldwide and is a principal bacterial cause of otitis media, pneumonia, and meningitis and an important cause of community-acquired bacteremia. An added significant challenge is the worldwide emergence of multiply antibiotic-resistant pneumococci. (See "Resistance of Streptococcus pneumoniae to beta-lactam antibiotics".)
The microbiology and pathogenesis of S. pneumoniae infection will be reviewed here. The clinical syndromes of pneumococcal pneumonia and meningitis as well as issues in vaccination against S. pneumoniae are discussed separately. (See related topic reviews.)
MICROBIOLOGY — S. pneumoniae is a gram-positive, alpha-hemolytic bacterium. It is fastidious, growing best in 5 percent carbon dioxide, and requires a source of catalase (eg, blood) to grow on agar plates. The inability to make catalase has clinical significance since, unlike other pyogenic organisms, catalase-negative bacteria generate hydrogen peroxide and are killed by the phagocytic cells of patients with chronic granulomatous disease. (See "Primary disorders of phagocyte number and/or function: An overview", section on 'Chronic granulomatous disease'.)
In the laboratory, pneumococci are identified by sensitivity to optochin and to lysis by bile salts (deoxycholate). The ability of deoxycholate and penicillin to dissolve the cell wall of the organism depends upon the presence of an autolytic enzyme in pneumococci [1]. During normal growth in broth, this enzyme is triggered when the culture reaches high density, resulting in a characteristic autolysis and death of all the bacteria in the culture. As an example, when a blood culture is allowed to grow for 24 to 48 hours without sampling or subculturing, the positive growth observed at six hours can be reversed by autolysis. The natural ability to undergo DNA transformation and autolysis in the stationary phase is an attribute that pneumococci share with Haemophilus influenzae and Neisseria meningitidis, two other common invasive bacteria.
The complete genomic sequence of S. pneumoniae serotype 4, a clinical isolate from a patient with meningitis, was placed online by the Institute for Genetic Research in November 1997. This was the first genome from a gram-positive bacterium to be sequenced in its entirety. A detailed annotation of 2236 potential coding regions, of which 64 percent have been assigned a potential biologic role, has been published [2]. The complete genome of an important laboratory strain R6 and a variety of clinical strains have also been described [3]. As new genomes are completed, updated sources for sequence analysis are available. Comparisons of these genomic sequences reveal a striking diversity within strains of S. pneumoniae. DNA composition of clinical isolates can vary by as much as 20 percent and still be classified as pneumococcus. This diversity is localized to regions of the chromosome that alter the virulence of the strain, referred to as pathogenicity islands [4,5].
PATHOGENESIS — Pneumococcus is a major cause of infection in children and older adults. Manifestations range from asymptomatic nasopharyngeal carriage to otitis media, pneumonia, sepsis, and meningitis. Acute cardiac events have also been described in patients with pneumococcal disease [6] and may result from the organism's ability to invade the myocardium and form microlesions that can lead to scarring, impaired cardiac contractility, and arrhythmias [7,8].
A number of features of S. pneumoniae mediate its ability to produce infection, including surface components that enhance virulence and provoke a host inflammatory reaction. The following discussion will emphasize the pathogenetic mechanisms of the organism and host responses. The steps involved in the bacteriologic and pathologic progression of pneumococcal pneumonia are discussed separately. (See "Pneumococcal pneumonia in patients requiring hospitalization".)
Capsule — The surface capsular polysaccharide of S. pneumoniae provokes a type-specific protective immune response and serves as the basis for serotyping of these organisms; 100 different pneumococcal serotypes have been identified. Prior to the routine use of pneumococcal conjugate vaccines, serotypes 6, 14, 18, 19, and 23 were the most prevalent, accounting for between 60 and 80 percent of infections depending upon the area of the world. Infections by strains included in vaccines are driven down in frequency and replaced by other serotypes over time.
The capsular polysaccharide is the major antiphagocytic surface element of pneumococci and the major protective antigen. The capsule locus has been sequenced [9]. A study has identified the galU gene of S. pneumoniae as an essential gene for the biosynthesis of capsular polysaccharide [10]. The gene has been cloned and sequenced; knockout mutants of pneumococcus types 1 and 3 lacking this gene are incapable of synthesizing a detectable capsule. The capsule is covalently linked to the glycan backbone of the cell wall.
