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Dr Alvin Fox
Emeritus Professor
University of South Carolina School of Medicine


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Logo image Jeffrey Nelson, Rush University, Chicago, Illinois  and The MicrobeLibrary


S. pneumoniae
Bile solubility test
Optochin susceptibility
Quellung reaction
Staphylococcus aureus
Staphylococcus epidermidis
Coagulase positive or Coagulase negative
Alpha, beta, gamma and delta cytotoxins
Toxic shock syndrome
Toxic shock toxin
Protein A

strep-pneu2.jpg (30090 bytes) Figure 1a  Streptococcus pneumoniae in spinal fluid. FA stain (digitally colorized). CDC/Dr. M.S. Mitchell 



strep-pneu.jpg (34733 bytes) Figure 1B  Scanning Electron Micrograph of Streptococcus pneumoniae. CDC/Dr. Richard Facklam  rrf2@cdc.gov 


Spneumo.jpg (85081 bytes) Figure 1C Encapsulated Streptococcus pneumoniae   Gloria J. Delisle and Lewis Tomalty, Queens University, Kingston, Ontario and The MicrobeLibrary

A spneu-opt.jpg (13412 bytes) smit-opt.jpg (17246 bytes) Figure 2 
It is difficult to distinguish normal alpha streptococci found in the mouth from the pathogenic Streptococcus pneumoniae. Both are alpha-hemolytic on blood agar and so must be distinguished using the "P" disk (optochin). 
S. pneumoniae (A) is sensitive while S. mitis (B) is resistant 
Pat Johnson, Palm Beach Community College, Lake Worth Florida



Figure 3
Photomicrograph of Streptococcus pneumoniae bacteria revealing capsular swelling using the Neufeld-Quellung test. CDC




Figure 4
Impact of seven valent pneumococcal vaccine on invasive pneumococcal disease in children under 5 years of age. CDC





Pneumococcal Disease

S. pneumoniae (figure 1) is a leading cause of pneumonia in all ages (particularly the young and old), often after "damage" to the upper respiratory tract (e.g. following viral infection). It also causes middle ear infections (otitis media). The organism often spreads causing bacteremia and meningitis. S. pneumoniae is α hemolytic and there is no group antigen.

Risk factors for pneumococcal disease in children (CDC)

  • Younger than 2 years of age
  • In group child care
  • Certain illnesses (sickle cell disease, HIV infection, and chronic heart or lung conditions)
  • Cochlear implants or cerebrospinal fluid (CSF) leaks (escape of the fluid that surrounds the brain and spinal cord)

Some American Indian, Alaska Native, and African American children may also be at increased risk.

Risk factors for pneumococcal disease in adults (CDC)

  • Chronic illnesses (lung, heart, liver, or kidney disease; asthma; diabetes; or alcoholism)
  • Conditions that weaken the immune system (HIV/AIDS, cancer, or damaged/absent spleen)
  • Living in nursing homes or other long-term care facilities
  • Cochlear implants or cerebrospinal fluid (CSF) leaks (escape of the fluid that surrounds the brain and spinal cord)
  • Smoking


There are more than 90 strains of pneumococcus bacteria. Seven serotypes (6A, 6B, 9V, 14, 19A, 19F, and 23F) accounted for most drug-resistant S. pneumoniae. These serotypes are covered by the PCV7 vaccine.

Disease and Symptoms

Pneumococcal pneumonia

According to CDC, as many as 400,000 hospitalizations from pneumococcal pneumonia occur each year in the United States. Pneumococci account for about 30% of adult community-acquired pneumonia. 

Pneumococcal pneumonia is the most common serious form of pneumococcal disease and can be mild to severe in all age groups. Complications include infection of the space between pleural membranes (empyema), inflammation of the pericardium, the sac surrounding the heart (pericarditis), and blockage of the airway that allows air into the lungs (endobronchial obstruction), with lung collapse (atelectasis) and collection of pus (abscess) in the lungs.

It is fatal in about five per cent of patients with non-invasive pneumococcal pneumonia, but the rate may be higher among elderly patients.

