© 2003 Blackwell Publishing Ltd, Helicobacter , 8, 202–206 202 Volume 8 • Number 3 • 2003 HELICOBACTER Blackwell Publishing Ltd. Mixed-Infection of Antibiotic Susceptible and Resistant Helicobacter pylori Isolates in a Single Patient and Underestimation of Antimicrobial Susceptibility Testing Jae J. Kim, * Jong G. Kim † and Dong H. Kwon ‡ * Department of Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea; † Internal Medicine, Korea University College of Medicine, Seoul, Korea; and ‡ Department of Medicine of Baylor College of Medicine and Veterans Affairs Medical Center, Houston, TX, USA ABSTRACT Antibiotic resistance among Helicobacter pylori has been increasing worldwide and has begun to affect the overall efficacy of current antibiotic regimens adversely. We examined 220 pairs of H. pylori isolates obtained from both the antrum and corpus of separate patients; 109 (50%) harbored antibiotic- resistant H. pylori: amoxicillin (0.5%), clarithro- mycin (5.9%), furazolidone (1.4%), metronidazole (45.5%), nitrofurantoin (1.4%), and tetracycline (6.8%). Heteroresistance among the two biopsy sites from each patient was present in 41 of the 109 patients (38%) with antibiotic resistant H. pylori (e.g. 34% with resistant strains would be mis- classified as susceptible if a biopsy of the antrum alone used for antimicrobial susceptibility testing). DNA fingerprinting genotype analysis was carried out on the 41 pairs of isolates with heteroresistance. While different patients had different fingerprinting patterns, each pair of isolates showed identical or similar fingerprinting patterns. These results suggest that antibiotic-resistant H. pylori typically develop from pre-existing susceptible strain rather than coinfection with a different strain. The minor dif- ferences in genotype (degeneration of genotype) seen reflect one of the processes for development of genetic diversity in H. pylori. No biopsy single site can be considered representative for antimicrobial susceptibility testing. Keywords. Helicobacter pylori, heteroresistance, mixed infection, fingerprinting genotype. H elicobacter pylori infection is recognized as a causal factor in the pathogenesis of chronic gastritis, peptic ulcer and gastric cancer [1– 4]. Although treatment regimens containing a proton pump inhibitor and combination of two or more antibiotics (amoxicillin, clarithromycin, metronidazole, or tetracycline) are considered to be the most efficacious, current antibiotic regimens produce only 60 –85% cure rates in clinical practice [5]. H. pylori resistance to amoxicillin, clarithro- mycin, metronidazole and tetracycline have all been reported [6–8] and failure of antibiotic treatment is often due to the presence of antibiotic-resistant H. pylori [5,9,10]. Although H. pylori from indi- vidual patients typically have either an antibiotic susceptible or resistant phenotype, both antibiotic- susceptible and -resistant H. pylori (i.e. heterore- sistance) can be present. Heteroresistance can represent infection with a single strain or infection with several different H. pylori strains [11–15]. These observations raise questions of which biopsy (antrum or corpus) is a representative of antibiotic susceptibility testing and how many biopsy specimens from a single patient is the most appropriate to detect antibiotic resistant H. pylori . Additionally, it is also unclear whether treatment failure related to antibiotic resistance is most often the result of pre-existing antibiotic- resistant H. pylori or development of a new antibiotic resistant strain de novo. In this study, we examined pairs of H. pylori isolates from the antrum and corpus of 220 patients to answer those questions raised. Materials and Methods Patients and H. pylori Culture Two hundred and twenty pairs of H. pylori iso- lates obtained from the antrum and corpus of Reprint requests to: Dong H. Kwon, PhD, Veterans Affairs Medical Center (111D), 2002 Holcombe Blvd., Houston, TX 77030, USA.