Catalase-Positive, Gram-Positive Cocci

Flashcards covering key vocabulary, characteristics, identification tests, and antimicrobial resistance mechanisms relating to Catalase-Positive, Gram-Positive Cocci, primarily Staphylococcus and Micrococcus species.

Catalase-Positive, Gram-Positive Cocci

A classification of bacteria, including Staphylococcus and Micrococcus, characterized by their Gram stain reaction and the presence of the catalase enzyme.

Staphylococcus aureus habitat

Normal microbiota of anterior nares, nasopharynx, perineal area, skin, and colonizer of mucosa.

Staphylococcus aureus mode of transmission

Endogenous strain (sterile site by traumatic introduction), direct contact (person-to-person, fomites), or indirect contact (aerosolized).

Staphylococcus epidermidis habitat

Normal microbiota of skin and mucous membranes.

Staphylococcus saprophyticus habitat

Normal microbiota of skin, genitourinary tract, and mucosa.

Micrococcus spp. habitat

Normal microbiota of skin, mucosa, and oropharynx.

Gram-positive cocci (GPC) microscopic morphology

'Perfectly round' cells, typically seen as single, pairs, or clusters resembling 'bunches of grapes'.

Gram-positive cocci (GPC) macroscopic morphology

Non-fastidious growth on BAP and CHOC, small to medium beige, white, yellow, or light gold colonies with creamy/buttery/smooth texture, and γ or β-hemolysis on BAP.

Catalase

An enzyme found in organisms exposed to oxygen that catalyzes the decomposition of hydrogen peroxide into water and oxygen, protecting from oxidative damage.

Catalase test purpose

Differentiates Staphylococcus species (positive reaction) from Streptococcus species (negative reaction).

Micrococcus spp. characteristics

Often found in the environment and normal skin flora, usually non-pathogenic but opportunistic, and produces a distinct yellow pigment on BAP.

Coagulase test

An enzyme test that clots plasma (fibrinogen to fibrin), used to differentiate Staphylococcus spp. into coagulase-positive and coagulase-negative groups.

Bound coagulase

A type of coagulase found on the surface of the cell that directly converts fibrinogen to fibrin.

Free coagulase

An extracellular coagulase that activates coagulase-reacting factor (CRF) to form a thrombin-like complex, converting fibrinogen to fibrin with a delayed reaction.

Coagulase-positive Staphylococci (human pathogens)

Includes Staphylococcus aureus (MSSA and MRSA), S. delphini, S. intermedius, S. lutrae, S. hyicus, S. pseudintermedius.

Coagulase-negative Staphylococci (CoNS)

A group of Staphylococcus species that are tube-coagulase negative, typically normal flora of skin and mucous membranes, less virulent than S. aureus but a significant cause of healthcare-associated infections.

Staphylococcus aureus

The primary human pathogen of the Staphylococcus genus, causing a wide range of infections in healthy and immunocompromised individuals, found on skin and mucosal membranes.

Staphylococcus aureus virulence factors

Enterotoxins, Toxic Shock Syndrome Toxin-1 (TSST-1), Exfoliative Toxin, Cytolytic Toxins, Exoenzymes, and Protein A.

Folliculitis

Superficial inflammation of hair follicle(s), usually resolved without complications but can progress to deeper infections.

Furuncle (boil)

Inflammation of a hair follicle or sebaceous gland that has progressed to an abscess or pustule.

Carbuncle

A larger and deeper lesion formed by the aggregation and interconnection of a cluster of furuncles.

Impetigo

A superficial skin infection involving the epidermis, characterized by vesicles that rupture and crust over ('honey crusted lesions'), common in children ages 2-5 years.

Protein A

A surface protein of S. aureus that binds the Fc portion of antibodies, preventing opsonization and phagocytosis, thereby decreasing immune clearance.

Hyaluronidase

An exoenzyme ('spreading factor') of S. aureus that hydrolyzes hyaluronic acid present in connective tissue, facilitating invasion.

Panton-Valentine Leukocidin (PVL)

A cytolytic toxin of S. aureus that is toxic to white blood cells, preventing clearance of infection and associated with more serious and invasive skin and soft tissue infections (SSTIs).

Enterotoxins (Staphylococcus aureus)

Heat and acid-stable toxins produced by S. aureus that damage the gut, causing diarrhea and vomiting; they can survive gastric acid and boiling.

Staphylococcal Food Poisoning

A toxin-mediated illness caused by S. aureus enterotoxins (primarily A, B, & D) in contaminated or inadequately refrigerated food, resulting in self-limiting symptoms.

Exfoliative toxin (epidermolytic toxin)

A serine protease produced by S. aureus that splits intracellular bridges of the epidermis, causing the skin's epidermal layer to 'slough off' (Scalded Skin Syndrome Toxin).

