Detection of transitory resistance in Streptococcus suis and Pasteurella multocida strains from swine origin in Cuba

Abstract

The acquired resistance requires a genetic change; either mutations or the acquisition by horizontal gene transfer. However, there are situations in which resistance is not driven by a genetic change and bacteria become transiently resistant to antibiotics. The transient and reversible resistance can be achieved by different mechanisms, such as the formation of biofilms or persistent cells, which are related to the physiological state of the bacteria when it is exposed to a stressful condition. Persistent cells represent a fraction in a bacterial population that begins a dormancy phase under adverse conditions. Unlike bacteria that resist antibiotics by genetic mechanisms, persistent cells are unable to grow in the presence of an antibiotic. Pasteurella multocida and Streptococcus suis are important pathogens in the respiratory disorders of swine production. These bacteria produce frequents infections that could be considered recurrent. This work was aimed at detecting events of transitory resistance in vitro in both species. Four strains, susceptible to ß-lactamic and quinolone, corresponding to each species, were selected after previously confirming their susceptibility according to the classical testing method.  P. multocida strains were analyzed for surviving cells after exposure to 200 and 400 µgml^-1 of Enrofloxacin and Ampicillin, respectively, while S. suis cells was treated with 100µgml^-1of Penicillin and 200µgml^-1Enrofloxacin during 24 and 48 hours. S. suis and P. multocida strains formed persistent cells under the action of both antibiotics until a detectable concentration of 1x10⁴ ufcml^-1. The level of persistence varies among the strains. This is the first time that the formation of persistent cells by P. multocida has been described and corroborates this behavior previously described by other authors in S. suis strains. The potentiation assay showed that it is possible to eradicate persistent cells in vitro through the combinations of aminoglycoside (Gentamicin) with glycerol and Gentamicin with arginine The manifestation of these transient resistance phenotypes not associated to genetic changes can explain the therapeutic failures and recurrence in respiratory infections, which usually occur subclinically, reducing lung capacity and decreasing gain in weight.

Key words:transient resistance, persistent cell, antibiotic tolerance, Pasteurella multocida, Streptococcus suis.