  Bacteria at fluid interfaces endure physical and chemical stresses unique to
these highly asymmetric environments. The responses of Pseudomonas aeruginosa
PAO1 and PA14 to a hexadecane-water interface are compared. PAO1 cells form
elastic films of bacteria, excreted polysaccharides and proteins, whereas PA14
cells move actively without forming an elastic film. Studies of PAO1 mutants
show that, unlike solid-supported biofilms, elastic interfacial film formation
occurs in the absence of flagella, pili, or certain polysaccharides. Highly
induced genes identified in transcriptional profiling include those for
putative enzymes and a carbohydrate metabolism enzyme, alkB2; this latter gene
is not upregulated in PA14 cells. Notably, PAO1 mutants lacking the alkB2 gene
fail to form an elastic layer. Rather, they form an active film like that
formed by PA14. These findings demonstrate that genetic expression is altered
by interfacial confinement, and suggest that the ability to metabolize alkanes
may play a role in elastic film formation at oil-water interfaces.
