Effect of pH and tenperature on antimicrobial activity of compounds produced by regional goat milk Lactic Aacid Bacteria

JUAN M. CASTRO; PATRICIA V. STAGNITTA; IRMA G. REZZA

Rosario

Congreso; V Congreso Argentino de Microbiología General; 2008

Sociedad Argentina de Microbiología General

Lactic Acid Bacteria (LAB) have been used as starters of food and beverage fermentation and contribute to the development of taste and flavors and retardation of food spoilage. LAB protective role is due not only to their capability to decrease the pH in the medium but to bacteriostatic and bactericidal substances secreted by them. These substances include: hydrogen peroxide, ethanol, carbon dioxide, bacteriocins or other similar compounds. In this work, our goal was to determine the effect of different pHs and temperatures on the antimicrobial activity against Listeria monocytogenes of cells-free supernatants obtained from regional LAB strains. For this study, we used three LAB strains isolated from regional raw goat milk and typified as Lactococcus lactis ssp lactis (L1), Lactococcus lactis ssp lactis 2 (L2) and Lactobacillus paracasei ssp paracasei 1 (L3), using API CH 50 commercial kit. The strains were grown in MRS broth during 48 h at 30ºC in anaerobiosis and showed inhibitory activity against Listeria monocytogenes 74902. Cells-free supernatants were obtained by centrifugation at 45000 rpm during 20 min at 20ºC and filtered through 0.2 μm pore filter. Aliquots of supernatants were subjected to different treatments: a) they were heated at 30, 60 and 100 º during 10, 30 and 90 min each; b) their pH was adjusted to 2, 5, 9 and 12 using HCl (10N) or NaOH (1N) as needed, during 2 h. After that, aliquots pHs were re-adjusted to 6.5 as in controls. The antimicrobial activity against L. monocytogenes 70902 was determined in the temperature- and pH-treated supernatants using the agar diffusion assay. Briefly, 20 ml of TSA plated in 10cm Petri dishes were inoculated with a Listeria monocytogenes suspension corresponding to 0.5 of Mc Farland. Wells of 5 mm diameter were cut and 30 ul of each treated supernatant were added to them and allowed to diffuse during 48 h at 30 ºC in aerobiosis. Macroscopic visualization of L. monocytogenes growing-inhibition zones showed 8.5, 8 and 7 mm diameters for the untreated supernatants from L1, L2 and L3, respectively. Interestingly, antimicrobial inhibitory activity was completely lost in the three cells-free supernatants subjected to the assayed pHs or heated at 60 and 100 ºC during any time, in comparison to controls. Additionally, L2 showed a decrease in the inhibitory activity when supernatant was heated at 30 ºC during 30 min. The results showed here would allow us to conclude that antimicrobial substances are susceptible to extreme pHs and temperatures treatments. These observations would contribute to the biochemical characterization of antimicrobial compounds secreted by L1, L2 and L3 strains which could have a potencial application as biopreservants or fermentation starters in food industry.