Characterization of bacteria of the genus Dietzia: an updated review

Abstract

The aim of this study was to characterize several aspects of species of the genus Dietzia, such as current taxonomic placement, morphological and growth characteristics, biochemical reactions, cellular lipid and fatty acid composition, the amino acids and sugars of whole-cell hydrolysates and the respiratory quinone system, and genomic guanine and cytosine (G + C) content. The species chosen for study were D. aerolata, D. alimentaria, D. aurantiaca, D. cerdiciphylli, D. cinnamea, D. kunjamensis, D. lutea, D. maris, D. natronolimnaea, D. papillomatosis, D. psychralcaliphila, D. schimae, and D. timorensis. The colony morphology study revealed that the colonies were small, smooth, circular and convex. Nitrate reduction, H₂S production, hydrolysis of urea, starch, and Tween 80, and the Voges–Proskauer and methyl red tests were performed for biochemical differentiation of the various Dietzia strains. Optimum growth temperature and pH for the different strains were 25–30 °C and 7–8, respectively. Among the strains studied, D. timorensis ID05-A0528^T had the lowest tolerance level to NaCl (7 %). This strain was also able to utilize a wide range of compounds as the sole carbon source. Short-chain mycolic acids were present in these bacteria. The cell wall contained meso-diaminopimelic acid, arabinose, and galactose; the glycan moiety of the cell wall contained acetyl residues. The major menaquinone was MK-8 (H₂). The G + C contents of the DNA ranged from 64.7 (D. alimentaria 72^T) to 73 mol % (D. maris DSM 43672^T). The most important phospholipids in these strains were diphosphatidylglycerol, phosphatidylglycerol, phosphatidylinositol mannoside, phosphatidylinositol, and phosphatidylethanolamine.

Introduction

Dietzia spp. (family Dietziaceae, suborder Corynebacterineae, order Actinomycetales) were originally isolated from various environments (Natarajan et al. 2005). The cells are Gram-positive, aerobic, short rod- and coccoid-like, non-motile, non-endospore-forming, non-acid fast, oxidase-positive, and catalase-positive (Koerner et al. 2009). At the time this article was being written 13 species of this genus had been identified, namely, D. maris, D. natronolimnaea, D. psychralcaliphila, D. cinnamea, D. kunjamensis, D. schimae, D. cerdiciphylli, D. papillomatosis, D. lutea, D. aerolata, D. timorensis, D. alimentaria and D. aurantiaca. Sequencing and PCR amplification of 16S ribosomal RNA (rRNA) gene sequences have become the basis for the rapid identification and classification of different Dietzia species as a phylogenetic tree (Fig. 1). This phylogenetic tree is a model of the evolutionary relationship between species based on homologous characters (Niwa et al. 2012).

Fig. 1

Phylogenetic analysis of different species of the genus Dietzia based on 16S rRNA gene sequence data using the neighbor-joining method (Saitou and Nei 1987). Bootstrap values (expressed as percentages of 1,000 replications) of >50 % are given at the nodes. Scale bar represents 0.1 substitutions per nucleotide position. The sequence of Tsukamurella paurometabola ATCC 8368^T was used as an outgroup (Kämpfer et al. 2012)

