This study examined 152 lactobacillus strains to develop reliable methods for distinguishing three closely related species: Lactobacillus plantarum, L. helveticus, and L. casei. Previous identification has been difficult and inconsistent. The researcher tested carbohydrate fermentation patterns, bile salt tolerance, Voges-Proskauer reaction, and acid production in yeastrel glucose litmus milk. Results showed 141 strains could be identified: 23 as L. helveticus, 35 as L. casei, and 83 as L. plantarum. L. helveticus fermented inositol, sorbose, glycerol, and rhamnose but not melibiose or raffinose, could not grow in bile salt, and produced rapid acid clots in milk. L. casei did not ferment those specific sugars and rarely tolerated bile salt. L. plantarum fermented melibiose and usually raffinose, tolerated bile salt, and produced slow acid clots. Combined with existing growth temperature and salt tolerance tests, these fermentation and physiological characteristics provide clear, reproducible differentiation of the three species suitable for identifying large numbers of strains.

Key findings

• L. helveticus consistently ferments inositol, sorbose, rhamnose, and glycerol but not melibiose or raffinose, distinguishing it from L. casei and L. plantarum
• L. plantarum ferments melibiose and usually raffinose while failing to ferment inositol, sorbose, and glycerol, providing reliable species identification
• L. casei cannot ferment melibiose, raffinose, rhamnose, glycerol, or typically inositol and sorbose, making it biochemically distinct from the other two species
• Bile salt tolerance and acid production rates in milk provide additional distinguishing characteristics: L. plantarum tolerates 4% bile salt and produces slow acid clots, while L. helveticus and L. casei are sensitive to bile salt and produce rapid acid clots
• Simple carbohydrate fermentation tests combined with existing temperature and salt tolerance tests enable clear, reproducible differentiation of three previously confused lactobacillus species