Changes in microstructure of Salmonella Typhimurium and Listeria monocytogenes exposed to hydroxycinnamic salts

Cambios en la microestructura de Salmonella Typhimurium y Listeria monocytogenes expuestas a sales hidroxicinámicas

D. Martínez-Arámburu, G.K. González-Quijano, L. Dorantes-Alvarez*, G. Aparicio-Ozores and E.O. López-Villegas

Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional. Prolongación de Carpio y Plan de Ayala. Col. Sto. Tomás, México, Distrito Federal, C.P. 11340. México. *Corresponding author. E-mail: ldoran@ipn.mx Tel. 57-29-60-00, X 62467.

Abstract

Bacteria are microorganisms that contain organelles, which have sizes in the nanoscale range. Certain stress conditions may induce morphological changes in bacteria as a strategy to survive. In the present work, the effects of two antimicrobials commonly found in plants were tested to determine the ultrastructure and morphology of Salmonella Typhimurium and Listeria monocytogenes. Kinetics of survival of bacteria were monitored, and ultrastructure changes were observed using scanning and transmission electron microscopy (SEM and TEM, respectively). The results show a bacteriostatic effect on Salmonella at sodium ferulate concentrations between 0.3 and 0.6%, which also caused elongated forms of the bacteria, which were observed by scanning electronic microscopy (SEM). The Feret diameter and surface area of the bacteria were determined using image analysis, where it was shown that these dimensions increased four times on average compared with those of the control cells. These data support the idea of multicellular aggregates. In the case of Listeria monocytogenes, the elongation of the cells occurred after 24 hours of exposure to 0.6% sodium ferulate. The number of cytoplasmic inclusions and the size of vacuoles increased after the first hour of exposure to 0.8% sodium caffeate and corresponded to a bactericidal effect of this antimicrobial. The elongation of the cells that occurred in bacteriostatic concentrations reverted when the bacteria were incubated in media without me antimicrobials, and me kinetics of growth became normal, which suggests a strategy to survive under stress conditions generated by the presence of antibacterial compounds.

Key words: ultrastructure, Salmonella Typhimurium, Listeria monocytogenes, morphological changes, phenolic acids.

Resumen

Las bacterias son microestructuras biológicas que contienen organelos en rangos nanométricos. Algunas condiciones de estrés pueden inducir cambios morfológicos en las bacterias como una estrategia de supervivencia. En el presente trabajo, el efecto de dos antimicrobianos comunmente encontrados en plantas fueron probados en la ultraestructura y en la morfología de Salmonella Typhimurium y Listeria monocytogenes. Las cinéticas de supervivencia de las bacterias fueron monitoreadas y los cambios en la ultraestructura fueron observados usando microscopía electrónica de barrido (SEM) y electrónica de transmision (TEM). Los resultados mostraron un efecto bacteriostático sobre Salmonella a concentraciones de 0.3 y 0.6% de ferulato de sodio que causaron cambios morfológicos en las bacterias y que fueron observados por SEM. El diámetro de Feret y el área de las células fueron determinados empleando análisis de imágenes. En el caso de Listeria monocytogenes la elongación celular ocurre después de 24 horas de exposición a 0.6% de ferulato de sodio. Las inclusiones citoplasmáticas y vacuolas incrementaron despues de 1 hora de incubación con 0.8% de cafeato de sodio. La elongación de las células causada en condiciones bacteriostáticas fue revertida cuando las bacterias fueron incubadas en medio sin los antimicrobianos y las cinéticas de crecimiento fueron normales, implicando un mecanismo de supervivencia bajo condiciones de estrés.

Palabras clave: microestructura, Salmonella Typhimurium, Listeria monocytogenes, cambios morfológicos, ácidos fenólicos.

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Acknowledgements

The authors thank CONACYT and PIFI-IPN for fellowships that support this Ph.D research in Food Science in the ENCB-IPN. This work was supported by IPN grants SIP 20150829 and 20141394.

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