Bacterial indicators and antibiotic resistance of Escherichia coli in groundwater
Abstract
The aim of this research was to investigate the groundwater quality in El Barreal basin (Córdoba, Argentina), through bacteriological analysis and antibiotic resistance of fecal bacteria indicators and their relationship with geochemical and land uses characteristics. Groundwater samples were collected in 36 wells and the following parameters were determined: major chemical components, heterotrophic plate counts, total and fecal coliforms, Escherichia coli and Pseudomonas aeruginosa. Antibiotic resistance of E. coli was analyzed using standard methods. The chemical and bacteriological analysis showed that more than 80 % of samples were unfit for human consumption. Bacteriological contamination was significant in 36 % of samples. The multivariate analysis between bacterial and geochemical variables explained local contamination conditions, evidenced by the arrival of bacteria and some typical indicators (NO -, Cl- y 3 HCO -) to groundwater. No significant correlation between NO - and bacterial counts was observed, which let us to 3 3 interpret that part of NO - contents can be supplied by inorganic fertilizers. Total and fecal coliforms were linked to a local 3 increase of Cl- and HCO - and a decrease of pH and dissolved oxygen, indicating the arrival and degradation of organic 3 matter into groundwater. The resistance pattern of total E. coli isolates (n=12) showed that the highest percentages were observed for antibiotics of animal use (ampicillin, tetracycline and cephalothin). The analysis of results revealed the impact of land uses demonstrating that livestock activities are the main punctual contaminant sources in this sedimentary aquifer.
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Introduction
Groundwater importance lies on its ability to act as a large reservoir of freshwater that provides “buffer storage” during periods of drought [1]. As was stated by the mentioned authors, regardless of its importance, groundwater is often misused, usually poorly understood and rarely well managed. In recent decades there has been increasing concern about the final destination of numerous chemical and biological pollutants (pathogenic bacteria, pesticides, fertilizers, industrial by-products and pharmaceuticals) that have a strong impact on water resources, including groundwater. Specifically in agricultural ecosystems there are potential polluting activities like pesticide and fertilizer uses, dairy farms and concentrated animal operations (CAFOs), especially for cows, pigs and poultry. Escherichia coli is the best indicator of fecal contamination, however the presence in the water does not provide definitive information about its possible origin [2]. This is why phenotypic methods, such as antibiotic resistance profiles of E. coli have been used as a tool to elucidate the origin of the contamination in various environments [3]. In rural areas, antibiotics are used in veterinary medicine in a prophylactic way (antiparasites and antibiotics) or as growth promoters (used in subtherapeutic doses), generating selective pressure in indicators of fecal contamination. The use of these compounds, coupled with the increase of animal husbandry in increasingly smaller areas, makes the contamination in rural areas considerably increasing [4].
The groundwater of the South of the Cordoba province (Argentina), almost entirely located in sedimentary aquifers, have been studied for different contamination problems [5]. The need to measure pollution indicators in groundwater is increasingly evident as they are useful for assessing the relationship between environmental variables and the causes and consequences of environmental changes. In this way, it is possible to contribute to the improvement of water resources and environment management. In this context, and taking into account as the main hypothesis that the intensive animal breeding affects in a punctual and more concentrated way the unconfined aquifer, the objective of this research was to carry out an integral study to evaluate the groundwater quality in the Barreal basin (Córdoba, Argentina). Bacteriological parameters and antibiotic resistance of bacteria indicative of fecal contamination were analyzed and linked to hydrogeological, hydrochemical and land use characteristics.
Conclusion
The relationship between bacteriological indicators and physical-chemical parameters with hydrogeological characteristics and land uses shows that the physical and chemical composition of groundwater is partially modified by human activities, which also induce the arrival of bacteria. This research shows that a high percentage of groundwater samples in the studied basin is unfit for human consumption. The results demonstrate the arrival of different pollutants to the groundwater, including some typical contamination tracers such as NO -, which show values above the regional natural background. Also, 3 high Cl- values were associated with bacteria, which allow to link pollution to livestock activity. The slight acidification of the medium and increase of HCO - associated to coliform bacteria in sectors of livestock activity also shows the local impact 3 of fecal residues.
The presence of indicator bacteria of fecal contamination which are resistant to ampicillin, tetracycline and cephalothin antibiotics confirmed that the bacterial contamination in water comes almost exclusively from animal waste. The bacteria arrival to the aquifer is highly variable depending on the contamination scenario and the hydrogeological characteristics of each sector of the basin, higher in areas where the water table is near the land surface.
From what has been explained, it is considered that livestock activity is the main source of punctual pollution in the sedimentary aquifer. However, there is a general diffuse chemical contamination, fundamentally represented by the varied NO - values in the whole basin, which may be fundamentally associated to the extended use of fertilizers. 3 The isolation of E. coli resistant to antibiotics in the unconfined aquifer of the Barreal basin represents an important contribution in the study of antibiotic resistance, since the selection, dissemination and persistence of resistant bacteria is an increasing problem that limits the effectiveness of antibiotic therapy.