Impact of engineered Streptococcus thermophilus trains overexpressing glyA gene on folic acid and acetaldehyde production in fermented milk

Impacto de linhagens de Streptococcus thermophilus com aumento da expressão do gene glyA na produção de ácido folico e acetaldeído em leite fermentado

INDUSTRIAL MICROBIOLOGY

Impact of engineered Streptococcus thermophilus trains overexpressing glyA gene on folic acid and acetaldehyde production in fermented milk

Impacto de linhagens de Streptococcus thermophilus com aumento da expressão do gene glyA na produção de ácido folico e acetaldeído em leite fermentado

Ana Carolina Sampaio Dória Chaves^I; Patrícia Ruas-Madiedo^II; Marjo Starrenburg^II; Jeroen Hugenholtz^II; Alda Luiza Santos Lerayer^I

^IInstituto de Tecnologia de Alimentos, Campinas, SP, Brasil

^IINIZO Food Research and Wageningen Center for Food Science (WCFS), The Netherlands

^{Correspondence}

ABSTRACT

The typical yogurt flavor is caused by acetaldehyde produced through many different pathways by the yogurt starter bacteria L. bulgaricus and S. thermophilus. The attention was focused on one specific reaction for acetaldehyde and folic acid formation catalyzed by serine hydroxymethyltransferase (SHMT), encoded by the glyA gene. In S. thermophilus, this enzyme SHMT also plays the typical role of the enzyme threonine aldolase (TA) that is the interconvertion of threonine into glycine and acetaldehyde. The behavior of engineered S. thermophilus strains in milk fermentation is described, folic acid and acetaldehyde production were measured and pH and counts were followed. The engineered S. thermophilus strains StA2305 and StB2305, have the glyA gene (encoding the enzyme serine hydroxymethyltransferase) overexpressed. These engineered strains showed normal growth in milk when it was supplemented with Casitione. When they were used in milk fermentation it was observed an increase in folic acid and in acetaldehyde production by StA2305 and for StB2305 it was noticed a significative increase in folic acid formation.

Key words: acetaldehyde; Streptococcus thermophilus; glyA gene; serine hydroxymethyltransferase and folic acid.

RESUMO

O acetaldeído, responsável pelo sabor e aroma característicos de iogurte, é produzido por diferentes vias metabólicas pelas bactérias lácticas: Streptococcus thermophilus (S. thermophilus) e Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus). Neste trabalho, a atenção foi focada especificamente na reação para a formação de acetaldeído e de ácido fólico, catalisada pela enzima serina hidroximetil transferase (SHMT), codificada pelo gene glyA. A enzima SHMT catalisa diversas reações e, no caso da bactéria S. thermophilus, ela exerce também a atividade característica da enzima treonina aldolase (TA), definida como a interconversão do aminoácido treonina em glicina e acetaldeído. Foram construídas linhagens de S. thermophilus (StA2305 e StB2305) com super expressão do gene glyA. Estas linhagens modificadas apresentaram crescimento normal quando o leite foi suplementado com hidrolisado de caseína (Casitione). Quando foram usadas para fermentação de leite, observou-se: aumento na produção de ácido fólico e acetaldeído por StA2305 e aumento significativo na formação de ácido fólico por StB2305.

Palavras-chave: Acetaldeído; Streptococcus thermophilus; serina hidroximetil transferase (SHMT); gene glyA e ácido fólico.

INTRODUCTION

Yoghurt is a product obtained through milk fermentation by a specific yoghurt starter culture consisting of a mixture of two species of lactic acid bacteria (LAB), Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus) and Streptococcus thermophilus (S. thermophilus). It is interesting to observe that in fermented milk there is a higher concentration of folic acid (up to 200 µg/L) in than in milk (from 20 to 60 µg/L). As well some others LAB, S. thermophilus is known to produce folic acid during growth in milk, the extent is strain dependent and the final concentration also depend on the L. bulgaricus strain used, once they consume this vitamin (10).

