Streptococcus Pneumoniae (Student notes)

This post may contain affiliate links, click here to learn more.

Streptococcus pneumoniae is a gram-positive coccus. Around 5-10% of us are asymptomatic carriers who can spread it among the population. 

It is a relatively common causative agent of bacterial infection in humans. Once it is able to take hold, it can cause everything from a mild upper airway infection to severe pneumonia and meningitis.

Although antibiotics are effective, antibiotic resistance is a problem. Fortunately, we have developed vaccines which can prevent serious infections. 

This bacteria should be on the curriculum for most microbiology courses. Continue reading below to get a high yield summary of everything you need to know for the boards and wards.

Laboratory features

Streptococcus Pneumoniae are gram +, encapsulated cocci, with a lanced-shape which have a tendency to appear in pairs when viewed under the microscope. 

Because of this, they were previously known as diplococcus pneumoniae 

Like other streptococci, it is quite fastidious, but can be grown on both blood and chocolate agars. 

When growing on blood agars, the bacterium shows a-hemolysis due to the release of an a-hemolysin. 

It will damage the RBC membranes, forming a green-ish zone of a-hemolysis around the colonies.

The bacteria also possess autolytic enzymes (autolysins) which will become activated when the bacteria grows in a stationary phase (like on agar plates). 

Because of this, autolysis of “older” colonies can be observed when cultivating SP.

In the laboratory, the bacteria can be distinguished from other streptococci, like the viridans streptococci (which also show a-hemolysis) by the fact that it is unable to grow on media containing the surfactant optochin.

In addition, the cells are unable to grow on media containing bile, which enables us to differentiate it from the enterococci.

Unlike many other streptococci, Streptococcus Pneumoniae cannot be grouped according to the Lancefield grouping, due to the nature of its capsule. 

However, like streptococcus pyogenes, their capsule contains many different antigens which is the basis for the classification of the 100 different serotypes.

Another biochemical feature of this bacteria is that swelling of its capsule is observed when exposed to anticapsular antibodies containing serum, in a reaction called the quelling reaction.

Virulence factors

Streptococcus Pneumoniae possesses many different virulence factors. That includes its capsule, pili, membrane bound proteins and enzymes as well as the toxin pneumolysin.

The capsule is its most important virulence factor. Is made up of polysaccharides and protects the cell from phagocytosis.

Not all Streptococcus Pneumoniae are piliated, but those that are, are more virulent, as the pili enables the bacteria to attach to the epithelial cells of the upper respiratory tract.

Of its membrane bound proteins, Choline-binding protein a is a major virulence factor as it allows the pneumococcus to attach to carbohydrates on epithelial cells of the nasopharynx.

As we mentioned, the bacteria express autolytic enzymes. These are expressed in the cell membrane as are able to hydrolyze components of its cell wall (autolysis). 

They are normally inactive, however, they can become activated by surface active agents, antibiotics, or by being kept in a stationary phase. 

Once activated, they can start the process of autolysis, resulting in cell lysis and release of intracellular virulence factors, like pneumolysin which is normally retained within the cytosol. 

Once released it can attack mammalian cell membranes by binding to its cholesterol molecules. 

This stimulates production of pro-inflammatory cytokines, inhibits the activity of immune cells and activates the complement system, causing tissue damage.


Streptococcus Pneumoniae is an obligate parasite of humans, and can be found in the nasopharynx of about 5-10% of the population who are asymptomatic carriers. 

Because of this, Pneumococcal infections can either be endogenous or exogenous.

Endogenous infections involve the spread of the bacterium from the nasopharynx of a carrier.

This usually occurs when their immune system becomes compromised, which can be due to malnutrition, alcoholism or viral infections etc.

The exogenous infections are usually caused by spread of the bacteria through respiratory droplets from an infected individual or a carrier.

Patients with sickle cell disease or those who have had their spleen removed are particularly at risk for infection as both leave them immunocompromised.

Upper airways

Streptococcus Pneumoniae most commonly causes local infections such as upper airway infections and otitis media.

Pneumococcal Otitis media is most commonly seen in children, and presents as a painful earache.


If the immune system is unable to halt the spread of the bacteria, it can spread down the respiratory tract, resulting in acute bacterial pneumonia. 

This is the leading cause of death in individuals infected with Streptococcus Pneumoniae, especially in older adults and immunosuppressed individuals.

This type of pneumonia is usually preceded by an upper or middle respiratory viral infection. 

This predisposes the individual to a pneumococcal infection of the lung parenchyma due to increased volume and viscosity of secretions and inhibition of mucociliary clearance. 

This makes it difficult for the lungs to clear, leaving a lot of pulmonary secretion which the bacteria can colonize and thrive in.


Streptococcus Pneumoniae can also spread to the bloodstream, causing bacteremia/sepsis and meningitis.

Once it passis into the blood stream, it is known as bacteremia. Once, here, the bacteria can be deposited in the meninges and cause meningitis.

As with pneumonia, individuals who are immunocompromised are at increased risk. This also includes individuals who are functionally or anatomically asplenic.

A common patient group who is functionally asplenic are individuals with sickle cell disease.

After the development of a vaccine against haemophilus influenzae. Streptococcus Pneumoniae become the most common cause of adult bacterial meningitis. 

Pneumococcal meningiti is a severe disease with a high mortality rate that requires aggressive and prompt treatment.


Streptococcus Pneumoniae can also cause ulcus serpens corneae, an infection of the cornea of the eye. 

It typically develops due to superinfection of the cornea with the bacteria after viral infections, such as smallpox.


As with most bacterial infections, it can be treated with antibiotics. Previously, it was usually treated with penicillin, but, increased resistance due to alterations of PBP has led to other AB’s being recommended. 

Most strains remain sensitive to 3rd generation cephalosporins, macrolides, fluoroquinolones and vancomycin, which are the agents of choice for invasive infections.


Pneumococcal infections can be prevented by vaccination. 2 types of vaccines have been developed. 

The first is the pneumococcal polysaccharide vaccine (PPV). It was introduced in the US in 1983 and immunizes against 23 of the most virulent serotypes. 

It is used for immunization of high-risk individuals older than 2 years of age

The second is the pneumococcal conjugate vaccine (PCV13). This one is very effective in infants and toddlers aged 6 weeks to 5 years. 

It is made up of 13 pneumococcal AG’s conjugated to a mutant, non-toxic diphtheria toxin. 

Although recommended predominantly to children, it is also recommended for adults over 65 years of age, adults with chronic diseases such as COPD and heart failure and patients who have undergone splenectomy.