Several strains identified as representing species of the genus Dietzia are potential human pathogens in immunocompetent (Pidoux et al. 2001) and immunocompromised (Bemer-Melchior et al. 1999; Yassin et al. 2006) patients. These bacteria also have many applications in a wide range of industries but particularly in the medical, chemical, and food industries. Click and Van Kampen (2010) reported that a number of Dietzia strains could be used as potential probiotics to inhibit Mycobacterium avium subsp. paratuberculosis under in vitro culture conditions. Compared to antimicrobioal drugs used for the same purpose, such Dietzia strains are associated with fewer medical complications. Some of the Dietzia species described to date have been found to degrade aliphatic hydrocarbons, such as n-alkanes (Nesterenko et al. 1982; Rainey et al. 1995; Yumoto et al. 2002; Alonso-Gutiérrez et al. 2011; Bihari et al. 2011). There have also been reports on the ability of Dietzia strains to degrade aromatic compounds, including benzoate (Maeda et al. 1998), carbazole, quinoline, fluoranthene (Kumar et al. 2011), phenanthrene (Brito et al. 2006; Al-Awadhi et al. 2007), naphthalene, and toluene (von der Weid et al. 2007; Bødtker et al. 2009). Iwaki et al. (2008) found that a Dietzia strain isolated from enrichment cultures was able to utilize cyclohexylacetic acid. The production of surface-active compounds related to hydrocarbon degradation has been reported (Nazina et al. 2003). Takeishi et al. (2006) reported the isolation of xylanolytic strains of the genus Dietzia from the hindgut and feces of Trypoxylus dichotomus larvae. Rashidi et al. (2009), who effectively biotransformed delta9-tetrahydrocannabinol (∆⁹-THC) using Dietzia sp. ENZHR1, concluded that alkane oxygenases of Dietzia spp. has the potential to play a significant role in the production of novel pharmaceuticals. Dietzia natronolimnaea HS-1 and D. maris NIT-D are the most promising sources for microbial production of carotenoid pigments, especially canthaxanthin, which are used in different industries, such as nutraceutical, cosmetics, and food and feed industries (Khodaiyan et al. 2007, 2008; Nasri Nasrabadi and Razavi 2010a, b, c; Gharibzahedi et al. 2012a,b, 2013).

Here, we present an overview on the characterization of bacteria of the genus Dietzia as a potential source of enzymes for use in the biodegradation, bioremediation, industrial fermentation, and carotenoid pigmentation.

Historical background

The genus Dietzia was first proposed by Rainey et al. (1995) to accommodate an actinomycetes previously classified as Rhodococcus maris (Nesterenko et al. 1982). R. maris was first known as Flavobactenum maris, but was subsequently classified as a species of the genus Rhodococcus (Rainey et al. 1995) based on the presence of similar morphological and chemotaxonomic characteristics with this genus, such as Gram-positive cells, lack of an aerial mycelium, cell-wall chemotype IV, mycolic acids (MAs) of the R. eiythropolis type, MK-8(H₂) as the major isoprenolog, fatty acids that included straight-chain saturated and mono-unsaturated fatty acids and tuberculostearic (10-metyloctadecanoic) acid, and a DNA guanine and cytosine (G + C) content of 73.2 mol% (Harrison 1929; Nesterenko et al. 1982).

This new classification was based on comparisons of 16S ribosomal DNA (rDNA) sequences, as well as on the unusual structure of polar lipids and the presence of short-chain MAs. This strain had been isolated from marine, soil and the skin and intestinal tract of a carp (Nesterenko et al. 1982; Rainey et al. 1995). Bemer-Melchior et al. (1999) were the first to report a case of bacteremia due to D. maris infection associated with a catheter in an immunocompromised patient presenting with septic shock and pneumothorax. D. maris has also been isolated from a bone biopsy specimen from a hospitalized patient associated with a total hip prosthesis replacement (Pidoux et al. 2001). Duckworth et al. (1998) isolated D. natronolimnaios strain 15LN1, as a secondary proposed member of the genus Dietzia, from an east African soda lake and compared its characteristics with D. maris. Using phenotypic characterization and the results of phylogenetic analyses based on 16S rRNA gene sequences, Yumoto et al. (2002) identified D. psychralcaliphila as another species of the genus Dietzia that can grow on a chemically defined medium containing n-alkanes as the sole carbon source. This facultatively psychrophilic alkaliphile was isolated from a drain of a fish product-processing plant. In 2006, D. cinnamea and D. kunjamensis were proposed as two additional members of the genus Dietzia (Mayilraj et al. 2006; Yassin et al. 2006). These strains were isolated from a perianal swab of a patient with a bone marrow transplant and a soil sample collected from Kunjam Pass (a cold desert of the Indian Himalayas), respectively. Li et al. (2008) studied the diversity of endophytic actinobacteria from important pharmaceutical plants and isolated two novel bacteria of the genus Dietzia, namely, D. schimae and D. cercidiphylli, from the surface-sterilized stems of Schima sp. and surface-sterilized roots of Cercidiphyllum japonicum, respectively, on tap water–yeast extract agar medium. Jones et al. (2008) also isolated an actinomycete from an immunocompetent patient suffering from confluent and reticulated papillomatosis. Using a polyphasic taxonomic approach, these authors found that this bacterium (D. papillomatosis) is a novel species of the genus Dietzia. In 2009, D. lutea, which was isolated from a soil sample collected from the eastern desert of Egypt, was introduced as the ninth member of the genus Dietzia (Li et al. 2009). During the period 2010–2012, Kämpfer et al. (2010), Yamamura et al. (2010), Kim et al. (2011), and Kämpfer et al. (2012) identified four new species of the Dietzia genus, including D. aerolata, D. timorensis, D. alimentaria, and D. aurantiaca. These species were isolated from the air of a duck barn, soil, a traditional salt-fermented seafood in Korea, and cerebrospinal fluid from a 24-year-old woman, respectively.

