Amoxycillin tolerance in Helicobacter pylori. J Antimicrob Chemother 1999

Resistance to amoxycillin in Helicobacter pylori has only recently been reported. To demonstrate
the existence of resistance, and to test for the presence of tolerance, 17 amoxycillinresistant
strains of H. pylori, first isolated in Sardinia (Italy) and the USA, were studied. Four
amoxycillin-sensitive strains were used as controls. Primary isolates of all test strains
exhibited amoxycillin resistance; -lactamase activity was not detected. Amoxycillin resistance
was lost after storage of strains at –80°C but could be rescued by plating these strains
on to amoxycillin gradient plates. MICs and MBCs from rescued isolates ranged from 0.5 to
32 mg/L and from 32 to >1024 mg/L, respectively. MBC/MIC ratios ³32 are characteristic of
antibiotic tolerance. The ratios of MBC/MIC of amoxycillin ranged from 32 to >1024 for the test
strains, indicating that these strains were tolerant to the antibiotic. Amoxycillin resistance does
occur in H. pylori. Amoxycillin susceptibility testing of H. pylori isolates in patients who fail
therapy should include determination of the MBC to detect tolerance.