Moraxella bovis, Moraxella ovis e Moraxella bovoculi: formação de biofilme e atividade de lisozima

Abstract

Infectious keratoconjunctivitis (IK) is the most important ocular disease in cattle and sheep. The disease represents significant economic losses in herds worldwide, since its aetiological agent is highly contagious. Moraxella bovis is considered mainly causative agent of infection, however M. ovis and M. bovoculi have also been recovered from animals with the disease. IK is documented to be a carrier disease, and these animals play a key role in maintaining the microorganism in a herd. However, it is still unclear which factors are related to the ability of Moraxella spp. survive in the conjunctiva and nasal mucosa of animals without promoting disease, as well as remain viable even after treatment with high antimicrobial dosages. In the current study, biofilm formation in M. bovis, M. ovis and M. bovoculi was considered as a factor associated with this maintenance of animals in the form of a carrier. In addition, the resistance capacity of Moraxella spp. to lysozyme, an important antibacterial enzyme present in the tear and acts as the first line of defense the invasion of the cornea by microorganisms. The resistance of microorganisms to lysozyme is associated with the ability to adhere to the cornea and to promote infection. In this research, it was evidenced that all the isolates of M. bovis (15/54), M. ovis (21/54) and M. bovoculi (18/54) tested (54/54) were able to form biofilm and were classified according to the intensity of biofilm formation. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of lysozyme were determined for 33 isolates of the three Moraxella species in planktonic cells. It was verified that MIC and MBC were quite variable intraspecies, of 79.12 μg ml-1 to values higher than 1266 μg ml-1, and there was no statistical difference in susceptibility to lysozyme among Moraxella spp. However, when tested in sessile forms, lysozyme did not have the capacity to eradicate the biofilms, but it promoted a reduction in the biofilms production. In addition, it was observed that in the consolidated biofilms there was no decrease after the addition of lysozyme, demonstrating that there is no action of this enzyme in previously established biofilms. The results of this research allow us to infer that the biofilm formation capacity of M. bovis, M. ovis and M. bovoculi and the resistance to lysozyme concentrations, equal to or greater than the physiological levels of the teat of cattle and sheep, may be linked not only to the ability to colonize the conjunctiva, but also the microorganism can remain at this site even after healing of the lesions and the infected animal will be considered a carrier and a reservoir of the etiologic agent of the disease in a herd.