Brief history of the taxonomy of Listeria
Taxonomy is a scientific discipline pertaining to the classification, nomenclature, and identification of organisms. Organisms are classified or grouped based on shared characteristics, previously unknown strains are placed in known taxa and assigned names following set nomenclature standards. The naming of organisms dates back as early as the great philosopher Aristotle (384-322BC). Modern taxonomy is based on the principles established by the Swedish botanist, Carolus Linnaeus (1707-1778). He introduced binomial nomenclature for naming organisms and established the Linnaean classification system whereby organisms are ranked hierarchically at species, genus, family, order, class, phylum and kingdom levels.
In the last few years, the taxonomy of the genus Listeria has undergone extensive modifications, mainly because of the now well-established DNA hybridisation techniques. Murray, Webb and Swann described Listeria monocytogenes as the causative agent for sepsis in rodents in 1926 and they named it Bacterium monocytogenesbecause the infection was monocytotic (Liu, 2013). The following year, Pirie isolated a similar bacillus from the infected liver of the African jumping mice, the gerbils and named it Listerella hepatolytica (McLauchlin, 1987). Up Until 1940, there was considerable confusion in the nomenclature of L. monocytogenes. J. H. Pirie (1940) chose Listerella as the generic name in honour of Lord Lister, the British surgeon who pioneered antiseptic surgery. However, this name had already been applied by Jahn in 1906 to a group of slime moulds (Mycetozoa) (J. H. Pirie, 1940). The generic name Bacterium as applied by Murray and collaborators was undesirable because the organism does not possess the characteristics of this genus. The resolution of the Committee on Nomenclature, Third International Congress for Microbiology, New York, 1939, was that in all duplications of generic names, only the one first applied should be considered valid, invalidating the generic name proposed by Pirie, and thus he suggested the name Listeria in 1940 (J. H. H. Pirie, 1940). Listeria was adopted in the sixth edition of Bergey’s Manual of Determinative Bacteriology and approved by the Judicial Commission on Bacteriological Nomenclature and Taxonomy, and it is now the official genus name (Rocourt & Buchrieser, 2007).
Listeria species known to date are as follows: L. monocytogenes (J. H. Pirie, 1940), Listeria grayi, Listeria innocua, Listeria welshimeri, Listeria seeligeri, Listeria ivanovii (Seeliger, Rocourt, Schrettenbrunner, Grimont, & Jones, 1984), Listeria marthii (Graves et al., 2010), Listeria rocourtiae (Leclercq et al., 2010), Listeria fleischmannii (Bertsch et al., 2013), Listeria weihenstephanensis (Lang Halter, Neuhaus, & Scherer, 2013), Listeria floridensis, Listeria aquatica, Listeria cornellensis, Listeria riparia and Listeria grandensis (den Bakker et al., 2014) and the recently published Listeria booriae and Listeria newyorkensis (Weller, Andrus, Wiedmann, & den Bakker, 2015). Additionally, two subspecies have been identified within L. ivanovii [subsp. ivanovii and subsp. londoniensis, (Boerlin et al, 1992 cited in Weller et al. (2015)], L. grayi [subsp. grayi and subsp. murrayi(Stuart & Welshimer, 1974)], and L. fleischmannii [subsp. fleischmannii and subsp. coloradonensis, (den Bakker, Manuel, Fortes, Wiedmann, & Nightingale, 2013)].
Listeria only had one species, L. monocytogenes, for many years. It was only in 1948 that Listeria denitrificans was added, followed by L. grayi in 1966, L. murrayi in 1971, L. innocua in 1981, L. welshimeri and L. seeligeriin 1983 and L. ivanovii in 1985. The newer members of the species were all added post-2009. The species are named after famous scientists in the sciences field who were mainly involved in microbiology related studies. The newer species are mostly named after the places they were first isolated.
Listeria species are ubiquitous, being found in most natural and urban environmental habitats. The wide distribution in the environment of the species isolated before 1985 is well documented. For example, the newly isolated species (2009 onwards) were from different environments as follows: L. weihenstephanensis was isolated from vegetation in a pond in Germany (Lang Halter et al., 2013). L. rocourtiaewas isolated from processed lettuce in Austria (Leclercq et al., 2010). L. booriae and L. newyorkensis were isolated from food processing environments in the USA (Weller et al., 2015). L. floridensis, L. aquatica, L. cornellensis, L. riparia and L. grandensis were isolated from natural and agricultural environments in the USA (den Bakker et al., 2014). L. fleischmannii was isolated from cheeses in Switzerland and Germany (Bertsch et al., 2013). L. marthii was isolated from different natural environments in the USA (Graves et al., 2010). This clearly shows that the Listeria species is adapted to various environmental conditions and not confined to specific environments.
