under the microscope
By Rodney E . Rohde , PhD , MS , SM ( ASCP ) CM SVCM , MBCM , FACSc
Abiotrophia defective : Quit Being So Difficult
Ultimately , A . defectiva and similar organisms require an approach through multiple paths for detection
– including cultures , serology , histopathology , and molecular methods , with blood cultures being the critical foundation .
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In
In this column , I will focus on the Abiotrophia defectiva as it relates to important and dangerous cases of endocarditis . Infective endocarditis ( IE ) refers to an infection involving the endocardial surface of the heart .
Most IE cases are caused by viridans Streptococci or Staphylococci species . Abiotrophia defectiva is known to cause less than 1 percent of cases of IE . Though rare , it can cause life-threatening complications such as septic embolization , destruction of heart valves , and heart failure especially if not detected and treated quickly . A . defectiva is a part of the normal flora of the oral cavity , the urogenital and the intestinal tracts . For both physicians and medical laboratory professionals , this difficult bacterium causes many headaches and difficulties as to diagnosis and treatment . Though rare , nutritionally variant Streptococci ( NVS ) are estimated to cause approximately 5 percent to 6 percent of all cases of IE , including being a major cause of blood culture-negative IE . Importantly , this can result in early misdiagnosis or non-diagnosis .
Abiotrophia and Granulicatella spp . often appear as Gram-positive cocci in pairs or chains but can be pleomorphic . Sometimes however , they can appear more like coccobacilli or stain as Gram variable . Gram stain pleomorphism can be particularly apparent with suboptimal nutritional conditions . Gram stains consistent with a streptococcal-like morphology that are not detected by common
used commercially available panels should raise suspicions for Abiotrophia or Granulicatella , among other organisms .
There have only been about 125 published cases of A . defective in the research literature . They do not synthesize pyridoxine , L-cysteine or other essential nutrients required for growth and depend on other bacteria ( satellite ) or enriched media for enhanced growth . The fastidious nature of this microbe often means that initial cultures appear to be sterile . A . defective is a pleomorphic organism , appearing as Gram-positive cocci , coccobacilli , and bacilli forms depending on the culture media . Literature suggests that due to the production of a considerable amount exopolysaccharides , the organism has a higher affinity for the endocardium and the ability to bind with fibronectin in the extracellular matrix , further contributing to their virulence .
Due to the difficulty in culturing this organism , one may need to utilize molecular diagnostic tools to raise sensitivity and specificity for quicker diagnosis , which can often lead to better patient outcomes . For example , Matrix-assisted laser desorption ionization – time of flight mass spectrometry ( MALDI-TOF MS ) with the Vitek MS using library v3.2 can assist in the detection of Abiotrophia defectiva . Clinical suspicion regarding this organism should arise in cases of culture-negative endocarditis and additional testing with supplemented media ( e . g ., chocolate agar ) should be performed to encourage growth of colonies . Further , susceptibility testing may be required via broth microdilution at a reference laboratory and interpreted by Clinical and Laboratory Standards Institute ( CLSI ) guidelines for proper treatment .
Ultimately , A . defectiva and similar organisms require an approach through multiple paths for detection – including cultures , serology , histopathology , and molecular methods , with blood cultures being the critical foundation . If surgery is considered , histopathologic analysis and culture of the explanted heart valve is critical because it may assist with confirmation of valve involvement and help inform duration of antimicrobials . Often culture of the valve is negative which may be due to the use of antimicrobial therapy prior to surgery . Abiotrophia and Granulicatella may also cause “ culture-negative ” endocarditis when antimicrobial therapy is administered during or prior to the drawing of blood cultures .
Rodney E . Rohde , PhD , MS , SM ( ASCP ) CM SVCM , MBCM , FACSc , serves as chair and professor of the Clinical Laboratory Science Program at Texas State University . Follow him on Twitter @ RodneyRohde / @ TXST _ CLS , or on his website : http :// rodneyerohde . wp . txstate . edu /
References :
Agrawal U , Prabhu MM . Abiotrophia defectiva : A Rare but Critical Cause of Infective Endocarditis . Cureus . 2019 ; 11 ( 12 ): e6492 . 2019 Dec 28 . doi : 10.7759 / cureus . 6492
Dumm RE , Wing A , Richterman A , Jacob J , Glaser LJ , Rodino KG . 2021 . The Brief Case : A variant on a classic — Abiotrophia defective endocarditis with discitis . J Clin Microbiol 59 : e03093-20 . https :// doi . org / 10.1128 / JCM . 03093-20 .