Symptoms include (CDC):

  • Fever and chills
  • Cough
  • Rapid breathing or difficulty breathing
  • Chest pain
  • Confusion or low alertness in older patients, rather than the more common symptoms listed above

Pneumococcal meningitis

Pneumococcal infection causes 13 to 19% of all cases of bacterial meningitis in the United States. An estimated 3,000 cases of pneumococcal meningitis occur each year. This is the most severe type of invasive pneumococcal disease. Ten per cent of children younger than 5 years old with pneumococcal meningitis die. Those that survive may have long-term problems, including hearing loss or developmental delay. The chance of death increases among elderly patients.

Symptoms include (CDC):

  • Stiff neck
  • Fever and headache
  • Pain when looking into bright lights
  • Confusion
  • In babies, meningitis may cause poor eating and drinking, low alertness, and vomiting.

Pneumococcal bacteremia and sepsis

About 12,000 cases of pneumococcal bacteremia occur each year in the United States. Asplenic patients who develop bacteremia may deteriorate very rapidly.

Bacteremia occurs in up to 25 to 30% of patients with pneumococcal pneumonia. The case-fatality rate is 5 to 7% and may be higher than 60% among elderly persons. About 4% of children with pneumococcal bacteremia die of the infection. The death rate increases among elderly patients.

Symptoms include (CDC):

  • Fever
  • Chills
  • Low alertness

Pneumococcal otitis media

Pneumococci commonly cause of acute otitis media. They are found in 28 to 55% of middle ear aspirates. By age 12 months, more than 60% of children have had at least one episode of acute otitis media. The sinuses can also be infected. These infections are usually mild. Some children develop repeated ear infections and may need ear tubes. It is likely that pneumococcal ear infections account for more than 10 million visits to doctors per year in the United States.

Symptoms include (CDC):

  • Ear pain
  • Red, swollen ear drum
  • Sometimes fever and sleepiness


Direct Gram staining or detection of capsular antigen in sputum can be diagnostic. The organism grows well on sheep blood agar.


Pneumococci are identified by solubility in bile. An autolysin (peptidoglycan-degrading enzyme) is released by bile from the cell membrane and binds to a choline-containing teichoic acid attached to the peptidoglycan. The autolysin then digests the bacterial cell wall resulting in lysis of the cell. If the cells are grown in ethanolamine instead of choline, ethanolamine is incorporated into the teichoic acid. The autolysin then cannot lyse the cell wall. Understanding how the autolysin works has led to the suggestion that antibiotics (including penicillin) work together with the autolysin in killing of pneumococci in vivo.

The organisms are also identified by susceptibility to optochin (ethyl hydrocupreine) (figure 2)


This is highly prominent in virulent strains (figure 1c) and its carbohydrate antigens vary greatly in structure among strains. The capsule is anti-phagocytic and immunization is primarily against the capsule. Capsular vaccines are available for susceptible individuals; immunity is serotype-specific. Using appropriate type-specific antisera, the capsule on isolated bacteria can be "fixed" and becomes visible microscopically (the Quellung reaction) which is useful in microbial identification (figure 3).

The organism also produces pneumolysin that degrades red blood cells under anaerobic conditions (observed as alpha hemolysis).

Complement activation by teichoic acid may explain the attraction of large numbers of inflammatory cells to the focal site of infection.


S. pneumoniae is transmitted person to person by contact with saliva and mucus.


Most strains of S. pneumoniae are susceptible to penicillin. However, resistance is quite common and 15% of invasive pneumococcal isolates are resistant to penicillin in some parts of the United States.


Pneumococcal vaccines, of which there are several types, are very good at preventing severe disease, hospitalization and death. Before the vaccine, there were about 700 cases of meningitis, 13,000 blood infections, and 200 deaths from pneumococcal disease each year among children younger than 5 years old in the United States. After vaccination started, these numbers dropped dramatically. Before introduction of the first vaccine, rates of invasive pneumococcal disease among children under five were approximately 80 cases per 100,000 with 10 cases per 100,000 population being pneumococcal meningitis (figure 4).

After the introduction of the PCV7 vaccine, rates of disease due to the seven serotypes in the vaccine dropped to less than 1 case per 100,000 by 2007.

The pneumococcal conjugate vaccine (PCV13 or Prevnar 13) provides protection against the 13 serotypes responsible for most severe illness in children.


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This page last changed on Thursday, March 03, 2016
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