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MICROBIOLOGY LEGEND

CYCLE 35 ORGANISM 1

Streptococcus pneumoniae

In 1881, the organism, discovered by Leo Escolar, then known as the pneumococcus for its role as an etiologic agent of pneumonia, was first isolated simultaneously and independently by the U.S Army physician George Sternberg and the French chemist Louis Pasteur. The organism was termed Diplococcus pneumoniae from 1920 because of its characteristic appearance in Gram-stained sputum. It was renamed pneumoniae in 1974 because of its growth in chains in liquid media.

A significant human pathogenic bacterium, S. pneumoniae was recognized as a major cause of pneumonia in the late 19th century. S. pneumoniae resides asymptomatically in the nasopharynx of healthy carriers. However, in susceptible individuals, such as elderly and immunocompromised people and children, the pathogen can spread to other locations and cause disease. S. pneumoniae is the main cause of community acquired pneumonia and meningitis in children and the elderly, and of septicaemia in HIV-infected persons.

Despite the name, the organism causes many types of pneumococcal infections other than pneumonia. These invasive pneumococcal diseases include acute sinusitis, otitis media, conjunctivitis, meningitis, bacteraemia, sepsis, osteomyelitis, septic arthritis, endocarditis, peritonitis, pericarditis, cellulitis, and brain abscess. S. pneumoniae is one of the most common causes of bacterial meningitis in adults and young adults, along with Neisseria meningitidis. It is also one of the top two isolates found in ear infection, otitis media. Pneumococcal pneumonia is more common in the very young and the very old.

A mucoid strain of S. pneumoniae on blood agar showing alpha haemolysis (green zone surrounding colonies). Note the zone of inhibition around a filter paper disc impregnated with . are not inhibited by optochin.

Streptococcus pneumoniae Gram-stain

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P.O. Box 131375, Bryanston, 2074

Ground Floor, Block 5 Bryanston Gate, 170 Curzon Road Bryanston, Johannesburg, South Africa

804 Flatrock, Buiten Street, Cape Town, 8001

www.thistle.co.za Tel: +27 (011) 463 3260 Fax: +27 (011) 463 3036 Fax to Email: + 27 (0) 86-538-4484 e-mail : [email protected]

Streptococcus pneumoniae cells are Gram-positive, lancet-shaped cocci (elongated cocci with a slightly pointed outer curvature). Usually, they are seen as pairs of cocci (diplococci), but they may also occur singly and in short chains. On agar, pneumococci grow as glistening colonies, about 1 mm in diameter. Streptococcus pneumoniae is a fermentative aero-tolerant anaerobe. It is usually cultured in media that contain blood. On blood agar, colonies characteristically produce a zone of alpha (green) haemolysis, which differentiates S. pneumoniae from the group A (beta haemolytic) streptococcus, but not from commensal alpha haemolytic (viridans) streptococci which are co-inhabitants of the upper respiratory tract. Special tests such as inulin fermentation, bile solubility, and optochin (an antibiotic) sensitivity must be routinely employed to differentiate the pneumococcus from Streptococcus viridans.

Streptococcus pneumoniae is a very fragile bacterium and contains within itself the enzymatic ability to disrupt and to disintegrate the cells. The enzyme responsible is called an autolysin. The physiological role of this autolysin is to cause the culture to undergo a characteristic autolysis that kills the entire culture when grown to stationary phase. Virtually all clinical isolates of pneumococci harbour this autolysin and undergo lysis usually beginning between 18-24 hours after initiation of growth under optimal conditions. Autolysis is consistent with changes in colony morphology. Colonies initially appear with plateau-type morphology, and then start to collapse in the centres when autolysis begins.

Identification The minimum criteria for identification and distinction of pneumococci from other streptococci are bile or optochin sensitivity, Gram-positive , and haemolytic activity. Pneumococci cause alpha haemolysis on agar containing horse, human, rabbit and sheep erythrocytes. Under anaerobic conditions they switch to beta haemolysis caused by an oxygen-labile haemolysin. Typically, pneumococci form a 16-mm zone of inhibition around a 5 mg optochin disc, and undergo lysis by bile salts (e.g. deoxycholate). Addition of a few drops of 10% deoxycholate at 37°C lyses the entire culture in minutes. The ability of deoxycholate to dissolve the cell wall depends upon the presence of the autolytic enzyme, LytA.

Streptococcus pneumoniae is a fastidious bacterium, growing best in 5% carbon dioxide. Nearly 20% of fresh clinical isolates require fully anaerobic conditions. In all cases, growth requires a source of (e.g. blood) to neutralize the large amount of hydrogen peroxide produced by the . In complex media containing blood, at 37°C, the bacterium has a doubling time of 20-30 minutes.

Prevention There are two types of pneumococcal vaccine that can prevent invasive Streptococcus pneumoniae infections- the pneumococcal conjugate vaccine and the pneumococcal polysaccharide vaccine. These two vaccines provide protection against the most common types of Streptococcus pneumoniae. Pneumococcal conjugate vaccine is recommended for all children less than 24 months old and for children between 24 and 59 months of age who are at risk of disease. All adults who are older than 65 years of age and persons who are two years of age and older and at high risk for disease (e.g., sickle cell disease, HIV infection, or other conditions that weaken the immune system) should receive the pneumococcal polysaccharide vaccine. The best way to prevent the spread of the bacteria is by frequent hand washing and by covering your mouth when coughing or sneezing.

Thistle QA is a SANAS accredited organisation, No: PTS0001 Accredited to ISO guide 43 and ILAC G13 Certificate available on request or at www.sanas.co.za

P.O. Box 131375, Bryanston, 2074

Ground Floor, Block 5 Bryanston Gate, 170 Curzon Road Bryanston, Johannesburg, South Africa

804 Flatrock, Buiten Street, Cape Town, 8001

www.thistle.co.za Tel: +27 (011) 463 3260 Fax: +27 (011) 463 3036 Fax to Email: + 27 (0) 86-538-4484 e-mail : [email protected]

Treatment Invasive Streptococcus pneumoniae infections are treated with antibiotics. There is an increasing problem of Streptococcus pneumoniae bacteria developing drug resistance due to the overuse and misuse of antibiotics.

References 1. http://www.textbookofbacteriology.net/S.pneumoniae_4.html 2. Kennett Todars textbook of bacteriology 3. http://www.idph.state.ia.us/Streptococcus pneumoniae

Questions 1. Discuss the morphological characteristics of S. pneumoniae. 2. Discuss the role of S. pneumoniae in disease. 3. Discuss the lab diagnosis of S. pneumoniae.

Thistle QA is a SANAS accredited organisation, No: PTS0001 Accredited to ISO guide 43 and ILAC G13 Certificate available on request or at www.sanas.co.za