Amoxycillin tolerance in Helicobacter pylori

Resistance to amoxycillin in Helicobacter pylori has only recently been rep orted. To demonstrate the existence of resistance, and to test for the pres ence of tolerance, 17 amoxycillin-resistant strains of H. pylori, first iso lated in Sardinia (Italy) and the USA, were studied. Four amoxycillin-sensi tive strains were used as controls. Primary isolates of all test strains ex hibited amoxycillin resistance; beta-lactamase activity was not detected. A moxycillin resistance was lost after storage of strains at -80 degrees C bu t could be rescued by plating these strains on to amoxycillin gradient plat es. 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 greater than or equal to 32 are characteristic of antibiotic tolerance. The ratios of MBC/MIC of amoxy cillin 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 p atients who fail therapy should include determination of the MBC to detect tolerance.