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Streptococcus pneumoniae (or pneumococcus) is a Gram-positive
coccus seen in pairs, which is typically a-haemolytic, but can be
Streptococcus pneumoniae has a polysaccharide capsule that
protects it from phagocytosis. There are over 90 highly
antigenic capsular serotypes and antibodies to specific types are
Pathogenicity features include:
- pro-inflammatory cell wall components (e.g. C-polysaccharide,
- IgA2 protease;
- pneumolysin, a cytotoxin that stimulates immune responses;
- adhesins that bind to cell surface carbohydrates (e.g. choline
binding protein A, pneumococcal surface protein A [PspA]);
- tissue damaging enzymes (e.g. neuraminidase, hyaluronidase).
Humans are the only host of S. pneumoniae. Carriage, which is
usually asymptomatic, is most common in the young or smokers
and is associated with overcrowding. Serotypes vary with country,
time and subject group.
Children under 1 year of age are vulnerable to acute pneumonia.
Complement deficiency, agammaglobulinaemia, HIV infection,
smoking, alcoholism and splenectomy predispose to severe infection.
The bacteria are able to adhere to pneumocytes and invade
the bloodstream by hijacking the platelet-aggregating factor receptor
pathway and produce complement-mediated damage to the
alveolus through the action of pneumolysin.
Antibiotic susceptibility and treatment
- Acute otitis media, sinusitis and acute pneumonia are the most
- Pneumococci cause between 50 and 75% of cases of communityacquired
pneumonia, up to 25-30% of which may develop bacteraemia.
- Bacteraemia is an important complication with a high mortality,
despite treatment (see Pyrexia of unknown origin and septicaemia
- Direct or haematogenous spread can give rise to meningitis,
which has a high mortality and is associated with brain damage.
This is now the commonest cause of meningitis in adults over 40
and the second commonest cause in children from populations
that have been vaccinated against Haemophilus influenzae type b
- Pneumococcus rarely causes cellulitis, abscesses, peritonitis and
- The mortality and incidence of sequelae are high.
Once universally susceptible to penicillin, significant numbers of
S. pneumoniae have developed resistance through a genetically
modified penicillin-binding protein gene (see Resistance to antibacterial agents
), and penicillin-
resistant clones have spread internationally. S. pneumoniae
is also susceptible to erythromycin, cephalosporins, tetracycline,
rifampicin and chloramphenicol, but multiple drug resistance is
growing. Penicillin is the treatment of choice for respiratory infection
but third generation cephalosporins are used for meningitis if
it is caused by less sensitive strains. Where high-level penicillin
resistance occurs, a glycopeptide (usually vancomycin) should be
Prevention and control
A conjugate vaccine incorporating up to 13 capsular serotypes has
been introduced and is highly immunogenic in young children.
Whilst this has led to a decline in invasive pneumococcal disease
in children and adults, there is some evidence that serotypes not
included in the vaccine are increasing.
There are a wide range of streptococci found in the oropharynx,
which can occasionally cause disease. Some of these species are
closely related to S. pneumoniae.
The a-haemolytic streptococci (S. oralis, S. sanguis, S. mutans and
S. salivarius) cause 40-60% of community-acquired native-valve
endocarditis. Infection may be of dental origin; while good evidence
is lacking, prophylaxis is recommended for at-risk patients
undergoing bacteraemia-inducing dental procedures such as
extraction or deep scaling (see Endocarditis, myocarditis and pericarditis
). Streptococcus bovis
bacteraemia and endocarditis is associated with underlying bowel
malignancy. Occasionally endocarditis is caused by nutritionally
deficient (pyridoxine-dependent) streptococci that can be missed
The 'Streptococcus milleri' group of organisms (S. anginosus, S.
intermedius and S. constellatus) colonize the mouth and gut. They
are sometime responsible for metastatic infection, causing brain,
lung or liver abscesses often as part of a mixed infection with
obligate anaerobes. Isolation of a member of the 'S. milleri' group
should prompt a thorough search for an occult abscess.
Other gram-positive cocci
A number of other Gram-positive cocci such as Leuconostoc and
Pediococcus are occasionally associated with infections, particularly
in immunocompromised individuals.
Alloiococcus otitidis is a slow-growing Gram-positive coccus that
produces lactic acid and has been associated with chronic otitis
media with effusion in children, particularly in the chronic phase
although its pathogenicity is not certain.
This organism is genetically closely related to S. pneumoniae but
does not have a capsule and may lack some of the common pathogenicity
determinants. It has been associated with isolation from
patients with chronic obstructive pulmonary disease.