First Report of Vancomycin-Intermediate Resistance in Sequence Type 72 Community Genotype Methicillin-Resistant Staphylococcus aureus

Abstract

Vancomycin-intermediate resistance has not been previously reported among sequence type 72 (ST72) methicillin-resistant Staphylococcus aureus (MRSA) isolates of SCCmec type IV (ST72-MRSA-IV), which are distinctive community genotype strains in Korea. We report the first case of vancomycin treatment failure due to development of vancomycin-intermediate resistance in infection caused by an ST72-MRSA-IV isolate.

CASE REPORT

A79-year-old man was admitted to the coronary care unit due to non-ST-elevation myocardial infarction and sepsis. Blood culture grew methicillin-resistant Staphylococcus aureus (MRSA), which was susceptible to most non-β-lactam agents, except erythromycin (vancomycin MIC, 1 mg/liter). There was no evidence of endocarditis in a transesophageal echocardiogram. Vancomycin was administered for 14 days after culture became negative. Follow-up blood culture 14 days after the end of treatment grew MRSA (vancomycin MIC, 1 mg/liter) again. At that time, the patient complained of back pain, and spine magnetic resonance imaging (MRI) revealed infectious spondylitis at T12-L1. Vancomycin was restarted, and the blood culture became negative 9 days later. Upon receiving vancomycin, his back pain was improved and his C-reactive protein (CRP) level decreased to 0.24 mg/dl until 30 days after restart of vancomycin, when back pain was aggravated and the CRP level suddenly elevated to 8.1 mg/dl. Blood culture grew MRSA with a vancomycin MIC higher than 2 mg/liter. A confirmative test using the Etest method in the clinical microbiology laboratory at the time of isolation revealed that this strain had a vancomycin MIC of 3 mg/liter. When we retested the frozen isolates using a broth microdilution test, the vancomycin MIC was 4 mg/liter. Vancomycin was switched to linezolid and continued for 4 weeks without any adverse effects. The patient's back pain was improved, and the CRP decreased to 0.03 mg/dl. At the time the manuscript for this article was submitted, the patient had been free of relapse for 3 months.

Antimicrobial susceptibility testing by MIC determination according to guidelines of the Clinical and Laboratory Standards Institute (2) showed that this vancomycin-intermediate S. aureus (VISA) strain was resistant to erythromycin and rifampin, whereas it was susceptible to gentamicin, ciprofloxacin, clindamycin, cotrimoxazole, tetracycline, fusidic acid, linezolid, ceftobiprole, and daptomycin. A MRSA strain isolated at the first episode of bacteremia and a VISA strain isolated at the breakthrough bacteremia were molecularly characterized and compared. Multilocus sequence typing (MLST) was carried out by PCR amplification and sequencing of seven housekeeping genes (arcC, aroE, glpF, gmk, pta, tpi, and yqiL) as previously described (5). The allelic profiles and sequence types (ST) were assigned by the MLST website (http://saureus.mlst.net/). SCCmec types were determined by the multiplex PCR method (13). Isolates were screened for the lukF-PV and lukS-PV genes encoding the components of the Panton-Valentine leukocidin (PVL) toxin, as previously described (11). Pulsed-field gel electrophoresis (PFGE) was performed as described previously (17). Both isolates were determined to be PVL-negative ST72 strains and carried SCCmec type IV. The PFGE patterns were analyzed using GelCompar II software (Applied Maths, Belgium), and compared to those of the VISA strains belonging to ST5 isolated from other two patients in the same hospital during 2009 to 2011: Mu3 (the reference heterogeneous VISA strain, kindly provided by K. Hiramatsu, Juntendo University, Japan), and Mu50 (reference VISA strain, kindly provided by K. Hiramatsu, Juntendo University, Japan). All of the isolates from the patient displayed the identical pulsotype, whereas they were different from the types of the reference strains (Fig. 1).

Fig 1.

Pulsed-field gel electrophoresis (PFGE) patterns of MRSA and VISA isolates from the patient and reference strains. Lane 1, K01-SAU-11-266 (the first strain of MRSA of ST72 and SCCmec type IV from the patient); lane 2, K01-SAU-11-298 (a VISA strain of ST72 and SCCmec type IV from the patient); lane 3, K01-SAU-11-300 (a VISA strain from the patient); lane 4, K01-SAU-11-075 (a VISA strain of ST5 and SCCmec type II from other patient A); lane 5, K01-SAU-09-67 (an MRSA strain of ST5 and SCCmec type II from other patient B); lane 6, K01-SAU-09-573 (a VISA strain of ST5 and SCCmec type II from other patient B); lane 7, Mu3 (reference hVISA strain); lane 8, Mu50 (reference VISA strain); M, lambda size marker.

In order to determine if an original isolate from the patient was heterogeneous VISA (hVISA), we conducted a population analysis profile (PAP) test as previously described (15). The first isolate from the patient was determined to be vancomycin-susceptible S. aureus (VSSA) with no heterogeneous population of VISA, and two isolates of VISA were again confirmed as VISA by this method. Unfortunately, the VSSA (vancomycin MIC, 1 mg/liter) strain isolated at the first relapse of bacteremia was not stored, and so it could not be tested for the presence of a heterogeneous population.

Since the first report of S. aureus with reduced susceptibility to vancomycin (MIC, 8 mg/liter) from Japan in 1997 (9), there have been increasing reports of VISA and hVISA worldwide. In Korea, the first case of VISA (MIC, 8 mg/liter) was reported in 2000 (10), and a subsequent nationwide surveillance study for VISA found 15 VISA strains through screening of 37,856 clinical isolates collected from 2001 to 2006 (1). VISA infections have been caused mostly by health care-associated MRSA clones (12), and all VISA strains reported in Korea had been ST5 MRSA of SCCmec type II (ST5-MRSA-II) or ST239-MRSA-III, which were health care-associated MRSA clones in this country (1, 14). Although community-associated MRSA (CA-MRSA) clones have emerged worldwide, development of the VISA phenotype in these clones has been uncommon. Recently, two cases of treatment failure caused by strains of USA300 CA-MRSA with intermediate vancomycin resistance were reported (7, 8). In particular, the VISA phenotype has never been reported among the strains of ST72-MRSA-IV, which has emerged as an important pathogen in the community and in hospitals in Korea during the past decade (16). The isolates from the patient showed susceptibility to most non-β-lactam agents, and this is a typical antibiogram seen in ST72-MRSA-IV in Korea (16).

The VISA isolates in our report showed different vancomycin MICs according to the test methods. The MIC from Etest analysis was lower than that from the broth microdilution method. Such differences in vancomycin MICs of S. aureus according to test method have been previously reported (4).

Antecedent vancomycin use and prior MRSA infection 2 or 3 months before the current infection have been reported to be independent risk factors for VISA (6). The cooperative effect of the clogging and cell wall thickening have been reported to enable VISA to prevent vancomycin from reaching its true target in the cytoplasmic membrane (3).

This is the first reported case of vancomycin treatment failure due to development of vancomycin-intermediate resistance in a patient with community genotype ST72-MRSA-IV infection. The development of vancomycin-intermediate resistance during vancomycin treatment in patients with infections by these community genotype MRSA strains poses difficult challenges for effective antimicrobial treatment and infection control.