Concentrating aqueous dispersions ofStaphylococcus Epidermidis bacteria by swelling of thermosensitive poly[(N-isopropylacrylamide)-co-(acrylic acid)] hydrogels

Abstract

A novel method for concentrating of bacteria dispersions without loss of bacterial viability has been developed. Bioseparation was facilitated by the swelling of a thermoresponsive hydrogel in a dilute bacterial dispersion. Hydrogels were prepared by free radical copolymerization in aqueous solution of N-isopropylacrylamide (NIPA) and acrylic acid (AA) with N,N ′-methylene bisacrylamide (BIS) as crosslinker. Hydrogels of a single comonomer composition and three concentrations of BIS were employed. The potential of the corresponding xerogels as bioseparation devices was assessed by swelling to equilibrium at 5°C in aqueous dispersions of the bacterium Staphylococcus Epidermidis. The resultant increase in concentration of dispersion, the concentrated dispersion being commonly termed the `raffinate', was measured by the spread plate technique. An efficiency of separation η was obtained from this increase, coupled with that calculated from the measured degree of swelling. It has been shown that high values of η can be achieved. It has also been shown that separation efficiency is dependent on the procedure employed in the sampling of the concentrated dispersion. Without agitation of the swollen hydrogel/raffinate prior to sampling no concentration increase was measured, i. e. η was 0%. However, η increased to values of 50–80% on agitation (via a vortex mixer) of the hydrogel/raffinate prior to raffinate sampling. The need for agitation and the reduction in the value of η from its theoretical maximum (100%) may indicate the adsorption of the bacteria onto the surface of the hydrogel. The effect of the bacterial dispersion on the swelling properties of the hydrogels was also investigated. It was found that the swelling ratio (= mass of hydrogel/mass of xerogel) and lower critical swelling temperature of the hydrogels in the bacterial dispersion and pure water were identical.