Microbiology
Summary
The name Staphylococcus aureus describes the appearance of this bacterium: it forms grape-like clusters (staphylo) and appears golden (aureus) on blood agar. It is a gram-positive bacterium, which means it retains the crystal violet stain in its cell wall during Gram staining. To differentiate Staph aureus from other bacteria, two primary tests can be conducted: the catalase test and the coagulase test. Staph aureus is catalase-positive and coagulase-positive. It is also a beta-hemolytic bacterium and can ferment mannitol.
Staph aureus can cause various diseases, both inflammatory and toxin-mediated. Inflammatory diseases include pneumonia, septic arthritis, and osteomyelitis, while toxin-mediated diseases involve scalded skin syndrome, toxic shock syndrome (TSS), and food poisoning. Methicillin-resistant Staph aureus (MRSA) is a strain that has acquired resistance to the antibiotic methicillin by altering its penicillin-binding proteins. Treatment for MRSA infections often involves the use of vancomycin, while nafcillin can be used for methicillin-sensitive Staph aureus infections.
Lesson Outline
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FAQs
Staphylococcus aureus, also known as Staph aureus, is a gram-positive, spherical bacterium that commonly resides on human skin and in the nasal passages. It is one of the primary pathogenic species within the Staphylococcus genus, and can cause various infections, ranging from mild skin infections to severe conditions, such as pneumonia and sepsis. Staph aureus can be distinguished from other Staphylococcus species through a positive catalase test, which indicates the production of the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen, and a positive coagulase test, which denotes its ability to produce the enzyme coagulase, leading to blood clot formation.
On mannitol salt agar, a selective and differential medium, Staph aureus colonies appear as bright yellow, due to their ability to ferment the mannitol sugar present in the medium, resulting in a drop in pH and subsequent color change. This characteristic helps to differentiate Staph aureus from other Staphylococcus species, which either grow as colorless colonies or do not grow at all since they cannot tolerate the high salt concentration in the medium.
Protein A, a cell wall component of Staph aureus, has the ability to bind to the Fc region of immunoglobulins, particularly IgG, which in turn prevents the activation of the complement system and phagocytosis, thus promoting bacterial survival within the host. Beta-hemolysis refers to the complete lysis of red blood cells, leading to the formation of a clear zone around Staph aureus colonies on blood agar. This is due to the secretion of hemolysins, toxins that destroy red blood cells and contribute to tissue damage during infection.
Scalded skin syndrome (SSS) or staphylococcal scalded skin syndrome (SSSS), and toxic shock syndrome (TSS) are serious conditions that stem from the release of exotoxins by specific strains of Staph aureus bacteria. SSSS typically impacts infants and young children, and it presents with widespread redness, blistering, and skin peeling, resembling scalded skin. On the other hand, TSS is a life-threatening ailment that can affect multiple organ systems, often linked to tampon use or surgical wounds. Both disorders result from the action of superantigens, which are exotoxins causing an exaggerated immune response leading to intense inflammation and tissue damage.
Methicillin-resistant Staph aureus (MRSA) and vancomycin-resistant Staph aureus (VRSA) refer to strains of Staph aureus that have acquired resistance to methicillin and vancomycin, respectively, which are commonly used antibiotics for treating Staphylococcus infections. These resistant strains are a major concern in healthcare settings as they limit the choice of effective antibiotics for treatment and increase morbidity and mortality rates. Both MRSA and VRSA can cause serious infections in patients, particularly those with weakened immune systems, and can spread within and between healthcare facilities, posing a significant public health threat.
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