The typical yoghurt flavour is caused by lactic acid, which imparts an acidic and refreshing taste, and a mixture of various carbonyl compounds like acetone, diacetyl, and acetaldehyde of which the latter is considered the major flavour component (6). In the yogurt bacterium S. thermophilus, the only enzyme with threonine aldolase activity (interconvertion of threonine into acetaldehyde and glycine) seems to be the serine hydroxymethyltransferase (SHMT, EC.2.1.2.1.) (1). This is an important enzyme involved in the metabolism of not only glycine and serine, but of folate in all organisms (5). Overexpression of the glyA gene (encoding for SHMT), showed an increase in TA activity and in acetaldehyde and folic acid formation when the engineered strains were grown on M17 medium (1).

The aim of this work was to investigate the role and the importance of SHMT in folic acid and acetaldehyde formation by using the engineered S. thermophilus strains for milk fermentation. These mutants could be interesting for the dairy Industry application due to its fuctional propriety (Higher folic acid content).

MATERIALS AND METHODS

Bacterial strains and culture conditions

The S. thermophilus strains used in this study are listed in Table 1. The strains were grown over-day at 42ºC in M17 medium (Oxoid) supplemented with 1% lactose at 42ºC and used to inoculate (1%) sterile skim milk and incubated at 42ºC overnight. Growth media (milk and M17) for the mutants were supplied with 5 mgmL^-1 of chloramphenicol.

Milk fermentation

Fermentation for each strain was carried out in duplicate in two independent replicated experiments; samples were taken at intervals of 2h (0h to 12h) and after 24h. The decrease of pH during this incubation period was continuously monitored (every 2 min) using two systems: Microprocessor pH/ION and Microbe System 6.05 (NIZO food research). Samples were taken for microbial counts (serial dilutions and deep-plated on M17-agar supplemented with 1% lactose). Plates were incubated at 42ºC/48h and results were expressed as log of colony-forming units. The acetaldehyde (kit according to the instructions of the suppliers Boehringer) and folic acid content were measured in triplicate after 24h. The folic acid produced during fermentation was measured using a microbiological assay (2,4,7,8). The samples were submitted to a conjugase treatment that allows the folic acid produced inside of the cells to go outside due to the beak down of its tail.

RESULTS AND DISCUSSION

Milk was inoculated with all four strains (Table 1) and incubated at 42ºC. Due to the poor growth of the mutants in milk observed (^{1A and 1C}), milk was supplemented with 0.2% casitone (Difco) in order to increase the available source of nitrogen and therefore improve growth (^{1B and 1D}). It is possible to observe in ^{Figs. 1A and 1C}, that StA2305 and StB2305 grew less (one log difference) than their respective parental strains and they also produced less lactic acid. When these four strains were grown on M17 medium, the acidification and growth rate of all of them were nearly identical (1). This result showed that the engineered strains were not able to grow and acidify milk in the same level than the parental strains but they can grow easily in a rich medium like M17, Probably, because in milk they had problems to break down the proteins in order to get the aminoacids they need to grow.

Milk fermented with StA2305 produced 22% more acetaldehyde them the wild type and for StB2305 and NIZOB130 there was no difference observed (Fig. 2). When they grow on M17, is observed an increase in acetaldehyde production on both glyA-overexpressing strains (1). The behaviour of StB2305 could be due to the poor growth observed. When the milk was supplemented by casitone there was a significative increase (around tree time) in acetaldehyde formation for all four strains, probably because of the improvement in the growth rate.

All the engineered strains showed a higher folic acid (FA) formation compared to their parental strains in all conditions (Table 2). In milk FA production was higher than in milk supplemented with casitone (except for AO54). In the supernatant there was a higher increase in FA content than in total folic acid in all samples.

Taken all results together it is possible to conclude that in the engineered S. thermophilus strains grew at the same rate and reached the same final pH as the wild type as long as the milk was supplemented with casitone. And StA2305 produced more acetaldehyde and folic acid when compared to its parental strain.

ACKNOWLEDGMENTS

A.C.S.D. Chaves was supported by a grant of FAPESP (SP-Brazil) and we gratefully acknowledge Dr. Willem de Vos for supporting this project.

This paper corresponds to an "extended abstract" selected for oral presentation in the 22^nd Brazilian Congress of Microbiology, held in Florianópolis, SC, Brazil, in November 17-20, 2003

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Publication Dates

• Publication in this collection
  30 Nov 2004
• Date of issue
  Nov 2003