S. pneumoniae (pneumococci) is a member of the S. mitis group (see Table 1) and are indistinguishable from them on the basis of 16SrRNA. Pneumococci are Gram-positive diplococci, often lancet shaped or arranged in chains, possessing a capsule of polysaccharide that permits typing with specific antisera. Pneumococci are readily lysed by surface-active agents, which probably remove or inactivate the inhibitors of cell wall autolysins. Pneumococci are nor mal inhabitants of the upper respiratory tract of 5–40% of humans and can cause pneumonia, sinusitis, otitis, bronchitis, bacteremia, meningitis, peritonitis, and other infectious processes.

Table1. Characteristics of Medically Important Streptococci

Morphology and Identification

 A. Typical Organisms

 The typical Gram-positive, lancet-shaped diplococci (Figure 1) are often seen in specimens of young cultures. In sputum or pus, single cocci or chains are also seen. With age, the organisms rapidly become Gram-negative and tend to lyse spontaneously. Autolysis of pneumococci is greatly enhanced by surface-active agents. Lysis of pneumococci occurs in a few minutes when ox bile (10%) or sodium deoxycholate (2%) is added to a broth culture or suspension of organisms at neutral pH. Viridans streptococci do not lyse and are thus easily differentiated from pneumococci. On solid media, the growth of pneumococci is inhibited around a disk of optochin; viridans streptococci are not inhibited by optochin (Figure 2).

Fig1. S. pneumoniae in sputum are seen as lancet shaped Gram-positive diplococci. Degenerating nuclei of polymorphonuclear cells are the large darker irregular red shapes (arrow). Mucus and amorphous debris are present in the background. Original magnification ×1000.

Fig2. A: Optochin inhibition and bile solubility of S. pneumoniae. The S. pneumoniae were grown overnight on 5% sheep blood agar. The optochin (ethyl hydrocupreine HCl) or P disk was placed when the plate was inoculated. The pneumococci are α-hemolytic with greening of the agar around the colonies. The zone of inhibition around the P disk is larger than 14 mm, indicating that the organisms are pneumococci rather than viridans streptococci. A drop of desoxycholate (“bile”) solution was placed on the overnight growth just to the right of the P disk area (arrow); after about 20 minutes at room temperature, the colonies of pneumococci were solubilized (bile soluble). B: The growth of viridans streptococci appears similar to the growth of pneumococci, but growth of the viridans streptococci is not inhibited by optochin. C: S. pneumoniae quellung reaction: a small amount of growth is mixed with saline, antisera against the capsule polysaccharide, and methylene blue stain. After incubation at room temperature for 1 hour, the reaction is observed under the microscope. The organisms are outlined in light blue. A positive reaction shows clumping because of cross-linking of the antibodies and pneumococci. The halo effect around the pneumococci is apparent capsular swelling. A negative control would show no clumping or capsular swelling. (Courtesy of H. Reyes.)

Other identifying points include almost uniform virulence for mice when injected intraperitoneally and the “capsule swelling test,” or quellung reaction (see below).

B. Culture

 Pneumococci form small round colonies, at first dome shaped and later developing a central depression with an elevated rim. Other colonies may appear glistening because of capsular polysaccharide production. Pneumococci are α-hemolytic on blood agar. Growth is enhanced by 5–10% CO₂.

C. Growth Characteristics

 Most energy is obtained from fermentation of glucose; this is accompanied by the rapid production of lactic acid, which limits growth. Neutralization of broth cultures with alkali at intervals results in massive growth.

D. Variation

 Pneumococcal isolates that produce large amounts of capsules appear as large mucoid colonies. Capsule production is not essential for growth on agar medium, and capsular pro duction is therefore lost after a small number of subcultures. The pneumococci will, however, again produce capsules and have enhanced virulence if injected into mice.

Antigenic Structure

 A. Component Structures

 The pneumococcal cell wall has peptidoglycan and teichoic acid, similar to other streptococci. The capsular polysaccharide is covalently bound to the peptidoglycan and to the cell wall polysaccharide. The capsular polysaccharide is immunologically distinct for each of the 91 types. C-polysaccharide that is found in the cell wall of all S. pneumoniae can be detected in the urine and cerebrospinal fluid (CSF) as useful diagnostic tests for pneumococcal infections.

