Question 30.2

65-year-old female with rheumatoid arthritis has been admitted to ICU with an acute confusional state following a two-day history of fever and headache.

Investigations reveal:

Blood culture:         Gram-positive bacilli at 24 hours.

a) Give the most likely causative organism.         (10% marks)

b) Give the specific therapy for this condition.          (10% marks)

c) List three risk factors for this condition.           (10% marks)

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College Answer


Diagnosis: Listeria monocytogenes                                                                                                                      



Benzylpenicillin 2.4 g IV, 4-hourly    


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

 c) Risk Factors:       

Extremes of age






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.), though for the latter, the simultaneous misinterpretation of shape and Gram stain would be a freakishly rare combination of errors
  • 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.


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. 


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


Schuchat, Anne, et al. "Role of foods in sporadic listeriosis: I. Case-control study of dietary risk factors." Jama 267.15 (1992): 2041-2045.

Rocourt, J. "Risk factors for listeriosis." Food Control 7.4 (1996): 195-202.

McKew, Genevieve, et al. "“Probable contaminants” no more: rapid identification of Gram-positive rods leads to improved clinical care." Journal of clinical microbiology 51.5 (2013): 1641-1641.

Von Graevenitz, A. "Antimicrobial therapy of infections with aerobic Gram‐positive rods." Clinical Microbiology and Infection 7.s4 (2001): 43-46.

Troxler, R., et al. "Natural antibiotic susceptibility of Listeria species: L. grayi, L. innocua, L. ivanovii, L. monocytogenes, L. seeligeri and L. welshimeri strains." Clinical microbiology and infection 6.10 (2000): 525-535.

Itoh, Naoya, et al. "Visual discrepancies in the identification of Haemophilus influenzae through gram staining blood culture and sputum smears." IDCases 33 (2023).