15.2 Gram-Positive Cocci & Rods

Staphylococcus: Catalase-Positive Cocci in Clusters

The genus Staphylococcus appears microscopically as Gram-positive cocci in grape-like clusters. All staphylococci are catalase-positive, which distinguishes them from streptococci (catalase-negative). The single most important subdivision within the genus is the coagulase test.

Coagulase-Positive: Staphylococcus aureus

S. aureus produces coagulase, an enzyme that converts fibrinogen to fibrin, forming a protective clot around the organism. It colonizes the anterior nares of about 30% of people. Major diseases include:

• Localized skin infections: folliculitis, furuncles, carbuncles, abscesses.
• Bullous impetigo (vs. streptococcal non-bullous impetigo).
• Food poisoning from heat-stable enterotoxin (rapid onset, vomiting dominant).
• Toxic shock syndrome (TSS) from TSST-1, a superantigen that binds MHC II and T-cell receptor, causing massive cytokine release, fever, rash (desquamation of palms/soles), and multi-organ failure.
• Scalded skin syndrome (SSSS) from exfoliative toxin A/B, mostly in infants.
• Osteomyelitis and septic arthritis — S. aureus is the most common cause.
• Endocarditis, especially in IV drug users (tricuspid valve).

Methicillin-resistant S. aureus (MRSA) carries the mecA gene encoding altered PBP2a, conferring resistance to all beta-lactams. Vancomycin, daptomycin, or linezolid are used for serious MRSA infections.

Coagulase-Negative Staphylococci (CoNS)

Staphylococcus epidermidis and S. lugdunensis are CoNS. They are normal skin flora and cause disease primarily through biofilm formation on foreign material — central venous catheters, prosthetic joints, prosthetic heart valves. S. epidermidis is the classic cause of nosocomial catheter-related bloodstream infection.

Streptococcus: Catalase-Negative Cocci in Chains

Streptococci are Gram-positive cocci in chains or pairs, catalase-negative. Two classification systems run in parallel: Lancefield grouping (carbohydrate antigens) and hemolysis pattern on blood agar.

Streptococcus pyogenes (Group A Strep, GAS)

S. pyogenes is the organism in the PA-CAT Bulletin of Information, rev. 20240815, sample item: a 13-year-old with sore throat and a rash with pink rings and clear centers (erythema marginatum) plus elevated ASO (anti-streptolysin O) antibodies points to rheumatic fever, a post-streptococcal sequela of untreated Group A pharyngitis. The organism is beta-hemolytic (complete clear hemolysis from streptolysin O and S) and bacitracin-sensitive.

GAS diseases split into suppurative (pharyngitis, pyoderma/impetigo, cellulitis, necrotizing fasciitis) and nonsuppurative (rheumatic fever, post-streptococcal glomerulonephritis). Rheumatic fever follows the modified Jones criteria: evidence of preceding GAS plus either two major criteria (carditis, polyarthritis, chorea, erythema marginatum, subcutaneous nodules) or one major plus two minor. Carditis is the most serious because it can cause chronic valvular damage (mitral stenosis classically).

Streptococcus agalactiae (Group B Strep, GBS)

GBS is beta-hemolytic, bacitracin-resistant, and CAMP-test–positive (a diffusible 'CAMP factor' synergizes with S. aureus sphingomyelinase to cause enhanced arrowhead hemolysis). It colonizes the maternal genitourinary tract and is the leading cause of neonatal sepsis and meningitis. Intrapartum intravenous penicillin (or ampicillin; cefazolin for penicillin-allergic, vancomycin for anaphylaxis) is given to colonized mothers to prevent early-onset disease. Screening is by rectovaginal culture at 35–37 weeks' gestation.

Streptococcus pneumoniae

S. pneumoniae is an alpha-hemolytic, lancet-shaped diplococcus that is optochin-sensitive and bile-soluble. Its polysaccharide capsule is the key virulence factor (over 90 serotypes). It causes lobar pneumonia (rust-colored sputum), otitis media, sinusitis, and meningitis. The conjugate pneumococcal vaccine (PCV) covers the most invasive serotypes in children.

Enterococcus: The Hardy Gram-Positive Coccus

Enterococcus faecalis and E. faecium are Gram-positive cocci in chains or pairs that grow under harsh conditions — in bile, in 6.5% NaCl, and at 45 °C — and are bile-esculin–positive. They are intrinsically resistant to cephalosporins (low-affinity PBPs) and have an important association: S. bovis group bacteremia/endocarditis correlates with colorectal neoplasia and warrants colonoscopy.

Vancomycin-resistant enterococci (VRE) carry vanA or vanB, which replace the D-Ala-D-Ala target with D-Ala-D-Lac, stripping vancomycin of its binding site. Linezolid and daptomycin are treatment options.

Gram-Positive Rods

Gram-positive rods split into spore-formers (Bacillus, Clostridium) and non-spore-formers (Corynebacterium, Listeria, Nocardia — note Nocardia is weakly acid-fast and branches).

Clinical Pearls for Gram-Positive Rods

• Anthrax eschar is painless and covered by a black necrotic scab; the capsule is the only non-polysaccharide bacterial capsule (poly-D-glutamate).
• Tetanus causes spastic paralysis (upward) and a trismus ('lockjaw'); botulism causes flaccid paralysis (downward). Both are toxemias — antibiotics alone do not reverse intoxication; antitoxin and supportive care are essential.
• Diphtheria toxin is encoded by a beta-corynephage; only toxigenic strains cause disease. The pseudomembrane bleeds if scraped.
• Listeria is one of the few bacterial causes of meningitis in neonates (with Group B strep and E. coli K1) and in the elderly/immunocompromised. It tolerates refrigeration, so cold enrichment and ready-to-eat foods (deli meats, soft cheeses, unpasteurized milk) are classic vehicles. Treatment is ampicillin (often plus gentamicin).