B. Quellung Reaction

When pneumococci of a certain type are mixed with specific antipolysaccharide serum of the same type—or with polyvalent antiserum—on a microscope slide, the capsule swells markedly, and the organisms agglutinate by cross linking of the antibodies (see Figure 2C). This reaction is useful for rapid identification and for typing of the organisms, either in sputum or in cultures. The polyvalent antiserum, which contains antibody to all of the types (“omniserum”), is a good reagent for rapid microscopic determination of whether or not pneumococci are present in fresh sputum. This test is rarely used because of the high reagent costs and the expertise required in assay performance and interpretation.

Pathogenesis

 A. Types of Pneumococci

 In adults, types 1–8 are responsible for about 75% of cases of pneumococcal pneumonia and for more than half of all fatalities in pneumococcal bacteremia; in children, types 6, 14, 19, and 23 are frequent causes.

B. Production of Disease

 Pneumococci produce disease through their ability to multi ply in the tissues. The virulence of the organism is a function of its capsule, which prevents or delays ingestion by phagocytes. A serum that contains antibodies against the type specific polysaccharide protects against infection. If such a serum is absorbed with the type-specific polysaccharide, it loses its protective power. Animals or humans immunized with a given type of pneumococcal polysaccharide are subsequently immune to that type of pneumococcus and possess precipitating and opsonizing antibodies for that type of polysaccharide.

C. Loss of Natural Resistance

 Because 40–70% of humans are at some time carriers of virulent pneumococci, the normal respiratory mucosa must possess great natural resistance to the pneumococcus. Among the factors that probably lower this resistance and thus pre dispose to pneumococcal infection are the following:

 1. Viral and other respiratory tract infections that damage sur face cells; abnormal accumulations of mucus (eg, allergy), which protect pneumococci from phagocytosis; bronchial obstruction (eg, atelectasis); and respiratory tract injury caused by irritants disturbing its mucociliary function.

 2. Alcohol or drug intoxication, which depresses phagocytic activity, depresses the cough reflex, and facilitates aspiration of foreign material.

3. Abnormal circulatory dynamics (eg, pulmonary congestion and heart failure).

 4. Other mechanisms, such as malnutrition, general debility, sickle cell anemia, hyposplenism, nephrosis, or complement deficiency.

 Pathology

 Pneumococcal infection causes an outpouring of fibrinous edema fluid into the alveoli followed by red blood cells and leukocytes, which results in consolidation of portions of the lung. Many pneumococci are found throughout this exudate, and they may reach the bloodstream via the lymphatic drainage of the lungs. The alveolar walls remain normally intact during the infection. Later, mononuclear cells actively phagocytose the debris, and this liquid phase is gradually reabsorbed. The pneumococci are taken up by phagocytes and digested intracellularly.

Clinical Findings

The onset of pneumococcal pneumonia is usually sudden, with fever, chills, and sharp pleural pain. The sputum is similar to the alveolar exudate, being characteristically bloody or rusty colored. Early in the disease, when the fever is high, bacteremia is present in 10–20% of cases. With antimicrobial therapy, the illness is usually terminated promptly; if drugs are given early, the development of consolidation is interrupted.

Pneumococcal pneumonia must be differentiated from pulmonary infarction, atelectasis, neoplasm, congestive heart failure, and pneumonia caused by many other bacteria. Empyema (pus in the pleural space) is a significant complication and requires aspiration and drainage.

From the respiratory tract, pneumococci may reach other sites. The sinuses and middle ear are most frequently involved. Infection sometimes extends from the mastoid to the meninges. Bacteremia from pneumonia has a triad of severe complications: meningitis, endocarditis, and septic arthritis. With the early use of chemotherapy, acute pneumococcal endocarditis and arthritis have become rare.

Diagnostic Laboratory Tests

 Blood is drawn for culture; CSF and sputum are collected for demonstration of pneumococci by smear and culture. CSF and urine can be used to detect pneumococcal C-polysaccharide by rapid immunochromatographic membrane assays. Serum antibody tests are impractical. All specimens should be sent to the microbiology laboratory as soon as possible after collection because pneumococci tend to autolyse and delay will significantly impact recovery by culture. Sputum may be examined in several ways.

