Mecanismos moleculares de resistencia antibiótica y papel epidemiológico del semental ovino en la agalaxia contagiosa

Laburpena

The present doctoral thesis is formed by six scientific articles which address the epidemiological role of rams in contagious agalactia and the molecular mechanisms behind the resistance of the etiological agents of this disease to macrolides, lincosamides and tetracyclines. The common goal of these subjects is to improve the preventive and control strategies of contagious agalactia. More specifically, the aims and methodology followed in the present doctoral thesis are: 1) To study the excretion of Mycoplasma agalactiae, the main causative agent of contagious agalactia, in the semen of naturally infected asymptomatic rams; 2) To determine the most efficient sample to detect carrier rams infected with M. agalactiae in flocks housed in artificial insemination centres; 3) To review recently published literature on other emergent mycoplasmoses of small ruminants whose symptoms are also associated to contagious agalactia; 4) To assess the antimicrobial susceptibility and the role of mutations altering the 23S ribosomal RNA and ribosomal proteins L4 and L22 in the acquisition of macrolide and lincomcyn resistance of field isolates and in vitro selected mutants of M. agalactiae; 5) To investigate the effect of mutations in the genes encoding 23S ribosomal RNA and ribosomal proteins L4 and L22 on the loss of susceptibility of field isolates and in vitro selected mutants of M. capricolum subsp. capricolum to macrolides and lincosamides; 6) To determine and to compare the tetracycline susceptibility of field isolates with different epidemiological features of the four mycoplasma species responsible for contagious agalactia and 7) To investigate the molecular mechanisms behind the decreased susceptibility to tetracyclines of contagious agalactia causative mycoplasmas. The obtained results allowed concluding that: 1) M. agalactiae is excreted intermittently in semen of asymptomatic naturally infected rams; 2) There are asymptomatic rams infected with M. agalactiae in all the studied artificial insemination centres; 3) Semen and nasal swabs are the most effective samples to detect asymptomatic rams infected with M. agalactiae; 4) An increasing development of resistance to tylosin can be observed in current field isolates of M. agalactiae; 5) Mutations altering 23S rRNA and ribosomal protein L22 are the molecular mechanisms responsible for resistance to macrolides and lincosamides in M. agalactiae and M. capricolum subsp. capricolum; 6) The results of tetracycline susceptibility of the four contagious agalactia causative mycoplasma species are reassuring, although M. agalactiae is the less susceptible species and resistance to these antibiotics is currently emerging in the field and 7) There is no simple relationship between mutations in the 16S rRNA encoding genes and decreased susceptibility to tetracyclines in the studied contagious agalactia causative mycoplasmas, which suggests the existence of other resistance mechanisms.