Actinobaculum schaalii: An Emerging Uropathogen?

A. schaalii is a rare uropathogen. We report urosepsis with Actinobaculum schaalii detected serendipitously in blood and urine culture in a 79-year-old with urinary tract obstruction. This paper illuminates the flaws in our current system in detecting A. schaalii and raises awareness among clinicians and laboratory teams.

1. Introduction

Actinobaculum schaalii is a small Gram-positive coccoid rod that requires CO2 for optimal growth. Also in the genus Actinobaculum are A. suis, A. urinale, and A. massiliense. A. suis is known to cause urinary tract infections (UTIs) in swine [1, 2] and human UTIs have been reported with A. urinale and A. massiliense [3-5]. A. schaalii, which is the focus of this paper, was first described as a cause of human UTIs in 1997, with subsequent reports [6-8]. Growing evidence suggests that A. schaalii may be a more common uropathogen in the elderly than previously reported, especially those with obstructive uropathy [7-13].

Most laboratories incubate urine cultures at 35°C in ambient air. Since A. schaalii grows slowly in aerobic conditions with no CO2 supplementation, this bacterium's isolation and identification from urine specimens by standard laboratory methods is rare. Most laboratories do not look specifically for Actinobaculum species. Even if A. schaalii grows, this organism may be overlooked either because of overgrowth of common uropathogens or the bacteria's resemblance to normal skin flora [7, 8, 11].

To address this problem, Bank et al. developed a TaqMan real-time quantitative PCR and assessed 252 clinical urine samples [9]. They found 22% of urine samples were positive for A. schaalii in the over 60-year-old patient population, whilst only 7% of samples were positive in the less than 60-year-old cohort [9]. Using the same diagnostic technique in a cohort of patients with kidney stones, they showed that more than 24% of urine samples were positive for A. schaalii, which was the only pathogen in more than 60% of those patients [13].

2. Case Report

A 79-year-old man with a previous history of nephrolithiasis and benign prostatic hyperplasia was admitted with fever, chills, nausea, vomiting, and diarrhea. On presentation, he had an elevated leukocyte count of 17.2 × 103 cells/μL (90% neutrophils) and an elevated creatinine and lactate (155 μmol/L and 2.6 mmol/L, resp.). His initial urinalysis tested positive for blood and leukocytes. An ultrasound of the left kidney showed left hydronephrosis and an obstructing stone in the left ureter. Empiric parenteral therapy with piperacillin-tazobactam was initiated and subsequently a left nephrostomy tube was placed. However, his condition deteriorated and the patient was transferred to the intensive care unit (ICU). A repeat urinalysis revealed 4+ polymorphonuclear cells, 4+ Gram-positive bacilli and was negative for nitrate. The initial urine culture was negative; however, an astute laboratory technologist noticed the discrepancy between the Gram-stain and the culture results. In consultation with the microbiologist, the technologist repeated the urine culture, including an anaerobic incubation. On repeat culture, the patient's urine culture grew 10 million CFU/L of a possible anaerobic Gram-positive bacillus with the blood culture also growing anaerobic Gram-positive bacilli. Due to the unusual nature of the cultures in this urosepsis patient, the samples were sent to the reference laboratory for microorganism identification. A. schaalii was identified from blood culture. Although the care providers were informed upon initial Gram-positive bacilli observation on smear, the final bacterial identification result was released 12 days after specimen receipt. Fortunately, A. schaalii is usually sensitive to pip-tazo and the patient became stable following a short stay in ICU. Subsequently, the patient's stone was unsuccessfully treated with extracorporeal shock wave lithotripsy (ESWL) and later definitively treated with an ureteroscopy and laser lithotripsy.

3. Discussion

A. schaalii is an emerging uropathogen mainly in elderly patients or patients with underlying urologic disease [7, 9, 13]. Although there are a few reports of nonurological infections [14, 15], the spectrum of infection primarily ranges from benign cystitis to severe pyelonephritis with urosepsis. A. schaalii has been reported in both inpatient and outpatient populations, either as an isolated organism or associated with other common uropathogens [8–11, 13]. Unfortunately, our routine laboratory methods will not identify A. schaalii from urine specimens as this bacterium requires CO2 and has sluggish growth in comparison to common uropathogens. For this reason, A. schaalii is identified from the blood rather than from the urine in 30% of identified cases [11].

Urologists are recommended to notify the laboratory if they are concerned about urinary infection with an unusual organism such as A. schaalii, based on risk factors or conflicting laboratory results. Accordingly, conventional laboratory methods must be modified to isolate A. schaalii from specimens upon such requests. In addition, laboratories should also consider A. schaalii as an etiology for UTI once there is no growth from the urine specimen, despite the presence of Gram-positive coccoid rods and/or leukocytes present on the urinalysis.

Another important issue relates to the susceptibility profile of A. schaalii is that it retains susceptibility to penicillins, 3rd generation cephalosporins, aminoglycosides, and nitrofurantoin. However, this organism is mainly resistant to ciprofloxacin and trimethoprim/sulfamethoxazole [11, 12], two commonly employed antibiotics in urology patients. To our knowledge, all of the A. schaalii reports and studies have been carried out in Europe with the exception of a single case report from North America [16]. Therefore, most of our clinicians are not aware of this potential uropathogen. We believe that a well-designed multicenter study is required to find out the prevalence and incidence of A. schaalii.