Background
Actinobaculum schaalii is a Gram-positive, facultative anaerobic, nonmotile coccoid rod, classified as a new genus in 1997 [
1]. The genus
Actinobaculum includes
A. schaalii, A. suis, A. massiliae, and
A. urinale and is closely related to the genera
Actinomyces and
Arcanobacterium [
1].
Actinobaculum schaalii grows slowly after 48 h in an anaerobic atmosphere at 37°C as tiny grey colonies, less than 1 mm in diameter, and shows weak β-hemolysis on agar plates containing 5% horse or sheep blood. It is catalase, oxidase, and urease negative and is easily overgrown by other bacteria, which are often found concomitantly. Because of its slow anaerobic growth and resemblance to the normal bacterial flora on skin and mucosa, A. schaalii is often overlooked or considered a contaminant. These difficulties regarding detection and identification impede evaluation of the clinical impact of this pathogen and of its potential to cause invasive infection.
A. schaalii has been reported to be responsible for urinary tract infections, mainly in elderly patients with underlying urological predispositions [
2‐
5]. It has also been recovered from other human clinical specimens than urine such as blood, but its pathogenic potential remains unknown.
Since 1999, A. schaalii is routinely screened at our hospital. The objective of this study was to determine the clinical significance of A. schaalii, identified in our microbiology laboratory over the last 11 years.
Results
40 specimens with detection of
A. schaalii were obtained from 27 different patients during the last 11 years. The patient's median age was 81 (19-101) years, and there was a slight male predominance (59.3%). Twenty-five patients (92.6%) had underlying diseases, of which cardiopathy was most commonly encountered (14, 51.9%). Twelve patients (44.4%) had a pathology of the genitourinary tract, mostly prostatic hyperplasia (5, 18.5%) and five patients (18.5%) had an urinary catheter or a double- J-catheter in place at the time of detection of
A.
schaalii. Only one patient (3.7%) was receiving immunosuppressive treatment due to renal transplantation (Table
1).
Table 1
Baseline characteristics and outcome
Age (years)
| |
Mean | 73 |
Median | 81 |
Range | 19-101 |
Gender
| |
Male | 16 (59.3%) |
Female | 11 (40.7%) |
Hospitalstay (days)
| |
Mean | 18 |
Median | 9 |
Range | 1-79 |
Immunosuppression
|
1 (3.7%)
|
Comorbidities
|
25 (92.6%)
|
Cancer | 7 (25.9%) |
Cardiopathy | 14 (51.9%) |
Chronic renal insufficiency | 4 (14.8%) |
Diabetes mellitus | 4 (14.8%) |
Neurological disorder | 13 (48.1%) |
Hypertension | 6 (22.2%) |
Chronic pulmonary disorder | 2 (7.4%) |
Chronic Vascular disorder | 4 (14.8%) |
Endocriological disorder (other than diabtes mellitus | 3 (11.1%) |
Chronic pancreatitis | 1 (3.7%) |
Underlying genitourinary tract pathology
|
12 (44.4%)
|
Nephrolithiasis | 1 (3.7%) |
Stenosis | 4 (14.8%) |
Double-J catheter | 2 (7.4%) |
Urinary catheter | 3 (11.1%) |
Prostatic hyperplasia | 5 (18.5%) |
Renal cysts | 1 (3.7%) |
Uterine and vaginal prolapse | 1 (3.7%) |
Renal transplantation | 1 (3.7%) |
Outcome
| |
Discharge | 17 (63.0%) |
Referal to another hospital | 3 (11.1%) |
Referal to a longterm-care facility | 5 (18.5%) |
Death | 2 (7.4%) |
A. schaalii was detected in 21 blood cultures, 7 deep tissue biopsies and 12 urine cultures. Its presence was interpreted as colonization of the urinary tract without clinical signs or symptoms of infection in five (18.5%) patients, accounting for 41.6% of all urine samples. Six patients (22.2%) suffered from urinary tract infections, six (22.2%) from abscesses (skin, intraabdominal, genitourinary tract, and surgical site infections) and 10 (37.0%) from bacteremia (Table
2).