The genus Listeria is composed of short (0.5 – 2 µm long), Gram-positive, non-sporulating, facultatively anaerobic rods that have a low G + C content. Cells are non-encapsulated exhibiting growth at temperatures ranging from 0 – 45oC, and at pH 6 – 9. Depending on the growing conditions, the cells can appear coccoid and motile. Listeria species are catalase-positive and oxidative-negative. According to Schleifer & Kandler, 1972, cited in Lang Halter et al. (2013), the cell walls contain meso-diaminopimelic acid variation A1γ. When grown at 37oC, the major fatty acids are anteiso-C17:0 and anteiso-C15:0. Menaquinones are the sole respiratory quinones as stated by Ludwig et al cited in Lang Halter et al. (2013).
Three of the Listeria species, that is, L. monocytogenes, L. ivanovii and L. seeligeri are haemolytic and harbour the Listeria virulence gene cluster. Of these species, only L. monocytogenes and L. ivanovii are pathogenic to animals and humans, with L. monocytogenes expressing pathogenicity in humans and L. ivanovii in ungulates (cattle and sheep). L. seeligeri is not pathogenic to humans even though it contains homologues of the virulence gene cluster (Lang Halter et al., 2013). Although L. monocytogenes is recognized as the causative agent of listeriosis in humans, rare cases of infection by L. innocua, L. ivanovii, and L. seeligeri have also been reported (Magalhães et al., 2014).
Batz (2005), Gillespie (2010) and Painter (2013) cited in Weller et al. (2015) stated that L. monocytogenes poses a significant threat to public health and Scallan et al (2011) cited in Orsi and Wiedmann (2016) mentioned that it is the third leading cause of foodborne deaths related to microbial contamination. It is the most commonly isolated member responsible for listeriosis (Leclercq et al., 2010). Major outbreaks of food-borne listeriosis have been recorded in different parts of the industrialised world. This has raised grave concerns in relation to public health. L. monocytogenes has been identified as the causative agent in all cases of human listeriosis. It mainly manifests in high-risk segments of the populations such as foetuses, neonates, infants, the elderly, pregnant women and immunocompromised people Farber and Peterkin, 1991 cited in Gormon and Phan-Thanh (1995). The ability of L. monocytogenes strains to survive in conditions of temperatures as low as 4oC have made it a bacterium that is significant in the food processing sector especially in ready-to-eat foods.
The relationship of Listeria spp to other bacteria was relatively unclear until the 1970s. It only received recognition in in 1930 when it was incorporated in the tribe Kurthia of the Corynebacteriaceae family. In 1974, it was unclearly placed with Erysipelothrix and Caryophanon after the Lactobacillaceae family. It was subsequently grouped with Lactobacillus, Erysipelothrix, Brochothrix, Renibacterium, Kurthia and Caryophanon in the section of “regular, non-sporing Gram-positive rods” in Bergey’s Manual of Determinative Bacteriology (Rocourt & Buchrieser, 2007).
Studies by Jones (1975) paved way for a broader understanding of the phylogenetic relationship between then known Listeria spp and various other genera. Wilkinson and Jones (1977) and Feresu and Jones (1988) cited in Rocourt and Buchrieser (2007) carried out studies that progressively led to a phylogenetic refinement in the positioning of Listeria. These works clearly distinguished Listeria from other genera such as Erysipelothrix and Brochothrix thermosphacta and its relatedness to Lactobacillus and Streptococcus. Based on these results, Wilkinson and Jones (1988) cited in Rocourt and Buchrieser (2007) suggested that Listeria, Gemella, Brochothrix, Streptococcus and Lactobacillus be classified in the family Lactobacillaceae.
Chemotaxonomic methods have shown that Listeria spp belong to the low G + C percent DNA content (<55%) group of Gram-positive bacteria. Lipoteichoic acids isolated from L. monocytogenes exhibit structural analogies with lipoteichoic acids from other bacteria. Rocourt and Buchrieser (2007) suggested that they may be used as taxonomic markers. According to Ruhland and Fiedler (1987) cited in Rocourt and Buchrieser (2007), lipoteichoic acids imply biochemical consistency and their absence in Coryneform bacteria and presence in Bacillus, Staphylococcus, Streptococcus and Lactobacillus cements the grouping of Listeria with the latter. Mara and Michalec (1977) cited in Rocourt and Buchrieser (2007) reported the absence of free mycolic acids in Listeria. The presence of menaquinones corroborates relatedness of Listeria and Brochothrix albeit distance from lactobacilli (Rocourt & Buchrieser, 2007).
