Dear Editor,
Aerococcus viridans is a catalase-negative gram-positive coccus that appears in clusters,
tetrads, or irregular arrangements [1]. This organism is generally considered as a
contaminant in clinical cultures, but is also infrequently reported as a clinically
significant isolate that causes endocarditis, bacteremia, spondylodiscitis, and urinary
tract infections [2
3
4
5]. Although most A. viridans strains were susceptible to penicillin and other commonly
used antibiotics, Uh et al [6] described a case of bacteremia caused by an A. viridans
strain showing high resistance to penicillin, erythromycin, clindamycin, and ceftriaxone.
In 2014, Zhou et al [7] reported a peritoneal dialysis-related infection caused by
vancomycin-resistant A. viridans harboring the vanA gene. We report a case of a vancomycin-resistant
A. viridans isolate obtained from an excisional biopsy wound. As far as we know, no
study regarding vancomycin-resistant A. viridans has yet been published in Korea.
A 77-yr-old farmer visited the emergency room complaining of severe chills. Swelling
in an external wound was observed on the inguinal areas where previous excisional
biopsy was performed. He had recently been diagnosed as having colon adenocarcinoma
and angioimmunoblastic T-cell lymphoma. On admission, his body temperature was 39.0℃.
Hematological investigation revealed a hemoglobin level of 9.2 g/dL, white blood cell
(WBC) count of 18.96×109/L (segmented neutrophils; 92.2%), and platelet count of 262×109/L.
Serum C-reactive protein (CRP) level (14.36 mg/dL, reference range: <0.30 mg/dL) was
elevated. Two aerobic and anaerobic blood culture sets were incubated in the BacT/Alert
3D system (bioMérieux, Durham, NC, USA). The mucus-like whitish aspirate from the
wound was plated onto 5% sheep blood agar (BD Diagnostic Systems, Sparks, MD, USA),
MacConkey agar (BD Diagnostic Systems), and fluid thioglycollate medium (BD Diagnostic
Systems).
Some colonies grew on the 5% sheep blood agar after 24 hr aerobic incubation at 35℃;
these colonies were identified as oxacillin-resistant Staphylococcus hemolyticus by
VITEK 2 (bioMérieux, Marcy l'Etoile, France). No growth was detected in the blood
cultures after five days of incubation. Thirteen days after hospital admission, chemotherapy
for lymphoma was initiated. Although WBC count and CRP level were within reference
ranges at that time, mucus-like discharge from the wound remained, and CRP started
to rise steadily. A. viridans and S. hemolyticus were repeatedly simultaneously isolated
from wound cultures. The identification probability of A. viridans by VITEK 2 was
98%. Antimicrobial susceptibility test by a MicroScan MICroSTREP plus panel (Beckman
Coulter, Brea, CA, USA) showed that A. viridans was susceptible to tetracycline, but
resistant to vancomycin, penicillin, cefotaxime, ceftriaxone, sulfamethoxazole/trimethoprim,
meropenem, and levofloxacin.
To confirm the species identification and antimicrobial susceptibilities, 16S rRNA
sequence analysis, minimal inhibitory concentration (MIC) determination for vancomycin
(Daewoong Lilly, Seoul, Korea) and teicoplanin (Narion Merrell Dow, Seoul, Korea),
and PCR to detect vanA and vanB genes were performed. MICs were performed by using
the broth microdilution method according to CLSI guidelines [8]. PCR amplification
of 16S rRNA was performed by using primers 16SF (5′-TAA YAC ATG CAA GTC GAR CG-3′)
and 608R (5′-TAT TAC CGC GGC TGC TGG CA-3′), and sequencing was conducted by using
the Big Dye Terminator Cycle Sequencing kit (Applied Biosystems, Foster City, CA,
USA) and an ABI PRISM 3730 genetic analyzer (Applied Biosystems). All sequences were
analyzed by using the basic local alignment search tool and ribosomal database project.
The 450-bp 16S rRNA gene sequence from our isolate showed 100% similarity to and 99%
query coverage with several A. viridans strains (GenBank accession no. KR140225.1,
LN998006.1, EU169542.1). The primers and PCR procedure for amplification of vanA and
vanB genes were previously described [9]. Each DNA product was tested by using gel
electrophoresis, where vanA gene was conclusively detected. A. viridans showed a high
degree of resistance to vancomycin (<128 µg/mL) and teicoplanin (64 µg/mL), indicating
potential acquisition of the vanA gene.
The physician added sulfamethoxazole/trimethoprim to teicoplanin and meropenem combination
therapy. Consequently, exudate lessened, and WBC count and CRP level also decreased
(Fig. 1).
It is difficult to distinguish simple colonization from real infection when A. viridans
is identified in a wound specimen. However, we paid attention to our A. viridans isolate
because this patient struggled with chemotherapy before A. viridans was isolated and
because the resistance may spread to other gram-positive cocci through transfer of
vanA from A. viridans [10].