The capsule is shed from the bacterial surface as it enters the respiratory tract. Contact with antimicrobial peptides triggers the autolytic enzyme LytA to cleave the cell wall, releasing the attached capsule while not killing the bacteria. This activity distinguishes triggering of the autolysin by penicillin, which results in bacterial lysis. Shedding of the capsule removes this key virulence determinant and reduces the ability of anticapsular antibody to opsonize the bacteria [11].
Adherence — Pneumococci, like other Streptococcus species, avidly adhere to epithelial cells of the nasopharynx (picture 1); this adherence does not usually produce a symptomatic infection. Like other streptococci that use fibrillar structures to contact human cells, a minority of pneumococci display pili to facilitate adherence [12].
The pneumococcus also exports over a dozen choline-binding proteins, which are noncovalently linked to the bacterial cell wall scaffold and serve as ligands to traffic the bacteria from one body site to another and disarm host defenses. These proteins recognize and bridge to human cell surface carbohydrates and proteins creating a direct contact between bacteria and human cells [13,14]. Multifunctional members of this class are CbpA and PspA.
Different, multiple ligand-receptor pairings occur at each body site [13]. For example, pneumococci have several ligands that bind fibronectin, extracellular matrix proteins, and complement. Sialic acid is a prominent receptor in the conjunctiva, Eustachian tube, and nasopharynx, while the disaccharide N-acetylgalactosamine b1-4 galactose is an important ligand in the lower respiratory tract. This sugar is recognized by a wide variety of pulmonary bacterial pathogens, suggesting this may be a basic "catch all" ligand for bacteria in the lung. Competitive inhibition of adherence by sugars has led to reduced pneumonia and bacteremia in animal models [15] and may spare the use of antibiotics in some clinical settings. Receptor analogs also might be designed to halt the progression of colonization or disease. Once bound to host receptors, intracellular signaling pathways are activated to induce bacterial uptake, cytoskeletal rearrangements, and inflammation.
Preceding influenza infection strongly predisposes to secondary invasive pneumococcal infection [16]. It appears that the influenza neuraminidase exposes receptors on lung cells for pneumococci that promote subsequent pneumococcal pneumonia [17].
Biofilm formation — Pneumococci form robust biofilms in the nasopharynx, sinus, and inner ear during colonization, sinusitis, and otitis media [18]. Biofilm formation is highly regulated depending on exogenous cues such as temperature. It has been tightly linked to the process of natural transformation whereby pneumococci become competent to exchange DNA in the environment.
Invasion — Pneumococci invade cells poorly, up to 10-fold less than other streptococci [19]. Clinical isolates exhibit a wide variability in invasive capacity [20]. Invasion is promoted by the cell wall, adhesins, and the cytotoxin pneumolysin; in comparison, invasion is inhibited by capsular polysaccharide.
Phosphorylcholine is a key component of the pneumococcal cell wall teichoic acid. Intact pneumococci use phosphorylcholine to tether to the host cell platelet-activating factor (PAF) receptor, thereby inserting the bacteria into the PAF receptor uptake pathway in an endocytic vacuole [21]. Bacterial phosphorylcholine recognition of the PAF receptor is an example of mimicry since the natural ligand PAF itself contains phosphorylcholine. This invasion pathway is shared by virtually all respiratory pathogens. In parallel, pneumococci can invade cells using the macropinocytosis uptake pathway, a receptor-independent uptake mechanism that nonspecifically translocates bulk contents [22].
To initiate uptake of pneumococci, human cells must become activated and upregulate the PAF receptor on their surfaces [21]. PAF receptors may also be upregulated in sickle cell disease, which may play an important role in the predisposition of such patients to invasive pneumococcal infection [23]. Conversely, blockade or genetic deletion of PAF receptors impairs cellular uptake and protects against invasive infection [21,23].
Once in the endocytic vacuole, pneumococci are subsequently transported to the basolateral cell surface, resulting in net transcytosis of the bacterium across the host cell and thus across epithelial and endothelial barriers (for instance, from the blood into the cerebrospinal fluid) [19]. Transcytosis without passing between cells or inducing cytotoxicity appears to be unique to pneumococci, as compared with other meningeal pathogens, and is initiated by binding to the PAF receptor [19]. Invasion is partially inhibited by PAF receptor antagonists [19]. Mice deficient in PAF receptor fail to develop pneumonia and meningitis in the face of bacteremia [24].