Table 2
Distribution of infections and colonisation with Actinobaculum schaalii
with urinary tract infection | 5 (18.5%) |
with intaabdominal infection | 3 (11.1%) |
with spondylodiscitis | 1 (3.7%) |
with pneumonia | 1 (3.7%) |
Abscess
|
6 (22.2%)
|
Abdominal | 1 (3.7%) |
Skin | 1 (3.7%) |
Surgical site infection (upper jaw and after bladder-stone extraction) | 2 (7.4%) |
Genitourinary tract | 2 (7.4%) |
Urinary tract infection
|
6 (22.2%)
|
with detection of A. schaalii in urine culture | 6 (22.2%) |
Colonisation (detection of
A. schaalii
in urine without clinical signs of infection
)
|
5 (18.6%)
|
A. schaalii affected the genitourinary tract infections in most cases (48.1%). The majority of patients (10/13, 76.9%) affected had underlying pathologies of the urinary tract. In three patients with positive blood cultures, diagnosed as urosepsis, Gram-positive rods (10
4-10
6 colonies per ml) were detected in urine cultures, however, weren't further identified as these cultures were polymicrobial. Abscesses of the genitourinary tract involved the scrotum in one patient and the renal pelvis in another (Table
2).
Surgical site infections occurred in two patients, one after an operation involving the upper jaw, the other after bladder-stone extraction.
Bacteremia with
Actinobaculum schaalii was associated with spondylodiscitis and pneumonia, in one case respectively, with intraabdominal infection in three cases and with urinary tract infections in 5 cases (Table
2). In the patient with spondylodiscitis,
Actinobaculum schaalii was also detected in the CT-guided biopsy of the disc. Underlying intraabdominal infections detected were occlusive ileus and diverticulitis.
Twenty-three (57.5%) specimens were monobacterial, the remaining 17 (42.5%) were polybacterial 7/7 deep tissue samples, five blood cultures, and five urine cultures (Table
3). In patients with polybacterial blood cultures, the other pathogens detected were
Pseudomonas
aeruginosa (
P. aeruginosa in 2 of 2 and
A. schaalii in 1 of 2 blood cultures) in a patient with urosepsis,
Finegoldia magna (
Finegoldia magna in 2 of 2 and
A. schaalii in 4 of 4 bloodcultures) in a patient with diverticulitis,
Clostridium clostridiforme and
Bacteroides
fragilis (both in 1 of 4 and
A. schaalii in 1 of 4 blood cultures) in a patient with obstructive ileus and
Veillonella (in 2 of 8 and
A. schaalii in 8 of 8 bloodcultures) in a patient with unclear intraabdominal infection. Of interest, the patient with
P. aeruginosa and
A. schaalii in the blood culture, also had Gram-positive rods (10
4 CFU/ml) and Gram-negative rods (10
5 CFU/ml) in the urine culture, however, these weren't further differentiated.
Table 3
Distribution of monobacterial and polybacterial isolates of Actinobaculum schaalii
monobacterial | 23 (57.5%) |
polybacterial | 17 (42.5%) |
Urine
|
12
|
monobacterial | 7 (58.3%) |
polybacterial | 5 (41.7%) |
Blood
|
21
|
monobacterial | 16 (76.2%) |
polybacterial | 5 (23.8%) |
Abscess
|
7
|
monobacterial | 0 (0.0%) |
polybacterial | 7 (100.0%) |
Six patients received no antibiotic treatment, in five of which detection of A. schaalii was interpreted as colonization. The remaining patient suffered from an abscess involving the skin, which was surgically drained. Most patients were treated with amoxicillin/clavulanic acid (10, 37.0%) followed by administration of a quinolone (7, 25.9%).
Median duration of antibiotic treatment was 15.5 days (range 3 to 84 days). The majority of patients could be discharged (17, 63.0%), two (7.4%) patients died due to metastatic carcinoma, the remaining were transferred to another hospital (3, 11.1%) or to a long-term-care facility (5, 18.5%).
