title: Limited detectability of linezolid-resistant Staphylococcus aureus by the Etest method and its improvement using enriched media.
abstract: The aim of this study was to evaluate Etest for detectability of linezolid-resistant meticillin-resistant Staphylococcus aureus (MRSA). The MIC of linezolid obtained by the Etest method in 18 linezolid-resistant strains of MRSA was compared with that obtained using standard agar and broth dilution methods according to Clinical and Laboratory Standards Institute guidelines. The mean linezolid MIC obtained by Etest in 18 linezolid-resistant strains of MRSA using Mueller-Hinton (MH) agar was 12.6-fold lower than that obtained by the agar dilution method, with the result that 78 % of the linezolid-resistant strains were incorrectly classified as linezolid-susceptible. The MIC of linezolid by Etest on brain-heart infusion (BHI) agar had a mean value 2.5-fold lower than that obtained by the agar dilution method, suggesting that replacing MH agar with BHI agar considerably improved the detectability of linezolid-resistant MRSA. Use of blood agar (MH agar supplemented with 5 % sheep blood) and 48 h of incubation resulted in 100 % agreement with the agar and broth dilution methods. Thus, this study revealed that the Etest on MH agar and BHI agar yielded false-negative results in a significant fraction of the linezolid-resistant MRSA. Hence, the use of blood agar and prolonged incubation is highly recommended for the accurate detection of linezolid-resistant MRSA using Etest.
authors.affiliation: Laboratory for Antimicrobial Agents, Kitasato University, Tokyo, Japan.
authors.forename: Yurika
authors.initials: Y
authors.lastname: Ikeda-Dantsuji
authors.affiliation: Laboratory for Antimicrobial Agents, Kitasato University, Tokyo, Japan.
authors.forename: Taiji
authors.initials: T
authors.lastname: Nakae
authors.affiliation: Department of Laboratory Medicine, Yamaguchi University Hospital, Yamaguchi, Japan.
authors.forename: Koichi
authors.initials: K
authors.lastname: Ariyoshi
authors.affiliation: Department of Laboratory Medicine, Yamaguchi University Hospital, Yamaguchi, Japan.
authors.forename: Hidekazu
authors.initials: H
authors.lastname: Mizuno
authors.affiliation: Department of Laboratory Medicine, Shimane University Hospital, Shimane, Japan.
authors.forename: Hidehiko
authors.initials: H
authors.lastname: Moriyama
authors.affiliation: Department of Laboratory Medicine, Hamamatsu University School of Medicine, Shizuoka, Japan.
authors.forename: Osanori
authors.initials: O
authors.lastname: Nagura
authors.affiliation: Department of Laboratory Medicine, Iwate Medical University School of Medicine, Morioka, Iwate, Japan.
authors.forename: Akira
authors.initials: A
authors.lastname: Suwabe
authors.affiliation: Department of Laboratory Medicine, Showa University School of Medicine, Tokyo, Japan.
authors.forename: Kunihiko
authors.initials: K
authors.lastname: Fukuchi
authors.affiliation: Department of Infection Control, St Mary's Hospital, Fukuoka, Japan.
authors.forename: Junichi
authors.initials: J
authors.lastname: Honda
authors.affiliation: Central Clinical Laboratory, Department of Infection Control and Laboratory Diagnostics, Internal Medicine, Tohoku University Graduate School of Medicine, Miyagi, Japan.
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Kaku
authors.affiliation: The Second Department of Internal Medicine, Nagasaki University School of Medicine, Japan.
authors.forename: Shigeru
authors.initials: S
authors.lastname: Kohno
authors.affiliation: Department of Infection Control and Prevention, Aichi Medical University, Aichi, Japan.
authors.forename: Hiroshige
authors.initials: H
authors.lastname: Mikamo
authors.affiliation: Department of Clinical Infectious Diseases, Showa University School of Medicine, Tokyo, Japan.
authors.forename: Yoshihito
authors.initials: Y
authors.lastname: Niki
authors.affiliation: Department of Infection Control and Prevention, Hyogo College of Medicine, Hyogo, Japan.
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Takesue
authors.affiliation: Department of Clinical Infectious Disease, Osaka University School of Medicine, Osaka, Japan.
authors.forename: Kazunori
authors.initials: K
authors.lastname: Tomono
authors.affiliation: Department of Laboratory Medicine, Nagasaki University School of Medicine, Nagasaki, Japan.
authors.forename: Katsunori
authors.initials: K
authors.lastname: Yanagihara
authors.affiliation: Laboratory for Antimicrobial Agents, Kitasato University, Tokyo, Japan.
authors.forename: Hideaki
authors.initials: H
authors.lastname: Hanaki
mesh_terms: D000081:Acetamides / Q000494:pharmacology*; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D003470:Culture Media / Q000737:chemistry*; D024881:Drug Resistance, Bacterial; D006801:Humans; D000069349:Linezolid; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects*; D008826:Microbial Sensitivity Tests / Q000379:methods; D023303:Oxazolidinones / Q000494:pharmacology*; D012680:Sensitivity and Specificity
chemical_list: D000081:Acetamides; D000900:Anti-Bacterial Agents; D003470:Culture Media; D023303:Oxazolidinones; D000069349:Linezolid