Comparison of the Transport of Tetracycline-Resistant and Tetracycline-Susceptible Escherichia coli Isolated from Lake Michigan

Abstract

It was recently reported that tetracycline could enhance the mobility of manure-derived Escherichia coli within saturated porous media (Walczak et al. (Water Research 45:1681-1690, 2011)). It was also shown, however, that E. coli from various sources could display marked variation in their mobility (Bolster et al. (Journal of Environmental Quality 35:1018-1025, 2009)). The focus of this research was to examine if the observed difference in the mobility of manure-derived tetracycline-resistant (tet(R)) and tetracycline-susceptible (tet(S)) E. coli strains was source-dependent. Specifically, E. coli were isolated from Lake Michigan, and the influence of tetracycline resistance on Lake Michigan-derived E. coli was investigated through column transport experiments. Additionally, a variety of cell morphology and surface properties were determined and related to the observed bacterial transport behavior. Our experimental results showed that, consistent with previous observations, the deposition rate coefficients of the tet(R)E. coli strain was ~20-100% higher than those of the tet(S)E. coli strain. The zeta potential of the tet(R)E. coli cells was ~25 mV more negative than the tet(S)E. coli cells. Because the surfaces of the E. coli cells and the quartz sands were negatively charged, the repulsive electrostatic double-layer interaction between the tet(R)E. coli cells and the quartz sands was stronger, and the mobility of the tet(R)E. coli cells in the sand packs was thus higher. The tet(R)E. coli cells were also more hydrophilic than the tet(S)E. coli cells. Results from migration to hydrocarbon phase (MATH) tests showed that about ~35% more tet(S)E. coli cells partitioned to the hydrocarbon phase. As it was previously shown that cell hydrophobicity could enhance the attachment of bacterial cells to quartz sand, the difference in cell hydrophobicity could also have contributed to the observed higher mobility of the tet(R)E. coli cells. The size of the tet(R) and tet(S)E. coli cells were similar, suggesting that the observed difference in their mobility was not size-related. Characterization of cell surface properties also showed that tet(R) and tetS E. coli cells differed slightly in cell-bound lipopolysaccharide contents and had distinct outer membrane protein profiles. Such difference could alter cell surface properties which in turn led to changes in cell mobility.

Fig. 1

PCR detection of tetB in the E. coli isolates. Lane 1: tet^S, lane 2: tet^R, lane M: 100 bp markers (New England Biolabs). The size of the tetB amplicon was 206 bp

Fig. 2

Breakthrough concentrations of the tet^R (a) and tet^S (b) E. coli strains in saturated porous media under ionic strength conditions of 1, 3, 10, 30, and 100 mM KCl. For clarity purpose, only one in five data points were shown

Fig. 3

Deposition rate coefficients (K_d, per minute) for the two E. coli strains under ionic strength conditions of 1, 3, 10, 30, and 100 mM KCl. The values of K_d were calculated using Eq. 1. Error bars present standard deviation of duplicate experiments. Some error bars are smaller than the size of symbols

Fig. 4

a Size (equivalent radius) of the bacterial cells suspended in 1, 3, 10, 30, and 100 mM of KCl. The equivalent size was calculated as $\sqrt{\frac{L_{\text{C}}\times W_{\text{C}}}{\pi }}$, where L_C and W_C represent the length and width of the cell, respectively. Error bars represent the standard deviation of a minimum of 35 measurements. b Zeta potential of the quartz sand and E. coli cells. Error bars present the standard deviation of triplicate measurements; each measurement contained a minimum of five runs. c Results of MATH test that were expressed as the fraction of bacterial cells that partitioned into the hydrocarbon phase. Error bars present the standard deviation of triplicate measurements. d Comparison of the LPS contents of the tet^R and tet^S E. coli cells suspended in 1, 3, 10, 30, and 100 mM KCl. The error bars represent standard deviation of triplicate extraction attempts

Fig. 5

a Protein contents of the tet^R and tet^S E. coli cells suspended in 1, 3, 10, 30, and 100 mM KCl. The error bars represent standard deviation of triplicate extraction attempts. b Outer membrane protein profiles of tet^S (lane 1) and tet^R (lane 2) strains. Molecular masses are shown on the left. The three proteins that are enriched in the tet^R strain are indicated with arrows