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The antibiotics are a very important class of chemotherapeutic agents that are capable to stop or even selectively destroy pathogens in man and animal. Antimicrobial agents affect susceptible organisms in various ways. The major mechanisms of action of antibiotics are: inhibition of cell wall synthesis, impairment of cell membrane function, inhibition of protein synthesis, inhibition of nucleic acids synthesis.

The enteric disease complex may have a great economic impact by decreasing the growth of the animals, the conversion-rate of the fodder and, implicitly, the quality of carcass. That results in low production values, in correlation with low costs of production and decreased profitability. A therapeutic protocol was established, being administered tiamullin in the period of maximum incidence ofthe disease –5 days before and after weaning in pig youth, and the same period of time for the fatten pigs. The average meat production increased with 37.04% in youth pigs and 29.23% in fat pigs, in comparison with the period before the therapy. The profit that was achieved in youth pigs was 95 %, in comparison with the investment (the medication) that was 5 %. The value of the investment was 27% in fat pigs, in comparison with the materialized profit of 73 %. The ratio between the total investment values (tiamullin medication) and the benefit obtained in posttherapeutic period was 1:3.76, for the enteric disease complex in swine.

A thoroughgoing study on resistance was achieved for the first time in insecticides, later on finding that, this can be extended also in anthelmintic substances. The resistance to anthelmintics evolved gradually from a simple medical curiosity, to an important economic problem. Many factors contribute to the appearance of resistance to anthelmintic substances. After epidemiological studies on various helmintic populations, it was concluded that there are four essential and distinguish factors that induce the appearance of parasitic resistance: operational factors, genetic factors, biological and ecological factors. In the last two decades there were conceived many tests to give emphasis to the phenomenon of resistance, but very few were accepted and validated at an international level. The motivation for this phenomenon consist in the fact that every of this methods have a certain sensibility, feasibility and profitableness. All these tests have positive aspects, and few negative aspects, that make themhard to be utilized (apparatus and high costs) or there are not enough sensible, so the obtained results are difficult to be interpreted. The use of molecular technique for the diagnosis of resistance to anthelmintics does not represent a viable option in the present, because the data about the intimate molecular mechanisms of resistance to anthelmintics are very few and there are not confirmed by the clinical reality.

The antibiotics represent the most important therapeutic arsenal in the fight against pathogen microorganisms. Even in the beginning of their use, there was registered bacterial resistance, phenomenon thatbecame an alarming subject in the last decades. There are some types of resistance to antibiotics that are influenced by many factors. The resistance term can be used as microbiological resistance and clinical resistance. The resistance to antibiotics can be a natural phenomenon or a gained one. The mechanisms of gaining resistance in bacteria are genetic or biochemical. Because of the fact that this is an increasing phenomena, data about antibiotic resistance is a permanent necessity in any part of the world.

Antibiotic therapy represents one of the most frequently used methods in medical practice, human or veterinary. The efficiency of this therapy is based on an appropriate choose of antibiotics, avoiding also the errors that may occur by their utilization. Sometimes, the association of antibiotics is recommended, in order to reduce the antibiotic resistance and also to increase the efficacy of antibiotics in veterinary practice.