Classification of lactococci
Written by MM
This article discusses the background of the change in classification of the lactic streptococci to the genus Lactococcus.
The lactic group of the genus Streptococcus originally included the species Str. lactis and Str. cremoris and a subspecies of Str. lactis, Str. lactis subsp. diacetylactis (Deibel and Seeley, 1974). However, even in the 1970s workers were suggesting that Str. lactis strains might be variants of Str. diacetylactis that were unable to ferment citric acid, since citrate permease – negative strains of Str. diacetylactis had been described (Lawrence, Thomas and Terzaghi, 1976).
Bacteria in this group were designated as the lactic streptococci. The designation 'lactic' was used by Sherman (1937) for mainly historical reasons, including the use of the term by Lister (1878) to describe a bacterium that we now know as Lc. lactis subsp. lactis.
Evidence was also accumulating to suggest that Str. cremoris, Str. lactis and Str. diacetylactis formed a phenotypically and genotypically continuous spectrum of variation (see Lawrence et. al., 1976; Lawrence and Thomas, 1979). The three so-called species were known to be similar in DNA base-composition (Knittel, 1965) indicating that they are closely related. Genetic relationships between the genus Streptococcus and the genera Lactobacillus and Leuconostoc were becoming apparent (London, 1976).
Differentiation of lactic streptococci to species level
Schemes for the differentiation of lactic streptococci to the species or subspecies level were developed by several groups (Sandine et al., 1959; Reiter and Møller-Madsen, 1963; Reddy et. al., 1972; Diebel and Seeley, 1974; Jones, 1978) and are generally still relevant for lactococci. The interpeptide bridge structure that was proposed by Schleifer and Kandler (1972), namely l-Lysine-D-isoasparagine has been confirmed (Schleiferr and Kilpper-Balz, 1987).
Rationale for assigning lactic streptococci to Lactococcus genus
The lactic streptococci have been assigned to the Lactococcus genus because of molecular and chemotaxonomic studies. This work has been reviewed (see Schleifer and Kilpper-Balz; 1987; Schleifer et al., 1992). Essentially the extent of DNA: DNA and DNA-rRNA hybridization, similarity between profiles produced by restriction mapping of chromosomal DNA and the nucleotide sequence of the 16S and 32S RNAs have formed the basis for the creation of the genus. Antisera against purified superoxide dismutase have been used to demonstrate similarity between lactococci but not streptococci or enterococci.
Because citrate utilisation is plasmid DNA coded and not coded on the cell chromosome, citrate utilisation and the associated diacetyl production are variable traits and the former Str. diacetylactis has not been given subspecies status.
This genus, which has only recently been established, contains one important starter species Lactococcus lactis. This species consists of Gram positive, catalase negative, homofermentative cocci that normally form short to long chains. L-lactic acid only is formed from lactose. Lactococcus lactis is divided into three subspecies: Lc. lactis subsp. lactis, Lc. lactis subsp. cremoris, and Lc. lactis subsp. hordinae. Only Lc. lactis subsp. lactis and Lc. lactis subsp. cremoris are used as starters. A biovariant of Lc. lactis subsp. lactis, designated variously as either Lc. lactis subsp. lactis biovar. diacetylactis or Cit+. Lc. lactis subsp. lactis is also recognised. The latter is identical to Lc. lactis subsp. lactis apart from its ability to utilise citrate in the presence of carbohydrate. More information on lactococci is availble in the article on the microbiology of starter cultures.
Current taxonomic status of Lactococcus lactis
There is currently uncertainity concerning the taxonomic status of L.lactis in particular the subspecies lactis and cremoris since two genomic populations can clearly be identified (Giovanna and Dellaglio, 2007). This area will be kept underreview.
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How to cite this article
Mullan, W.M.A. (2011).
[On-line]. Available from: https://www.dairyscience.info/cheese-starters/201-lactic-streptococci.html . Accessed: 22 August, 2019.
Revised March 2013; November 2017.