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The impact of biological interactions on the transport and attachment of Salmonella typhimurium was investigated using cells grown to the mid- and late-exponential phases under a range of ionic strength (IS) conditions, ion valence (KCl vs. CaCl2), and the presence of natural organic matter (NOM). A parallel plate flow chamber was used for the observation of bacterial cell attachment kinetics onto a glass surface with an optical microscope. The physicochemical interactions occurring between the bacteria and surface, as well as the contribution of growth phase and NOM were evaluated. Results showed that none of the cells harvested at the mid- and late-exponential phases attached to glass surfaces at low IS condition (10(-3) M KCl), whereas maximum attachment was observed at high IS (10(-1) M KCl) for both growth phases. Meanwhile, the presence of NOM reduced the attachment of the Salmonella cells significantly under all conditions. Without NOM, attachment efficiencies (alpha) in KCl were similar at both growth phases; however, in the presence of the divalent ions, alpha decreased as the cells aged. In the presence of NOM, alpha was significantly lower at the late-exponential phase than the mid-exponential phase. Cellular characterization suggested that bacterial cell surface heterogeneity and functional groups due to growth phase is one of the primary contributors to the observed attachment trends, affected by both NOM and ion valence. These trends indicate the complex role of NOM in the fate and transport mechanisms of Salmonella, coupled with ion valence and growth phase.