The bacterium Staphylococcus aureus, often shortened to “Staph,” is a common microorganism found on the skin and in the nasal passages of many healthy individuals. While often harmlessly colonizing the body, this organism can cause a wide range of infections, from simple skin boils to serious conditions like pneumonia or sepsis. To accurately study and identify this microbe, scientists and clinical microbiologists grow it in a controlled setting, typically on a Petri dish containing nutrient-rich agar. Observing the growth characteristics on these plates provides the first visual clues about the microbe’s identity, guiding the laboratory toward a definitive diagnosis.
The Distinct Look of Staphylococcus Aureus Colonies
When S. aureus is grown on a general-purpose medium, such as nutrient agar or blood agar, its colonies display a macroscopic morphology that aids in initial identification. After incubation, the colonies typically appear round, raised, and have a smooth, glistening surface with an entire edge. They are small to moderate in size, generally ranging from 1 to 3 millimeters in diameter.
The most notable visual feature is the characteristic pigmentation, which gives the bacterium its name (aureus means “golden” in Latin). Most strains produce colonies that range in color from pale cream or gray to a deep, opaque golden-yellow. This golden color is usually more pronounced when the culture is grown aerobically and is a strong indicator of S. aureus, although some strains may be white.
When grown on blood agar, pathogenic strains of S. aureus typically produce a clear zone of complete red blood cell destruction, known as beta-hemolysis. This clear area is caused by the release of toxins called hemolysins, which dissolve the blood cells in the medium. The combination of the smooth, golden colony and the clear surrounding zone of beta-hemolysis offers a preliminary profile for the organism.
Specialized Media for Isolation and Differentiation
Since mixed samples require a specialized approach, microbiologists utilize Mannitol Salt Agar (MSA) to isolate Staphylococcus species and differentiate S. aureus from other Staphylococci. MSA is both a selective and differential medium.
The selective property of MSA comes from its high salt concentration, typically around 7.5% sodium chloride. This high salinity inhibits the growth of most other bacteria, allowing only salt-tolerant organisms, such as the Staphylococcus group, to thrive. S. aureus colonies growing on this medium remain round and smooth, typically measuring 2 to 3 millimeters in diameter.
The differential property allows for the distinction between S. aureus and other species, such as Staphylococcus epidermidis. The medium contains the sugar alcohol mannitol and the pH indicator phenol red, which gives the uninoculated agar a light red or reddish-pink color.
S. aureus ferments the mannitol sugar, producing acidic byproducts that lower the pH of the surrounding medium. This drop in pH causes the phenol red indicator to change color, turning the agar from red to a bright yellow in the area immediately surrounding the growing colonies. Conversely, Staphylococci that cannot ferment mannitol, like S. epidermidis, do not produce acid; therefore, the medium remains red or pink, with the colonies appearing small and often colorless. The yellow color change on MSA provides a strong indication of S. aureus, but further confirmation is required.
Confirming Identity: The Pathogen Signature
The final and most definitive step in identifying S. aureus is a biochemical test for the enzyme coagulase. This test is considered the standard for distinguishing S. aureus from other Staphylococci.
S. aureus is described as being coagulase-positive, a trait that directly relates to its ability to cause infection.
The coagulase test involves mixing a suspension of the cultured bacterial colony with rabbit or human plasma. The coagulase enzyme converts the soluble protein fibrinogen into insoluble fibrin. This conversion results in the formation of a visible clot or gel in the test tube, which is interpreted as a positive result.
The formation of this fibrin clot in the host body is thought to help shield the bacteria from the host’s immune defenses. The ability to clot plasma is a defining characteristic of S. aureus and confirms its identity beyond visual appearance. This biochemical reaction separates the potentially pathogenic S. aureus from the many other coagulase-negative Staphylococcus species that commonly reside on the skin.