Regulatory mechanisms — Pneumococci use numerous regulatory mechanisms to change their surfaces in response to new host environments. Spontaneous phase variation changes gene expression when the bacteria transit between the mucosal surface and the bloodstream, altering the surface coat to avoid host defenses. As an example, the amount of surface phosphorylcholine in pneumococci declines when the organism enters the bloodstream; phosphorylcholine is useful in attaching to the lung but detrimental in the blood, where C-reactive protein can bind and opsonize the bacterium. Other pulmonary pathogens, such as H. influenzae, Pseudomonas, and Mycoplasma, also can modulate the amount of surface phosphorylcholine [25].
Pneumococci change the proteins they express after exposure to heat shock, as occurs when they translocate from the nasal mucosa to the lungs, the brain, and the blood during infection. The S. pneumoniae heat shock protein, ClpP, appears to regulate the expression of pneumococcal virulence proteins, including pneumolysin and the capsular polysaccharide. In a murine model, ClpP was required for colonization of the nasopharynx and for survival in host macrophages [26]. Immunization of mice with ClpP elicited a protective immune response against fatal systemic challenge with S. pneumoniae, making ClpP a potential vaccine candidate for human pneumococcal disease.
Pneumococci also can sense the density of other pneumococci in the environment and establish communication between the bacterial cells using small peptides similar to eukaryotic hormones [27]. These peptides signal all the bacteria, as a unified group, to undergo DNA transformation, adherence, or autolysis.
Pneumococci secrete a potent cytotoxin, pneumolysin, which binds to cholesterol and can indiscriminately form pores in membranes, thereby killing any cell [28]. In addition, pneumolysin promotes intra-alveolar replication of pneumococci, the penetration of pneumococci from the alveoli into the interstitium, and dissemination of the organisms into the bloodstream [29]. These effects are attenuated in pneumolysin-deficient strains [29]. Pneumolysin is an important factor leading to neuronal loss during meningitis [30].
Host inflammatory response — Gram-negative bacilli stimulate the host inflammatory response via endotoxin. A major unanswered question is how gram-positive bacteria, which do not have endotoxin, initiate this response. Pneumococci produce at least three hemolysins, one being the widely active cytolysin pneumolysin. When pneumococcal cell wall, cytoplasm, and capsular polysaccharide are compared for inflammatory capacity, cell wall has the highest specific activity [14]. This activity can be diminished by the overlying capsule on the native bacteria. The signs and symptoms of infection induced by cell wall mimic those of intact bacteria in animal models of meningitis, pneumonia, and otitis media, suggesting that the cell wall is an important virulence determinant [14]. Clinical strains with defects in the release of cell wall fragments induce an attenuated pattern of disease [31].
The teichoic acid and lipoteichoic acid of the cell wall contribute strongly to host defense responses associated with acute inflammation. These components have the following effects [14]:
●Activate the alternative pathway of the complement cascade
●Bind the acute-phase reactant C-reactive protein
●Activate procoagulant activity on the surface of endothelial cells
●Upon binding to epithelial and endothelial cells and macrophages, induce production of cytokines, nitric oxide, and chemokines
●Initiate the influx of neutrophils
Cell walls are recognized by the innate immune system in several ways. Cell wall-binding proteins in serum bind specific wall fragments and present them to human cell receptors [32]. The most prominent receptor on many cell types is Toll-like receptor 2 [33]. Downstream signaling leads to production of cytokines to initiate inflammation, with interleukin 1 being prominent. Evidence suggests that a cell wall-binding protein, Nod-2, also exists in the human cell cytoplasm [34-36]. Inflammatory signaling by various components of the cell wall leads to very diverse symptoms of infection such as induction of slow wave sleep, killing respiratory ciliated cells, and promoting blood-brain barrier permeability. Viable pneumococci also induce apoptosis in several cell types in the brain and lung [37].
The factors mediating the influx of neutrophils are in part organ specific. As an example, P-selectin mediates rolling or slowing of neutrophils, while intercellular adhesion molecule-1 (ICAM-1) contributes to the firm adhesion and emigration of neutrophils. Neutrophil emigration into the peritoneum during S. pneumoniae-induced peritonitis is markedly reduced in mice with mutations in either P-selectin or ICAM-1 and is abolished in double mutants [38]. In contrast, neutrophil emigration into the alveoli during S. pneumoniae-induced pneumonia is not impaired in double mutants. This is consistent with the observation that, in the lungs, pneumococci induce neutrophil efflux in two ways: one dependent upon the CD18 family of leukocyte adhesion molecules and the other by an unknown mechanism independent of CD18 [39].
