Answer to November 2015 Photo Quiz

Answer: Actinobaculum schaalii. The Gram stain of material from the abscess showed many Gram-positive cocci and Gram-negative coccobacilli. The organism that grew in aerobic culture initially presented as a pleomorphic Gram-positive organism with both coccoid and rod-shaped forms. After extended incubation, the Gram stain morphology appeared more consistently diphtheroid-like, with some branching. Based on the Gram stain morphology, the negative result for catalase, and the negative results for pyrrolidonyl arylamidase (PYR) and leucine aminopeptidase (LAP), the isolate was presumptively identified as Actinobaculum species. Matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) mass spectroscopy using the Vitek MS RUO system with Saramis SuperSpectra database version 4.09 (bioMérieux, Durham, NC) provided a good identification of Actinobaculum schaalii, with an 80.5% confidence value and a reference species proposed as a single choice. The identification was corroborated at the Georgia Public Health Laboratory using DNA sequence analysis of a 401-bp segment of the 16S rRNA gene, which yielded 99.6% homology with A. schaalii strain NR_040859.1 (http://www.ncbi.nlm.nih.gov/GenBank/index.html).

The genus Actinobaculum was first described in 1997, at which time A. schaalii was designated the type species and Actinobaculum suis (formerly Actinomyces suis) was transferred from the genus Actinomyces. Actinobaculum urinale and Actinobaculum massiliense were added in 2003 and 2006, respectively. The phylogenetic relationships of Actinobaculum are not yet clearly established, but existing phylogenetic studies show the genus to be closely related to the genera Actinomyces, Arcanobacterium, Trueperella, and Mobiluncus (1). Actinobaculum grows best under anaerobic or microaerophilic conditions and requires at least 5% CO₂. These Gram-positive coccoid rods have a tendency to branch and may decolorize on a Gram stain. Actinobaculum grows very slowly, producing pinpoint gray colonies <1 mm in diameter, and may show weak beta-hemolysis on agar containing 5% horse or sheep blood after 2 to 5 days of incubation.

A. suis is known to cause actinomycosis of the mammary gland, urinary tract infections, and abortions in sows. The Actinobaculum species that have been found in humans probably belong to the commensal flora of the oral cavity and genitourinary tract and may be present on surrounding skin (2). The elderly appear to be at the greatest risk for colonization (2). Prevalence of infection and pathogenic potential are undoubtedly underestimated, and there are increasing numbers of reports of Actinobaculum infections appearing in peer-reviewed literature. A few reports implicate A. urinale and A. massiliense as causes of chronic cystitis in elderly women. In addition, A. massiliense has been reported as a cause of superficial skin infection (3).

A. schaalii is the most frequently reported species and has primarily been recovered from urine, typically as a cause of unexplained pyuria or urinary tract infection unresponsive to treatment with ciprofloxacin or trimethoprim in elderly patients with underlying urological conditions. A survey of randomly selected urine specimens showed that 22% of 155 urine samples from patients over 60 years of age were positive for A. schaalii at quantities of >10⁴ CFU/ml using real-time PCR (4). Urinary tract infections caused by A. schaalii in adults can progress to urosepsis. Other serious illnesses attributed to A. schaalii include osteomyelitis, endocarditis, necrotizing fasciitis, bacteremia, abscesses, and epididymitis (4, 5, 6, 7, 8). A. schaalii infections, particularly in the urinary tract in both adult and pediatric populations, are frequently reported as polymicrobial (4, 9, 10).

Review of the literature identified six cases of A. schaalii infections in children under the age of 15 years (6, 10, 11). The most common presentation was cystitis, and in nearly all cases, a preexisting urogenital abnormality was identified. All of the reported children were diaper dependent, suggesting that, similar to what was observed for the adult population, colonization with A. schaalii is an important risk factor for development of infection.

Resemblance to normal flora of the skin and mucosa and difficulties identifying Actinobaculum using conventional biochemical tests or test systems that do not include this organism in the identification database have likely led to underrecognition or misidentification of this opportunistic pathogen. In addition, many clinical laboratories incubate urine cultures in ambient air for 24 to 28 h. These conditions will fail to detect Actinobaculum, which requires at least 5% CO₂, and several days of incubation may be needed to produce visible colonies. It is recommended that Actinobaculum be considered in cases of unexplained chronic pyuria that does not respond to treatment with ciprofloxacin or trimethoprim. Discrepancies between direct Gram stain and culture results under aerobic conditions may suggest Actinobaculum infection. Analysis of urine sediment revealed the presence of polymorphonuclear leukocytes in most cases, but the organism does not possess the enzyme needed to reduce nitrate to nitrite, and the result for dipstick urinalysis is therefore negative for nitrites (7).

Susceptibility testing for A. schaalii has not been standardized, and most reports of susceptibility results do not provide MICs or describe the methods or interpretive criteria used. In studies where conditions and MICs or interpretive criteria were specified, A. schaalii isolates were determined to have low MICs for penicillin, amoxicillin, ceftriaxone, cefuroxime, gentamicin, vancomycin, clindamycin, linezolid, tetracycline, and nitrofurantoin. Isolates may also be susceptible to rifampin, erythromycin, and doxycycline, but the numbers of organisms tested have been small. A. schaalii is described as intrinsically resistant to metronidazole and colistin, and ciprofloxacin and trimethoprim-sulfamethoxazole have reduced activity (1). Isolates may test as susceptible to levofloxacin and moxifloxacin in vitro, but the use of these agents is not recommended (1).

Treatment with a β-lactam antibiotic is generally recommended (1, 4). Vancomycin and clindamycin have also been successfully used in several cases (1). The optimal duration of antimicrobial drug treatment is not clearly defined, but treatment for at least 2 weeks is recommended since failure has been experienced with treatment of shorter duration (1, 4, 6, 12).

A. schaalii was the only organism recovered in culture from the abscess in the patient in this case. It is not known if the infection in this case was polymicrobial, since anaerobic cultures were not performed. The Gram-negative coccobacilli that were seen in addition to Gram-positive cocci on the direct Gram stain may have been a second organism or decolorized Actinobaculum. However, since A. schaalii was present in a high quantity, is known to be recovered from groin skin, and has been reported in association with abscesses, this organism was considered a pathogen in this patient. Intravenous vancomycin was continued throughout the patient's hospital stay, and he was discharged 5 days after admission, with wound care instructions and oral clindamycin for completion of 10 days of antibiotic therapy.

(See page 3391 in this issue [doi:10.1128/JCM.02376-13] for photo quiz case presentation.)