Clinical Characteristics of Actinomyces viscosus Bacteremia

Background and Objectives: Actinomyces species are part of the normal flora of humans and rarely cause disease. It is an uncommon cause of disease in humans. The clinical features of actinomycosis have been described, and various anatomical sites (such as face, bones and joints, respiratory tract, genitourinary tract, digestive tract, central nervous system, skin, and soft tissue structures) can be affected. It is not easy to identify actinomycosis because it sometimes mimics cancer due to under-recognition. As new diagnostic methods have been applied, Actinomyces can now more easily be identified at the species level. Recent studies have also highlighted differences among Actinomyces species. We report a case of Actinomyces viscosus bacteremia with cutaneous actinomycosis. Materials and Methods: A 66 years old male developed fever for a day with progressive right lower-leg erythematous swelling. Blood culture isolates yielded Actinomyces species, which was identified as Actinomyces viscosus by sequencing of the 16S rRNA gene. In addition, we searched for the term Actinomyces or actinomycosis cross-referenced with bacteremia or “blood culture” or “blood stream” from January 2010 to July 2020. The infectious diseases caused by species of A. viscosus from January 1977 to July 2020 were also reviewed. Results: The patient recovered well after intravenous ampicillin treatment. Poor oral hygiene was confirmed by dental examination. There were no disease relapses during the following period. Most cases of actinomycosis can be treated with penicillin. However, clinical alertness, risk factor evaluation, and identification of Actinomyces species can prevent inappropriate antibiotic or intervention. We also compiled a total of 18 cases of Actinomyces bacteremia after conducting an online database search. Conclusions: In summary, we describe a case of fever and progressive cellulitis. Actinomyces species was isolated from blood culture, which was further identified as Actinomyces viscosus by 16S rRNA sequencing. The cellulitis improved after pathogen-directed antibiotics. Evaluation of risk factors in patients with Actinomyces bacteremia and further identification of the Actinomyces species are recommended for successful treatment.


Introduction
The first case of actinomycosis was described in cattle by a pathologist in 1877 [1]. Then, shortly afterward in 1890, Actinomyces israelii was discovered in humans [2]. These bacteria are Gram-positive, filamentous, and rod-shaped. They are mostly facultative anaerobic organisms, which normally colonize the oral cavity, gastrointestinal tract, colon, and vagina [3,4]. However, only a handful of species are known to be associated with human infections such as A. israelii, A. meyeri, A. neuii, and A. turicensis [3,4]. Odontomyces viscosus was first found in the periodontal plaque of hamsters in 1958. The species was classified and named "Actinomyces viscosus" [5,6]. The two closely related species A. viscosus and A. naeslundii showed high phenotypic and serological relatedness and could be isolated from dental plaque and mucosa samples [7]. A. naeslundii Serotype II was renamed "Actinomyces oris" in 2009 [8], and human strains which have been assigned to A. viscosus are likely members of A. oris [9]. Despite being a commensal bacterium in the majority of human adults with teeth, A. viscosus has rarely been reported to cause disease [10].
Actinomyces was redefined to include catalase-positive organisms in 1969 [11]. Unlike other species which are catalase-negative and indole-positive, these colonies are catalasepositive and indole-negative. Actinomyces species can be cultured on blood agar (BA) with selective nutritional requirements [12]. In the 1980s, a new standard of 16S rRNA gene sequencing for bacterial identification was introduced and also used to establish the genotypic taxonomy [13][14][15]. In order to identify bacteria at the genus and species level, we used 16S rRNA gene sequencing technology to quickly perform reliable identification of bacteria in this case.
Evidence of actinomycosis infection is based on accurate identification of Actinomyces species. Therefore, accuracy is a key factor in preventing unnecessary invasive intervention and facilitates the selection of proper treatment [16,17]. The most common pathogenic actinomycosis species include A. viscosus and A. meyeri [2][3][4]12]. According to some reviews, A. israelii and A. meyeri have been identified as frequently encountered specimens from periappendiceal abscesses and abdominal actinomycosis [12,18].
Cutaneous actinomycosis is uncommon in clinical practice [19] and is usually a secondary infectious process with an underlying focus in deeper tissues [20], or it may appear as a result of hematogenous spread from an actinomycotic lesion elsewhere in the body [4]. In cutaneous actinomycosis, the commonly found causative organisms were A. meyeri and A. viscosus according to previous reports [4,12]. Oral hygiene is a recognized risk factor for the development of cutaneous actinomycosis [21]. According to a literature review of original clinical studies on Actinomyces, this species can be a source of invasive disease when superadded by periodontal disease and poor oral hygiene, leading to the development of infections. Although oral cervicofacial actinomycosis is the common form, Actinomyces can also result in infection of the thoracic, abdominopelvic, cutaneous, musculoskeletal system, pericardium, and central nervous system, as well as in disseminated disease [3].
In the present review, we report a case with A. viscosus bacteremia and cellulitis in a patient with poor oral hygiene, and we gathered this information to provide a comprehensive and microbiologically consistent overview of the A. viscosus bacteremia and cutaneous actinomycosis in human infections.

