title: Cell wall thickening is associated with adaptive resistance to amikacin in methicillin-resistant Staphylococcus aureus clinical isolates.
abstract: OBJECTIVE
  Methicillin-resistant Staphylococcus aureus (MRSA) infection is increasing and causing global concern. The mechanism of MRSA resistance to amikacin is poorly understood. We report on the first matched-pair study to reveal that the phenotypic cell wall thickening of MRSA is associated with adaptive resistance to amikacin.
  
  
  METHODS
  Two MRSA strains (CY001 and CY002) were isolated from blood and synovial fluid samples, respectively, from a 12-year-old male patient with osteomyelitis. The strains were subjected to a matched-pair study, including antimicrobial agent susceptibility determination, molecular typing, morphological observation and in vitro resistance induction.
  
  
  RESULTS
  Both strains are Panton-Valentine leucocidin-positive, multilocus sequence type 59, staphylococcal cassette chromosome mec type IV and spa type 437 MRSA with identical PFGE profiles. The drug susceptibility spectra of the two isolates are similar. However, CY001 is resistant to amikacin (CY001-AMI(R); MIC = 64 mg/L), contrary to the susceptible CY002 (CY002-AMI(S); MIC = 8 mg/L). CY001-AMI(R) may have developed adaptive resistance, because it lacks aminoglycoside-modifying enzymes and has an altered growth curve. Interestingly, CY001-AMI(R) has a thicker cell wall (36.43 ± 4.25 nm) than CY002-AMI(S) (18.15 ± 3.74 nm) in the presence of amikacin at its MIC. The thickened cell wall can also be observed in an in vitro-induced strain (CY002-AMI(R)) in the presence of amikacin at its MIC (36.78 ± 3.41 nm); this strain was obtained by gradually increasing the amount of amikacin. However, the cell wall-thickened strains cultured in the presence of amikacin are still susceptible to vancomycin.
  
  
  CONCLUSIONS
  Cell wall thickening is associated with adaptive resistance in MRSA and alternative antibiotics can be used to treat patients when adaptive resistance to amikacin has developed.
authors: Department of Microbiology, College of Basic Medical Sciences, Third Military Medical University, Chongqing, China.
authors: Wenchang
authors: W
authors: Yuan
authors: Qiwen
authors: Q
authors: Hu
authors: Hang
authors: H
authors: Cheng
authors: Weilong
authors: W
authors: Shang
authors: Nan
authors: N
authors: Liu
authors: Ziyu
authors: Z
authors: Hua
authors: Junmin
authors: J
authors: Zhu
authors: Zhen
authors: Z
authors: Hu
authors: Jizhen
authors: J
authors: Yuan
authors: Xia
authors: X
authors: Zhang
authors: Shu
authors: S
authors: Li
authors: Zhijin
authors: Z
authors: Chen
authors: Xiaomei
authors: X
authors: Hu
authors: Jianfeng
authors: J
authors: Fu
authors: Xiancai
authors: X
authors: Rao
mesh_terms: D000583:Amikacin / Q000494:pharmacology*; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001769:Blood / Q000382:microbiology; D002473:Cell Wall / Q000187:drug effects* / Q000648:ultrastructure*; D002648:Child; D024881:Drug Resistance, Bacterial; D016521:Electrophoresis, Gel, Pulsed-Field; D006801:Humans; D008297:Male; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects* / Q000302:isolation & purification / Q000648:ultrastructure*; D008826:Microbial Sensitivity Tests; D046529:Microscopy, Electron, Transmission; D058889:Molecular Typing; D010019:Osteomyelitis / Q000382:microbiology; D013203:Staphylococcal Infections / Q000382:microbiology; D013582:Synovial Fluid / Q000382:microbiology
chemical_list: D000900:Anti-Bacterial Agents; D000583:Amikacin