[Epidemiological investigation of "beta-lactam antibiotic induced vancomycin-resistant MRSA (BIVR)"]

Mu3 strain with heterogeneous intermediated-resistance to vancomycin (hetero-VISA) reported in 1997, also have possessed a character of an antagonistic effect of beta-lactam antibiotics and vancomycin. Mu3 is only strain which satisfies the definition of hetero-VISA in Japan. But, MRSA with antagonistic effects of beta-lactam antibiotics and vancomycin, was reported by many institutions. To separate hetero-VISA, we called "beta-lactam antibiotic induced vancomycin-resistant MRSA (BIVR)". But the detection rate of clinical isolated BIVR in Japan is unknown, we reported on the detection method and the epidemiological investigation for BIVR. Mu 3 agar containing 4 micrograms/mL of vancomycin is used to detect BIVR. Mu3 strains were spread on the agar, BIVR can grow around the paper disc impregnated with ceftizoxime or grow on the whole surface on Mu 3 agar after incubation. The detection rate of BIVR was 45 in 717 (6.3%) clinical isolated strains. In detected strains as BIVR, the number of strains with grown on the whole surface of Mu3 agar showing a high resistance to vancomycin were 10 strains (1.4%). Besides, from 106 strains of blood isolates MRSA, BIVR were detected 16 strains (15.1%), from 611 strains of non-blood isolates MRSA, BIVR were detected 29 strains (4.7%) (P < 0.0001). In BIVR strains grown on the whole surface of Mu3 agar, the number of BIVR strains isolated from blood were 8 in 106 (7.5%), from non-blood were 2 in 611 (0.3%) (P < 0.0001). On one side, hetero-VISA were not detected from all of BIVR growing on the whole surface of Mu3 agar. As a result, detection method and the definition of BIVR were quite different from those of hetero-VISA. An existence of BIVR in Japan was confirmed, we thought that the high detection rate of BIVR isolated from blood compared with that of non-blood showed the pathogenecity of BIVR which contribute to MRSA infections.