Characterisation of the activity of biosurfactants produced by pseudomonas species isolated from foods
Fernandes, Nathalie Madalena Melro
Spoiled food products were screened for biosurfactant-producing Pseudomonas strains, which were then evaluated for antimicrobial activity and the ability to grow on two model hydrocarbons. Strains isolated from 14 different spoiled food products were screened for biosurfactant production using the drop collapse assay, of which 5.6% tested positive. None exhibited emulsifying activity. The strains were isolated predominantly from leafy vegetable products, and bottled mineral water and low-fat milk; the latter two of which had the lowest APC and PC, respectively. The sources of biosurfactant-producers suggest an attachment role for biosurfactants, rather than one of directly increasing the availability of hydrophobic nutrients to spoilage bacteria. Biosurfactant-producing strains were evaluated for antibacterial activity against potentially pathogenic and food spoilage bacteria using the spot-on-lawn assay, of which 56% exhibited activity against only Gram-positive bacteria, predominantly B. cereus ATCC 10702. Strains found to be active against B. cereus ATCC 10702 were subjected to treatments with protease, heat and organic solvents in order to determine their stability, and 6 of those isolates were further characterised by TLC. The combined data suggested that the majority of compounds produced by the strains isolated in this study produce cyclic lipopeptides, some of which may be novel. Identification by 16S rRNA sequencing identified the antibacterial strains as Pseudomonas (8), B. pumilis (9) and Proteus vulgaris (1). Phylogenetic analysis of the sequences also showed that the strains are closely related to other bacteria with biosurfactant-producing, antimicrobial and biodegradative activities. The ability of 8 biosurfactant-producing strains (3 Pseudomonas, 1 Proteus, 4 B. pumilis) and 2 consortia (Gram-negative and -positive) to grow in minimal media supplemented with n-hexadecane (MMH) or mineral motor oil (MMMO) was evaluated. All of the strains and both consortia reached high cell numbers (>7 – 9 log CFU/ml); however, no significant differences (P<0.05) were observed between individual strains inoculated into either medium. The Gram-negative strains and consortium grew at lower rates in MMMO than in MMH, and when compared with the Gram-positive strains and consortium. P. fulva 381C grew at a significantly lower (P<0.05) rate when grown in MMMO. Furthermore, the consortia did not achieve significantly higher (P<0.05) cell numbers or grow better than individually inoculated strains in either medium. This study is the first to document antibacterial activity by P. fulva, and hydrocarbon and n-alkane utilisation by strains of P. fulva and Proteus vulgaris, respectively.