LAUSR

&NA; Bacterial biosurfactants have a wide range of biological functions and biotechnological applications. Previous analyses had suggested a limit to their reduction of aqueous liquid surface tensions (&ggr;Min), and here we confirm this in an analysis of 25 Pseudomonas spp. strains isolated from soil which produce high‐strength surfactants that reduce surface tensions to 25.2 ± 0.1‐26.5 ± 0.2 mN m‐1 (the surface tension of sterile growth medium and pure water was 52.9 ± 0.4 mN m‐1 and 72.1 ± 1.2 mN m‐1, respectively). Comparisons of culture supernatants produced using different growth media and semi‐purified samples indicate that the limit of 24.2‐24.7 mN m‐1 is not greatly influenced by culture conditions, pH or NaCl concentrations. We have used foam, emulsion and oil‐displacement behavioural assays as a simple and cost‐effective proxy for in‐depth biochemical characterisation, and these suggest that there is significant structural diversity amongst these surfactants that may reflect different biological functions and offer new biotechnological opportunities. Finally, we obtained a draft genome for the strain producing the highest strength surfactant, and identified a cluster of non‐ribosomal protein synthase genes that may produce a cyclic lipopeptide (CLP)‐like surfactant. Further investigation of this group of related bacteria recovered from the same site will allow a better understanding of the significance of the great variety of surfactants produced by bacterial communities found in soil and elsewhere. &NA; Graphical Abstract Figure. Bacterial biosurfactants appear to have a limit to aqueous liquid surface tension activity of ˜24 mN m‐1, which is approached by a number of pseudomonad surfactants showing significant behavioural diversity.