Laboratory diagnosis, Treatment and Prevention of Streptococcus pneumoniae


Streptococcus pneumoniae (pneumococcus) is a Gram-positive, lancet-shaped bacterium that causes various infections, such as pneumonia, meningitis, otitis media, sinusitis, and bacteremia. Laboratory diagnosis of pneumococcal infection is important for timely and appropriate treatment and prevention.

The main methods for laboratory diagnosis of Streptococcus pneumoniae are:

  • Microscopy: This involves examining the specimen under a microscope after staining with Gram stain or other special stains. Gram staining of sputum, cerebrospinal fluid (CSF), or other body fluids can show lancet-shaped Gram-positive cocci in pairs or chains . This can provide a presumptive diagnosis of pneumococcal infection, but it is not specific enough to differentiate it from other streptococci or enterococci. A more specific test is the quellung reaction, which involves mixing fresh emulsified sputum with antiserum against pneumococcal capsular polysaccharide. This causes capsule swelling and increased refractility of the bacteria under the microscope . The quellung reaction can also be used to identify the serotype of pneumococcus by using specific antisera.
  • Culture: This involves growing the bacteria on suitable media and observing their colony morphology and hemolytic pattern. Sputum, blood, CSF, or other specimens are plated on blood agar and incubated at 37°C in the presence of 5–10% carbon dioxide. After overnight incubation, gray colonies with alpha-hemolysis (partial hemolysis) are observed . These colonies can be further identified by performing biochemical tests, such as optochin sensitivity test, bile solubility test, and inulin fermentation test . Optochin sensitivity test involves placing a disk impregnated with optochin (ethylhydrocupreine dihydrochloride) on a blood agar plate inoculated with the isolate. A zone of inhibition around the disk indicates sensitivity to optochin and presumptive identification of pneumococcus . Bile solubility test involves adding a drop of bile or sodium deoxycholate to an isolated colony and observing for lysis. Pneumococci are lysed rapidly by bile due to the presence of an autolytic enzyme that breaks down their cell wall . Inulin fermentation test involves inoculating the isolate into a broth containing inulin (a polysaccharide) and observing for acid production. Pneumococci can ferment inulin and produce acid, unlike other streptococci .
  • Animal inoculation: This involves injecting the specimen into mice and observing for signs of infection and death. Sputum or blood specimens containing few pneumococci can be isolated by intraperitoneal inoculation in mice. Pneumococci can be demonstrated in the peritoneal exudate and heart blood of the mice, which die 1–3 days after inoculation . This method is rarely used nowadays due to ethical and practical issues.
  • Antigen detection: This involves detecting pneumococcal antigens in body fluids using immunological methods. Pneumococcal C polysaccharide is excreted in urine and can be detected using a commercially available immunoassay . This test can be useful for diagnosing pneumococcal pneumonia in adults, especially when sputum culture is negative or unavailable. The counterimmunoelectrophoresis (CIEP) is another test that can detect pneumococcal capsular polysaccharide antigen in CSF, blood, or urine for diagnosis of meningitis, bacteremia, or pneumonia . Latex agglutination test using latex particles coated with anti-CRP antibody can detect C-reactive protein (CRP), which is an acute phase protein produced by the liver in response to inflammation. CRP can be used as a prognostic marker in acute cases of pneumococcal pneumonia and other infectious diseases.
  • Antibody detection: This involves measuring specific antibodies against pneumococcus in serum using serological methods. The indirect hemagglutination, indirect fluorescent antibody test, and enzyme-linked immunosorbent assay (ELISA) are some of the methods that can demonstrate specific pneumococcal antibodies in invasive pneumococcal diseases . These tests can be useful for epidemiological studies and evaluation of vaccine efficacy.
  • Nucleic acid-based tests: These involve amplifying and detecting pneumococcal DNA or RNA in specimens using molecular methods. Nucleic acid probes and polymerase chain reaction (PCR) assays are some of the methods that can identify S. pneumoniae isolates in culture or directly from clinical specimens . These tests are more sensitive and specific than conventional methods and can also provide information on serotype, genotype, and antibiotic resistance.

These are some of the main methods for laboratory diagnosis of Streptococcus pneumoniae infection. Depending on the type and source of specimen, one or more methods may be used to confirm the diagnosis and guide the treatment.