A. Stained Smears

A Gram-stained film of rusty-red sputum shows typical organisms, many polymorphonuclear neutrophils, and many red blood cells.

B. Capsule Swelling Tests

Fresh emulsified sputum mixed with antiserum causes capsule swelling (the quellung reaction) for identification of pneumococci.

C. Culture

The culture is created by inoculating sputum to blood agar and incubating the plate in CO2 at 37°C. A blood culture is also usually obtained.

 D. Nucleic Acid Amplification

Tests Several manufacturers have included S. pneumoniae on panels for identification of positive blood culture bottles and several of these assays are FDA cleared. Also, in development are panel tests for meningitis and separate molecular panels for direct detection of S. pneumoniae in respiratory samples obtained from specimens in patients suspected of having community acquired or health care-associated pneumonia.

 E. Immunity

Immunity to infection with pneumococci is type specific and depends both on antibodies to capsular polysaccharide and on intact phagocytic function. Vaccines can induce pro duction of antibodies to capsular polysaccharides.

Treatment

Over the last several decades, pneumococci have become increasingly more resistant to a broad range of antimicrobial agents. Penicillin G can no longer be considered the empiric agent of choice. Around 15% of pneumococci from nonmeningeal sources are penicillin resistant (minimum inhibitory concentration [MIC] ≥ 8 µg/mL). High-dose penicillin G appears to be effective in treating pneumonia caused by pneumococci with MICs to penicillin below 8 µg/mL (resistant breakpoint) but would not be effective in treatment of meningitis caused by the same strains. Some penicillin-resistant strains are resistant to cefotaxime. Resistance to tetracycline, erythromycin, and fluoroquinolones also occurs. Pneumococci remain susceptible to vancomycin. Because resistance profiles are not predictable, routine susceptibility testing using a method that can determine MIC values for isolates from sterile sites should be performed for all pneumococcal infections.

Epidemiology, Prevention, and Control

 Pneumococcal pneumonia accounts for about 60% of all bacterial pneumonias. In the development of illness, predisposing factors (see earlier discussion) are more important than exposure to the infectious agent, and a healthy carrier is more important in disseminating pneumococci than a sick patient.

It is possible to immunize individuals with type-specific polysaccharides. Such vaccines can probably provide 90% protection against bacteremic pneumonia. A polysaccharide vaccine containing 23 types (PPSV23) is licensed in the United States. A pneumococcal conjugate vaccine contains capsular polysaccharides conjugated to diphtheria CRM197 protein. The current conjugate vaccine is a 13-valent one (PCV13, Prevnar 13, Wyeth Pharmaceuticals). PCV13 contains the polysaccharide conjugates of the serotypes found in the PCV7 vaccine (4, 6B, 9V, 14, 18C, 19F, 23 F) plus serotypes 1, 3, 5, 6A, 7F, and 19A. It is recommended for all children as a four-dose series at 2, 4, 6, and 12–15 months of age. Children younger than 24 months of age who began their vaccination with PCV-7 and who have received one or more doses can complete the series with PCV-13. Older children and those with underlying medical conditions who were fully vaccinated with PCV-7 should receive a single dose of PCV-13.

Adults 19 years of age or older with immunocompromising conditions should receive both PPSV23 and PCV13. The schedule for vaccine administration depends upon the timing and type of prior vaccination. The reader is referred to the latest recommendations published by the Centers for Disease Control and Prevention for current guidelines and schedules (http://www.cdc.gov/vaccines/schedules/downloads/adult/ adult-combined-schedule.pdf). In 2014, in addition to the existing recommendation to receive PPSV23, persons more than 65 years of age should now also receive one dose of PCV13. See above-mentioned guidelines for complete information.

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مواضيع ذات صلة

Morphology and Identification of Enterobacteriaceae

Clinical Findings of Enterococci

Anaerobic Media

Specimen Processing of Anaerobic Bacteriology

Specimen Collection and Transport of Anaerobic Bacteriology

Gonococcal Arthritis

Nongonococcal Bacterial Arthritis

Acute Bacterial Arthritis

Streptococcus pyogenes

Treatment of Staphylococci

Enzymes and Toxins of Staphylococci

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