Granulicatella adiacens abscess: Two rare cases and review

Introduction

Granulicatella adiacens is a nutritionally variant streptococcus (NVS) which requires pyridoxal or other additional agents to be incorporated into standard media for its successful laboratory isolation.[1] These bacteria were transferred from Streptococcus to a separate genus, Abiotrophia[2] and later on, this genus was divided into the genera Abiotrophia and Granulicatella (species Granulicatella adiacens, Granulicatella elegans, and Granulicatella balaenopterae) on the basis of 16S rRNA gene sequencing.[3]

Despite being a part of the oral, gastrointestinal, and urogenital commensal flora, G. adiacens is rarely implicated in infections due to their difficult isolation and differentiation techniques as compared to other streptococci. These have been mostly reported to cause bacteremia and endocarditis and less commonly seen in device associated infections such as postinstrumentation meningitis, infections of breast implants, and of peritoneal dialysis-related peritonitis.[1] There are very few reports of localized pyogenic infections, and most of them are in patients with underlying pathology like diabetes or other systemic illnesses. Here, we are reporting two cases of subcutaneous abscesses caused by G. adiacens. To the best of our knowledge, this is the first case report from India describing subcutaneous abscesses due to G. adiacens in healthy adult persons.

Case Reports

Discussion

Nutritionally variant streptococci (NVS) are named so as they grow best on media supplemented with thiol or pyridoxal.[1] With the increased use of commercial media like BacT Alert^® culture bottles that contain pyridoxal and L-cystein, their isolation rate has increased. In most of the cases we reviewed, NVS were isolated from automated culture bottles.[56789] However, on subculture from these positive bottles growth is generally either delayed or/and very faint. Colonies can be identified by satellitism around colonies of Staphylococcus aureus, typical Gram stain picture of pleomorphism and increased growth on thiol containing media. Phenotypic ID is technically demanding and unreliable but automated ID systems like Vitek^® or API Strep^® panels have been successfully used as an easy alternative with good sensitivity and specificity. 16S rRNA gene sequencing is confirmatory but is feasible only in reference labs.

Clinically, Nutritionally variant streptococci (NVS) has been associated with up to 2.3% of cases of streptococcal bacteremia and up to 5% cases of streptococcal endocarditis.[10] Localized pyogenic infections are rare. There are a few isolated reports of septic arthritis, vertebral osteomyelitis and discitis. Out of 101 NVS isolates studied by Christensen and Facklam, the clinical diagnosis was endocarditis in 58% of patients, septicemia or bacteremia in 26% of patients and only single patient had scrotal abscess.[1]

In 2007, CLSI published guidelines (M45) for AST testing for NVS along with other fastidious microorganisms.[4] It recommended broth microdilution method (BMD) using cation-adjusted Mueller–Hinton broth supplemented with 2.5%–5% lysed horse blood and 0.001% pyridoxal HCl. The lack of disc diffusion breakpoints, FDA approved automated susceptibility systems, and limited availability of BMD in clinical laboratories makes it difficult to accurately report susceptibility pattern of these bacteria. E test on different media has been compared with BMD and has shown variable results in different studies.[1112]

There are only a few studies on AST pattern of NVS and most of these have used breakpoints for Streptococcus viridans to interpret their data.[10] In the largest study on susceptibility pattern of NVS, Alberti et al. tested 132 isolates (37 A. defectiva, 90 G. Adiacens, and 5 G. elegans) by BMD using CLSI M45 guidelines.[13] Only 38.9% of G. adiacens isolates were found susceptible to Penicillin, however, a large number (47.8%) of isolates were intermediate susceptible. Susceptibility to other antibiotics was cefotaxime (18.9%), ceftriaxone (43.3%), erythromycin (52.2%), clindamycin (84.5%), and levofloxacin (91.9%) but all G. adiacens isolates were 100% susceptible to meropenem and vancomycin. Both of our isolates were also susceptible to penicillin, cefotaxime, ceftriaxone, clindamycin, levofloxacin, and vancomycin except erythromycin resistance shown by one.

We could not find any well documented Indian research study on AST pattern of NVS except for a few case reports and a case series comprising of seven cases. In most of these reports, NVS was grown in BacT alert® bottle, identified by Vitek® and AST was performed either by disc diffusion method or Vitek® and interpreted using CLSI breakpoints for viridans streptococci [Table 1]. We also could not perform BMD due to nonavailability of required media supplements. Erythromycin resistance was reported in 7 out of 12 isolates reviewed by us, but penicillin resistance was less with only 2 isolates being resistant. Both of our isolates were also penicillin susceptible, but one was erythromycin resistant.[14]

Table 1 Granulicatella adiacens-case reports from India

On the basis of susceptibility data and clinical outcome of various cases, it is recommended to treat NVS related endocarditis like enterococci with penicillin and aminoglycoside.[6] However, pyogenic infections have been treated successfully with various antibiotics such as penicillin, cefotaxime, and erythromycin. Linezolid or vancomycin resistance has never been reported so far.

Conclusion

NVS is an important but under-reported cause of bacteremia and endocarditis and a rare cause of pyogenic infections. The use of automated culture and ID systems can definitely help in better isolation of these fastidious bacteria. More studies are required using standard methods of ID and AST to assess their susceptibility pattern, especially from India.