The Occurrence of Antibiotic Resistant Bacteria in Manure, Manure-Applied Fields, and Pets on the Farm

The Occurrence of Antibiotic Resistant Bacteria in Manure, Manure-Applied Fields, and Pets on the Farm

Summary

Antibiotic resistant bacteria (ARB) analysis of manure, soil, compost, and dog feces samples collected from 32 farms covering three animal species (swine, beef, turkeys) and two types of antibiotic use (subtherapeutic use, no subtherapeutic use) showed a strong trend in higher antibiotic resistance in manure microbes isolated from farms that use antibiotics subtherapeutically. However, there was no difference in ARB in soil or dog feces samples taken from either type of farms. These results suggest that subtherapeutic antibiotic feeding may be increasing ARB in manure samples but this resistance does not spread to soil or dogs microbes.

Objectives/Performance Targets

1. To quantify the extent of antibiotic resistant bacteria in manure and manure-applied fields for three different types of animal (swine, beef, and turkeys) production systems.

2. To determine whether or not the use of antibiotics as feed additives in animal production systems is promoting the emergence of antibiotic resistant bacteria (ARB).

3. To assess whether or not microbial antibiotic resistance permeates to domestic pets that live on these farms.

4. To identify manure and soil management practices that may lessen the impact of antibiotic use on antibiotic resistance on the farm.

Accomplishments/Milestones

For each animal species (swine, beef, turkeys) four to five farms that use antibiotics as feed additives and four to five farms that do not use antibiotics as feed additives were selected for our study. From each farm, samples of manure, compost (wherever available), soil (where manure from the farm has been applied) and dog feces were collected in pre-sterilized polypropylene containers. Samples were transported to the laboratory and screened for the presence of antibiotic resistant bacteria (ARB). Antibiotic resistance was checked for three different antibiotics, namely tetracycline, tylosin and monensin. Additionally, samples from turkey farms were checked for virginiamycin. Suspensions of all samples were plated on bacteriological media containing one of the named antibiotics. As control, samples were simultaneously plated on plates without added antibiotic. After inoculation, all plates were incubated for 24 hrs at 37C. After incubation, numbers of colony forming units (CFUs) in antibiotic containing plates as well as in control plate were counted.

Objectives 1 & 2: General trends show higher percent of ARB in manure samples collected from farms where antibiotics are fed sub-therapeutically than where antibiotics are not added as feed additive. Among the three antibiotics used for screening of ARB, higher resistance was found against tylosin in all types of manure samples. This could be because tylosin is the most widely used antibiotic for sub-therapeutic feeding and is used for all types of food animals. Resistance to tetracycline was found to be higher in samples of swine and turkey manure. Tetracycline is generally used during the initial growth of these animals. Some level of resistance was also found in manure from those farms that do not use any antibiotics. This could possibly be due to the fact that these producers have only recently (3-4 years) eliminated antibiotics as feed additives. ARB was highest in swine manure followed by turkey manure and beef cattle manure.

Very low levels of ARB was found in soil samples for tetracycline, suggesting limited transfer of this resistance from manure to soil bacteria. This may be partially because tetracycline is very tightly held by the soil (unpublished results) and thus may not be available to react with soil microbes. Resistance to monensin in soil samples was found to be higher than tetracycline. Except in one case where some tylosin and monensin resistance was observed, no ARB could be isolated from the soil samples taken from fields where manure has never been applied.

Objective 3: No direct correlation could be established between the antibiotic usage and the percentage of ARB from dog fecal samples because ARB was about the same for both types of farms (those that used or did not use antibiotics as feed additives). Furthermore, the ARB was also high in the dog fecal samples collected from the farms that had no food animals and where manure had never been applied to soil.

Further work is continuing on increasing our database by adding additional farms. We are also in the process of identifying the isolated bacterial strains and then testing some of them for minimum inhibitory concentration (MIC) using Sensititer plates. We are also analyzing selected microbes for the presence or absence of antibiotic resistance genes. This to make sure that the absence of antibiotic resistance in soil, and dog fecal microbes, is mainly due to the absence of these genes and not due to lack of their expression. During our sample collection, we have also been gathering information on farm and manure management practices at each of the farms. We will be analyzing this database to see if a particular management practice might be more amenable to reducing ARB in manure and the environment in general.

Impacts and Contributions/Outcomes

During the preliminary identification of bacterial isolates, we have noticed the presence of certain bacterial species like Providencia rettegeri and Burkholdia cepacia in manure and dog fecal samples. These species have never been isolated from farm environments before and both of these are known human pathogens. This suggests the possible role of antibiotic feeding in the emergence and spread of antibiotic resistant traits to potential human pathogenic bacterial species. This needs further investigation, as there are other factors, which also play an important role in the emergence of these antibiotic resistant bacteria.

Collaborators: