Staphylococcus and Micrococcus Flashcards
In taxonomy, the term "aureus" is derived from Latin and means "golden."
Briefly describe important characteristics of the Staphylococcus genus.
How are Staphylococci clinically grouped?
Staphylococci are clinically grouped into two categories:
(i) Coagulase positive Staphylococci
(ii) Coagulase negative Staphylococci
Name one coagulase positive species of Staphylococcus.
Staphylococcus aureus
Name two coagulase negative Staphylococcus species.
S. epidermidis and S. saprophyticus
How is S. aureus transmitted from person to person?
direct contact, fomites
List pyrogenic exotoxins produced by S. aureus.
◾ Enterotoxins (A-F)
◾ Toxic shock syndrome toxin-1 (TSST-1)
◾ Epidermolytic toxins A and B
Briefly discuss how the following contribute to pathogenicity of S. aureus.
(a) Protein A
(b) Teichoic acids
(c) Capsule
(a) Protein A: binds to the Fc region of immunoglobulin G (IgG) antibodies. It thus prevents opsonization and phagocytosis.
(b) Teichoic acids: Teichoic acids can trigger an immune response in the host, leading to inflammation. They are also capable of producing septic shock, contributing to the severity of infections caused by S. aureus.
(c) Capsule: it is antiphagocytic
Briefly discuss 7 enzymes that facilitate invasiveness of S. aureus.
(a) coagulase: converts fibrinogen to fibrin, leading to clot formation. The clots enable S. aureus to evade the host immune system by creating a protective barrier around the bacteria, preventing phagocytosis and immune detection.
(b) haemolysins: these lyse red blood cells. The lysis of red blood cells results in release of nutrients such as iron, which are essential for bacterial growth.
(c) leukocidin: targets and destroys white blood cells.
(d) fibrinolysin [aka. staphylokinase]: dissolves fibrin clots. This allows S. aureus to spread from the initial site of infection to other parts of the host.
(e) lipase: breaks down lipids. This enables S. aureus to colonize and invade fatty tissues such as the skin and sebaceous glands.
(f) hyaluronidase: breaks down hyaluronic acid, a component of the extracellular matrix. This facilitates the spread of S. aureus through connective tissues.
(g) β-Lactamase: breaks down β-lactam antibiotics, such as penicillin.
Explain how the following contribute to the pathogenicity of staphylococcus aureus.
(a) biofilms
(b) small-colony variants
(c) chemotaxis inhibitory protein
(a) biofilms: these are structured communities of bacteria encased in a self produced extracellular matrix. They protect S. aureus from host defenses and antimicrobial agents, making infections difficult to treat.
(b) small-colony variants: these are a subpopulation of bacteria with a slow-growing phenotype. SCVs are associated with persistent and recurrent infections. They can survive in hostile environments, such as within host cells, and are often less susceptible to antibiotics, contributing to the chronic nature of some S. aureus infections.
(c) chemotaxis inhibitory protein: inhibits the chemotaxis of immune cells, such as neutrophils, to the site of infection
Outline clinical features of Staphylococcal infection.
(1) Pyogenic diseases
Local lesions of skin: impetigo, furuncles (boils) and carbuncles (boils clustered together), folliculitis, eyelid infection (styes)
Deep infections: septicemia, endocarditis, osteomyelitis, arthritis, post-surgical wound infections
(2) Toxin-mediated diseases
(a) Gastroenteritis
Due to ingestion of preformed toxin in food.
Patients present with vomiting and water non-bloody diarrhea which resolves within 24 hours.
(b) Toxic shock syndrome
Mediated by TSST-1.
Characterised by fever, erythematous skin rash, desquamation of the palms of the hands and the soles of the feet and shock.
(c) Staphylococcal scalded skin syndrome (SSSS)
Mediated by exfoliative or epidermolytic toxin-characterized by fever, and large erythematous rash that sloughs off the body.
What specimens are used in the laboratory testing of S. aureus infection?
Pus and swabs from infected sites, sputum, blood, nasal swabs from carriers
What is the clonal morphology when S. aureus is cultured on:
(a) blood agar
(b) mannitol salt agar
Explain what would be observed if MacConkay agar is inoculated with S. aureus.
Colonies of S. aureus would not be observed.
Reason: MacConkey agar is selective for Gram-negative bacteria and inhibits the growth of Gram-positive bacteria
What are the appropriate growth conditions when culturing S. aureus?
Temperature: 37°C
Duration: 12 - 18 hours
Incubate in air.
What is one advantage of using mannitol salt agar over blood agar in the culturing of S. aureus?
Mannitol salt agar contains a high concentration of salt (7.5% NaCl), which inhibits the growth of most bacteria except for staphylocci. This makes it highly selective for S. aureus.
Blood agar supports the growth of a wide range of bacteria, making it less selective.
