College Answer

a)

Diagnosis: Listeria monocytogenes                                                                                                                      

b)

Treatment:                                                                                                                                             

Benzylpenicillin 2.4 g IV, 4-hourly    

Or

Trimethoprim+sulfamethoxazole 160+800 mg IV, 6-hourly (in case of penicillin allergy)

 c) Risk Factors:       

Extremes of age

Pregnancy

Immunosuppression

Malignancy

Discussion

a)

The question is designed poorly. Gram-positive rods - in which bottle? Cultured how? An excellent 2001 article by Von Graevenitz  allows us to consider the following as realistic differentials:

• Misidentified organism
  • Gram-positive cocci misidentified as bacilli: typically, diplococci (eg. Streptococcus sp.) or coccobacilli (eg. Haemophilus sp.)
• Contaminants from the skin
  • Corynebacterium sp., eg. C.jeikeum (aerobic; a skin saprophyte)
  • Propionibacterium acnes. (aerobic)
  • Numerous others, eg. Arcanobacterium sp., Actinomyces sp., etc
• Clinically insignificant contaminants from other sources:
  • Genitourinary source, eg. when the culture is staken from a groin stab - for example, Lactobacillus sp. (aerobic) or Gardenerella vaginalis (anaerobic)
  • Oral source or unrelated oral bacteraemia, eg. Rothia dentocariosa
• CNS infection
  • Listeria monocytogenes (aerobic)
  • Nocardia asteroides (aerobic)
• Severe systemic infection with septic encephalopathy
  • Clostridium species, eg. C.perfringens, C.difficile (anaerobic) 
• Rare and unlikely pathogens:
  • Bacillus anthracis (aerobic)
  • Corynebacterium diphteriae (aerobic)
  • C.tetani  and C.botulinum (anaerobic)

But of course, if the college asks about a gram-positive rod, it's always going to be Listeria m., because the college loves Listeria.

b)

Listeria is equisitely sensitive to penicillin. The high dose recommended by the college answer seems to come from the Sanford recommendations for treating listerial meningitis. Cotrimoxazole is a valid alternative.

According to Troxler et al (2000), all Listeria species are innately resistant to cephalosporins and aztreonam. They are also all innately sensitive to penicillins of all kinds, as well as aminoglycosides cabapenems glycopeptides and lincosamides. In a theoretical real-life situation, one's panic in the face of an unknown CNS infection often leads to the choice of a vancomycin/meropenem combination: if it turns out to be Listeria, this combination would still be appropriate, even if it is inelegantly overpowered. 

c)

From J Rocourt (1996) and Schuchat et al (1992): 

• Patient risk factors
  • Malignancy
  • Pregnancy
  • Organ transplantat recipient
  • HIV/AIDS
  • Corticosteroid therapy
  • Anything that interferes with T-cell mediated immunity
  • Extremes of age (neonates, the elderly)
• Food risk factors
  • ​Soft cheese
  • Food "purchased from store delicatessen counters"
  • Undercooked chicken
  • Broadly speaking, foods which are "ready to eat", stored at refrigeration temperature for prolonged periods