Necessity and usefulness of detection by PCR of mecA and aac(6')/aph(2'') genes for identification of arbekacin-resistant MRSA

Arbekacin (ABK)-resistant bacteria (43 strains) isolated as MRSA by regular clinical procedures in Japanese clinics in 1998 were characterized in terms of taxonomic properties, aminoglycoside resistance, and mecA and aac(6')/aph(2'') genes linking with ABK-resistant MRSA. Taxonomically the 43 strains fell into Staphylococcus aureus (33 strains) and Enterococcus (10 strains). As to ABK resistance, the 13 strains of MRSA clinically reported as high ABK resistants (128 μg/ml or higher) did not show clear high ABK resistance except for 2 strains when their ABK resitance was tested using 0.5% NaCl containing nutrient agar. We designed and established the primers and conditions for PCR to detect the above two resistance genes. PCR analysis of DNAs from the 43 strains clearly indicated that only 33 strains identified taxonomically as S. aureus possessed meca indicating MRSA and 23 out of them possessed aac(6')/aph(2''). The other 10 strains of MRSA lacking aac(6')/aph(2'') were ABK-sensitive. Thus, there were a good correlation between ABK resistance and aac(6')/aph(2'') existence. Based on these, it was conclusive that the appropriate ABK resistance test as well as the detection of meca and aac(6')/aph(2'') genes by PCR are necessary and useful to avoid false ABK-resistant MRSA strains.