STEPTRACKERMICROBIOLOGY + NEUROLOGY | GRAM-POSITIVE RODS | HIGH-YIELD REVIEW

Clostridia

Spore-forming, obligate anaerobic gram-positive rods — four species, four toxin stories: spasm, paralysis, gangrene and colitis.

FOCUS ON

C tetani: puncture wound → tetanospasmin (blocks GABA/glycine in Renshaw cells) → trismus, risus sardonicus, opisthotonos. C botulinum: preformed toxin (canned food) or spores (infant honey) → blocks ACh release → descending flaccid paralysis. C perfringens: α-toxin (lecithinase) → gas gangrene; also late-onset food poisoning (reheated meat). C difficile: toxins A and B → pseudomembranous colitis after antibiotics (clindamycin, fluoroquinolones, cephalosporins) → oral vancomycin or fidaxomicin.

ORGANISM · DISEASE

TOXIN · MECHANISM

CLINICAL PICTURE

DIAGNOSIS · TREATMENT

C tetani — tetanus

Tetanospasmin travels retrograde → cleaves synaptobrevin in inhibitory interneurons → ↓ GABA, glycine

Trismus (lockjaw), risus sardonicus, opisthotonos, spasms, autonomic instability

Clinical · TIG + toxoid, wound debridement, benzodiazepines, metronidazole

Neonatal tetanus

Contaminated umbilical stump

Poor suck, spasms in first weeks

Maternal toxoid vaccination prevents

C botulinum — foodborne

Preformed toxin blocks ACh release (SNARE cleavage)

Symmetric descending paralysis: diplopia, ptosis, dysarthria, dysphagia, then limbs and breathing; dilated pupils; no fever, normal sensation

Home-canned food · equine antitoxin, ventilation

Infant botulism

Spores colonize gut → toxin made in vivo

"Floppy baby": constipation first, weak cry and suck, hypotonia

Honey, soil · human botulism immune globulin

Wound botulism

Spores in contaminated wounds

Same paralysis

Injection drug use ("black tar" heroin)

C perfringens — gas gangrene

α-toxin (lecithinase, phospholipase C) destroys membranes

Rapid myonecrosis, crepitus, severe pain, bronze skin, hemolysis, shock after trauma/surgery

Double zone of hemolysis · debridement, penicillin + clindamycin, hyperbaric O₂

C perfringens — food poisoning

Spores survive cooking → germinate in reheated meat → enterotoxin in gut

Watery diarrhea 8–16 h later, no vomiting, self-limited

Reheated meat, gravy

Necrotizing enteritis (pig-bel)

β-toxin of C perfringens type C

Bloody diarrhea, bowel necrosis

Low trypsin diets

C difficile colitis

Toxin A (enterotoxin) and B (cytotoxin) glucosylate Rho GTPases

Watery diarrhea, cramps, fever, leukocytosis after antibiotics or PPIs; pseudomembranes; toxic megacolon

Stool PCR/toxin · fidaxomicin or oral vancomycin · recurrent → fecal transplant, bezlotoxumab

C septicum

Spontaneous gas gangrene

Myonecrosis without trauma

Linked to colon cancer, neutropenia

Shared biology

Gram ⁺ rods, spores, obligate anaerobes

Spores resist alcohol gels

Soap-and-water handwashing for C difficile

Mechanism — antibiotics to colitis

Antibiotic (clindamycin, FQ, cephalosporin) / PPI

→

Normal colonic flora wiped out

→

C difficile spores germinate

→

Toxins A and BActin disruption, cell death

→

PseudomembranesFibrin, neutrophils, mucus

Clostridia at a glance

SPECIES

SOURCE

PARALYSIS / LESION

KEY DRUG

C tetani

Rusty nail, soil

Spastic

TIG, toxoid

C botulinum

Canned food, honey

Flaccid, descending

Antitoxin

C perfringens

Trauma, reheated meat

Myonecrosis

Penicillin + clindamycin

C difficile

Antibiotics, hospital

Colitis

Fidaxomicin, oral vanc

How it's tested

COMMON CONFUSIONBotulism is descending and symmetric with intact sensation and cranial nerves first; Guillain-Barré is usually ascending with areflexia and sensory symptoms.

THE TRAPIV vancomycin does not treat C difficile — it doesn't reach the colon lumen. Oral vancomycin or fidaxomicin does.

YOUR APPROACHSpasm vs flaccid paralysis vs gas in tissue vs diarrhea after antibiotics → species → toxin → treatment.

If you see… think…

Lockjaw and back arching after stepping on a nail → tetanus

Diplopia, dysphagia, descending weakness after home-canned beans → botulism

Constipated floppy infant fed honey → infant botulism

Crepitus, bronze skin, rapid myonecrosis after a car crash → C perfringens gas gangrene

Diarrhea 12 h after reheated stew, no vomiting → C perfringens enterotoxin

Diarrhea + leukocytosis after clindamycin → C difficile

Yellow plaques on colonoscopy → pseudomembranous colitis

First-line C difficile treatment → fidaxomicin or oral vancomycin

Spontaneous gas gangrene, no trauma → C septicum (look for colon cancer)

Double zone of hemolysis → C perfringens