Acidophilic and Acid Tolerant Actinobacteria as New Sources of Antimicrobial Agents against Helicobacter Pylori.

About half of the world's population is infected by Helicobacter pylori, which is related to various diseases. The increase in the resistance of H. pylori to antibiotics is alarming and requires new medication candidates. In this study, 83 acidic soil samples (pH 3.9-6.8) were collected from tea and rice farms, located in the semitropical strip in the north of Iran (Lahijan and Fooman cities, Gilan Province). After various pretreatments, including dry heating (120 oC, 10 min), exposure to electromagnetic waves (800 Hz, 3 min), and centrifuging (2950 g, 15 min), 33 acidophilic or acid-tolerant actinobacteria were isolated and their potentials as a source of active metabolites against H. pylori were investigated. According to phenotypic and molecular identification tests, the actinobacterial isolates were classified into Streptomyces and Kitasatospora genera. Among 10 strains that had anti-H. pylori activity, the highest potentials were seen in the strains UTMC 3061 and UTMC 3318. The minimum inhibitory concentrations (MIC) of the related metabolites were 125 and 62.5 µg/ml, respectively. In the checkerboard test, the metabolites of these actinobacteria showed synergism with clarithromycin and reduced its MIC from 1 to 0.5 µg/ml. However, no synergism was seen between the metabolites and amoxicillin or metronidazole. The gas chromatography-mass spectrometry (GC-MS) analysis of the metabolites showed some antimicrobial agents, including carbamic acid, maltol, 2.4-di-tert-butylphenol, methyl dimendone, prolylleucyl, and oleamide. The strains UTMC 3061 and UTMC 3318 showed 99.41 and 100% similarity in 16S rRNA gene sequence to Streptomyces spinoverrucosus and Streptomyces cirratus, respectively. Their metabolites showed good antibiotic activity and limited toxicity and can be considered as promising sources of natural products against H. pylori.