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JAC Advance Access first published online on January 11, 2008
This version published online on January 22, 2008
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkm504
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© The Author 2008. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Helicobacter pylori amoxicillin heteroresistance due to point mutations in PBP-1A in isogenic isolates

Mario José Matteo1, Gabriela Granados1, Martín Olmos2, Andrés Wonaga3 and Mariana Catalano1,*

1 Departamento de Microbiología, Parasitología e Inmunología, Facultad de Medicina, Universidad de Buenos Aires, Buenos Aires, Argentina 2 Servicio de Endoscopía, Hospital General de Agudos Juan A Fernández, Buenos Aires, Argentina 3 Servicio de Gastroenterología, Hospital Escuela ‘Don José de San Martín’ Facultad de Medicina, Buenos Aires, Argentina

Received 19 September 2007; returned 8 October 2007; revised 27 November 2007; accepted 30 November 2007

* Correspondence address. Departamento de Microbiología, Facultad de Medicina, Universidad de Buenos Aires, Paraguay 2155, Piso 12 (1121), Buenos Aires, Argentina. Tel: +54-11-5950-9500-2181; E-mail: catalano{at}fmed.uba.ar

Objectives: To investigate the Helicobacter pylori amoxicillin resistance rate, the occurrence of heteroresistance, and their related molecular mechanisms.

Methods: Eighty-seven H. pylori-positive patients were included: 45/87 with single biopsy and 42/87 with multiple biopsies. MICs were determined, and sequencing analysis of pbp1A gene and the variable regions of seven hop porins was performed in resistant and susceptible isolates. Clonal relationships were determined by lspA-glmM-RFLP and by random amplification of polymorphic DNA–PCR. An isogenic amoxicillin-susceptible isolate was transformed with pbp1A PCR products from the resistant isolates.

Results: Amoxicillin-resistant (MIC 2 mg/L) and amoxicillin-susceptible (MIC 0.06 mg/L) isolates, belonging to the same strain, were observed in different biopsies in one patient (inter-niche heteroresistance). Isolates from the remaining patients were amoxicillin-susceptible. Sequencing analysis of the pbp1A of two amoxicillin-resistant isolates and their susceptible partners revealed the same two point mutations: (i) in the third PBP motif of the resistant isolates (C1667G); (ii) a nonsense mutation at the 3' end of the gene. Replacement of pbp1A of a susceptible isolate by pbp1A from a resistant isolate increased the transformants MICs (2 mg/L). A similar MIC was observed when a pbp1A DNA fragment including both point mutations was transformed. Transfer of the smallest fragment (C1667G region only) yielded slightly lower MICs (0.5–1 mg/L). Identical hop gene sequences were observed in paired susceptible and resistant isolates.

Conclusions: A low resistance rate was observed. However, inter-niche heteroresistance could hinder amoxicillin resistance detection when only one biopsy is obtained. Alteration in PBP-1A seems to be enough to reach an MIC of 2 mg/L in our resistant isolates.

Key Words: beta-lactams , resistance , Gram-negative , bacilli


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