Gram-positive cocci - Part 1: Staphylococcus aureus

Flashcards covering the classification of Gram-positive cocci and the identity, structure, enzymes, and laboratory identification of Staphylococcus aureus.

Coccus

A round or spherical bacterium (plural: cocci).

Gram-positive

Bacteria that retain the purple Gram dye because they possess a thick peptidoglycan mesh wall that traps the dye.

Normal flora

The harmless microbes that live on the human skin and body linings all the time.

Catalase

An enzyme that breaks hydrogen peroxide into water and oxygen ($2 H_2O_2 \rightarrow 2 H_2O + O_2$); the test separates Staphylococcus (positive) from Streptococcus and Enterococcus (negative).

Coagulase

An enzyme that clots blood plasma; it is used to distinguish Staphylococcus aureus (positive) from other skin staphylococci (negative).

Free coagulase

A form of coagulase released into surroundings that joins with a plasma factor to form staphylothrombin, converting fibrinogen to fibrin; detected by the tube test within $1$ to $4$ hours.

Bound coagulase (clumping factor)

A form of coagulase localized on the cell surface that grabs fibrinogen directly to stick cells together; detected by the slide test within seconds.

Staphylococci arrangement

Irregular grape-like clusters formed because cells divide in several random planes.

Streptococci arrangement

Chains formed because cells divide in one plane and stay stuck together in a row.

Capsule

A slippery outer coat of sugars (polysaccharides) that acts as a virulence factor by making the organism hard for white blood cells to grab and swallow.

Virulence factor

Any feature or characteristic of a microbe that helps it cause disease.

Beta-haemolytic

Colonies surrounded by a clear zone on blood agar, indicating the complete breakdown of red blood cells.

Staphylococcus aureus identity

A Gram-positive coccus in clusters that is catalase-positive, coagulase-positive, a facultative anaerobe, salt-tolerant, and mannitol-fermenting.

Anterior nares

The front of the nostrils; the primary site where S. aureus normally lives in humans.

Slime (gelatinous) layer

A soft, water-soluble film of sugars and proteins that allows bacteria to glue onto plastic devices such as catheters and artificial valves.

Pentaglycine bridge

A short chain of five glycines that forms cross-links between L-lysine and D-alanine in the S. aureus peptidoglycan wall.

Teichoic acid

A sugar-phosphate polymer threaded through the wall of S. aureus that binds host fibronectin to aid in attachment to surfaces.

Protein A

A surface protein on S. aureus that binds the Fc tail of IgG antibodies backwards, blocking opsonisation and helping the microbe avoid phagocytosis.

MSCRAMM

A family of wall-anchored surface proteins (Microbial Surface Components Recognizing Adhesive Matrix Molecules) that glue S. aureus to host proteins like fibrinogen, fibronectin, and collagen.

Nuclease (DNase)

An enzyme that degrades DNA, specifically cutting up the DNA webs thrown out by neutrophils to help S. aureus escape from the immune response.

Hyaluronidase

An enzyme that dissolves hyaluronic acid (the glue between cells), enabling S. aureus to spread through host tissue.

Fibrinolysin (staphylokinase)

An enzyme produced by S. aureus that dissolves fibrin clots, allowing the organism to break out of a walled-off lesion.

Penicillinase

A beta-lactamase enzyme that destroys the beta-lactam ring of penicillin, providing antibiotic resistance.

Mannitol-salt agar

A selective and differential medium where high salt suppresses most growth and S. aureus ferments mannitol to turn the medium yellow.

NAAT (Nucleic-Acid Amplification Test)

A molecular method that copies and detects bacterial DNA, often used to screen carriers for MRSA.

MALDI-TOF

A mass spectrometry method that identifies an organism based on its unique protein fingerprint.

Staphylothrombin

The active clotting enzyme formed when free coagulase joins with coagulase-reacting factor in plasma.

Opsonisation

The process of coating a microbe with antibody or complement so that white blood cells can more easily recognize and swallow it.