Discussion
A. schaalii caused an invasive infection in 81.5% of our patients and its presence was interpreted as colonization of the urinary tract in five patients. Infections of the genitourinary tract were by far the most common manifestation of invasive disease. The ability of this pathogen to cause urinary tract infections has also been reported in the literature previously. Beguelin et al. described the clinical features of 20 patients and found urinary tract infections to be the most common manifestation (11/20, 55%) [
10]. The authors further described a high rate of underlying urinary tract pathology as detected in our study. We detected
A. schaalii in the blood cultures of 10 patients, only half of which were associated with urinary tract infections. The majority of the remaining patients had underlying intraabdominal infections and one patient suffered from spondylodiscitis. Therefore, detection of this pathogen in blood cultures should prompt the clinician to search also for abdominal or other foci if no association with the genitourinary tract can be found.
Beguelin et al. described the results of antimicrobial resistance testing in their study, although there are no recognized minimal inhibitory concentration (MIC) interpretative standards for
A. schaalii [
10]. They found that most of their strains showed high MIC levels to ciprofloxacin (>32 mg/L) and to trimethoprim/sulfamethoxazole (>32 mg/L), however, all of their 20 isolated tested, showed low MIC level to beta-lactams (≤ 0.19 for penicillin) [
10]. Similar results were described by Cattoir et al. after testing 48 clinical isolates [
11]. The majority of our patients were treated with betalactam-antibiotics, seven however, all with urinary tract infections, were treated with quinolones, possibly without in vitro activity. We therefore conclude, that susceptebilty testing should be routinely performed, when
A. schaalii is detected in relevant clinical specimens and interpretative standards for interpretation of MICs should be developed. Empirically initiated treatment for urinary tract infections often consists of quinolones or trimethoprim/sulfamethoxazole and may therefore, not be effective.
A. schaalii was often detected together with other bacteria (17 of 40 cultures, 42.5%), especially in urine cultures (5 of 12, 41.7%) and in all abscesses (7 of 7, 100%). In contrast, Beguelin et al. described a much lower rate of polybacterial cultures, with 18 out of 21 (86%) specimens being monobacterial [
10]. As this pathogen is difficult to detect in the laboratory and is easily overgrown by other bacteria, awareness of its existence is crucial as not to underestimate its presence. Bank et al. analyzed 252 urine samples for presence of
A. schaalii by a real- time quantitative PCR they developed and found 41 of 252 samples (16%) to be positive with bacterial concentrations >10
4 CFU/mL [
3]. Of 155 urine samples from patients >60 years of age, 34 (22%) were PCR positive, of which 31 (91%) - the vast majority - harbored other common uropathogenic bacteria, in addition to
A.
schaalii. In our study three patients with
A. schaalii in their blood cultures had Gram-positive rods (10
4-10
6 colonies per ml) in urine cultures drawn at the same time, however, these weren't further identified as cultures were polymicrobial. One of the patients described in our study with
P. aeruginosa and
A. schaalii in blood cultures, also had Gram-positive rods (10
4/ml) and Gram-negative rods (10
5/ml) in the urine culture drawn at the same time. It is plausible that the pathogens detected in the urine culture, were
A. schaalii and
P. aeruginosa, respectively, demonstrating that
A. schaalii has the ability to invade the bloodstream even in the presence of a well-known uro-pathogen as
P. aeruginosa.
As our data demonstrate that this pathogen invaded the bloodstream in the presence of other bacteria in primary foci, we conclude that
A. schaalii is not just an innocent bystander, but has the potential to become an invasive pathogen, leading to severe invasive infection as sepsis, spondylodiscitis [
12], and endocarditis [
13].
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
All authors read and approved the final manuscript. ST-S wrote the manuscript and analyzed the data RF performed and supervised the microbiological analyses. MW supervised and edited the manuscript. DG performed and supervised the microbiological analyses. AFW initiated the study, analysed the data, supervised the whole project and edited the manuscript.