Tolerance of Lactobacillus kefir to lead, cadmium, nickel and zinc ions

ESTEBAN GERBINO; CARASI PAULA; TYMCZYSZYN E; GOMEZ-ZAVAGLIA A.

Rosario, Argentina

Congreso; IX Congreso Argentino de Microbiología General; 2013

SAMIGE

Heavy metals such as lead, cadmium, nickel and zinc are natural elements with a high toxicity (depending on the concentration, persistence and speciation). To overcome this problem, microorganisms have evolved coping strategies to either transform the element to a less-harmful form or bind the metal intra- or extracellularly, thereby preventing any harmful interaction in the host cell. The interactions between Lactic Acid Bacteria (LAB) and metal ions are very poorly investigated. Therefore, the objective of this work was to investigate the influence of heavy metal ions (Pb2+, Cd2+, Ni2+ and Zn2+) on the growth of two strains of Lactobacillus kefir. L. kefir strains CIDCA 8348 (aggregating) and JCM 5818 (non-aggregating) were used. Bacteria were grown in de MRS broth [De Man, J.O. et al, 1960] containing different concentrations of metal ions ranging from 0 to 10 mM and incubated at 30ºC for 76 h. The bacterial growth was determined by measuring the absorbance at 600 nm. The lag time and the EC50 (concentration of metal ion that produces 50% inhibition of bacterial growth) were determined from growth kinetics. Optical and transmission microscopy observations for each strain were carried out using the EC50. The metal ions uptake was determined using atomic absorption spectrometry. Both L. kefir strains were able to grow in the presence of all the metal ions assayed, although the tolerated levels were different for each one. Broadly, L. kefir 8348 CIDCA has lower lag times that L. kefir JCM 5818, ie. it adapts faster to growth in the presence of Pb2+, Zn2+, Ni2+, and Cd2+. Cadmium was the most toxic metal (EC50 was about 0.01 mM in both strains). EC50 for Zn2+ was greater than for Ni2+ (concentrations ranging from 7 mM to 3.43 mM), which means that the Ni2+ was the less toxic metal. L. kefir CIDCA 8348 grown in the presence of Cd2+, Zn2+ and Ni2+ depicted shorter rods, some of them thicker than the control. When this strain was grown in the presence of Pb2+, besides thickening, it became a non-aggregating strain. On the other hand, L. kefir JCM 5818 (non-aggregating), became aggregative when grown in the presence of Pb2+, Zn2+ and Ni2+. In turn, in the presence of Cd2+ and Zn2+ shorter bacilli were observed. The tolerance of L. kefir CIDCA JCM 8348 and 5818 was good compared to other genera of LAB, Cd2+ being the most toxic metal ion. Considering their GRAS status (Generally Recognized as Safe), these results support the potential use of BAL to sequestrate traces heavy metals in products for human and animal consumption.