A 45-year-old male has been in ICU for 10 days for necrotising pancreatitis. He has been treated for eight days with vancomycin, meropenem and caspofungin in appropriate dosages. He has been febrile and hypotensive for 24 hours and has had a change in vascular access.

The following three scenarios describe different potential results from his blood cultures:

Scenario 1 :

His blood cultures from the previous day become positive with a Gram-negative bacillus. The line tips show no growth.

List four likely identities for the Gram-negative bacillus, AND give an appropriate choice of antimicrobial for each.   (60% marks)

Scenario 2:

His blood cultures from the previous day become positive with a Gram-positive coccus. The line tips show no growth.

List three likely identities for the Gram-positive coccus, AND give an appropriate choice of antimicrobial for each.   (30% marks)

Scenario 3:

His blood cultures from the previous day become positive with a yeast.

Give the  likely identity for the yeast, AND suggest an appropriate antimicrobial agent.

(10% marks)

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College answer

a) Scenario 1

  • Stenotrophomonas maltophilia- environmental organism with low virulence Treatment is cotrimoxazole, ticarcillin clavulanic acid
  • Multi-resistant Acinetobacter baumanii
    Low virulence overall- though recent cases of high virulence community acquired cases in USA
    Treatment is complex- Colistin, Tigecycline
  • Multi-resistant E.coli
  • Multi-resistant K. pneumonia (or metalloprotein betalactamase secreting GNB) – virulent with high mortality- combination treatment which includes carbapenem, colistin, rifampicin and tetracycline – new agents such as avibactam and cefiderocol show promise
  • Multiresistant Pseudomonas aeruginosa
  • Metalloprotein beta-lactamase secreting GNB
    Acceptable answer
    Treatment will depend on extended susceptibilities- colistin and amikacin are potential options

b) Scenario 2

  • Vancomycin resistant Enterococcus faecalis
  • Vancomycin resistant Enterococcus faecium
  • Staphylococcus aureus with intermediate susceptibility to Vancomycin (VISA)
  • Vancomycin resistant Staphylococcus aureus (VRSA) (not yet reported in Australia but candidates should get credit if they mention it)

c) Scenario 3

  • The likely yeast in this setting is Candida glabrata (would accept Kruzei or Tropicalis, or other resistant organism. Simply stating caspofungin resistant organism did not score marks), Scedesporium acceptable
  • Treatment would be with Amphotericin

Discussion

Scenario 1 :

His blood cultures from the previous day become positive with a Gram-negative bacillus. The line tips show no growth.

List four likely identities for the Gram-negative bacillus, AND give an appropriate choice of antimicrobial for each.   (60% marks)

So, there are Gram-negative rods in the blood, and the lines are clean. The bugs you choose to answer this question would probably be at least to some extent resistant to carbapenems. The colleges offers a selection of classics, such as S.maltophila and A.baumanii. All the other choices offered by the college are simply familiar Gram-negative bugs prefaced with the words "Multi-Resistant".

Other possible appropriate bugs with antibiotics listed in parentheses are offered below.

  • Bacteroides fragilis (metronidazole)
  • Pseudomonas maltophilia (a fluoroquinolone, eg. ciprofloxacin)
  • Pseudomonas cepacia (a fluoroquinolone, eg. ciprofloxacin)

Scenario 2:

His blood cultures from the previous day become positive with a Gram-positive coccus. The line tips show no growth.

List three likely identities for the Gram-positive coccus, AND give an appropriate choice of antimicrobial for each.   (30% marks)

So, there are Gram-positive cocci in the blood, and the lines are clean. The cocci would have to be resistant to vancomycin. Or, as an unwelcome detour into the territory of common sense, the trainee could sacrifice 10% of the grade here by suggesting that the peripheral blood cultures might be contaminated by a coagulase-negative staph (surely that would attract zero marks because the model answer does not countenance such a possibility). Anyway, the college examiners appear to have lost interest in giving antibiotic recommendations after the first scenario. For those of us still paying attention, the appropriate antibiotics are listed in parentheses.

  • Enterococcus faecium, or E.faecalis i.e. VRE  (linezolid, or daptomycin, or tigecycline)
  • Vancomycin-intermediate MRSA, i.e. VISA (tigecycline, or telavancin, or ceftaroline)
  • VRSA (tigecycline, or telavancin, or ceftaroline)
  • Leuconostoc mesenteroides (tigecycline)

Scenario 3:

His blood cultures from the previous day become positive with a yeast.

Give the  likely identity for the yeast, AND suggest an appropriate antimicrobial agent.

(10% marks)

This would have to be a yeast sufficiently resistant to caspofungin so as to survive an eight-day course and still be causing systemic fungaemia (yeastaemia?). In actual fact, this could be any albicans or non-albicans Candida with an Fks1 mutation (conferring a 1000-fold decrease in susceptibility).  Treatment would be with something like amphotericin, the old go-to of fungicidal therapy.

References

References

Perlin, David S. "Resistance to echinocandin-class antifungal drugs.Drug Resistance Updates 10.3 (2007): 121-130.

Kumar, Randhir, et al. "METHICILLIN AND VANCOMYCIN RESISTANCE AMONG STAPHYLOCOCCUS AUREUS STRAINS ISOLATED FROM PATIENTS ATTENDING TERTIARY CARE HOSPITAL IN EASTERN BIHAR." JOURNAL OF EVOLUTION OF MEDICAL AND DENTAL SCIENCES-JEMDS 6.12 (2017): 914-917.