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Episode #9 – Arts & Grafts

9 Cover Art

Summary

Dr. Tara Vijayan explains her approach to positive blood cultures and how to evaluate for possible aortic graft infection.

Credits

Host: Sara Dong

Guest: Tara Vijayan

Writing/Producing/Editing/Cover Art: Sara Dong

Infographics: Sara Dong

Our Guest

Tara Vijayan, MD

Twitter

She is an associate professor of medicine in the division of infectious diseases at UCLA. She currently serves as the Medical Director for the Antimicrobial Stewardship Program and also carries several other roles within the School of Medicine including Associate Director of Scientific Foundations of Medicine, Director of the Health Equity Pathway for the Internal Medicine Residency and Director of the Medical Education Pathway for the Multicampus Fellowship in Infectious Diseases. Her passions are health equity, implementation science and combating the global threat of antimicrobial resistance. Outside of UCLA she loves doing puzzles, reading and hiking with her two wildlings, her husband and her 10-year-old hound.

Culture

Tara’s picks!

Trevor Noah’s Born a Crime

Schitt’s Creek

Consult Notes

Consult Q

GPCs in blood cxs

One-liner

50 yo M with history of type A aortic dissection s/p Bentall procedure who was found to have Staph lugdunensis bacteremia and aortic graft infection.

Key Points

Jump to:

Staph lugdunensis!

  • S.lugdunensis has a special position within the coagulase-negative Staphylococci (CoNS) due to it’s pathogenicity and clinical significance.  Unlike other CoNS, S.lugdunensis is known for causing severe and aggressive disease similar to S.aureus.
  • First identified in 1980s and is named after Lyon, France where organism first isolated (Lugdunum = Latin name of Lyon) 
  • Community-acquired and a part of normal skin flora.  Often associated with lower body and extremities — particularly in areas like pelvis, perineum, groin, lower extremities, axilla.  Can be found in nasal cavity 

Clinical Presentations of S.lugdunensis

Micro details for S.lugdunensis

  • Gram positive, catalase positive, coagulase negative
  • May demonstrate hemolysis on BAP
  • Colonies: unpigmented, cream-colored, pale yellow to golden
  • Pearl:  Use of latex agglutination tests for clumping factor to distinguish S.aureus from CoNS → can lead to false ID of S.aureus as it can also agglutinate S.lugdunensis
  • Nowadays, MALDI TOF can accurately identify CoNS to spp level 

 

Why is S.lugdunensis so pathogenic / virulent?

Antibiotic susceptibilities in S.lugdunensis

S.lugdunensis isolates are usually known to be susceptible to most antibiotics, unlike many other CoNS (such as S.epidermidis).  We discussed this on the show, and here are some notes and references:

Penicillin resistance was reported <4% in 1990s, but appears to be increasing (10-25% range)

    • Resistance has been reported as high as 45% in the USA  and 87% in Taiwan 
    • In this paper, PCN susceptibility ranged 48.5-62.5%, lower than published estimates >70% .  If isolate was PCN R, majority with had MIC >=2

Oxacillin/Methacillin

    • Resistance to ox/methicillin and most b-lactam antibiotics is conferred by mecA gene, which is acquired by horizontal transfer of mobile genetic elements called staphylococcal cassette chromosome mec elements (SCCmec)
    • mecA in S.lugdunensis is uncommon.  Ranges from ~3-5% with exception of Taiwan (reported occurrence 21%)
      • CoNS isolates that harbor mecA gene can have oxacillin MICs in 0.5-2 range → which is much lower than oxacillin MICs observed in mecA-positive isolates of S.lugdunensis and S.aureus
      • The CLSI oxacillin breakpoint for S.lugdunensis was modified in 2005 to more precisely reflect presence of mecA 
      • CLSI recommends S.lugdunensis MICs be interpreted using S.aureus oxacillin and cefoxitin breakpoints (rather than those used by other CoNS)
        • S.lugdunensis isolates with oxacillin MIC <=2 mcg/mL = S; those >=4 mcg/mL = R 
        • In contrast to other CoNS for which breakpoint for ox S <=0.25 and for oxacillin R >=0.5
    • Other resistance mechanisms are also rare, but include a novel mecA homologue, mecC
      •  MICs for strains with mecC are typically in resistant range for cefoxitin and/or oxacillin
      • mecC resistance cannot be detected by tests directed at mecA or PBP2A 

Here is a great paper on beta-lactam resistance in S.lugdunensis: McHardy IH, Veltman J, Hindler J, Bruxvoort K, Carvalho MM, Humphries RM. Clinical and Microbiological Aspects of β-Lactam Resistance in Staphylococcus lugdunensis. J Clin Microbiol. 2017;55(2):585-595. doi:10.1128/JCM.02092-16

Other resources and those mentioned above:

The ASM Editors in Conversation podcast covered susceptibility testing for Staph other than S.aureus not too long ago, check out here

Is this a contaminant?!  How to approach possible blood culture contaminants

Ascertain details on the blood cultures

  • How many bottles/sets were positive? Site of stick? Aerobic/anaerobic bottle? Can also ask about gram stain shape to get clue of organism
  • Often there is focus on the number of positive bottles or the ratio (of positive bottles to total bottles), which may or may not be helpful — our episode emphasized how multiple positive sets is concerning in particular! 
  • Longer time to positivity is often suggestive of contaminant, although this is tough to utilize unless extreme (such as cultures are positive days later)

How is the patient doing clinically?

  • Do they have signs or symptoms that would correlate with infection with the bacteria?
  • Do they have risk factors, such as hardware or prosthetic valve/graft/catheter/other material, that may require more consideration of pathogens such as CoNS?

Microbiology! 

  • See the infographic for a reference of organisms that often come into question regarding contaminant or not.  
  • Organisms that should always be considered clinically significant in blood culture:
      • S.aureus
      • S.pneumoniae
      • Grp A and B Strep
      • H.influenzae
      • Gram negatives: Enterobacterales, Pseudomonas, anaerobes
      • Fungi, mold

A few references/readings:

Nuclear medicine imaging can be a resource in diagnosis of complicated infections, such as in our case from this episode.

Thinking about nuclear medicine imaging for endovascular graft infections in particular:

    • CT angiography may be a poor tool, especially if patient recently had surgery
    • Tagged WBC scans have been evaluated.  As Tara mentioned in the show, the limited resolution may decrease its sensitivity, especially for low-grade infections
    • FDG-PET/CT has been used successfully with high sensitivity and specificity
    • Talk to your local nuclear medicine imaging team.  Choices are often limited by what is available.

Some references:

Other miscellaneous mentions and notes:

Episode Art & Infographics

Goal

Listeners will be able to identify Staphylococcus lugdunensis as a pathogen with unique characteristics similar to S.aureus

Learning Objectives

After listening to this episode, listeners will be able to:

  • Describe common clinical presentations of S.lugdunensis
  • Interpret S.lugdunensis antibiotic susceptibilities 
  • Explain an approach to determining whether a positive blood culture organism is a contaminant
  • Discuss the utility of nuclear medicine imaging in the diagnosis of vascular graft infection

Disclosures

Our guest (Tara Vijayan) as well as Febrile podcast and hosts report no relevant financial disclosures

Citation

Vijayan, T., Dong, S. “#9: Arts & Grafts”. Febrile: A Cultured Podcast. https://player.captivate.fm/episode/3c0e8b47-011c-4237-ab6a-5c14c6585134

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