Case Presentation
A 66 years old male with hypertension and bipolar disease under treatment with lithium, fludiazepam, and seroquel came to our emergency department due to fever up to 38.9 • C (102 • F) without chills for a day. The patient complained of an erythematous painful swelling over the right lower leg, which developed gradually during the previous week. The patient also mentioned nausea with vomiting for 2 days, which did not seem to be related to meals. Otherwise, no other symptoms were mentioned. There was no traumatic history, no bug bite, and no exposure to livestock. On physical examination, blood pressure was 131/66 mmHg, heart rate was 91/min, respiratory rate was 18/min, and body temperature was 38.9°C. Erythematous change measuring around 10 × 20 cm over the anterior tibial region with tenderness was found, but there were no excoriated skin lesions in this region. Laboratory tests showed notable leukocytosis with neutrophil predominance. C-reactive protein level was 17.9 mg/dL, procalcitonin level was 14.5 ng/mL, serum creatinine level was 1.55 mg/dL, and ClCr level was 51 mL/min. Two sets of blood cultures were obtained and yielded Actinomyces sp. Further sensitivity tests showed susceptibility to clindamycin, penicillin, ampicillin-sulbactam, cefoxitin, cefmetazole, and carbapenem, but resistance to metronidazole. Clindamycin 600 mg Q8H was administered for Actinomyces bacteremia and cellulitis. The erythematous swelling of the right lower leg improved gradually, and antibiotic treatment was shifted to sole intravenous ampicillin 2 g every 6 h on admission day 10 due to clinical improvement. There was no bacterial growth in the following blood culture.
Due to poor oral hygiene, a dentist was consulted for evaluation. Full-mouth gingivitis with plaque deposition and easy bleeding on probing was found during the dental examination. The patient then completed 7 days of intravenous ampicillin and was discharged with oral ampicillin 500 mg every 6 h. Further 16s rDNA sequencing for identification of pathogens revealed Actinomyces viscosus. The primers used for amplification of the 16S ribosomal DNA gene were 27F/1525R, 8F2/806R, and fD1modF/16S1RR-B. The amplification products obtained by PCR were sequenced, and the sequences obtained (791 bp) were compared to known 16S ribosomal DNA sequences in the GenBank database of the National Center for Biotechnology Information using the BLASTN algorithm (http://www.ncbi.nlm.nih.gov/blast accessed on 7 September 2021). The closest match was obtained with Actinomyces viscosus (GenBank accession number NR_113030; maximal score 848, E value 0.0, and maximal identity 84% (600/711)). There was no evidence of any diseases during follow-up.

Literature Review
A review of the English-language literature on Actinomyces bacteremia was conducted. Key search terms were Actinomyces OR actinomycosis cross-referenced with bacteremia OR "blood culture" OR "blood stream" in Pub Med/NCBI and other similar databases from January 2010 to July 2020. The infectious diseases caused by species of A. viscosus from January 1977 to July 2020 were also reviewed. All of the relevant information obtained from the literature review is presented in tables.

