Food-fermenting lactic acid bacteria (LAB) are generally considered to be non-toxic and non-pathogenic. Some species of LAB, however, can produce biogenic amines (BAs). BAs are organic, basic, nitrogenous compounds, mainly formed through decarboxylation of amino acids. BAs are present in a wide range of foods, including dairy products, and can occasionally accumulate in high concentrations. The consumption of food containing large amounts of these amines can have toxicological consequences. Although there is no specific legislation regarding BA content in many fermented products, it is generally assumed that they should not be allowed to accumulate. The ability of microorganisms to decarboxylate amino acids is highly variable, often being strain specific, and therefore the detection of bacteria possessing amino acid decarboxylase activity is important to estimate the likelihood that foods contain BA and to prevent their accumulation in food products. Moreover, improved knowledge of the factors involved in the synthesis and accumulation of BA should lead to a reduction in their incidence in foods.

Agmatine deiminase pathway genes in Lactobacillus brevis are linked to the tyrosine decarboxylation operon in a putative acid resistance locus In lactic acid bacteria (LAB), amino acids and their derivatives may be converted into aminecontaining compounds designated biogenic amines, in pathways providing metabolic energy and/ or acid resistance to the bacteria. In a previous study, a pathway converting tyrosine to tyramine was detected in Lactobacillus brevis and a fragment of a gene possibly involved in the production of another biogenic amine, putrescine, from agmatine, was detected in the same locus. The present study was carried out to determine if Lb. brevis actually harbours two biogenic amineproducing pathways in the same locus and to investigate the occurrence of the two gene clusters in other bacteria. Sequencing of the DNA locus in Lb. brevis revealed a cluster of six genes that are related to previously reported genes of agmatine deiminase pathways but with marked differences such as two genes encoding putative agmatine deiminases rather than one. Heterologous expression of encoded enzymes confirmed the presence of at least one active agmatine deiminase and one amino acid transporter that efficiently exchanged agmatine and putrescine. It was concluded that the Lb. brevis gene cluster encodes a functional and highly specific agmatine deiminase pathway. Screening of a collection of 197 LAB disclosed the same genes in 36 strains from six different species, and almost all the positive bacteria also contained the tyrosine catabolic pathway genes in the same locus. These results support the hypothesis that the agmatine deiminase and tyrosine catabolic pathways belong to a genomic region that provides acid resistance and that is exchanged horizontally as a whole between LAB. INTRODUCTIONLactic acid bacteria (LAB) can produce metabolic energy or increase their acid resistance by using catabolic pathways that convert amino acids or their derivatives into aminecontaining compounds designated biogenic amines (Molenaar et al., 1993; Fernández & Zú ñiga, 2006). In bacteria, biogenic amines may serve physiological functions such as cell proliferation control, but they are more generally released into the medium as the end products of the catabolic pathways (Tabor & Tabor, 1985;Griswold et al., 2006). In fact, excretion of the amine is an essential step in the pathways (see below). LAB carrying amino acid catabolic pathways are an important source of biogenic amines in foods (Ten Brink et al., 1990;Stratton et al., 1991;Silla Santos, 1996). There is interest in avoiding the proliferation of these bacteria because ingestion of foods containing large quantities of biogenic amines may cause human health issues such as headache, palpitations, flushing or vomiting (Silla Santos, 1996). Recently the genes of diverse pathways producing biogenic amines were identified in LAB (for a review, see Fernández & Zú ñiga, 2006). Interestingly, the pathways are strain specific rather than species specific, suggesting that horizonta...

Although Lactococcus is one of the most extensively studied lactic acid bacteria and is the paradigm for biochemical studies of citrate metabolism, little information is available on the regulation of the citrate lyase complex. In order to fill this gap, we characterized the genes encoding the subunits of the citrate lyase of Lactococcus lactis CRL264, which are located on an 11.4-kb chromosomal DNA region. Nucleotide sequence analysis revealed a cluster of eight genes in a new type of genetic organization. The citM-citCDEFXG operon (cit operon) is transcribed as a single polycistronic mRNA of 8.6 kb. This operon carries a gene encoding a malic enzyme (CitM, a putative oxaloacetate decarboxylase), the structural genes coding for the citrate lyase subunits (citD, citE, and citF), and the accessory genes required for the synthesis of an active citrate lyase complex (citC, citX, and citG). We have found that the cit operon is induced by natural acidification of the medium during cell growth or by a shift to media buffered at acidic pHs. Between the citM and citC genes is a divergent open reading frame whose expression was also increased at acidic pH, which was designated citI. This inducible response to acid stress takes place at the transcriptional level and correlates with increased activity of citrate lyase. It is suggested that coordinated induction of the citrate transporter, CitP, and citrate lyase by acid stress provides a mechanism to make the cells (more) resistant to the inhibitory effects of the fermentation product (lactate) that accumulates under these conditions. Many bacteria can utilize citrate under fermentative conditions. The citrate pathway has been extensively studied in enterobacteria (4, 26). In all known citrate fermentation pathways, after its uptake into the cell, citrate is split into acetate and oxaloacetate by the enzyme citrate lyase. In Klebsiella pneumoniae the citrate-specific fermentation genes form a cluster of two divergent operons (4, 26). This cluster includes the genes citDEF, encoding the citrate lyase subunits ␥, ␤, and ␣, and the citS gene, encoding a citrate H 2Ϫ /Na 1ϩ proton motive force-dissipating transporter. Associated with this cluster, K. pneumoniae contains the oadGAB genes, encoding the biotin oxaloacetate decarboxylase, which allows growth with citrate as the sole carbon and energy source (Fig. 1). In Escherichia coli, the cit cluster includes the genes encoding citrate lyase and the CitT citrate/succinate antiporter (22) (Fig.