Staphylococcal Scalded Skin Syndrome (SSSS)

A toxin-mediated infection caused by exfoliative toxin, leading to profuse peeling of the epidermal layer, revealing moist-red middle epidermis, primarily in babies.

Toxic Shock Syndrome Toxin-1 (TSST-1)

An S. aureus enterotoxin (F) acting as a superantigen, causing endothelial and epidermal cell death through an aggressive immune response (T-cell proliferation, cytokine overproduction), responsible for most menstruating-associated TSS.

Staphylococcal Toxic Shock Syndrome

A toxin-mediated illness associated with tampons, characterized by high fever, rash, watery diarrhea, vomiting, dehydration, and hypotension, potentially fatal due to multiple organ system failure.

CoNS virulence factors

Biofilm formation (extracellular polysaccharide layer) and the ability to acquire resistance genes.

Staphylococcus epidermidis (SEPI) infection

Commonly associated with infections related to indwelling catheters and other medical devices.

Staphylococcus saprophyticus (SSAP) infection

A common cause of community-acquired urinary tract infections (UTIs), particularly in young, sexually active females.

Staphylococcus lugdunensis (SLUG) infection

Linked to serious infections such as native valve endocarditis.

Mannitol Salt Agar (MSA)

A selective and differential agar medium that isolates and differentiates Staphylococcus spp. based on their ability to ferment mannitol, indicated by a color change from red to yellow.

Catalase Test procedure

To perform, place a drop of H2O2 on a slide, transfer a bacterial colony to it, and observe for immediate bubble formation; Staphylococcus and Micrococcus spp. are positive.

Slide Coagulase Test

Detects bound coagulase (clumping factor) by combining rabbit plasma with a suspect colony on a slide and checking for immediate agglutination. Negative results consistent with S. aureus morphology should be confirmed with the tube method.

Tube Coagulase Test

The gold standard test for S. aureus, detecting free (extracellular) coagulase by observing plasma coagulation within 4-6 hours (or up to 20-24 hours).

Modified oxidase test (Microdase disk)

Detects the enzyme oxidase, which is positive for Micrococcus spp. only (most Staphylococcus spp. are negative), helping to differentiate between the two genera.

Bacitracin susceptibility test (0.04 U)

Differentiates Micrococcus spp. (susceptible, >10mm zone) from Staphylococcus spp. (resistant, <10mm zone).

Novobiocin susceptibility test (for CoNS)

Test for presumptive identification of S. saprophyticus in urine cultures; S. saprophyticus is resistant (

PYR Test

Detects the pyrrolidonyl peptidase enzyme, producing a red color if positive. Used to differentiate S. aureus (negative) from S. lugdunensis (positive) and S. haemolyticus (positive).

Beta-lactam antibiotics

A class of antibiotics that inhibit bacterial cell wall synthesis by binding to and irreversibly inactivating penicillin-binding proteins (PBPs), leading to bacterial lysis.

Beta-lactamase enzyme

An enzyme produced by microorganisms that hydrolyzes the beta-lactam ring of antibiotics, preventing them from binding to PBPs and inhibiting cell wall synthesis.

Zone Edge test ('Beach and cliff effect')

A phenomenon used to confirm the presence of beta-lactamase enzymes in bacterial isolates by observing the characteristic inhibition zone edge around antibiotic disks.

PBP 2a

An altered penicillin-binding protein, encoded by the mecA gene, which is the primary mechanism of resistance in MRSA, rendering all beta-lactam antibiotics ineffective.

mecA gene

A gene, carried on a mobile DNA element (SCCmec), that produces altered PBP 2a, conferring resistance to beta-lactam antibiotics and responsible for widespread antibiotic resistance in MRSA.

Community Acquired MRSA (CA-MRSA)

MRSA infections acquired in community settings, often associated with skin and soft tissue infections, and typically carrying SCCmec type IV elements and genes for toxins like PVL.

Healthcare-Associated MRSA (HA-MRSA)

MRSA infections acquired in healthcare settings, associated with invasive infections like bloodstream infections or surgical site infections, typically carrying larger SCCmec elements (types I-III) with additional resistance genes.

Cefoxitin (FOX) 30ug disk diffusion test for MRSA

A screening method where a zone diameter < 19mm indicates resistance and > 20mm indicates susceptibility for MRSA.

Inducible Clindamycin Resistance

A mechanism of macrolide resistance where resistance to clindamycin is activated by the presence of erythromycin, typically identified in erythromycin-resistant S. aureus strains.

D-Test (for inducible clindamycin resistance)

A disk diffusion test performed when an organism shows clindamycin susceptibility but erythromycin resistance; a flattened D-shaped zone of inhibition around the clindamycin disk indicates inducible resistance.