Physicochemical and biochemical characterization of biosurfactants released by Lactobacillus strains

Biosurfactants from Lactobacillus casei subsp. rhamnosus 36 and ATCC 7469, Lactobacillus fermentum B54 and Lactobacillus acidophilus RC14 were isolated from bacteria in their mid-exponential (4-5 h) and stationary growth phases (18 h) and physicochemical and biochemical properties of the freeze-dried biosurfactants from both growth phases were compared. The mid-exponential and stationary phase biosurfactants were similar in their surface activities, but the latter had a lower and better defined critical micelle concentration. In particular, the stationary phase biosurfactant from L. acidophilus RC14 reached a low liquid surface tension of 39 mJ m(-2) in phosphate buffered saline, with a critical micelle concentration of 1.0 mg ml(-1). All biosurfactants consisted of a mixture of protein and polysaccharides, possibly containing bound phosphate groups, but the stationary phase biosurfactants were richest in protein, as concluded from Fourier transform infrared spectroscopy on biosurfactants in KBr pellets and from X-ray photoelectron spectroscopy on biosurfactants deposited on gold-coated glass slides. Infrared spectra of mid-exponential phase biosurfactants showed an absorption band around 2500 cm(-1), which was absent in the spectra of stationary phase biosurfactants and was assigned to nitrogen-containing compounds with >NH2+, >(NH+)-, or =(NH+)- groups. Amino acid analysis of the hydrolyzed mid-exponential and stationary phase biosurfactants from L. casei subsp. rhamnosus 36 demonstrated a high occurrence of alanine, which is possibly present as the free amino acid. The L. acidophilus RC14 mid-exponential phase biosurfactant also had a high alanine content, but the L. acidophilus RC14 stationary phase biosurfactant had an approximately four-fold lower alanine occurrence. We suggest that the protein-rich biosurfactants released by certain Lactobacillus strains in their stationary growth phases may interfere with uropathogen adhesion, for which we propose the name ''surlactin''.