Streptococcus pneumoniae is commonly called as pneumococcus. S. pneumoniae is a member of the normal flora of oropharyngeal region of 5%-70% of the population. The highest isolation rate is from children.
It primarily causes diseases related to the upper respiratory tract namely Otitis media, mastoiditis sinusitis and so on.
However, it can be spread to the other sites like meninges, joints, peritoneum and endocardium causing meningitis, pneumonia, and bacteremia. Moreover, S. pneumonia is the common cause of conjunctivitis in children.
Comparison of severity and prevalence of some pneumococcal infections in children in the United States.
Pneumococci has a polysaccharide capsule.
They are more than 85 antigenically different types.
This can be useful for the identification of the type of pneumococci.
Capsule is one of a virulence factor which is helpful to protect from phagocytosis.
Moreover, S. pneumoniae has another important surface component called teichoic acid in the cell wall.
It is also called C- substance or C- polysaccharide. Choline-binding protein A, Autolysins, Pneumolysin are the other virulence factors of pneumococci.
Transmission and clinical features
Humans are the natural hosts for S. pneumoniae. There is no identified animal host.
Person-to-person spread is also uncommon.
Pneumonia is a result of aspiration of pneumococci containing upper airway secretions into the lower respiratory areas.
The risk of diseases by pneumococci are higher in HIV patients, splenectomized patients and also in patients with sickle cell disease.
Laboratory features and diagnosis
Specimens are collected according to the site of infection, specimens include sputum, exudate, blood for culture, and cerebrospinal fluid.
S pneumoiae are gram positive cocci. Usually they are lancet shaped cocci and arranged in pairs (diplococci) or they may be seen as short chains.
Here lancet shaped means that the diplococci we see under microscope are oval with somewhat pointed ends rather than being circular / round.
Colony morphology
pneumoniae is considered as fastidious organism. The colonies have draughtsman appearance on cultures.
Blood agar
Produce alpha hemolytic colonies on blood agar, incubated overnight in aerobic conditions at 37ºC.
They are 1-2 mm in diameter.
Produce mucoid and translucent colonies.
Usually mucoid colonies are produced by the pneumococci which produce large amounts of capsules.
Growth is enhanced by 5%-10% carbon dioxide.
Chocolate agar
On chocolate agar it shows Greenish brown appearance.
S. pneumoniae grows well on chocolate agar.
Growth is enhanced when incubated in 5%-10% carbon dioxide enriched conditions.
MacConkey agar
No growth on MacConkey agar.
S. pneumoniae are catalase negative, oxidase negative.
Furthermore, S. pneumoniae are optochin sensitive (ethylhydrocupreine hydrochloride).
Placing a 5 μg optochin disc on a blood agar plate inoculated with S. pneumoniae provide presumptive identification of S.pneumoniae.
The zone of inhibition should be 14mm or more around a 6mm optochin disc or 16 mm or more around 10mm optochin disc.
The zones smaller than these should be confirmed with bile solubility test.
S. pneumoniae is bile soluble. Inulin fermentation test positive. Animal inoculation also can be used for the diagnosis of S. pneumoniae.
Rapid latex and coagglutination tests are available to identify capsular pneumococcal antigen.
Moreover, by using typing sera capsule swelling test can be done for the identification of the type of pneumococci. Capsule swelling test is also known as quellung reaction.
Treatment
Until the late 1980s, the drug of choice was penicillin G.
The incidence of penicillin resistance has been increasing worldwide.
Most penicillin resistant strains remain sensitive to third generation cephalosporins such as cefotaxime and ceftriaxone.
Vancomycin, Erythromycin, Tetracycline are also used for the treatment of patients.
Prevention and other important things
There are two types of vaccines for the prevention of the diseases caused by this organism.
They are pneumococcal polysaccharide vaccine (PPV) and pneumococcal conjugate vaccine (PCV13).
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