16S and 23S rRNA analysis of L. monocytogenes has elucidated the position of Listeria with regard to the other genera of Gram-positive bacteria. Collins et al cited in Rocourt and Buchrieser (2007) deduced that Listeria is remote from Lactobacillus and should not be in the Lactobacillaceae family and that the Listeria-Brochothrix subline should be a different family, Listeriaceae. 23S rRNA sequencing has affirmed the dissimilarities between Lactobacillus and Listeria and the similarities with Bacillus and Staphylococcus (Rocourt & Buchrieser, 2007).
According to den Bakker et al. (2014), the species of the genus Listeria can be put into four clades by use of 16S rRNA gene and amino acid sequence phylogenies as follows:
- L. monocytogenes and related species (Listeria sensustricto: L. marthii, L. innocua, L. welshimeri, L. seeligeri, and L. ivanovii),
- L. rocourtiae, L. weihenstephanensis, L. cornellensis, L. grandensis and L. riparia,
- L. fleischmannii, L. floridensis and L. aquatica, and
- L. grayi.
They also state that the L. rocourtiae and L. grayi clades are sister groups based on 16S rRNA gene sequence analysis.
Sensustricto and sensulato
Chiara et al. (2015) and Orsi and Wiedmann (2016) categorised the species in the genus Listeria into two groups. Listeria sensu stricto, comprising L. monocytogenes, L. seeligeri, L. marthii, L. ivanovii, L. welshimeri and L. innocua and Listeria sensu lato, consisting of L. grayi and all the other 10 remaining species. These researchers based their separation on the relatedness of the species to L. monocytogenes which was the first Listeria species to be classified and its importance to the food processing sector.
The members of this group of species are easily recognisable by the following characteristics. They can grow at temperatures as low as 4oC; motility at temperatures around 30oC; catalase-positive; unable to reduce nitrate to nitrite as well as showing a positive reaction to Voges-Proskauer test which gives an indication of the glucose fermenting ability via the butane diol pathway. All sensu stricto species are able to ferment D-arabitol, α-methyl D-glucoside, cellobiose, D-fructose, D-mannose, N-acetylglucosamine, maltose, and lactose but are unable to ferment inositol, L-arabinose, and D-mannitol (Orsi & Wiedmann, 2016). The species can be differentiated from each other based on standard biochemical tests. For example, L. marthii is unique in its ability to ferment both D-mannitol and D-xylose but it is also the only sensu stricto unable to ferment sucrose.
The species in sensu lato have all been recently described and their distribution in the environment is yet to be comprehensively studied, except for L. grayi. The sensu lato species are catalase-positive, non-capsulated, non-sporulating rods. The sensu lato species can be distinguished from the sensu stricto by standard biochemical tests as well. They are non-motile except for L. grayi. Initial studies carried out by Lang Halter et al. (2013); Leclercq et al. (2010) found the presence of motility in L. rocourtiae and L. weinhenstephanensis but later studies found that they were non-motile. Weller et al. (2015) carried out subsequent analyses that revealed the absence of motility in all sensu lato species at temperatures ranging from 4 to 37oC, except for L. grayi.
L. grayi and L. murrayi
The position of L. grayi and L. murrayi in the genus Listeria has courted controversy for decades, with various suggestions having been proposed. Rocourt, Boerlin, Grimont, Jacquet, and Piffaretti (1992) proposed that L. grayi and L. murrayi be assigned to a single taxon, L. grayi, because based on the DNA-DNA hybridisation, multilocus enzyme electrophoresis, and rRNA restriction fragment length polymorphism techniques they used, they found a high level of relatedness between the two species. They share specific chemotaxonomic properties which allow for their distinction from the other Listeria spp. They can only be distinguished from each other based on nitrate reduction data.
A high level of DNA-DNA homology was observed between L. grayi and L. murrayi by Stuart and Welshimer with a subsequent proposal that they should be classified in their own genus. However, they also discovered a low DNA relatedness between L. monocytogenes and L. grayi and L. murrayi. They proposed the removal of L. grayi and L. murrayi and transfer to a new genus “Murraya” as “Murraya grayi subsp. grayi” and “Murraya grayi subsp murrayi” (Stuart & Welshimer, 1974).
