LACTIC ACID BACTERIA ISOLATED FROM EGGPLANT ARE ABLE TO INHIBIT THE GROWTH OF Escherichia coli.

TOLEDO LAURA; PERATO MARISA; RODRIGUEZ VAQUERO MARIA JOSÉ; SAGUIR FABIANA

Mar del Plata

Congreso; VIII Congreso Argentino de Microbiología General (SAMIGE)- Sociedad Argentina de Microbiología General; 2012

Eggplant fruit (Solanum melongena L.) is of considerable economic importance in many parts of the world including warm temperature regions. Its consumption is growing as consumer health and diet-related education expands. Although vegetables carry a natural generally non-pathogenic microbiota, they can be contaminated with human pathogens from many different sources such as Enterobacteria. Consumers demand for safer foods without synthetic chemical preservatives. Thus, the use of lactic acid bacteria (LAB) has taken great importance nowadays due to its capacity for controlling pathogenic and spoilage microorganisms. However, its application in fresh fruits and vegetables preserving has not been enough developed yet. The aim of this work was to isolate LAB from eggplant cultivated in north of Argentine and to evaluate their ability to inhibit the growth of Escherichia coli in order to establish their potential application in bioprotect of vegetables. For LAB isolation, eggplants were washed with sterile distilled water, each washing water sample, diluted if necessary, was plated on MRS agar pH 5.5 containing chloramphenicol and incubated anaerobically for 72h at 30°C. Isolates were identified as LAB by morphological and phenotypic characteristics and screened for antibacterial activity using E. coli ATCC 35218 as indicator organism. Antimicrobial activity was evaluated by the spot test in agar medium and by using cell-free supernatant (SN) added to BHI medium (10%) inoculated (7%) with an overnight culture of the pathogenic bacterium. Bacterial growth was measured spectrophotometrically at 540 nm for 24 h at 37°C and by counting the viable cells number. From the total of isolated colonies 50 strains that showed positive Gram reactions, absence of mobility, absence of spore formation, absence of catalase activity and fermentative metabolism of glucose were considered as LAB. They were divided in four groups according to their morphological and physiological features. By comparison of the antimicrobial spectra of the LAB isolates in agar medium, six strains correspond to homofermentative rods and belonging to group 1 that gave the most significant positive results were selected for further studies in BHI medium. In this condition, the strain of E. coli grew of about 5 log cycles for 24 h. The individual addition of SN from selected LAB, in general, decreased the viable cells and the growth rate of the pathogenic bacterium between 0.39 and 2.32 log cycles and 5 and 61%, respectively. This effect was most pronounced for SN from SB5 strain. Thus, the variations in the inhibitory effect of the tested LAB on the growth of the E. coli population suggested that the inhibitory compounds were not identical. On the other hand, the results obtained demonstrated that the fermentation of vegetables by homofermentative rods from eggplants would provide an aggregate benefit of inhibiting the growth of pathogenic bacteria in fermented foods.