Aerococcus Viridans: Case Report, Microbiology, and Literature Review

Background

Aerococcus is a genus of microaerophilic Gram-positive cocci that are α-hemolytic, catalase and oxidase negative, facultatively anaerobic, and leucine aminopeptidase positive. Unlike other bacteria, it divides on 2 planes at right angles, which results in tetrads and irregular clusters [1]. The colonies are morphologically similar to Viridans streptococci and enterococci. Biochemical characteristics are also similar, and many commercially available systems for species determination (based on biochemical reactions) have not been specific. There have been reports of A. sanguinicola incorrectly assigned as A. viridans, and some systems have poorly differentiated between Granulicatella and A. urinae [2–4]. Thus, species identification with biochemical reactions is inappropriate for Aerococci.

On the other hand, 16S rRNA gene sequencing clearly distinguishes most Aerococci [3]. Christensen and colleagues reported that 0.8% of all urine specimens (cultured during a 4-month period in a Denmark hospital) yielded growth of “Aerococcus-like” organisms [4]. These similarities may have led to a misidentification of Aerococcus, and subsequent misconception that the bacteria are a rare cause of human infections. Fortunately, with the introduction of matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDITOF), Aerococci are more readily identified and acknowledged as human pathogens. Among the different species, A. urinae is the most common cause of a urinary traction infection, along with A. sanguinocol and A. viridans [3,4].

A. viridans was first described as a potential human pathogen in 1967 [1]. The bacteria have a fastidious growth, and they are often confused with other strains of Streptococci or Staphylococci. A. viridans strains are widely distributed in healthcare and marine environments (e.g., causing fatal infections in lobsters) [5]. Prior studies conducted on Aerococci as an etiologic agent of infection have been performed on isolates from urinary specimens. The infected patients are typically older than 65 years of age, predominantly female, with underlying risk factors for urinary tract infection.

Case Report

INVESTIGATION:

A set of 2 blood cultures was obtained prior to initiating antimicrobial therapy. Using a continuous monitoring automated blood culture system (BD BACTEC™), 10 mL of blood was inoculated into aerobic and anaerobic culture bottles. Upon receipt in the laboratory, the samples were incubated at 35°C until signal positive or until the end of day 5. In this patient’s case, a positive signal was observed on day 2. The bottles were unloaded from instrument and Gram’s staining and cultures were performed using standard microbiological protocol [6]. The blood samples were sub-cultured on 5% sheep blood agar, and this resulted in 1–2 mm, circular, α-hemolytic colonies. The Gram-positive cocci were arranged in clusters, and were catalase negative and bile esculin negative. The isolates were subjected to identification using MALDI-TOF MS instrument (Bruker Daltonics, Bremen, Germany). The isolates were confirmed to be A. viridans (identification score ≥2.2). Antibiotic susceptibility testing was performed with Etest®strips (bioMérieux) to generate a minimum inhibitory concentration (MIC). MIC interpretation was done using the Clinical Laboratory Standards Institute guidelines (CLSI) break point [7]. The isolate of A. viridans recovered from the blood specimen was susceptible to vancomycin and meropenem, but resistant to cefotaxime, ceftriaxone, levofloxacin, linezolid, penicillin, tetracycline, and sulfamethoxazole/trimethoprim.

The urine sample also underwent microbiological testing. On wet mount, microscopic examination revealed numerous white blood cells and bacteria (per high-power field). The semi-quantitative culture on a cysteine lysine electrolyte deficient (CLED) agar also demonstrated significant bacterial growth, with a colony count >105 CFU/mL. The colonies were small (2 mm) and yellowish in color. The isolate was sub-cultured on 5% sheep blood agar, revealing 2 mm, circular, α-hemolytic colonies. On Gram staining, these colonies were identified as Gram-positive cocci of approximately 1 μm in size, arranged in clusters, catalase-negative, and bile esculin-negative. MALDI-TOF MS instrument (Bruker Daltonics, Bremen, Germany) positively identified the isolate. Antibiotic susceptibilities were identified using test strips (bioMérieux) to generate a MIC and interpreted using the CLSI break point [7]. Antimicrobial susceptibility results of both urinary and blood isolates of A. viridans were identical.

Repeat blood cultures from the first 2 hospital days were negative. A transthoracic echocardiogram (TTE) did not find any valvular vegetation.

TREATMENT:

On day 3 of hospitalization, piperacillin/tazobactam was discontinued. Two days later, the patient was discharged to a long-term acute care hospital, where she completed a 14-day course of vancomycin.

OUTCOME AND FOLLOW-UP:

The patient had a resolution of her fever and other initial symptoms.

Discussion

Gram-positive cocci of Aerococcus are morphologically similar to viridans Streptococci. The clinical significance of Aerococci is often disregarded or underreported, due to their fastidious growth. Isolates can also be misidentified and considered as insignificant contaminants. Aerococci are widely distributed in hospital environments, and the bacteria can lead to serious infections (e.g., bacteremia, endocarditis, and urinary traction infection) [8]. In our case, we believe that the patient’s nursing home residence was an environmental source of infection.

Urinary traction infections due to Gram-positive bacteria are easily overlooked due to the limited availability of urine culture-based assays in hospital microbiology laboratories. Notable Gram-positive uropathogens include Staphylococcus saprophyticus, Enterococcus faecalis, and Streptococcus agalactiae. Other previously underreported (but emerging) Gram-positive uropathogens include Aerococcus, Actinobaculum, Corynebacterium, and Gardnerella [9]. Based on current evidence, Gram-positive bacteria remain important uropathogens. Alas, molecular tools (e.g., amplification and sequencing of 16S rRNA) are not commonly found in clinical microbiology laboratories, but these are required for the accurate identification of Aerococci. In our case, the rapid clearance of bacteremia and a negative TTE decreased the likelihood for endocarditis diagnosis; therefore, a transesophageal echocardiogram was unnecessary. Aerococci typically exhibit high MICs or resistance to ciprofloxacin, clindamycin, gentamicin, sulfamethoxazole/ trimethoprim, and tetracycline [10,11]. Although a few case reports have highlighted resistance to penicillin and vancomycin [12,13], empiric antibiotic recommendations are to utilize penicillin and sulfonamides. In our case report, the isolate was sensitive to vancomycin and meropenem, but it was resistant to all oral antibiotics and first-line drugs for urinary traction infections (with the exception of fosfomycin, which was untested). Prompt and accurate identification of Aerococcus is necessary to initiate appropriate antibiotics and avoid life-threatening systemic infection. If left untreated infections with A. viridans can lead to severe complications such as acute pyelonephritis, bacteremia, urosepsis, and death.

Conclusions

FUTURE PERSPECTIVES:

The role of Aerococci in normal flora and the incidence of bacteriuria in asymptomatic patients need to be further delineated.