Rocourt et al. (1992) carried out studies to re-assess and re-evaluate the genomic relatedness between Listeria grayi and Listeria murrayi using DNA-DNA hybridisation, multilocus enzyme electrophoresis, and rRNA restriction fragment length polymorphism techniques. They concluded the species should be assigned a single genus, confirming the data published since 1973. They observed that there were high levels of similarity between the strains of these two species and suggested that they be assigned the name L. grayi.
DNA-DNA hybridisations carried out by Stuart and Welshimeri in 1974 identified L. monocytogenes as heterogenous. As they had only one labelled DNA in their collection, the total number of DNA hybridisation groups could not be ascertained. Further DNA hybridisation techniques discovered five DNA relatedness groups that had previously been identified as L. monocytogenes. Genomic group 1 contained the type strain L. monocytogenes (sensu stricto). Genomic group 2 contained haemolytic strains belonging to L. monocytogenes serovar 5. Upon further investigation of the strains, using DNA-DNA hybridisation and rRNA gene restriction patterns, two subspecies L. ivanovii subsp ivanovii (ribose-positive) and L. ivanovii subsp. londoniensis (ribose-negative) were labelled. Genomic group 3 contained non-haemolytic and non-pathogenic (for mice) strains corresponding to L. innocua. Genomic group 4 contained non-haemolytic and non-pathogenic strains now known as L. welshimeri. Genomic group 5 included haemolytic and non-pathogenic strains and was subsequently named L. seeligeri (Rocourt & Buchrieser, 2007).
In 1975, Ivanov as cited in Seeliger et al. (1984), suggested the name Listeria bulgarica for what was then known as Listeria monocytogenes serovar 5. In 1982, Seeliger et al cited in Seeliger et al. (1984) recommended that L. monocytogenes serovar 5 strains be recognised either as a subspecies perhaemolytica or as a distinct species which could be named either Listeria perhaemolytica or L. ivanovii. The authors were merely expressing their opinions but did not actually propose a name change for the species in question. Studies carried out showed that L. monocytogenes serovar 5 was pathogenic for mice and was reported as the etiological agent of abortion in sheep by Macleod & Watt in 1974, cited in Seeliger et al. (1984). Thus, the name L. ivanovii was proposed.
Listeria denitrificans / Jonesisdenitrificans
On the basis of DNA-DNA hybridization, DNA base composition, and determination of 16S rRNA cataloguing data and the results of numerous other chemotaxonomic studies, Listeria denitrificans was excluded from the genus and renamed Jonesiadenitrificans (Boerlin, Rocourt, & Piffaretti, 1991) (Stuart & Welshimer, 1973). The results obtained confirmed that this species was not a member of the genus Listeria. L. denitrificans was found to be phylogenetically similar to sub-branches of the actinomycetes subdivision (Rocourt, Wehmeyer, & Stackebrandt, 1987). Studies carried out later revealed that the closest relatives of Jonesiawere Dermobacter hominis and Brachybacterium faicium (Rocourt & Buchrieser, 2007).
Results from over 30 years of research demonstrated that Listeria is a robust taxon with clearly distinct characteristics from neighbouring taxa. Numerical studies, chemotaxonomy, rRNA and DNA sequencing analyses showed that it does not belong to the Coryneform family of bacteria. The exact phylogenetic position is still debatable.
The identification of new Listeria species and changes in the taxonomy of Listeria can have considerable impacts on the food industry and test kit manufacturers. There is no doubt there will be a progressive evolution of taxonomic studies that will open new avenues in taxonomy.
Genomic studies are relevant and have revolutionised taxonomy. They offer a definitive classification, including that of previously unknown strains. The study of the pathogenic and non-pathogenic strains of Listeria are being done to understand genome evolution and the evolution of virulence characteristics therein. The information obtained from such studies will allow for the development of genetic and genomic criteria for pathogenic strains, including the development of assays that specifically detect pathogenic Listeria strains (Henk C. den Bakker et al., 2010). Pallen (2007) cited in Henk C. den Bakker et al. (2010) reiterates that an understanding of the genomic content of non-pathogenic strains of pathogenic species is necessary to understand the evolution of virulence-associated genes and to facilitate identification of putative virulence genes.
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