Physiological consequences of the S-layer of Aeromonas salmonicida in relation to growth, temperature, and outer membrane permeation

S-layers are paracrystalline protein multimers that cover the entire cell surface of many bacterial species. The presence of an S-layer in Aeromonas salmonicida (also known as A-layer) predisposed this bacterium to apparently unrelated physiological consequences: inhibition of growth at 30°C, enhanced cell filamentation at 37°C, and enhanced uptake of the hydrophobic antibiotics streptonigrin and chloramphenicol. Growth inhibition or enhanced filamentation was not observed when the native A-layer was missing or its arrangement altered, as in Ca²⁺-limited or Ca²⁺- and Mg²⁺-limited cells, in A-layer-negative (A^-) cells with an artificially reconstituted A-layer, or in mutants unable to correctly assemble this layer. A-layer-positive cells (A⁺) were far more sensitive to the intracellularly acting antibiotics streptonigrin and chloramphenicol than were A^- cells, and streptonigrin-resistant mutants were predominantly A^-. Hemin, a compound known to specifically bind to the A-layer, alleviated streptonigrin toxicity to A⁺, but not A^-, cells. As well, (Ca²⁺- and Mg²⁺-limited cells, or mutants harboring A-layer defects had a reduced sensitivity to streptonigrin, and A^- cells with reconstituted A-layers remained resistant to streptonigrin and chloramphenicol. Thus, the presence of a native A-layer arrangement on the cell surface, and not the mere presence of A-layer protein subunits, predisposed A. salmonicida toward the aforementioned physiological consequences. The A-layer is suggested to specifically effect these consequences, in particular the permeation of streptonigrin or chloramphenicol, by a specific interaction of A-layer subunits with the outer membrane.