Staphylococcus aureus is a common bacterium found on human skin and in nasal passages. While often harmless, it can cause infections if it enters the body through breaks in the skin. Laboratories often grow Staphylococcus aureus on specialized plates for study and identification. This article explores the visual characteristics of Staphylococcus aureus on laboratory media and their significance for managing bacterial infections.
Understanding Agar Plates
Agar plates are a fundamental tool in microbiology, providing a controlled environment for microorganisms to grow. Agar is a jelly-like substance derived from seaweed that solidifies at room temperature, offering a stable, non-nutritive base. When combined with specific nutrients, it becomes a culture medium that supports bacterial proliferation. A typical agar plate consists of nutrient-rich agar poured into a shallow petri dish, creating a solid surface for bacterial colonies. Growing bacteria on these plates allows scientists to isolate individual species, observe their growth patterns, and prepare them for further analysis.
How Staphylococcus aureus Appears on Agar
When grown on common laboratory media, Staphylococcus aureus exhibits distinct characteristics for preliminary identification. On a Blood Agar Plate (BAP), colonies appear round, raised, and opaque. These colonies measure between 1 to 2 millimeters in diameter after 18 to 24 hours of incubation.
On Blood Agar, a notable feature is beta-hemolysis, a clear zone around the colonies. This indicates complete lysis of red blood cells by bacterial toxins. On Nutrient Agar, Staphylococcus aureus colonies are also round, convex, smooth, and glistening, appearing 2 to 4 millimeters in diameter.
The color of Staphylococcus aureus colonies can vary, but they often develop a golden-yellow pigment, especially on rich media like tryptic soy agar or brain heart infusion agar. This is where the “aureus” (meaning golden) in its name originates. While this golden color is characteristic, some strains may appear white or off-white. Pigment production is enhanced by aerobic incubation at around 22°C.
Identifying Staphylococcus aureus in the Lab
Visual appearance on an agar plate provides an initial clue, but laboratories employ additional tests for definitive identification of Staphylococcus aureus. A Gram stain is a common first microscopic examination, revealing S. aureus as Gram-positive cocci, spherical bacteria staining purple. These cocci are arranged in irregular, grape-like clusters, a distinctive pattern resulting from cell division in multiple planes.
The catalase test differentiates Staphylococcus species from other Gram-positive cocci like Streptococcus. When a bacterial colony is mixed with hydrogen peroxide, Staphylococcus aureus produces rapid bubbling. This is due to the catalase enzyme, which breaks down hydrogen peroxide into water and oxygen.
The coagulase test is an important step for identifying Staphylococcus aureus, distinguishing it from less pathogenic Staphylococcus species, often called coagulase-negative staphylococci (CoNS). Staphylococcus aureus produces coagulase, an enzyme that clots plasma, observed as a firm clot in a test tube with rabbit plasma. This clotting ability is a virulence factor.
Mannitol Salt Agar (MSA) is another selective and differential medium used for identification. This agar contains a high concentration of salt, which inhibits most bacterial growth but allows salt-tolerant staphylococci to grow. Staphylococcus aureus can ferment mannitol, producing acid that turns the phenol red indicator in the agar from red to yellow around the colonies.
Why Identifying Staphylococcus aureus Matters
Accurate identification of Staphylococcus aureus is significant in healthcare and public health. This bacterium is a significant human pathogen, capable of causing a wide spectrum of infections. These range from localized skin and soft tissue infections like boils and abscesses to more severe conditions such as pneumonia, sepsis, and osteomyelitis. Rapid and precise identification guides appropriate antibiotic treatment, as many S. aureus strains, particularly Methicillin-Resistant Staphylococcus aureus (MRSA), are resistant to common antibiotics.
Beyond individual patient care, identifying Staphylococcus aureus is important for infection control, especially in hospital settings where transmission can be a significant concern. Knowing its presence allows for measures to prevent its spread, protecting vulnerable patients. Public health surveillance also relies on accurate identification to track the prevalence of S. aureus infections and monitor the emergence and spread of antibiotic-resistant strains, informing strategies to combat these public health threats.