Gram-Positive Bacteria – Exam 3 Quick Notes

Low vs High G+C Gram-Positives

• Two phyla distinguished by genomic $\text{G+C}$ content.
• Low \text{G+C}<60\% → Phylum Firmicutes (tough cell wall; many endospore formers). • High \text{G+C}>60\% → Phylum Actinobacteria (thicker walls, often with mycolic acid).
• Tenericutes (Mollicutes) = Firmicutes that lost cell wall.

Firmicutes – Key Groups & Facts

• Clostridium (Obligate anaerobic, spore formers)
• Energy: fermentation / Stickland reaction.
• C. tetani → tetanospasmin; blocks $\text{GABA/glycine}$ release → spastic paralysis (lockjaw). Vaccine = toxoid booster every $10$ yrs.
• C. botulinum → botulinum toxin; blocks acetylcholine release → flaccid paralysis (floppy baby). No routine vaccine; equine antitoxin.
• C. perfringens: $\alpha$-toxin gas gangrene (deep wounds) + enterotoxin food poisoning.
• C. difficile: post-antibiotic colitis, toxins $A,B$; treat with fecal transplant if severe.

• Bacillus (Aerobic spores; non-communicable)
• B. cereus: reheated rice / buffet foods → emetic or diarrheal illness.
• B. anthracis: soil spores; edema + lethal toxins. Types: cutaneous (most common), inhalation (most lethal), GI, injectable. Vaccine for military/lab.

• Staphylococcus (Facultative; skin/mucosa)
• S. epidermidis: normal flora; biofilm infections on catheters.
• S. aureus: coagulase $+$, $\beta$-hemolytic, DNase $+$; opportunistic SSTI → pneumonia, sepsis.
• MRSA = methicillin-resistant S. aureus; major nosocomial threat.

• Listeria monocytogenes
• Cold-tolerant foodborne pathogen (soft cheese, deli meat, ice cream).
• Risk groups: pregnant, neonates, elderly, immunocompromised.

• Lactobacillus
• Strict fermenters (lactic acid); probiotics; normal oral, GI, vaginal flora; produce bacteriocins.

• Streptococcus (Chains; Lancefield groups)
• S. pyogenes (Group A, $\beta$-hemolytic): strep throat → scarlet fever, rheumatic fever, glomerulonephritis.
• S. pneumoniae: encapsulated diplococci → lobar pneumonia, otitis media.
• Enterococcus faecalis (formerly Group D Strep): UTIs, endocarditis; salt tolerant.

Actinobacteria – Key Groups & Facts

• Actinomycetes (e.g., Actinomyces, Streptomyces)
• Filamentous, produce aerial spores; source of $\approx70\%$ of antibiotics.
• Actinomyces: oral/GI flora; poor hygiene/trauma → actinomycosis (cervicofacial abscess).

• Streptomyces
• Soil; make geosmin. Drugs: erythromycin, streptomycin, neomycin, avermectins (ivermectin).

• Corynebacterium diphtheriae
• Lysogenic phage encodes diphtheria toxin → throat pseudomembrane; vaccine = toxoid (DTaP/TDaP).

• Mycobacterium
• Acid-fast (mycolic acid); slow growth.
• M. tuberculosis: airborne; survives in macrophages; rising $\text{MDR/XDR}$ TB.
• M. leprae: chronic infection via respiratory droplets/close contact → peripheral nerve damage.
• Vaccine: BCG (live M. bovis) in high-TB regions.

• Propionibacterium (Cutibacterium)
• Anaerobic; skin/GI; produce propionic acid. P. acnes → acne; P. freudenreichii → Swiss cheese holes.

• Gardnerella vaginalis
• Normal vaginal flora; overgrowth (loss of Lactobacillus) → bacterial vaginosis.

• Bifidobacterium
• Pioneer infant gut colonizer (breast-fed); probiotics (e.g., Activia); ferment to acetic + lactic acids.

Tetanospasmin vs Botulinum Toxin (Board Alert)

• Both = zinc endopeptidase SNARE-cleaving neurotoxins.
• Tetanospasmin: enters wound → retrograde to CNS → blocks inhibitory transmitters → spastic paralysis.
• Botulinum toxin: ingested/wound → acts peripherally → blocks acetylcholine release → flaccid paralysis.
• Vaccine: Yes (tetanus toxoid) / No (botulinum).
• Antitoxin: TIG / Equine botulinum antitoxin.

Quick Recall Checklist

• Phylum split: <60\% vs >60\% $\text{G+C}$.
• Spore formers: Clostridium (anaerobe), Bacillus (aerobe).
• Neurotoxins: tetanospasmin (spastic), botulinum (flaccid).
• Gas gangrene: C. perfringens $\alpha$-toxin.
• MRSA = hospital SSTI threat.
• Diphtheria pseudomembrane; TB acid-fast; Leprosy nerve lesions.
• Actinomycetes ⇒ majority antibiotics; Streptomyces smell of soil.
• Listeria cold food; Lactobacillus probiotics.