High surface hydrophobicity of autoaggregating Staphylococcus aureus strains isolated from human infections studied with the salt aggregation test

Abstract

A total of 209 strains of Staphylococcus aureus isolated from infections and 23 strains from nose cultures of healthy laboratory personnel were compared for relative surface hydrophobicity in the salt aggregation test (Lindahl et al., Biochim. Biophys. Acta 677:471-476, 1981). In the standard method, bacterial cell suspensions from blood agar-grown cultures were tested for visible aggregation by "salting out" in serial dilutions of ammonium sulfate (0.1 to 1.6 M [final concentration]). Bacteria were defined as extremely hydrophobic when showing autoaggregation in saline or in 0.002 M sodium phosphate buffer (pH 6.8). Using this definition, we found a large number of strains isolated from various infections to be very hydrophobic: 123 of 135 strains from patients with septicemia (91%), 54 of 60 strains from wound infections (90%), and 12 of 14 strains from urinary tract infections (86%). In contrast, only 9 of 23 strains from nose cultures of healthy carriers (39%) were autoaggregating. A total of 12 autoaggregating strains were grown on various solid and liquid media. Only growth on hematin agar was found to completely suppress surface hydrophobicity as revealed by our salt aggregation test method, and growth in liquid media prevented the expression of hydrophobicity in most strains. Growth at 20 or 42 degrees C or under anaerobic conditions did not affect hydrophobicity. Cells harvested from various phases of growth did not differ significantly in surface hydrophobicity. Heating washed cell suspensions at 56 degrees C did not affect the salt aggregation test values, whereas heating the cell suspensions at 80 and 100 degrees C caused a significant decline in hydrophobicity. The addition of ethylene glycol (25% [vol/vol] final concentration) prevented the autoaggregation of 10 of the 12 strains. Likewise, treating the cell suspensions with proteolytic enzymes decreased the surface hydrophobicity, indicating that surface proteins contribute to high surface hydrophobicity of autoaggregating strains.