Influence of pretreatment and experimental conditions on electrophoretic mobility and hydrophobicity of Cryptosporidium parvum oocysts

Abstract

Surface properties of Cryptosporidium parvum oocysts were investigated by using electrophoretic mobility and hydrophobicity measurements. Oocysts purified from calf feces by several sucrose flotation steps and deionized water (DI) washes (DIS method) had an electrophoretic mobility (neutral surface charge) near 0.0 m2 V-1 s-1 over a pH range of 2 to 10. The mean electrophoretic mobility of oocysts stored in DI containing a mixture of antibiotics had a lower standard deviation (sigma = 0.36) than that of oocysts stored in DI without antibiotics (sigma = 0.53); their electrophoretic mobility remained unchanged up to 121 days after collection. The electrophoretic mobility of oocysts purified on a cold Percoll-sucrose gradient after the feces was defatted with ethyl acetate (EAPS method) varied linearly with pH from 0.0 m2 V-1 s-1 at pH 2.4 to -3.2 x 10(-8) m2 V-1 s-1 at pH 10 (sigma = 0.52), thus displaying the negative surface charge at neutral pH observed by other researchers. The hydrophobicity of oocysts and two types of polystyrene beads was measured as a function of ionic strength by adhesion to polystyrene. Oocysts were purified by the DIS method. The ionic strength of the suspending solution was varied from 0 to 95 mmol liter-1. Two-week-old oocysts exhibited strong adhesion ( approximately 85%) at ionic strengths of 0 to 10 mmol liter-1 and moderate adhesion ( approximately 20%) at ionic strengths of 20 to 95 mmol liter-1. Two-month-old oocysts exhibited high adhesion ( approximately 60 to 80%) at all ionic strengths. These results show that adhesion properties governed by the electrophoretic mobility of purified C. parvum oocysts can be altered by the method of purification and that hydrophobicity can change as oocysts age.

FIG. 1

Electrophoretic mobility versus pH for DIS-purified C. parvum oocysts without antibiotics (a), DIS-purified C. parvum oocysts with antibiotics (b), and EAPS-purified C. parvum oocysts with antibiotics (c). The squares represent individual electrophoretic mobility measurements, and the dashed lines represent the least-squares regression lines through the data.

FIG. 2

Data and Lowess plots for adhesion of 12- to 17-day-old DIS-purified C. parvum oocysts (squares and solid line) and >60-day-old DIS-purified C. parvum oocysts (circles and dashed line) to polystyrene at different ionic strengths.

FIG. 3

Adhesion of 6.0-μm-diameter uncharged polystyrene microspheres (electrophoretic mobility = 0.0 m² V⁻¹ cm⁻²) to polystyrene at different ionic strengths.

FIG. 4

Adhesion of 4.5-μm-diameter carboxylated polystyrene microspheres (electrophoretic mobility = −68 m² V⁻¹ cm⁻²) to polystyrene at different ionic strengths.