Attenuating the acute host response to pneumococci has direct clinical application in improving the outcome of disease. A major change in the therapy of meningitis arose from the observation that pneumococcal cell wall pieces are as bioactive as intact bacteria [40]. This observation provided an explanation for an increased host inflammatory response during antibiotic therapy and a rationale for reducing this unwanted side effect. Over the first few hours of antibiotic therapy of bacterial meningitis, the leukocyte density in cerebrospinal fluid can increase one to two orders of magnitude [41]. This burst is sufficient to injure host tissues, as evidenced by the significant attenuation of damage upon the inhibition of leukocyte recruitment [42]. The use of steroids during the early phase of antibiotic therapy to inhibit this response has become accepted for pneumococcal meningitis in adults. (See "Dexamethasone to prevent neurologic complications of bacterial meningitis in adults".)
To truly affect the outcome of meningitis, the bacterial host interactions that cause neuronal death must be interrupted. In this context, the ability of pneumococci to induce human cell apoptosis is important. Evidence indicates that the inflammatory response to pneumococci leads to caspase-dependent human cell death, while direct toxicity of pneumolysin and hydrogen peroxide cause caspase-independent human cell death [30,43,44]. Both mechanisms may be inhibited in vitro and in vivo by treatment with a phosphatidylcholine analog, citicoline [45].
SUMMARY
●Streptococcus pneumoniae occupies an important position in the history of microbiology (see 'Introduction' above):
•The organism was first identified in 1881.
•Its role in causing lobar pneumonia was appreciated by the late 1880s.
•The central role of antibody in host defense against extracellular organisms was first described for the pneumococcus.
•The first recognition that antibody directed to the capsular polysaccharide of a bacteria could be protective was shown for the pneumococcus; this observation forms the basis for many bacterial vaccines.
•The discovery of DNA as the material for genetic exchange was made from the pneumococcus.
●Despite the extensive study of this pathogen and the availability of a vaccine covering 23 different serotypes, S. pneumoniae is a major invasive pathogen of children and older adults and is a principal cause of otitis media, pneumonia, bacteremia, and meningitis. An added significant challenge is the worldwide emergence of multiply antibiotic-resistant pneumococci. (See 'Introduction' above.)
●S. pneumoniae is a gram-positive, alpha-hemolytic bacterium. It is fastidious, growing best in 5 percent carbon dioxide, and requires a source of catalase (eg, blood) to grow on agar plates. The inability to make catalase has clinical significance since, unlike other pyogenic organisms, catalase-negative bacteria generate hydrogen peroxide and are killed by the phagocytic cells of patients with chronic granulomatous disease. (See 'Microbiology' above.)
●A striking feature of S. pneumoniae is the tremendous diversity among different strains. DNA composition of clinical isolates can vary by as much as 20 percent and still be classified as pneumococcus. This diversity is localized to regions of the chromosome that alter the virulence of the strain. (See 'Microbiology' above.)
●The surface capsular polysaccharide of S. pneumoniae provokes a type-specific protective immune response and serves as the basis for serotyping of these organisms; 100 different pneumococcal serotypes have been identified. The capsular polysaccharide is the major antiphagocytic surface element of pneumococci and the major protective antigen. (See 'Capsule' above.)
●Pneumococci, like other Streptococcus species, avidly adhere to epithelial cells of the nasopharynx; this adherence does not usually produce a symptomatic infection. A minority of pneumococci display pili to facilitate adherence (picture 1). The pneumococcus also exports several proteins, which are noncovalently linked to the bacterial cell wall scaffold, in order to traffic from one body site to the other. These proteins recognize and bridge to human cell carbohydrates or the receptor for platelet-activating factor (PAF), creating a direct contact between bacteria and human cells. (See 'Adherence' above.)
●To initiate uptake of pneumococci, human cells must become activated and upregulate the PAF receptor on their surface. (See 'Invasion' above.)
●Pneumococci use many regulatory mechanisms to change their surfaces in response to new host environments. (See 'Regulatory mechanisms' above.)
●Pneumococci secrete a potent cytotoxin, pneumolysin, which binds to cholesterol and can indiscriminately form pores in membranes, thereby killing any eukaryotic cell. (See 'Regulatory mechanisms' above.)
●The teichoic acid and lipoteichoic acid of the cell wall contribute strongly to host defense responses associated with acute inflammation. These components have the following effects (see 'Host inflammatory response' above):
•Activate the alternative pathway of the complement cascade
•Bind the acute-phase reactant C-reactive protein
•Activate procoagulant activity on the surface of endothelial cells
•Induce production of cytokines, nitric oxide, and PAF upon binding to epithelial and endothelial cells and macrophages
•Initiate the influx of neutrophils
