Biomedical subjects

Li-Chong Xu

Publications and source records attributed to Li-Chong Xu.

6 recordsLinked to original sources

PubMed2006

Interaction forces measured using AFM between colloids and surfaces coated with both dextran and protein.

Both proteins and polysaccharides are biopolymers present on a bacterial surface that can simultaneously affect bacterial adhesion. To better understand how the combined presence of proteins and polysaccharides might influence bacterial attachment, adhesion forces were examined using atomic force microscopy (AFM) between colloids (COOH- or protein-coated) and polymer-coated surfaces (BSA, lysozyme, dextran, BSA+dextran and lysozyme+dextran) as a function of residence time and ionic strength. Protein and dextran were competitively covalently bonded onto glass surfaces, forming a coating that was 22-33% protein and 68-77% dextran. Topographic and phase images of polymer-coated surfaces obtained with tapping mode AFM indicated that proteins at short residence times (<1 s) were shielded by dextran. Adhesion forces measured between colloid and polymer-coated surfaces at short residence times increased in the order protein+dextran < or = protein < dextran. However, the adhesion forces for protein+dextran-coated surface substantially increased with longer residence times, producing the largest adhesion forces between polymer coated surfaces and the colloid over the longest residence times (50-100 s). It was speculated that with longer interaction times the proteins extended out from beneath the dextran and interacted with the colloid, leading to a molecular rearrangement that increased the overall adhesion force. These results show the importance of examining the effect of the combined adhesion force with two different types of biopolymers present and how the time of interaction affects the magnitude of the force obtained with two-polymer-coated surfaces.

Colloids↗

PubMed2006

Adhesion forces between functionalized latex microspheres and protein-coated surfaces evaluated using colloid probe atomic force microscopy.

Proteins are important in bacterial adhesion, but interactions at molecular-scales between proteins and specific functional groups are not well understood. The adhesion forces between four proteins [bovine serum albumin (BSA), protein A, lysozyme, and poly-d-lysine] and COOH, NH2 and OH-functionalized (latex) colloids were examined using colloid probe atomic force microscopy (AFM) as the function of colloid residence time (T) and solution ionic strength (IS). For three of the proteins, OH-functionalized colloids produced higher adhesion forces to proteins (2.6-30.5 nN; IS=1 mM, T=10s) than COOH- and NH2-functionalized colloids (1.6-6.8 nN). However, protein A produced the largest adhesion force (8.1+/-1.0 nN, T=10 s) with the COOH-functionalized colloid, demonstrating the importance of specific and unanticipated protein-functional group interactions. The NH2-functionalized colloid typically produced the lowest adhesion forces with all proteins, likely due to repulsive electrostatic forces and weak bonds for NH2-NH2 interactions. The adhesion force (F) between functionalized colloids and proteins consistently increased with residence time (T), and data was well fitted by F=ATn. The constant value of n=0.21+/-0.07 for all combinations of proteins and functionalized colloids indicated that water exclusion and protein rearrangement were the primary factors affecting adhesion over time. Adhesion forces decreased inversely with IS for all functional groups interacting with surface proteins, consistent with previous findings. These results demonstrate the importance of specific molecular-scale interactions between functional groups and proteins that will help us to better understand factors colloidal adhesion to surfaces.

Adsorption↗

PubMed2005

Residence time, loading force, pH, and ionic strength affect adhesion forces between colloids and biopolymer-coated surfaces.

Exopolymers are thought to influence bacterial adhesion to surfaces, but the time-dependent nature of molecular-scale interactions of biopolymers with a surface are poorly understood. In this study, the adhesion forces between two proteins and a polysaccharide [Bovine serum albumin (BSA), lysozyme, or dextran] and colloids (uncoated or BSA-coated carboxylated latex microspheres) were analyzed using colloid probe atomic force microscopy (AFM). Increasing the residence time of an uncoated or BSA-coated microsphere on a surface consistently increased the adhesion force measured during retraction of the colloid from the surface, demonstrating the important contribution of polymer rearrangement to increased adhesion force. Increasing the force applied on the colloid (loading force) also increased the adhesion force. For example, at a lower loading force of approximately 0.6 nN there was little adhesion (less than -0.47 nN) measured between a microsphere and the BSA surface for an exposure time up to 10 s. Increasing the loading force to 5.4 nN increased the adhesion force to -4.1 nN for an uncoated microsphere to a BSA surface and to as much as -7.5 nN for a BSA-coated microsphere to a BSA-coated glass surface for a residence time of 10 s. Adhesion forces between colloids and biopolymer surfaces decreased inversely with pH over a pH range of 4.5-10.6, suggesting that hydrogen bonding and a reduction of electrostatic repulsion were dominant mechanisms of adhesion in lower pH solutions. Larger adhesion forces were observed at low (1 mM) versus high ionic strength (100 mM), consistent with previous AFM findings. These results show the importance of polymers for colloid adhesion to surfaces by demonstrating that adhesion forces increase with applied force and detention time, and that changes in the adhesion forces reflect changes in solution chemistry.

