A healthy 17-day-old male infant without a family history of immunodeficiency syndrome was consulted by his family physician for cough, nasal discharge, and a fever on the third day of illness. He had not started any vaccines because of his age (<1 month). He was diagnosed with acute pharyngitis along with exudation on the back of the larynx. Although cefotaxime treatment was initiated, wheezing, difficulty in vocalizing, and cyanosis on crying were observed on the fourth day. On the fifth day, laryngoscopy revealed a downward spread of exudation and laryngeal edema, which led to diagnosis of acute epiglottitis (Figure 1A, B). Because airway management was required, he was referred to our pediatric intensive care unit and immediately started on artificial respiration.
At the time of admission, the patient's laboratory tests showed the following: white blood cell count 4,700/muL (band cell: 35%, segmented cell: 31%, eosinophil: 5%, monocyte: 8%, lymphocyte: 18%, atypical lymphocyte: 3%), hemoglobin level 11.1 g/dL, platelet count 24.4x104/muL, and C-reactive protein level 6.65 mg/dL. Other laboratory findings were as follows: serum albumin 2.4 g/dL, total bilirubin 0.48 mg/dL, sodium 137 mEq/L, potassium 3.3 mEq/L, aspartate aminotransferase 14 U/L, alanine aminotransferase 8 U/L, lactate dehydrogenase 260 IU/L, urea nitrogen 3.9 mg/dL, and creatinine 0.19 mg/dL. His IgG, IgA, IgM, C3, C4, and CH50 levels were within the normal ranges (IgG: 686 mg/dL, IgA: 19 mg/dL, IgM: 52 mg/dL, C3: 84 mg/dL, C4: 15 mg/dL, and CH50: 32.5 U/L). His arterial blood gas under 1.0 of the fraction of inspired oxygen showed a pH of 7.428, pCO2 of 42 mmHg, pO2 of 114.0 mmHg, HCO -3 of 27.2 mmol/L, and base excess of 3.1 mmol/L. Rapid antigen test for respiratory syncytial and adenoviruses was negative (Capilia  RSV Neo and Adeno; TAUNS Laboratories, Inc., Shizuoka, Japan). MRSA was detected from the posterior nasal cavity swab culture and the pharyngeal swab culture at the site attached to the exudate, but skin swab culture and blood culture were negative. Owing to the detection of MRSA, he was treated with vancomycin. The fever disappeared on the first day of admission, and he was extubated on the eighth day of admission. Figure 1C, D shows his laryngoscopic images after extubation. Finally, the patient was discharged on the 18th day of admission.
Gram staining and culture using blood and chocolate agar were performed to identify the organisms using swabs obtained from the posterior nasal cavity or pharynx of the patient. Because Sta. aureus was detected, the culture using MRSA screen media (CHROMagar ; Kanto Chemical Co., Inc., Tokyo, Japan) was performed to identify MRSA. Determination of the minimum inhibitory concentration of each antibiotic for the isolated strain was performed using a RAI-SUS  system with the RSMSE panel (Nissui Pharmaceutical Co., Ltd, Tokyo, Japan). The inoculum was adjusted to yield a cell density of 1x105 colony-forming units/mL. The plates were incubated for 18-24 hours at 35 C and then analyzed automatically. Result of the antimicrobial susceptibility test for the isolated strain is shown in Table 1.
Bacterial genomic DNA of the isolated strain was extracted using a DNeasy Blood & Tissue Kit (Qiagen) with lysostaphin (Wako). Genomic DNA was used as a template for PCR-based screening assays. MRSA SCCmec type (I-V) screening was conducted using a PCR-based assay as previously described. We detected mecA gene and class B mec gene complex and type 2 ccr gene complex by multiplex-PCR methods. These results indicated that SCC-mec type of the isolated strain was type IV. Staphylococcal virulence genes including the Panton-Valentine leukocidin (PVL) gene (luks-pv), two exfoliative toxin (ET) genes (eta and etb), and toxic shock syndrome toxin 1 (TSST-1) gene (tst-1) were detected by PCR assay using primers that have been reported previously. The results of the MRSA genome analysis of the patient sample revealed a PVL-negative and TSST-1-positive SCCmec type IV clone (Table 2).