Background: Staphylococcus aureus is one of the main microorganisms that causes bovine mastitis, and its well-known virulence characteristics and interactions with the environment are used to aid the design of more efficient therapies. Objectives: To determine whether the virulence traits, such as antibiotic resistance and biofilm-forming and internalization abilities, of S. aureus isolated from bovine mastitis are related to dairy production system types. Methods: The study was performed in the Mexican states of Guanajuato and Michoacan. Semi-intensive dairy farms (SIDFs) and family dairy farms (FDFs) (454 and 363 cows, respectively) were included. The 194 milk samples from mastitis affected quarters were collected and 92 strains of S. aureus were isolated and identified by biochemical and molecular tests. Antibiotic resistance, biofilm and internalization assays were performed on 30 randomly selected isolated strains to determine virulence traits, and these strains were equally allocated to the 2 dairy production systems. Results: All 30 selected strains displayed a high degree of resistance (50%-91.7%) to the antibiotics tested, but no significant difference was found between SIDF and FDF isolates. S. aureus strains from SIDFs had an average biofilm forming capacity of up to 36% (18.9%-53.1%), while S. aureus strains from FDFs registered an average of up to 53% (31.5%-77.8%) (p > 0.05). Internalization assays revealed a higher frequency of internalization capacity for strains isolated from FDFs (33.3%) than for those isolated from SIDFs (6.7%) (p > 0.05). fnbpA gen was detected in 46.6% of FDF strains and 33.3% of SIDF strains, and this difference was significant (p < 0.05). Conclusions: Our findings show that the virulence traits of S. aureus isolates analyzed in this study, depend significantly on several factors, such as phenotype, genotype, and environmental conditions, which are significantly related to dairy production system type and daily management practices.

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En este trabajo se describen los patrones de resistencia a antibióticos de aislados de Staphylococcus aureus obtenidos de vacas con mastitis, así como su susceptibilidad in vitro a nisina, quitosano y al compósito nisina/quitosano. A partir de muestras de leche obtenidas de vacas enfermas se aislaron e identificaron 53 S. aureus, a los cuales se les analizó el perfil de resistencia a antibióticos. Los resultados evidenciaron una alta frecuencia de resistencia hacia el grupo de los beta-lactámicos, principalmente a penicilina, dicloxacilina y ampicilina. Asimismo, se detectó multirresistencia en algunos aislados, ya que mostraron tolerancia a 3 o 4 grupos de antibióticos, destacando los aislados AMC-9 (resistente a tres grupos) y AMC-23 (resistente a cuatro grupos). También se resalta que todos los aislados fueron susceptibles a levofloxacina. Por otra parte, se analizó la susceptibilidad in vitro de los aislados a nisina, quitosano y al compósito nisina/quitosano, mediante la determinación de la concentración mínima inhibitoria. Los resultados indican que el 100 % de los aislados fue susceptible a quitosano y al compósito nisina/quitosano y el 47.1 % a la nisina. El efecto del quitosano y del compósito nisina/quitosano sobre los aislados multirresistentes AMC-9 y AMC-23 fue bactericida. Estos resultados revelan el potencial de los compuestos antimicrobianos quitosano y nisina/quitosano para ser utilizados como agentes terapéuticos contra la mastitis bovina.