Effects of substratum wettability and molecular topography on the initial adhesion of bacteria to chemically defined substrataab

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Abstract
Self‐assembled monolayers (SAMs) constructed from alkanethiols, whose surface composition and wettability can be rigorously characterized, were used to investigate bacterial adhesion to surfaces. Mixtures of methyl‐terminated and hydroxyl‐terminated alkanethiols were used to produce a series of SAMs with different wettabilities for water and hexadecane. Also, mixtures of 11‐carbon and 18‐carbon SAMs were used to determine the influence of molecular topography on bacterial adhesion and surface wettability. In particular, attachment to mixed chain‐length SAMs in which hydroxyl‐terminated, 11‐carbon alkanethiols were mixed with methyl‐terminated, 18‐carbon alkanethiols was evaluated, to determine whether masking of the polar hydroxyl groups by the longer alkane chains affected bacterial attachment to these surfaces. Substratum wettability was evaluated by measuring the contact angles of water and hexadecane. The kinetics of attachment of an estuarine pseudomonad (MI‐1A) to these surfaces in a laminar flow chamber were measured over periods of 120 min. The numbers of cells attached after 120 min and the percentages of attached cells that adsorbed or desorbed between successive measurements were quantified by phase‐contrast microscopy and digital image processing. SAMs exhibited various substratum compositions and wettabilities, ranging from hydrophilic (contact angle of water ≤ 10°), hydroxyl‐terminated monolayers to hydrophobic (contact angle of water > 100°), methyl‐terminated monolayers. With both types of mixed‐SAMs, the greatest numbers of attached cells occurred on hydrophobic surfaces (∼4.5 x 104 cells mm‐2). In addition, the desorption of cells between successive measurements (ca 2 min) decreased with increasing substratum hydrophobicity. The relationship between substratum wettability and adhesion was also influenced by substratum topography. There were greater numbers of attached cells and less desorption with the SAMs composed of nonpolar polymethylene chains overlying a plane of hydroxyl groups than with SAMs composed of hydroxyl‐ and methyl‐terminated alkanethiols with equal chain‐lengths. This study indicates that not only bulk surface chemistry, but also molecular topography, influence bacterial adhesion.