Reversible Bacterial Depletion Aggregation and Restabilization by Water-Soluble Polymers

Depletion forces are ubiquitous in soft materials and biological systems; however, their role in producing depletion aggregation of bacterial cells is becoming recognized as an important factor impacting bacterial growth and infection. The concentration of the depletant has been quantified only in limited cases. To provide a broader perspective, this work compares depletion aggregation of flagella-free Escherichia coli by several different macromolecules relevant to food and biomedical applications, to provide perspective on the concentration range of each polymer that produces aggregation: both the onset of depletion aggregation and restabilization. The macromolecules include hyaluronic acid, poly(acrylic acid) (PAA), guar, and albumin, in comparison to previously published results with poly(ethylene oxide) and the same E. coli. The reversibility of aggregation is established to distinguish depletion from other potential aggregation mechanisms. We report that due to the varying size of the macromolecular depletants, the depletant concentration needed to produce bacterial aggregation can vary by 2 orders of magnitude, with guar producing aggregation below 0.03 wt% while about 1 wt% albumin can be tolerated in solution before aggregation occurs. At reduced ionic strengths approaching freshwater conditions, polyanionic depletants produced aggregation below concentrations of 0.01 wt%. Restabilization at higher polymer concentrations was seen, consistent with concentrations exceeding the polymer overlap concentration. Restabilization occurred more readily with more dilute cell suspensions. Concentrated cell suspensions did not restabilize at the same high (∼1 wt%) polymer concentrations. Low molecular weight 5K PAA produced aggregation only in a narrow window. Data for the minimum depletant concentration needed to produce depletion aggregation were collapsed by considering the effective depletant volume fraction based on a hydrated random coil or, for albumin, globule.