Antibiotic Susceptibility of Bacteria Isolated from Livestock-exposed and Unexposed Areas in Perlite-rich Soil in Thailand

Antibiotic resistant bacteria are present in various environments and are concerning for public health. Antibiotic resistance genes in soil may proliferate throughout different ecosystems and be acquired by pathogens that pose health risks to humans and animals. This study investigated the antibiotic resistance of cultivable bacteria isolated from livestock-exposed and unexposed areas in a former perlite mining area in the Fa-La-Mee Mountain range, Lopburi, Thailand. Soil samples were collected in five different locations. Soil bacteria were isolated and tested for susceptibility against seven antibiotics, followed by 16S rRNA gene-based identification. A total of 56 bacterial isolates were isolated from the perlite-rich soil samples, including 34 isolates from the livestock-exposed areas and 22 isolates from the unexposed areas. Most of the isolates were resistant to cefepime (26.5% in exposed areas and 22.7% in unexposed areas) and ampicillin (23.5% in exposed areas and 22.7% in unexposed areas). 16S rRNA gene sequences revealed that most of the resistant bacteria isolated from perlite-rich soil were Bacillus spp. A multidrug-resistant isolate of Bacillus cereus resistant to ampicillin, trimethoprim/sulfamethoxazole, and cefepime was found in a livestock-unexposed area. The majority of the culturable antibiotic-resistant bacteria isolated from perlite-rich soil were cefepime-resistant Bacillus. Interestingly, the pattern of antibiotic resistance in exposed and unexposed areas was not different, which indicated that antibiotic resistance in perlite-rich soil is likely caused by other factors, such as physicochemical characteristics and/or the microbial population of the soil rather than by antibiotics used in livestock husbandry.