[Objectives] The purpose of this study was to estimate whether 1) the inpatients colonized with MRSA, but not infected, could transmit the bacterium to other inpatients, and 2) active surveillance could reduce nosocomial transmission in a ward.
[Methods] MIC-antibiogram types of 5592 strains of MRSA were constructed using MIC values of antibiotics (CEZ, CDTRP, CTX, IPMCS, EM, CLDM, CAM, LVFX, CZOP, FOM, GM, MEPM, FMOX, PCG, ABPC, MINO, SBTAB, CMZ and AMK). All the types thus obtained were plotted in a graph with a time-line. When the same MIC-antibiogram type was found by two or more inpatients that had an overlapping period of stay at the ward, we defined as a nosocomial transmission. To confirm the accuracy of MIC-antibiogram typing method, a part of the strains were simultaneously analyzed by spa-typing and MLST.
[Results] During the 6.2-year study period in the Kochi Medical School Hospital (12 wards), a total of 27,466 inpatients were admitted, and from 35.6% of them, bacteriological culture was taken. Active surveillance was performed only in the NICU. A total of 5592 MRSA strains were isolated from 16.1% inpatients received the cultures. The average hospitalization period of inpatients (except patients with only one culture) was 128 days (2-2112 days, 784 patients). The average admission was 2288 inpatients / ward. Among them, 21.9 % were estimated by MIC-antibiogram typing as having acquired MRSA by nosocomial transmission.
According to the ratios between the rate of clinical test or active surveillance-coverage of the inpatients and the rate of MRSA isolation, the wards could be divided into 2 groups. The rate of the patients receiving a bacteriological test was proportional to the rate of MRSA isolation in each ward in the groups (group 1, r=0.99, group 2, r=0.96). The rate of clinical test-coverage of inpatients in a ward correlated well with the rate of nosocomial transmission (r=0.92), however, number of inpatients with MRSA did not correlate with the rate of nosocomial transmission (r=0.49). The rate of bacteriological test-coverage of inpatients was 2.2 times higher in group 1 (including NICU) (p<0.05). On the contrary, the rate of MRSA isolation in a unit-rate of the clinical test-coverage of the inpatients was 2.3 times higher in group 2 (p<0.001).
[Conclusions] MRSA could be transmitted nosocomially from the inpatients without any suspicion of infectious diseases to others, and a higher rate of clinical cultures and/, or active surveillance significantly reduced nosocomial transmission of MRSA.