title: Trends in the gentamicin and arbekacin susceptibility of methicillin-resistant Staphylococcus aureus and the genes encoding aminoglycoside-modifying enzymes.
abstract: It is generally accepted that methicillin-resistant Staphylococcus aureus (MRSA) is also resistant to aminoglycoside antibiotics. We investigated trends of gentamicin and arbekacin susceptibilities and the prevalence of the genes encoding aminoglycoside-modifying enzymes (AMEs) for a total of 218 strains of MRSA isolated from blood specimens obtained from 1978 through 2002 in one hospital. The minimum inhibitory concentrations of gentamicin at which 50% of the strains were inhibited (MIC(50)) were > or =128 and 32 microg/ml for isolates obtained from 1978 to 1984 and from 1985 to 1989, respectively, and 0.5 microg/ml for isolates obtained from 1990 to 2002. The MIC(90) of gentamicin was consistently > or =128 microg/ml. Investigation of the occurrence of AME revealed that the MIC(50) of gentamicin was highly correlated with the presence of aac(6')/aph(2'') encoding aminoglycoside acetyl/phosphotransferase. The MIC(50) of arbekacin was 2 microg/ml for strains isolated in 1978-1984 and </=0.5 microg/ml for strains isolated from 1985 to 2002. The MIC(90) of arbekacin was 8 microg/ml for the strains isolated in 1978-1989 and 1 to 2 microg/ml for strains isolated in 1990-2002. Though it has been established that AAC(6')/APH(2'') modifies arbekacin, the trend of arbekacin resistance was not necessarily consistent with the presence of this enzyme. However, the prevalence of both aac(6')/aph(2'') and aph(3')-III in the strains isolated from 1978 through 2002 was correlated with the MIC(90) values of arbekacin. Thus, it is most likely that APH(3')-III, in addition to AAC(6')/APH(2''), is somehow involved in arbekacin resistance in S. aureus. Our results imply that gentamicin- and arbekacin-resistant MRSAs have consistently decreased for the past 25 years and that this finding is, most likely, attributable to the declining prevalence of genes encoding for AMEs.
authors: Laboratory of Infectious Diseases, Graduate School of Infection Control Sciences, Kitasato University, 5-9-1 Shirokane, Minato-ku, Tokyo 108-8642, Japan. p98224@st.pharm.kitasato-u.ac.jp
authors: Kazunari
authors: K
authors: Barada
authors: Hideaki
authors: H
authors: Hanaki
authors: Shinsuke
authors: S
authors: Ikeda
authors: Yoshio
authors: Y
authors: Yamaguchi
authors: Hiroyuki
authors: H
authors: Akama
authors: Taiji
authors: T
authors: Nakae
authors: Takashi
authors: T
authors: Inamatsu
authors: Keisuke
authors: K
authors: Sunakawa
mesh_terms: D000368:Aged; D000617:Aminoglycosides / Q000378:metabolism*; D000900:Anti-Bacterial Agents / Q000378:metabolism; D003428:Cross Infection / Q000453:epidemiology / Q000235:genetics* / Q000382:microbiology; D003982:Dibekacin / Q000031:analogs & derivatives / Q000378:metabolism; D024901:Drug Resistance, Multiple, Bacterial / Q000235:genetics*; D005839:Gentamicins / Q000378:metabolism; D006765:Hospitals, Convalescent; D006801:Humans; D016106:Methicillin Resistance / Q000187:drug effects* / Q000235:genetics; D008826:Microbial Sensitivity Tests; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000235:genetics; D014041:Tokyo / Q000453:epidemiology
chemical_list: D000617:Aminoglycosides; D000900:Anti-Bacterial Agents; D005839:Gentamicins; D003982:Dibekacin; C031986:arbekacin