Emery Haley, PhD, Scientific Writing Specialist
Actinotignum schaalii (previously known as Actinobaculum schaalii)
Clinical Summary
- A. schaalii is a nitrite-negative, biofilm-forming, gram-positive microorganism.
- A. schaalii is fastidious and cannot grow in standard urine culture conditions.
- A. schaalii is associated with complicated UTIs in young children, older adults, and individuals with urinary tract abnormalities.
- In symptomatic UTI patients, A. schaalii:
- Is not a contaminant (is found in catheter-collected urine specimens).
- Is viable (can grow out in expanded culture conditions).
- Is pathogenic (associated with elevated urine biomarkers of infection).
- Reported severe complications of A. schaalii UTI include pyelonephritis, hyperammonemia, bacteremia, and urosepsis.
Bacterial Characteristics
Gram-stain
Gram-positive
Morphology
Coccobacillus (intermediate between round and rod-shaped)
Growth Requirements
Fastidious (slow growing, prefers blood-agar)
Anaerobe/microaerophile
Nitrate Reduction
No
Urease
Negative
Biofilm Formation
Yes
Pathogenicity
Colonizer or Pathobiont
Clinical Relevance in UTI
A. schaalii was first identified as a human pathogen and classified as Actinobaculum schaalii in 1997 when the novel Actinobaculum genus was described.[1] In 2015 the Actinobaculum genus was split into Actinobaculum and Actinotignum, with A. schaalii being reclassified as Actinotignum schaalii.[2]
A. schaalii has been recognized for over a decade as a resident of the human urogenital microbiome.[3] By 2016 it had also been reported as the cause of at least 121 urinary tract infections,[4] predominantly in older adults,[5,6] young children, [7,8] and individuals with urinary tract abnormalities [9–12]. A. schaalii has also been reported to participate in the formation of polymicrobial biofilms on indwelling catheters.[13]
A. schaalii UTIs are likely significantly underdiagnosed due to the numerous challenges in the identification of A. schaalii.[4,14] Firstly, A. schaalii lacks nitrate reductase activity, so screening strategies involving urinalysis for nitrite positivity will be false-negative.[15,16] Secondly, growing A. schaalii in culture requires an anaerobic atmosphere with at least 5% CO2, particular nutritional requirements (blood agar medium), and extended incubation times of 48-72 hours.[14] These conditions are not typically used in clinical laboratories performing standard culture techniques for UTI diagnosis. Thirdly, even when this organism does grow in culture, it is often overgrown by other faster-growing species or dismissed as an irrelevant gram-positive commensal organism of the urogenital microbiome and labeled as a “contaminant”.[14] Instead, A. schaalii UTI is most often diagnosed by advanced proteomic techniques, such as Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS), or advanced molecular techniques, including polymerase chain reaction (PCR) and sequencing.[4,11,14,17]
A study using expanded quantitative urine culture (EQUC), a technique growing a larger urine volume with additional nutritional media, different atmospheric conditions, and longer incubations, found that the organism was viable.[18] In a study of older adult males and females with clinically suspected complicated UTI, A. schaalii was detected in both midstream voided and in-and-out-catheter collected specimens indicating that it was truly present in the bladder, not simply a contaminant picked up during voiding.[19] Furthermore, elevated markers of immune system activation in the urinary tract have been measured from the same clinical urine specimens in which A. schaalii was detected, indicating that the presence of A. schaalii was associated with an immune response to urinary tract infection.[20–22]
Critically, A. schaalii UTIs have also been reported to be resistant to several antibiotics [12] and to result in severe complications, including hyperammonemia [23] and progression to pyelonephritis,[10,24] bacteremia, [24,25] and even urosepsis [26]. Therefore, rapid and accurate diagnosis of A. schaalii as a causative pathogen in cases of cystitis is crucial for the prevention of urosepsis in at-risk individuals, including young children, older adults, individuals with urinary tract abnormalities, and individuals with significant comorbidities.[4]
Together, these findings demonstrate the value of detecting this organism and indicate that A. schaalii should be seriously considered as a uropathogen when detected in any individual with UTI symptoms.
Treatment
Evidence of Efficacy (Checkmarks) [4,27–29]: Amoxicillin/Clavulanate, Ampicillin/Sulbactam, Ampicillin, Ceftriaxone, Doxycycline, Gentamicin, Levofloxacin, Linezolid, Nitrofurantoin, and Vancomycin.