Gram-Positive Cocci (Student Notes)

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

There are a wide variety of gram-positive cocci in nature. Common to all is that they are seen as small, round, blue/purple stained bacteria under the microscope when stained with gram staining.

In the medical field, 3 families of gram-positive cocci are of particular interest due to their ability to cause diseases in humans. These are the streptococcaceae, staphylococcacea and enterococcacea families.

Let’s review the individual families for what you need to know for your average medical microbiology course.

The staphylococcacea family

The staphylococcaceae are a bacterial family of great medical importance.

This is largely due to it being the family of the bacteria staphylococcus genus which includes staphylococcus aureus. The causative agent of many common bacterial infections in humans.

A distinctive feature of the staphylococci is their ability to perform the catalase reaction due to their expression of the catalase enzyme.

This enables us to separate them from other gram-positive cocci in the lab with a catalase test. This is a chemical reaction performed by the catalase enzyme that allows the bacteria to decompose hydrogen peroxide (H2O2) when applied.

The reaction that liberates O2 and heat, which can be seen as air bubbles at the site of the reaction.

As mentioned, Staphylococcus aureus is the most important member of this family. However, other important memebers include:

Staphylococcacea family
Staphylococcus Aureus
Staphylococcus Epidermidis
Staphylococcus Haemolyticus
Staphylococcus Saprophyticus
Staphylococcus Hominis

With the exception of Staphylococcus Aureus. The other members of the family are most often non-pathogenic members of the bacterial flora in the skin and mucosal membranes.

Staphylococcus Aureus is more virulent (pathogenic). And while it might also colonize the skin and mucosa without causing infections (carrier state), it is not considered part of the normal flora.

In addition to being more virulent. Staphylococcus aureus separates itself from the others by being coagulase-positive.

This is a biochemical feature due to its expression of coagulase enzymes. This enables them to produce an agglutination  (clumping) effect when exposed to certain reactants such as fibrinogen.

When exposed to a fibrinogen-containing solution. The coagulase enzyme will convert soluble fibrinogen to fibrin, which is insoluble, creating a clumping effect known as agglutination.

Because of this, the staphylococci that do not express coagulase enzymes are referred to as coagulase-negative staphylococci

The Streptococcaceae family

The streptococcaceae are another bacterial family of great importance. It is home to the streptococcus genus, which has several pathogenic bacteria.

Most have heard about strep throat. This is because it is caused by streptococcus bacteria, more precisely streptococcus pyogenes.

They can easily be distinguished from staphylococcus species by being catalase-negative. Also, they tend to prefer more nutrient-rich environments and are not as easily cultivated.

That being said, they do like blood agar, which is a nutrient agar containing 5-10% mammalian blood. When growing on these they can show different patterns of hemolysis (red blood cell destruction). This forms the basis of their categorization based on their hemolytic properties.

The first is alpha hemolysis where the bacteria cause oxidization of iron in hemoglobin molecules within red blood cells. The result is a greenish color on the blood agar around the bacterial colonies.

The second type is beta-hemolysis which causes complete rupture of red blood cells in the agar. This results in clear areas around the bacterial colonies.

The third type is gamma-hemolysis, wich is actually no hemolysis.

Streptococcus pnuemoniaeStreptococcus pyogenesStreptococcus bovis
Viridans streptococcus (various)Streptococcus agalactiae Enterococci (formerly)

As you can see, Enterococci is listed under gamma hemolytic streptococci. The fact is that it used to be classified as a member of the Streptococcus genus. But is now considered its own family of enterococcaceae.

The Enterococcaceae family

Formerly a member of the streptococcaceae, the enterococci now belong to their own bacterial family. Their morphology is quite similar to that of Streptococcus.

Both are gram + cocci, however, but the enterococci are slightly elongated (oval) and are aggregated in shorter chains when viewed under the microscope.

As the name implies, the enterococci are members of the normal gastrointestinal flora. The clinically most important species are E. faecalis and E. faecium.

They are categorized as facultative pathogens as they do not cause any harm if contained within the gut lumen. However, if they are able to penetrate through the gut wall or are displaced to other mucosal membranes they can cause severe infections.

The enterococci can penetrate the gut wall through lesions/ulcers or due to decreased immune presence and displacement is mostly due to fecal contamination due to inadequate hygiene.

Once outside the gut, they can cause infections such as UTIs, cystitis, peritonitis, and bacteremia, which in turn can result in bacterial meningitis and endocarditis.

These infections can become very severe as the enterococci are naturally resistant to cephalosporin and sulphonamide antibiotics.

This makes treating enterococci infections harder, and treatment often requires the administration of a combination of different Antibiotics.

Because of this, enterococci have become a common agent in hospital-acquired infections (nosocomial infections).