In this research, tetracycline resistant (tet R ) and tetracycline susceptible (tet S ) Escherichia coli isolates were retrieved from dairy manure and the influence of tetracycline resistance on the transport of E. coli in saturated porous media was investigated through laboratory column transport experiments. Experimental results showed that tet R E. coli strains had higher mobility than the tet S strains in saturated porous media. Measurements of cell surface properties suggested that tet R E. coli strains exhibited lower zeta potentials than the tet S strains. Because the surface of clean quartz sands is negatively charged, the repulsive electrostatic double layer (EDL) interaction between the tet R cells and the surface of sands was stronger and thus facilitated the transport of the tet R cells. Although no difference was observed in surface acidity, cell size, lipopolysaccharides (LPS) sugar content and cell-bound protein levels between the tet R and tet S strains, they displayed distinct outer membrane protein (OMP) profiles. It was likely that the difference in OMPs, some potentially related to drug efflux pumps, between the tet R and tet S strains led to alteration in cell surface properties which in turn affected cell transport in saturated porous media. Findings from this research suggested that manure-derived tet R E. coli could spread more widely in the groundwater system and pose serious public health risks.

The spread of antibiotic-resistant bacteria in the environment is raising serious public health concerns, and manure is being increasingly recognized as a major source of antibiotic-resistant bacteria. In this research, we isolated Escherichia coli and enterococci from manure produced in a Wisconsin, USA family dairy farm to determine their resistance to six representative antibiotics. The average densities for E. coli and enterococci were 6.37(±4.38)×10 7 colony formation units (CFU)g −1 and 1.60(±1.57)× 10 4 CFU g −1 , respectively. The E. coli isolates were found to be resistant to cephalothin, ampicillin, tetracycline, and erythromycin. In addition to these four antibiotics, the Enterococcus isolates were also resistant to gentamicin and ciprofloxacin. Additionally, we examined the survival and growth of E. coli and enterococci in dairy manure over a period of 3 days. While the densities of enterococci remained stable over the study period, the concentrations of E. coli on average increased by 1.5 log10 units. Further tests of the bacterial antibiotic resistance over time showed no significant changes in the prevalence of antibiotic resistance. This result indicated that slightly aged manure could represent a larger source of antibiotic-resistant E. coli than fresh manure and the accumulation of antibiotic-resistant E. coli and enterococci in the agricultural fields must be accounted for in the modeling of the spread of antibioticresistant bacteria in the environment.

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 (tetR) and tetracycline-susceptible (tetS) 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 tetR E. coli strain was ~20–100% higher than those of the tetS E. coli strain. The zeta potential of the tetR E. coli cells was ~25 mV more negative than the tetS 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 tetR E. coli cells and the quartz sands was stronger, and the mobility of the tetR E. coli cells in the sand packs was thus higher. The tetR E. coli cells were also more hydrophilic than the tetS E. coli cells. Results from migration to hydrocarbon phase (MATH) tests showed that about ~35% more tetS 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 tetR E. coli cells. The size of the tetR and tetS 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 tetR 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.

In this research, we investigate the effects of starvation on the transport of Escherichia coli K12 in saturated porous media. Particularly, we examine the relationship between the starvation effects and the pH and ionic strength of the electrolyte solutions used for cell starvation. E. coli K12 was cultured using either Luria-Bertani Miller (LB-Miller) broth, which contained 10 g/L of NaCl, or LB-Luria broth, which contained 0.5 g/L of NaCl. As both types of broths had similar pH (~7.2) they differed in ionic strengths. The bacterial cells were harvested at late-exponential phase and resuspended in buffered (pH =7.2) and non-buffered (pH = 5.7) electrolyte solutions that had ionic strengths of 8.4 mM or 168 mM, respectively. Column transport experiments were performed following 4, 25 and 52 hours of cell starvation to evaluate the temporal changes in cell mobility. Our results showed that starvation led to a significant increase in the mobility of E. coli K12, particularly between 4 and 25 hours, when both pH and ionic strength of the electrolyte solution were different from those of the growth media. The size, viability and surface properties (e.g., zeta potential, hydrophobicity, LPS sugar content, outer membrane protein profiles) of the bacterial cells were determined and related to the observed temporal variation patterns of cell mobility. We found that starvation in electrolyte solutions that had different pH and ionic strength from the growth media significantly lowered cell viability, which may be related to the temporal change in cell mobility under these specific conditions.

Bacterial cells that enter the groundwater system commonly experience desiccation stresses (i. e., bacterial cells are directly exposed to air) when traveling through the unsaturated layer of soil. Little is known about the effects of desiccation on the transport of bacterial cells in the groundwater system. In this research, we investigated the transport of desiccated and non-desiccated Escherichia coli K12 (ATCC 10798) cells through saturated sand packs using laboratory column transport experiments. Cell desiccation was performed at 25°C under relative humidity (RH) levels of 22%, 53%, 75%, and 97%, respectively, and the desiccation duration was 22 h. Our results showed that desiccation reduced the viability of E. coli cells under all RH levels and increased the transport of E. coli cells under ≥75% RH levels. The increase in the transport of the desiccated E. coli cells was not related to changes in cell size or cell zeta potential. Desiccation under high (i.e., ≥75%) RH levels, however, led to lower cell hydrophobicity, which was found to be positively correlated with cell transport.