Bacillus cereus
Causes vomiting and diarrhea following consumption of reheated rice / fried rice due to germination of spores.
Can also cause eye infections post-surgery (e.g., for cataracts). What the USMLE will do is tell you a patient had recent eye surgery + now has an infection caused by a gram-positive rod, where B. cereus is the only gram-positive rod listed. This is how you can arrive at the answer (i.e., if you know the taxonomy), even if you haven’t heard of B. cereus causing an eye infection before.
Bacillus anthracis
Causes pulmonary and cutaneous anthrax.
Pulmonary anthrax will be a hemorrhagic mediastinitis. Cutaneous anthrax presents as a black eschar with surrounding edema.
Anthrax produces edema factor, which functions as an adenylate cyclase enzyme, thereby increasing cAMP.
USMLE will usually mention the patient is a postal worker. They are very buzzy this way, and I’ve seen this on NBME.
Anthrax is aka “wool-sorter disease,” because its spores can survive on the dry hides of farm animals due to their protein structure. Anthrax is the only organism with a protein capsule (poly-D-glutamic acid).
Clostridium tetani
Causes spastic paralysis.
Tetanus toxin of Clostridium tetani inhibits presynaptic SNARE protein, resulting in decreased release of presynaptic neurotransmitters GABA and glycine, which are normally inhibitory.
Can present as opisthotonos (arched back) and trismus (lock-jaw).
Presents in two patients on USMLE: 1) neonate born at home whose umbilical cord was cut with a kitchen knife + tied with twine; 2) random dude who cut himself in back yard.
DTaP given at 2, 4, 6, months, then again at 15-18 months, then again at 4-6 years.
School-age kids require booster at 11-12 years, followed by booster every 10 years thereafter.
Pregnant women should get DTaP at 27-36 weeks to protect neonate from pertussis.
For cuts/wounds post-vaccine:
- 0-5 years post-vaccine: no Tx is necessary.
- 6-9 years post-vaccine: if clean wound: no Tx; if dirty wound: give Td booster.
- 10+ years post-vaccine: if clean wound: give Td booster; if dirty wound: IVIG + vaccine.
- In other words, only ever give IVIG if it’s a dirty wound + has been 10+ years.
Tetanus is a toxoid vaccine (inactivated toxin/protein).
Clostridium botulinum
Causes flaccid paralysis.
Botulin toxin of Clostridium botulinum inhibits presynaptic SNARE protein, resulting in decreased release of presynaptic acetylcholine, which is normally stimulatory at muscles.
Can present as floppy baby syndrome; can also cause cranial nerve palsies.
USMLE can be weird about the answer, where I’ve seen them write on an NBME exam something along the lines of “prevents acetylcholine from binding to its receptor” as the MOA of the toxin, even though this isn’t technically the direct effect.
Acquired as spores in honey in infants under 1; acquired as pre-formed toxin from canned goods in anyone older.
NBME exam wants you to know that administering the toxin does not change the effect of strength of the effect of acetylcholine binding to its receptor. This is because the toxin isn’t a competitive inhibitor + only decreases endogenous ACh release; this has no impact on any ACh administered exogenously.
Clostridium difficile
Diarrhea (pseudomembranous colitis) ~7-10 days after commencing oral antibiotics.
Antibiotics kill off normal bowel flora, allowing C. difficile to overgrow.
C. difficile is not normal flora, however. It is acquired via consumption of spores.
USMLE doesn’t care about which antibiotics per se cause pseudomembranous colitis; you just need to know any antibiotics in general can technically cause it if the patient has been inoculated with spores prior. But you could be aware that clindamycin is a classic agent known to increase risk, since it is a very powerful agent that essentially deletes your GI flora, leaving C. diff without competition for growth.
Can be watery or bloody diarrhea on NBME. Can also cause LLQ cramping (not RLQ as with Yersinia, which causes pseudo-appendicitis).
There is NBME Q where they say 28-year-old with LLQ cramping and bloody diarrhea 7 days after starting oral antibiotics –> answer = C. diff; wrong answer is Yersinia.
Diagnose with stool AB toxin test; stool culture is wrong answer.
If toxin test is already performed, if they ask how to further confirm the diagnosis, the answer is colonoscopy.
Never perform colonoscopy if a patient has toxic megacolon (which is possible with C. diff). Toxic megacolon will present as fever and distended abdomen, often with SIRS vitals (means abnormal vitals). Do abdominal x-ray to diagnose toxic megacolon.
Treat with oral vancomycin.
Vancomycin has poor oral bioavailability, so is given IV for things like endocarditis and meningitis. But for C. diff infection, that’s a good thing because we want it to stay within the GI tract. If USMLE asks why vancomycin is given orally for C. diff, the answer can be something like “has poor oral bioavailability,” which on the surface sounds like a bad thing when reading answer choices, but as I already said, this is favorable when we are treating C. diff.
Other fancy Abx like fidaxomicin, rifaximin, etc., I’ve never seen on NBME.
Clostridium perfringens
Causes watery/secretory diarrhea following consumption of poultry.
Causes gas gangrene (CO2 gas) due to production of a-toxin/phospholipase; presents as black skin / crepitus (subcutaneous emphysema).
Can also cause emphysematous cholecystitis (air in gall bladder wall).
C. perfringens is known to be associated with necrotizing fasciitis, which is deep infection along fascial planes. I reiterate that crepitus is HY. If the question specifically tells you that there is no crepitus + the infection started from some sort of scratch or puncture wound, the answer will be Group A Strep or S. aureus instead.
Listeria monocytogenes
Can be described as gram-positive rod with tumbling motility.
Can grow at very low temperatures (i.e., 0-4 degrees C).
USMLE will say neonate who has pneumonia, meningitis, or sepsis caused by gram-positive rod (if gram-positive coccus, that’s GBS; if gram-negative rod, that’s E. coli).
Can be acquired by pregnant women via deli meats and soft cheeses.
Can cause some obscure condition called granulomatosis infantiseptica, which presents as black skin lesions in the neonate.
Corynebacterium diphtheriae
Causes diphtheria, which presents in unvaccinated or immigrant children as a grey pseudomembrane in the posterior oropharynx. I’ve seen this grey pseudomembrane detail show up in one Q that wasn’t diphtheria, but the association is 9/10 times diphtheria.
Can cause myocarditis if untreated.
Diphtheria toxin inhibits protein translation by inhibiting elongation factor 2 (EF2).
Vaccine is a toxoid (inactive toxin).
Nocardia
Weakly acid-fast on staining; aerobic.
Can cause pulmonary infection resembling TB.
Can occasionally cause disseminated disease leading to osteomyelitis and meningitis.
SNAP –> Sulfonamides for Nocardia; Actinomyces use Penicillin.
Actinomyces
Not acid-fast; anaerobic.
Causes formation of yellow sulfur granules.
Causes draining sinus tracts in the oral cavity.
SNAP –> Sulfonamides for Nocardia; Actinomyces use Penicillin.