Results
We compiled a total of 18 cases of Actinomyces bacteremia from the online database search. The clinical and microbiological characteristics of these cases are summarized in Table 1.
The average age of the patients was 52.5 years, ranging widely from 23 weeks gestational age to 90 years old. Eight patients (44%) were male. Seven patients (39%) had an underlying systemic condition, which resulted in a relatively immunocompromised status (one alcoholic liver cirrhosis, four diabetes mellitus, one preterm labor, and one monoclonal gammopathy). Seven patients (39%) had previous exposure to invasive procedures or implantations (two IUD, one TVOR, one colonoscopy, one surgery, one endoscope, and one central-line placement). Eleven (61%) patients had fever. Fifteen (83%) patients had a primary site of infection; five (33% = 5/15) of these were gynecology-genital organ infection (including testicular abscess), followed by three (20%) urinary tract infections, two (13%) pulmonary infections, two (13%) soft-tissue infections, and one case each of (7%) meningitis, abdominal infection, and endocarditis.
Most of the treatments included penicillin-based antibiotic administration. Alternative antibiotics included third-generation cephalosporin (ceftriaxone for example) or clindamycin. Broad-spectrum antibiotics, such as carbapenems, were used in severe or comorbid patients. Three patients (17%) required further surgical intervention (endocarditis, necrotizing soft-tissue infection, and pelvic inflammatory disease with abscess). Most of the patients had favorable outcomes, and only a few patients with multiple comorbidities died or suffered from morbidity.
We also reviewed previous infectious disease with the species Actinomyces viscosus as a causative organism from January 1977 to July 2020, using the search term "Actinomyces viscosus" OR "A. viscosus", as shown in Table 1B. Fifteen studies were found with a total of 19 relevant human cases. The clinical and microbiological characteristics of these cases are summarized in Table 2. The 19 cases comprised 10 men and nine women. The medium age of the patients was 35 years, ranging in age from gestational age of 23 weeks to 81 years old. Six patients (32%) had a relatively immunocompromised status (multiple myeloma, alcoholism, pancreatic cancer, acute lymphoblastic leukemia under chemotherapy, and psoriatic arthritis under methotrexate). Eleven patients (57%) suffered from fever. As to the acquired specimens, in four patients (21%), Actinomyces viscosus was found from blood culture. Seven patients (37%) needed biopsy or tissue culture for Actinomyces viscosus isolation. Other specimens included pus, drainage or discharge from submandibular, neck, chest wall, and breast abscess, subdural empyema, pleural fluid, percutaneous transtracheal aspiration, and vitreous washings. Most of the specimens were diagnosed by biochemical analysis, and only two specimens were analyzed using PCR (in the year 2005) or 16s sequencing (in the year 2018). Only one patient suffered from subdural empyema, resulting in death.  Discharge to another healthcare facility [32] Abbreviation: GERD = gastroesophageal reflux disease; ALL= acute lymphocytic leukemia; COPD = chronic obstructive pulmonary disease; HELLP = hemolysis, elevated liver enzymes, and low platelets.

