Estrategias en la reducción del consumo de antibióticos en el ganado porcino

Abstract

In veterinary medicine, research on alternatives to antibiotics use which will limit the selection and the spread of antibiotic resistance, is currently a priority. Current and forthcoming restrictions in antibiotics use also encourage the research and finding of strategies which provide an effective alternative in infection treatment and control. These strategies will also guarantee and promote the economic viability of farms and the animal health and welfare. Among the existing strategies for the control and prophylaxis of infectious diseases in pig production, one of the most important in livestock production with relevant antimicrobial use, stands out the use of natural compounds such as organic acids (OAs) or essential oils (EOs) as feed additives and the promotion and implementation of strict biosecurity measures. Further research on these strategies is needed in order to optimise their effectiveness. This Doctoral Thesis includes four studies focusing on different aspects of the in vitro antimicrobial activity of alternative compounds to antibiotics and disinfectant agents against different microorganisms relevant as enteric pathogens for pigs and with a high prevalence of antimicrobial resistance. In a first study, the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC) of different OAs, EOs and single EO compounds were determined against a collection of isolates recovered from outbreaks of enteric disease on pig farms, as well as against some strains from type culture collections of Escherichia coli, Salmonella enterica and Clostridium perfringens species. The potential synergistic effect of compounds with higher antibacterial activity when combined was also evaluated through the determination of the fractional inhibitory concentration (FIC) index, as well as the activity of their volatile compounds and the progression of their antibacterial effect as a function of concentration after a short exposure. The results obtained show the potential use of this type of compounds against pathogenic bacteria regardless of their antibiotic resistance profile. In a second study, the mechanism of action of two single EOs compounds, thymol and cinnamaldehyde, was studied in depth using techniques such as flow cytometry (FC), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The results obtained allow us to conclude that the main cellular target of thymol is the cell wall, since it caused significant changes in functional groups of carbohydrate and polysaccharide, as well as an increase of the membrane permeability in the three bacterial species tested. In contrast, cinnamaldehyde caused minimal alterations in the cell wall and its target at the molecular level differed between Gram-negative bacteria, E. coli and S. enterica, and C. perfringens isolates. In a third study, the cytotoxic effect of different concentrations of OAs and single EOs compounds was determined using a fluorescent cell viability assay. The antiviral activity of these compounds at a concentration with low cytotoxicity was also evaluated against a cell culture-adapted porcine epidemic diarrhea virus (PEDV) strain. Our results showed that formic acid was the only one of the compounds tested with antiviral activity which could be associated with an inhibitory effect on viral replication. The last study revealed, using a standard dilution-neutralization method, that most of the commercial disinfectants tested were effective against a collection of Brachyspira hyodysenteriae isolates, reducing bacterial counts by more than five logarithmic units. Finally, the efficacy of cleaning and disinfection protocols used on two farms with swine dysentery was also evaluated. The persistence of B. hyodysenteriae after cleaning of the pens and even after disinfection highlights the importance of revising the cleaning and disinfection protocols on pig farms.