Gram-Positive Bacteria Laboratory Identification Algorithm

Vocabulary flashcards summarizing tests, organisms, and key characteristics used in the laboratory algorithm for identifying gram-positive bacteria.

Gram-Positive Cocci

Spherical purple/blue bacteria on Gram stain; include Staphylococcus and Streptococcus/Enterococcus groups.

Gram-Positive Bacilli

Rod-shaped purple/blue bacteria; include Listeria, Bacillus, Clostridium, Corynebacterium, Cutibacterium.

Catalase Test

Enzyme test that separates catalase-positive Staphylococcus from catalase-negative Streptococcus/Enterococcus.

Coagulase Test

Detects ability to clot plasma; coagulase-positive identifies Staphylococcus aureus.

Hemolysis Patterns

Appearance of red-blood-cell lysis on blood agar: β (complete/clear), α (partial/green), γ (none).

β Hemolysis

Complete clearing around colonies; seen in Staphylococcus aureus, Group A & B streptococci.

α Hemolysis

Partial (green) hemolysis; seen in Streptococcus pneumoniae and Viridans streptococci.

γ Hemolysis

No hemolysis; typical of some Enterococcus species and nonhemolytic streptococci.

Novobiocin Sensitivity Test

Differentiates Staphylococcus epidermidis (sensitive) from Staphylococcus saprophyticus (resistant).

Staphylococcus aureus

Coagulase-positive, β-hemolytic Staph; important pathogen causing skin, systemic, toxin-mediated diseases.

Staphylococcus epidermidis

Coagulase-negative, novobiocin-sensitive skin commensal; forms biofilms on prosthetic devices.

Staphylococcus saprophyticus

Coagulase-negative, novobiocin-resistant; common cause of urinary tract infections in young women.

Streptococcus pyogenes (Group A)

β-hemolytic, bacitracin-sensitive, PYR-positive streptococcus; causes pharyngitis, rheumatic fever, cellulitis.

Streptococcus agalactiae (Group B)

β-hemolytic, bacitracin-resistant streptococcus; neonatal sepsis/meningitis culprit.

Bacitracin Sensitivity

Antibiotic disk test distinguishing Group A Strep (sensitive) from Group B Strep (resistant).

PYR Test

Detects pyrrolidonyl aminopeptidase; positive in Streptococcus pyogenes and Enterococcus species.

Streptococcus pneumoniae

α-hemolytic, encapsulated diplococcus; optochin-sensitive & bile soluble; causes meningitis, pneumonia, otitis.

Viridans Streptococci

α-hemolytic, optochin-resistant, non-encapsulated group (e.g., S. mutans, S. mitis, S. gallolyticus) inhabiting oral/GI flora.

Optochin Sensitivity

Disk test distinguishing Streptococcus pneumoniae (sensitive) from Viridans streptococci (resistant).

Bile Solubility Test

S. pneumoniae lyses in bile whereas Viridans streptococci do not; confirms pneumococcus.

Enterococcus (E. faecalis/E. faecium)

Gram-positive cocci that may be α- or γ-hemolytic, PYR-positive, grow in 6.5 % NaCl; cause UTIs, endocarditis.

Growth in 6.5 % NaCl

Salt-tolerance test; positive in Enterococcus species, negative in many streptococci.

Listeria monocytogenes

Aerobic motile gram-positive rod; crosses placenta, causes neonatal meningitis; tumbling motility at room temp.

Bacillus species

Aerobic spore-forming gram-positive rods; include B. anthracis (anthrax) and B. cereus (food poisoning).

Clostridium species

Anaerobic spore-forming gram-positive rods; produce potent toxins (e.g., tetanus, botulism, difficile colitis).

Corynebacterium diphtheriae

Aerobic gram-positive rod; produces diphtheria toxin; club-shaped 'Chinese letters' on stain.

Cutibacterium (Propionibacterium)

Anaerobic gram-positive rod of skin flora; implicated in acne and device infections.

Nocardia

Aerobic, weakly acid-fast branching filamentous bacteria; cause pulmonary or cutaneous infections in immunocompromised.

Actinomyces israelii

Anaerobic, non-acid-fast branching filament; causes cervicofacial abscesses with sulfur granules.

Branching Filamentous Gram-Positives

Collective term for Nocardia (weakly acid fast) and Actinomyces (not acid fast).

Aerobic vs. Anaerobic Classification

Gram-positive bacilli: Bacillus, Listeria, Nocardia are aerobic; Clostridium, Actinomyces, Cutibacterium are anaerobic.