Bacterial Adhesion↗

PubMed2005

Interaction forces between colloids and protein-coated surfaces measured using an atomic force microscope.

Bacterial surfaces contain proteins, polysaccharides, and other biopolymers that can affect their adhesion to another surface. To better understand the role of proteins in bacterial adhesion, the interactions between two different model colloids (glass beads and carboxylated latex microspheres) and four proteins covalently bonded to glass surfaces were examined using colloid probes and an atomic force microscope (AFM). Adhesion forces between an uncoated glass colloid probe and protein-coated surfaces, measured in retraction force curves, decreased in the order poly-D-lysine > lysozyme > protein A > BSA. This ordering was consistent with the relative calculated charges of the proteins at neutral pH and the zeta-potentials measured for glass beads and latex microspheres coated with these proteins. When the glass bead was coated with a protein (BSA), overall adhesion forces between the protein-coated colloid and the protein-coated surfaces were reduced, and the adhesion force for each protein decreased in the same order observed in experiments with the uncoated glass bead. When latex colloid probes were coated with BSA, adhesion forces were significantly larger than those measured with BSA-coated glass colloid probes under the same conditions, demonstrating that the nature of the underlying colloid can affect the measured interaction forces. In addition, the adhesion forces measured with the BSA-coated latex colloid increased in a different order (BSA < lysozyme < protein A < poly-D-lysine) than that observed using the BSA-coated glass colloid. It was also found that increasing the solution ionic strength consistently decreased adhesion forces. This result is contrary to the general observation that bacterial adhesion increases with ionic strength. It was speculated that conformational changes of the protein produced this decrease in adhesion with increased ionic strength. These results suggest the need to measure nanoscale adhesion forces in order to understand better molecular scale interactions between colloids and surfaces.

Adsorption↗

PubMed2002

Anaerobic electrochemical corrosion of mild steel in the presence of extracellular polymeric substances produced by a culture enriched in sulfate-reducing bacteria.

The corrosion of mild steel in a seawater medium containing extracellular polymeric substances (EPS) produced by sulfate-reducing bacteria (SRB) was studied by electrochemical experiments and atomic force microscopy (AFM). Under anaerobic conditions, the corrosion of mild steel increased up to 5-fold in the presence of a 1% (w/w) EPS solution but in the absence of SRB. The enhanced corrosion is mainly due to the oxidizing power of EPS with a reduction potential of E1/2 at -0.54 V (saturated calomel electrode), which is 0.4 V above that of hydrogen reduction. The electrochemical reduction of EPS provides a couple to iron oxidation, as demonstrated by H-shaped cell experiments in which the steel sample and EPS are not in physical contact but are ionically connected via the solution and electronically connected through an external wire. Fourier transformation infrared spectroscopy and X-ray photoelectron spectroscopy showed that EPS derived from SRB are comprised of 60% proteins, 37% polysaccharides, and 3% hydrocarbons. The XPS results showed that, upon corrosion, polysaccharide components were mostly converted to hydrocarbons.

Biofilms↗

PubMed2002

Effects of toxic metals and chemicals on biofilm and biocorrosion.

Microbes in marine biofilms aggregated into clusters and increased the production of extracellular polymeric substances (EPS), by over 100% in some cases, when the seawater media containing toxic metals and chemicals, such as Cd(II), Cu(II), Pb(II), Zn(II), AI(III), Cr(III), glutaraldehyde, and phenol. The formation of microbial cluster and the increased production of EPS, which contained 84-92% proteins and 8-16% polysaccharides, accelerated the corrosion of the mild steel. However, there was no quantitative relationship between the degree of increased corrosion and the toxicity of metals/chemicals towards sulfate-reducing bacteria, or the increased EPS production.

Biofilms↗