Occurrence and characterization of bacillus licheniformis and bacillus subtilis in yam powder products from Vietnam as a health risk factor

Yam (Dioscorea alata L.) is a carbohydrate-rich tuber crop widely consumed in tropical countries. This includes Vietnam, where both fresh yam and yam powder are part of traditional diets. Despite its popularity, the microbiological safety of yam-derived products has received limited scientific attention. This study investigated the microbial contamination and characterized Bacillus licheniformis and Bacillus subtilis isolated from commercially available yam powder products in Vietnam. The results showed that while B. cereus, E. coli, S. aureus, or Salmonella spp. were not detected, varying levels of total aerobic bacteria, yeasts, and molds were present in most samples. B. licheniformis was the predominant species, isolated from five out of seven powder samples, while B. subtilis appeared in two. It should be noted that all B. licheniformis strains harbored the licA, licB, and licC genes responsible for lichenysin synthesis and displayed multidrug resistance to antibiotics such as erythromycin, amoxicillin-clavulanate, and cephalosporins. In contrast, B. subtilis strains lacked diarrheagenic toxin genes (nhe, hbl, cytK) and showed higher susceptibility to most antibiotics. These findings suggest that microbial contamination may occur during processing or storage and highlight the urgent need for updated microbial guidelines, targeted hygienic practices, and routine toxin gene screening to ensure safety and quality of yam-based food products in Vietnam.