Discussion
Actinomyces species are opportunistic pathogens and capable of causing disease, which often invade the body following the disruption of mucosal barriers [4,8].
There are nine Actinomyces species that are commonly found in the oral cavity, namely, A. israelii, A. viscosus, A. odontolyticus, A. naeslundii, A. georgiae, A. gerensceriae, A. meyeri, A. radicidentis, and A. graevenitzii [52,53]. A. israelii is known as the most common bacterium associated with classical actinomycosis, which has been linked with dental abscesses and oral infections [12]. In this study, we reviewed cases of Actinomyces bacteremia in databases and found that 15 of 18 patients had at least one risk factor.
According to the analysis, seven patients were relatively immunocompromised (one alcoholic liver cirrhosis, four diabetes mellitus, one preterm labor, and one monoclonal gammopathy). Seven cases had a procedure-related risk factor (two IUD, one TVOR, one colonoscopy, and one surgery), and four cases had evidence of poor oral hygiene. The diagnostic method and basic characteristics of Actinomyces bacteremia cases reported from 2010 January to 2020 July are summarized in Tables 1 and 2. A phenotypic test is an easily applied, rapid, and cost-efficient tool for identification of Actinomyces species. The classical method is based on phenotypic tests such as gas production from glucose, urease, catalase, and acid production [8]. However, it is difficult to determine the taxon when species have the same characteristics [4]. With the low cost of 16S rRNA sequencing in recent years, it is possible to use this reliable and accurate approach currently available in clinical practice. In this study, we used 16S sequencing to confirm the pathogen, which was identified as Actinomyces viscosus.
In general, Actinomyces species are susceptible to penicillin and beta-lactam antibiotics [54]. Some studies reported that actinomycosis can be treated successfully with ceftriaxone [23,27,54], but several case reports noted that some isolates were resistant to ceftriaxone, such as A. europaeus and A. graevenitzii [12,54]. A retrospective assessment of antimicrobial susceptibility testing of isolates showed that the Actinomyces spp. were also susceptible to carbapenems, tetracyclines, clindamycin, and vancomycin as alternative treatments [54]. However, another interesting observation was that Actinomyces isolates were resistant to doxycycline [12] and clindamycin, showing a high range from 30% to 80% with poor susceptibility rates [55]. Additionally, in the present study, it was found that A. meyeri and A. odontolyticus were resistant to tetracycline and vancomycin; in most Actinomyces spp., isolates showed high resistance to metronidazole and quinolones as "intrinsic resistance" [55]. Therefore, the susceptibility profiles of Actinomyces are important as they help to inform the selection of an appropriate treatment [54]. In our study, the patient recovered well after intravenous ampicillin treatment.
A recent study on the healthy human oral microbiome showed that Actinomyces species are part of the oral flora in Taiwanese populations [56]. Human isolates of Actinomyces viscosus showed high phenotypic and serological relatedness to A. naeslundii [57]. Therefore, we used 16S rRNA sequencing analysis for identification of A. viscosus; it was also possible to differentiate it from other closely related species in the genera Arcanobacterium and Actinobaculum [58,59]. The oral cavity could be considered a causative factor in human actinomycosis [60]. The isolation of Actinomyces spp. from blood culture is traditionally regarded as clinically significant [61]. However, the possibility of contaminants does exist [62]. Sound clinical judgment based on the presence of risk factors is essential to interpretate the results. According to the literature review of original clinical studies on Actinomyces, this species can become pathological when superadded by periodontal disease and poor oral hygiene, leading to the development of infections. The mucosal barrier is disrupted by triggering factors such as plaque, tooth cavities, and periodontitis in the case of oral infections [19,63,64].
In this study, our patient had poor oral hygiene and chronic periodontitis, which were considered risk factors. A. viscosus is an opportunistic pathogen in the oral cavity. This may have been the source of bacteremia in our patient, who had hematogenous spreading, which led to cellulitis of the right lower leg. Furthermore, immunocompromised patients, such as those with diabetes, human immunodeficiency virus (HIV), etc., patients who have undergone a surgical or invasive procedure, and patients with local tissue damage, are also traditionally considered to be at greater risk for actinomycosis.

Conclusions
Actinomycosis is considered a curable disease that can be easily treated with penicillin and amoxicillin as the first-line treatment. However, chronic granulomatous clinical presentation, selective cultivation of an environment, and species differences in susceptibility profiles may result in clinical confusion, which may lead to treatment failure. In summary, we describe a case with fever and progressive cellulitis. The Actinomyces species was isolated from blood culture, which was further identified as A. viscosus by 16S rRNA sequencing. The cellulitis improved after pathogen-directed antibiotics. Evaluation of the risk factors of a patient with Actinomyces bacteremia and further identification of Actinomyces species may increase the likelihood of successful treatment. Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Institutional Review Board Statement:
The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Institutional Review Board (IRB) of Taichung Veterans General Hospital, Taiwan (protocol code: CE21007A on 11 January 2021).

Informed Consent Statement:
Informed consent was obtained from all subjects involved in the study.
Data Availability Statement: All the data are available from the corresponding author upon reasonable request.