Staphylococcus aureus is a bacterium commonly found on the skin and in the nasal passages of approximately 30% of the human population. This organism often exists as a harmless colonizer, forming part of the normal human microbiota without causing symptoms. However, S. aureus is also an opportunistic pathogen responsible for a wide range of infections, from minor skin abscesses to life-threatening conditions like pneumonia and sepsis. Identifying this specific species is foundational in clinical and research settings, as its presence often dictates the need for treatment, particularly due to the rise of antibiotic-resistant strains. Accurate identification relies on a precise set of observations, starting with how the organism appears to the naked eye on laboratory media.
Macroscopic Identification (Colony Morphology)
When grown on common laboratory media, such as a Blood Agar Plate (BAP), S. aureus produces distinct colonies that allow for presumptive identification. After incubation for 18 to 24 hours, the colonies are typically medium to large, exhibiting a smooth, slightly raised, and glistening surface with an entire, well-defined border.
A defining visual characteristic is the pigmentation, which gives the species its name; aureus is Latin for “golden.” Most strains produce a characteristic golden-yellow or deep cream color due to the production of the carotenoid pigment staphyloxanthin. This pigment contributes to the bacterium’s ability to survive by neutralizing reactive oxygen species produced by the host immune system.
The most telling macroscopic feature on a BAP is the presence of beta-hemolysis, which is the complete lysis, or clearing, of the red blood cells surrounding the colony. This appears as a transparent zone, indicating the organism secretes hemolysins that destroy red blood cells. While not all strains are strongly hemolytic, the combination of golden pigmentation and complete hemolysis provides a strong preliminary indication of S. aureus.
Microscopic Structure and Cellular Arrangement
Once a colony is selected from a culture plate, the next step in identification involves viewing the bacteria under a microscope after a Gram stain procedure. Staphylococcus aureus is classified as a Gram-positive organism, meaning it retains the primary crystal violet dye due to its thick peptidoglycan cell wall. As a result, the individual cells appear a deep violet or purple color under the microscope.
The shape of the individual cells is spherical, which classifies them as cocci, and they typically measure about 0.5 to 1.5 micrometers in diameter. Their arrangement on a stained slide is highly characteristic and is the source of the genus name, Staphylococcus. The name is derived from the Greek word staphyle, meaning “bunch of grapes,” which accurately describes the irregular, non-uniform clusters formed by the dividing cells.
This grape-like clustering distinguishes Staphylococci from other spherical bacteria, such as Streptococci, which tend to form chains. The non-motile nature of the organism, lacking flagella, is another physical trait observed under the microscope. Observing these specific morphological and staining characteristics provides a foundational classification, leading to the definitive biochemical tests.
Defining Characteristics (Biochemical Testing)
Biochemical testing is required to differentiate S. aureus from other Staphylococcus species and related organisms. The initial test often performed is the Catalase test, which differentiates the genus Staphylococcus from Streptococcus and Enterococcus. S. aureus produces the enzyme catalase, which rapidly breaks down hydrogen peroxide into water and oxygen gas. A positive result is indicated by the immediate and vigorous formation of bubbles when hydrogen peroxide is added to a colony.
The most definitive characteristic for identifying S. aureus is the presence of the enzyme coagulase. This test is fundamental because it separates pathogenic, coagulase-positive S. aureus from coagulase-negative staphylococci (CoNS). Coagulase functions as a virulence factor by converting the host’s plasma protein fibrinogen into insoluble fibrin, causing the formation of a protective fibrin clot.
The coagulase test is performed in two main ways. The slide test detects bound coagulase, also called clumping factor, which remains attached to the bacterial cell wall and causes cell clumping when mixed with plasma. The tube test detects free coagulase, an extracellular enzyme that clots the plasma in a test tube, which is considered the gold standard for full confirmation of S. aureus. This clotting action allows the bacteria to wall itself off from immune defenses, enabling the formation of abscesses and persistent infections.
Beyond coagulase, the bacterium expresses Protein A, a cell-surface protein that contributes significantly to its ability to evade the immune system. Protein A binds to the Fc region of immunoglobulin G (IgG) antibodies, effectively coating the bacterial cell. This immunological disguise prevents the antibodies from functioning correctly and interferes with opsonization, a process where immune cells recognize and engulf the bacteria. The detection of Protein A, along with clumping factor, is often utilized in rapid latex agglutination tests for clinical identification.