title: High cephalosporin resistance due to amino acid substitutions in PBP1A and PBP2X in a clinical isolate of group B Streptococcus.
abstract: OBJECTIVE
  Group B Streptococcus (GBS; Streptococcus agalactiae) has been regarded as uniformly susceptible to penicillins. However, we recently reported the existence of GBS with reduced penicillin susceptibility (PRGBS), with amino acid substitutions in penicillin-binding protein (PBP) 2X. Although most PRGBS show high MICs of ceftizoxime (4-64 mg/L) and cefotaxime (0.12-1 mg/L), those for strain B1 are exceptionally high (ceftizoxime MIC ≥256 mg/L and cefotaxime MIC 2 mg/L). We previously found an amino acid substitution (G539S) neighbouring the conserved K540TG motif in PBP1A in addition to the PRGBS-specific amino acid substitution Q557E in PBP2X of B1. The aim of this study was to reveal the effect of the amino acid substitutions in PBP1A and PBP2X of B1 on the high cephalosporin resistance.
  
  
  METHODS
  A ceftizoxime competition assay was performed to reveal the PBPs that are the main targets of ceftizoxime. We generated two allelic exchange mutants from β-lactam-susceptible GBS BAA-611. BAA-611 (B1PBP2X) contained the PBP2X gene derived from B1 and BAA-611 (B1PBP2X, B1PBP1A) contained both the PBP2X and the PBP1A gene derived from B1. These allelic exchange mutants and strain B1 were subjected to susceptibility testing.
  
  
  RESULTS
  The ceftizoxime competition assay revealed that PBP1A and PBP2X were the main targets of ceftizoxime. Although the MICs of ceftizoxime and cefotaxime for BAA-611 (B1PBP2X) were 64 and 0.5 mg/L, respectively, BAA-611 (B1PBP2X, B1PBP1A) showed high cephalosporin resistance (ceftizoxime MIC ≥256 mg/L and cefotaxime MIC 2 mg/L) comparable to B1.
  
  
  CONCLUSIONS
  The high cephalosporin resistance of GBS was caused by amino acid substitutions in PBP1A and PBP2X.
authors: Department of Bacteriology, Nagoya University Graduate School of Medicine, 65 Tsurumai-cho, Showa-ku, Nagoya, Aichi 466-8550, Japan. koujikim@med.nagoya-u.ac.jp
authors: Kouji
authors: K
authors: Kimura
authors: Jun-ichi
authors: J
authors: Wachino
authors: Hiroshi
authors: H
authors: Kurokawa
authors: Mari
authors: M
authors: Matsui
authors: Satowa
authors: S
authors: Suzuki
authors: Kunikazu
authors: K
authors: Yamane
authors: Noriyuki
authors: N
authors: Nagano
authors: Keigo
authors: K
authors: Shibayama
authors: Yoshichika
authors: Y
authors: Arakawa
mesh_terms: D019943:Amino Acid Substitution; D018550:Cephalosporin Resistance; D008826:Microbial Sensitivity Tests; D050505:Mutant Proteins / Q000235:genetics / Q000378:metabolism; D020125:Mutation, Missense; D046915:Penicillin-Binding Proteins / Q000235:genetics* / Q000378:metabolism*; D010458:Peptidyl Transferases / Q000235:genetics* / Q000378:metabolism*; D013292:Streptococcus agalactiae / Q000187:drug effects* / Q000201:enzymology*
chemical_list: D050505:Mutant Proteins; D046915:Penicillin-Binding Proteins; C072223:PBP 2x protein, Streptococcus; D010458:Peptidyl Transferases
keywords: GBS; PBPs; PRGBS; antibiotic resistance; group B streptococci with reduced penicillin susceptibility; penicillin-binding proteins