*4.1. Consequences of the Oral Administration of Antibacterial Agents*

In the present study, one week after the first administration period (d21; AT stage), the amount of resistance was higher compared to the *E. coli* isolates before treatment. A higher rate of enrofloxacin-resistant *E. coli* after oral administration of antibacterial agents was observed in several studies [25,33,34]. Chuppava et al. [25] stated that a single treatment for five days with enrofloxacin led to markedly reduced ratios of susceptible *E. coli* isolates in cloacal swabs and manure samples. The highest proportion of cloacal swabs with resistant *E. coli* was found directly after treatment. Afterwards, a decrease in resistance to enroflaxacin was seen. Therefore, our results are in agreement with those of Chuppava et al. [25] who described a very rapid occurrence of FQ resistance among the commensal *E. coli* after enrofloxacin treatment in poultry. Nevertheless, no difference in resistance in *E. coli* isolates was found between the AT and ET stages after two consecutive treatments with enrofloxacin in this study because MIC values were already very high after one-time treatment.

As expected, after treatment with enrofloxacin, increased MIC values above 2 μg/mL occurred in *E. coli* isolates (T2) with high detection rates up to the end of the trial. Scherz et al. [27] showed that a long-term exposure (21 days) of the commensal flora of poultry to enrofloxacin leads to an amplification and selection of resistant—*E. coli* isolates. These isolates persist in the commensal microbiota. The transmission of *E. coli* isolates of animal origin between the animals in the same pen as well as into the environment may contribute directly to the spread of resistant bacteria in general and may also be a problem for public health [35].

Medication is the main reason for occurrence of resistance to antibacterial agents in *E. coli* [31]. Oral group treatments led to an environmental contamination with antibacterial agents. The application procedure itself or excreted feces from treated animals can be the source [36]. Due to the fact that the metabolic rate of antibiotics is low, 90% of the administered dose is excreted via feces [5]. Avian intestines can act as potential reservoirs of *E. coli* [37]. Thus, there is a higher risk for resistance to antibacterial agents spreading from birds to other birds or from birds to the environment. In other European countries, the higher occurrence of FQ resistance in broilers compared to turkeys has been

suggested to depend on an overall use-dependent higher exposure to FQ [3]. It should be noted, however, that the fattening period in turkeys takes much longer under field conditions. Therefore, the resistance situation in the present investigations at the end of the experiment is not comparable with the resistance situation occurring within the normal fattening duration.

In agreement with our data, Jurado et al. [34] and Chuppava et al. [25] found a significant increase in the frequency of resistance to ampicillin in *E. coli* isolates from poultry after orally administering enrofloxacin. These findings may be due to the coselection of β-lactam resistance genes. As the transmissible genetic elements were not analyzed in our study, further studies are recommended in order to confirm the role of such elements in the spread of resistance genes in poultry for *E. coli*.
