Although studies have revealed that contaminated cattle drinking water is an important vehicle in the persistence and dissemination of E. coli O157:H7 on cattle farms, highly effective,
Inactivation of Enterohemorrhagic Escherichia coli in Rumen Content- or Feces-Contaminated Drinking Water for Cattle
Tong Zhao, Ping Zhao, [...], and Michael P. Doyle
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ABSTRACT
Cattle drinking water is a source of on-farm Escherichia coli O157:H7 transmission. The antimicrobial activities of disinfectants to control E. coli O157:H7 in on-farm drinking water are frequently neutralized by the presence of rumen content and manure that generally contaminate the drinking water. Different chemical treatments, including lactic acid, acidic calcium sulfate, chlorine, chlorine dioxide, hydrogen peroxide, caprylic acid, ozone, butyric acid, sodium benzoate, and competing E. coli, were tested individually or in combination for inactivation of E. coli O157:H7 in the presence of rumen content. Chlorine (5 ppm), ozone (22 to 24 ppm at 5°C), and competing E. coli treatment of water had minimal effects (<1 log CFU/ml reduction) on killing E. coli O157:H7 in the presence of rumen content at water-to-rumen content ratios of 50:1 (vol/wt) and lower. Four chemical-treatment combinations, including (i) 0.1% lactic acid, 0.9% acidic calcium sulfate, and 0.05% caprylic acid (treatment A); (ii) 0.1% lactic acid, 0.9% acidic calcium sulfate, and 0.1% sodium benzoate (treatment B); (iii) 0.1% lactic acid, 0.9% acidic calcium sulfate, and 0.5% butyric acid (treatment C); and (iv) 0.1% lactic acid, 0.9% acidic calcium sulfate, and 100 ppm chlorine dioxide (treatment D); were highly effective (>3 log CFU/ml reduction) at 21°C in killing E. coli O157:H7, O26:H11, and O111:NM in water heavily contaminated with rumen content (10:1 water/rumen content ratio [vol/wt]) or feces (20:1 water/feces ratio [vol/wt]). Among them, treatments A, B, and C killed >5 log CFU E. coli O157:H7, O26:H11, and O111:NM/ml within 30 min in water containing rumen content or feces, whereas treatment D inactivated approximately 3 to 4 log CFU/ml under the same conditions. Cattle given water containing treatment A or C or untreated water (control) ad libitum for two 7-day periods drank 15.2, 13.8, and 30.3 liters/day, respectively, and cattle given water containing 0.1% lactic acid plus 0.9% acidic calcium sulfate (pH 2.1) drank 18.6 liters/day. The amounts of water consumed for all water treatments were significantly different from that for the control, but there were no significant differences among the water treatments. Such treatments may best be applied periodically to drinking water troughs and then flushed, rather than being added continuously, to avoid reduced water consumption by cattle.
Escherichia coli O157:H7 has emerged in the last 10 years as an important food-borne pathogen (12, 15, 22, 30, 32, 36), with an estimated 73,000 cases of E. coli O157 infection annually in the United States (23). Cattle are a major reservoir of E. coli O157:H7 (2, 4, 5, 7), and cattle water troughs are important sources of the pathogen on farms (2, 4, 5, 7, 17, 20, 21, 27, 28, 37). Studies have further revealed that when present in cattle drinking water, the pathogen was disseminated to other cattle consuming the contaminated water (19, 29).
Genomic subtyping by pulsed-field gel electrophoresis of E. coli O157:H7 isolates from farms revealed that a single O157:H7 strain was dominant among isolates from cohort and noncohort cattle, water, and other positive samples (i.e., from feed, flies, a pigeon, etc.) on a farm (29). This information demonstrates that drinking water is an important vehicle for disseminating E. coli O157:H7 on the farm and that methods for treatment of drinking water on farms are needed for reduction of the pathogen.
Studies indicate that E. coli O157:H7 can survive in cattle drinking water for a long time (up to 12 months) (13, 19, 29). A variety of treatments have been evaluated for efficacy in killing E. coli O157:H7 in drinking water contaminated with cattle feces (9, 10, 18, 24). The results revealed that most had minimal effects on killing the pathogen, in part because these treatments were neutralized by organic materials present in feces. The objective of this study was to identify practical treatments to eliminate or control E. coli O157:H7 in drinking water by simulating on-farm conditions.
MATERIALS AND METHODS
Bacterial strains. Five isolates of E. coli O157:H7, i.e., 932 (a human isolate), E009 (a beef isolate), E0018 (a cattle isolate), E0122 (a cattle isolate), and E0139 (a deer jerky isolate); five isolates of E. coli O26:H11, i.e., strains DEC10E (a cattle isolate), DEC9E (a cattle isolate), DEC10B (a cattle isolate), 3079-97 (a human isolate), and 3183-96 (a human isolate); and five strains of E. coli O111:NM, i.e., strains 3208-95 (a human isolate), 0944-95 (a cattle isolate), 3287-97 (a human isolate), 4543-95 (a cattle isolate), and 0073-92 (a cattle isolate), were used as five-strain mixtures specific to each serotype. To facilitate the enumeration of these bacterial isolates, all strains were selected for resistance to nalidixic acid (50 μg/ml) according to procedures described previously (37). Each strain was grown individually in 10 ml of tryptic soy broth (TSB) (Becton Dickinson Microbiology Systems, Sparks, MD) containing 50 μg of nalidixic acid (NA) (Sigma Chemical Co., St. Louis, MO) per ml (TSB-NA) for 16 to 18 h at 37°C with agitation (150 rpm). The bacterial cells were sedimented three times by centrifugation (4,000 × g; 20 min); washed in 0.1 M phosphate-buffered saline (PBS), pH 7.2; and resuspended in PBS. The cell suspensions were adjusted with PBS to an optical density at 630 nm of 0.5 (approximately 108 CFU/ml). Five strains of the same serotype were combined at approximately equal cell numbers, which, for each individual strain and the five-strain mixture, were enumerated on tryptic soy agar (TSA) and sorbitol MacConkey agar (SMA) or MacConkey agar plates (all obtained from Becton Dickinson Microbiology Systems).
Rumen contents and feces. Rumen contents or feces from three different cattle were combined and used as a mixture. Rumen contents were collected from beef cattle at slaughter, and feces were collected from cattle on a beef farm, held at 4°C, and used within 7 days. Different samples obtained from the same slaughterhouse or farm were used for different trials.
Treatment with competing bacteria. A five-strain mixture of E. coli O157:H7 at 105 CFU/ml and a mixture of three strains of competing bacteria (E. coli 271, 786, and 797) (37) antagonistic to E. coli O157:H7 at 107 CFU/ml were added to different flasks containing a mixture of water and rumen content at ratios of 100:1, 50:1, 25:1, 10:1, and 5:1 and held at 21°C.
Chlorine and chlorine dioxide treatments. Standard chlorine solutions obtained from HACH Company (Loveland, CO) were freshly diluted for each experiment in deionized water to the required concentration according to a method described previously (39). The free-chlorine concentrations in the diluted chlorine solutions were determined with a Digital Titrator (HACH Co.). The E. coli O157:H7 suspension (1 ml) at 108 to 109 CFU/ml was added to 199 ml of water containing rumen content at ratios of 100:1, 50:1, 25:1, and 10:1 (vol/wt) and 5 ppm chlorine solution (4 ppm chlorine is the maximum residual disinfectant level allowed in drinking water by the Environmental Protection Agency) at 21°C and stirred with a magnetic stir bar in a 500-ml Erlenmeyer flask. Studies with chlorine dioxide were conducted using similar procedures.
Ozone treatments. Ozone was produced by a laboratory scale ozone generator (model H-50; Hess Machine International, Ephrata, PA) equipped with an oxygen concentrator (model AS-12; AirSep, Buffalo, NY), and ozone concentrations (ppm) were measured by the indigo colorimeter method. Ozonated (22 to 24 ppm at 5°C) water was mixed within 5 min with rumen content at ratios of 100:1, 50:1, 25:1, 10:1, and 5:1. Milli-Q water (Milli-Q Synthesis A10; Millipore Corp., Billerica, MA) was used as the control. One milliliter of a mixture of five strains of E. coli O157:H7 (108 CFU/ml) was mixed with 199 ml of the ozonated water with rumen content at 5°C and sampled at 0 to 20 min.
Chemical treatments. Chemicals, including lactic acid (0.05 to 0.5%; Fisher Scientific, Fair Lawn, NJ), hydrogen peroxide (0.5%; Sigma Chemicals Inc., St. Louis, MO), sodium benzoate (0.1%; Fisher Scientific), acidic calcium sulfate (0.9 to 4.5%; Mionix Inc., Naperville, IL), caprylic acid (0.05 to 1.5%; Aldrich Chemicals Inc., Milwaukee, WI), butyric acid (0.5 to 4%; Aldrich Chemicals Inc.), propionic acid (0.5 to 4%; Sigma Chemicals Inc.), and chlorine dioxide (10 to 1,000 ppm; Aldrich Chemicals Inc.), were evaluated separately or as a combination. The concentrations used for each chemical evaluated were based on the results of previous studies conducted with the chemicals and E. coli O157:H7 in deionized water. The chemicals were diluted to appropriate concentrations in Milli-Q water (Milli-Q Synthesis A10; Millipore Corp.) initially tested with the pure cultures of E. coli O157:H7. The effective chemical or combination of different chemicals was further tested for killing effects at 21°C on E. coli O157:H7 in tap water containing rumen content at the different ratios described above.
Enumeration of nalidixic acid-resistant E. coli. At predetermined sampling times, 1.0 ml of the treated bacterial suspension was removed and mixed with 9.0 ml of neutralizing buffer (Becton Dickinson Microbiology Systems). Bacteria were serially (1:10) diluted in 0.1% peptone water, and 0.1 ml of each dilution was surface plated onto TSA containing 50 μg NA/ml (TSA-NA) and SMA containing 50 μg NA/ml (SMA-NA) in duplicate and incubated at 37°C for 24 h. Colonies typical of E. coli O157:H7 (sorbitol negative) we
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