Adaptação e adaptação cruzada de Listeria spp. a óleos essenciais de plantas condimentares e ao estresse ácido

Abstract

Listeria monocytogenes is the species of pathogenic bacteria that causes the infection listeriosis, which can lead to death. This bacteria is considered persistent in processing environments, presenting resistance to several sanitizing agents used in the food industry, mainly due to the use of these agents in sub-inhibitory concentrations. Looking for alternatives, natural substances such as the essential oils have been suggested due to their multiple targets in bacterial cells. Several essential oils have high antimicrobial activity and broad spectrum of action against foodborne pathogens. However, little is known about the physiological responses of bacteria when exposed to sublethal concentrations of essential oils, which leads to the need to evaluate more than one bacterial strain. The objective of this work was to evaluate the adaptive and cross adaptive capacity of L. monocytogenes and L. innocua to the essential oils of thyme, oregano and nutmeg and to acid stress. The minimum bactericidal concentrations (MBC) of the essential oils were determined using the microdilution technique. The tested concentrations were 4; 2; 1; 0.5; 0.2; 0.1 and 0.05. To determine the minimum inhibitory pH and minimum growth pH, lactic acid was used for adjusting pH to 6.0; 5.5; 5.0; 4.5; 4.0; 3.5; 3.0 and 2.5. The microorganisms were incubated at 37 °C for 24 hours. After incubation, aliquots of the cultures were plated and incubated at 37 °C for 24 hours. The adaptive capacity of the strains to essential oils and the acid stress, as well as their cross-adaptation between the essential oils and acid stress, were evaluated by exposing the cultures of L. monocytogenes and L. innocua for 6h at concentrations of 1/4 and 1 / 8 of the CMB of the essential oils and minimum growth pH. Cells recovered by discarding the supernatant were cultured in the presence of the essential oils (CMB / 2; CMB; 1.2CMB; 1.4CMB; 1.6CMB; 1.8CMB and 2CMB). The minimum inhibitory pH and minimum growth were 4.0 and 4.5, respectively, for both strains used. The CMBs of the essential oils for both strains were 0.1% for thyme and oregano and 0.2% for nutmeg. The adaptation and crossadaptation of L. monocytogenes and L. innocua to the essential oils and acid stress was observed. Although it is a viable alternative to antimicrobials commonly used in industry, essential oils should be used in adequate concentrations.