Staphylococcus aureus is a bacterium commonly found on the skin and in the nasal passages of healthy individuals, yet it remains a leading cause of severe infections worldwide. This microbe can cause a range of clinical issues, from minor skin infections to life-threatening conditions like pneumonia and sepsis. In the microbiology laboratory, identifying this pathogen begins with observing its growth characteristics on solid culture media. The distinct way S. aureus colonies appear on the agar plate helps laboratory professionals quickly differentiate it from less harmful bacteria.
Culturing S. aureus: Selecting the Right Media
Microbiologists use a combination of media types to isolate and provisionally identify S. aureus from a clinical sample. The Blood Agar Plate (BAP) is a foundational, general-purpose medium that supports the growth of staphylococci. BAP is also differential because it contains sheep blood, allowing for the observation of the bacteria’s ability to lyse red blood cells.
Selective media are employed to isolate staphylococci while inhibiting the growth of most other bacteria. Mannitol Salt Agar (MSA) is a prime example, incorporating a high concentration of salt (typically 7.5% sodium chloride) that only salt-tolerant organisms can endure. S. aureus is halotolerant and grows well on MSA, which simultaneously tests its ability to ferment the sugar mannitol. If fermentation occurs, the medium changes color from red to yellow, providing a differential marker.
Colony Morphology and Pigmentation
The physical appearance of S. aureus colonies on a non-selective medium like Blood Agar provides specific morphological indicators. Individual colonies are usually medium to large, reaching 1 to 4 millimeters in diameter after 18 to 24 hours of incubation. Their shape is typically round and convex, possessing a dome-like or slightly raised profile.
The colony surface generally appears smooth, moist, and glistening, often described as having a “buttery” consistency. A distinguishing feature is the production of a characteristic golden-yellow or orange pigment. This coloration is the origin of the species name aureus, the Latin word for “golden.”
The golden color is caused by the carotenoid pigment staphyloxanthin. This pigment serves a protective function for the bacterium, acting as an antioxidant that helps the microbe survive the reactive oxygen species produced by the host’s immune cells.
Hemolysis: The Key Reaction on Blood Agar
The most visually striking reaction of S. aureus occurs on the Blood Agar Plate, where the bacterium demonstrates its ability to break down red blood cells, a process termed hemolysis. This lytic activity is mediated by secreted protein toxins, known as hemolysins, which act as virulence factors. Hemolysis is typically categorized into three types based on the extent of red blood cell destruction.
S. aureus is most frequently characterized by beta-hemolysis, which represents the complete lysis of red blood cells in the surrounding agar. This results in a clear, transparent zone or halo directly around the colony where the agar has lost its opaque red color. This complete clearing indicates the potent action of the bacterial toxins.
Multiple toxins contribute to this effect, including alpha-hemolysin, a pore-forming toxin that creates holes in the red blood cell membrane. The presence of a clear zone of beta-hemolysis is a significant marker for presumptive identification in the clinical laboratory setting.
Initial Differentiation from Other Staphylococci
The combined visual cues of pigmentation and hemolysis allow microbiologists to make a strong initial differentiation of S. aureus from other related staphylococcal species. For instance, Coagulase-Negative Staphylococci (CoNS), such as Staphylococcus epidermidis, present a distinct contrast. S. epidermidis colonies typically appear porcelain-white and are generally non-hemolytic, exhibiting either no hemolysis (gamma-hemolysis) or, rarely, weak hemolysis.
The presence of golden pigmentation and a clear zone of beta-hemolysis points strongly toward S. aureus, distinguishing it from most other staphylococci. This visual assessment serves as a crucial screening step, but definitive identification still requires biochemical confirmation.
The gold standard for confirmation is the coagulase test, which determines the bacterium’s ability to clot plasma. S. aureus is coagulase-positive, a trait correlated with its pathogenic potential and distinguishing it from coagulase-negative species that lack this enzyme.