Characterization of the different species of Dietzia

Phenotypic characteristics

Morphological characteristics of Dietzia colonies and cells

The colonies of different Dietzia strains tested were characterized by their small size, circular shape, and convex form. The surface of the convex portion of the colonies was smooth. However, D. papillomatosis N 1280^T and D. natronolimnaea CBS 107.95^T had colonies with a shiny surface. The color of D. aerolata Sj14a^T, D. aurantiaca CCUG 35676^T, and D. maris DSM 43672^T colonies was deep orange (Table 1), while those of D. kunjamensis K30-10^T, D. cercidiphylli YIM 65002^T, D. papillomatosis N 1280^T, D. lutea DSM 45074^T, D. timorensis ID05-A0528^T, and D. cinnamea IMMIB RIV-399^T were red, reddish-orange, orange, orange–yellow, orange–yellow and yellow, respectively. The colonies of other Dietzia strains also had a different color (Table 1).

Table 1 Colony morphology and cell morphology characteristics of the type strains of Dietzia species

Table 1 also shows the cell morphology of the type strains of Dietzia species. The cells varied in shape from rods (D. alimentaria 72^T, D. aurantiaca CCUG 35676^T), short rods (D. maris DSM 43672^T, D. natronolimnaea CBS 107.95^T), coccoids (D. aerolata Sj14a^T, D. aurantiaca CCUG 35676^T), and rods and coccoids (D. kunjamensis K30-10^T, D. lutea DSM 45074^T, D. timorensis ID05-A0528^T). The cells of short, rod-shaped cells of D. cercidiphylli YIM 65002^T and D. schimae YIM 65001^T also exhibited snapping division and produce V-forms. Moreover, D. papillomatosis N 1280^T cells showed snapping division and V-forms and a rod–coccus life cycle (Table 1). Generally, the mean cell diameter of the type strains of Dietzia species was approximately 1.1–1.5 μm. According to currently available data, the cells of D. lutea DSM 45074^T and D. papillomatosis N 1280^T were the largest and smallest, respectively, of all Dietzia strains (Table 1).

Biochemical reactions

Nitrate reduction, H₂S production, hydrolysis of urea, starch, and Tween, and the Voges–Proskauer (VP) and methyl red (MR) tests are the seven main biochemical reactions used to discriminate the different Dietzia strains. Li et al. (2009) showed that D. lutea DSM 45074^T is negative for all of these phenotypic characteristics. D. cinnamea IMMIB RIV-399^T, D. maris DSM 43672^T, and D. papillomatosis N 1280^T were able to reduce nitrates to nitrogen and hydrolyze urea. In contrast, D. timorensis ID05-A0528^T tested negative for nitrate reduction and urea hydrolysis (Table 2). D. natronolimnaea CBS 107.95^T and D. psychralcaliphila ILA-1^T were positive for H₂S production and hydrolysis of urea, but they could not reduce nitrates to nitrogen. A dissimilar behavior was observed for D. schimae YIM 65001^T. D. cercidiphylli YIM 65002^T and D. kunjamensis K30-10^T also showed two different behaviors in terms of the studied phenotypic properties (Table 2). The tested strains of D. alimentaria 72^T, D. cercidiphylli YIM 65002^T, D. kunjamensis K30-10^T, and D. lutea DSM 45074^T were not able to hydrolyze starch. In contrast, D. cinnamea IMMIB RIV-399^T, D. maris DSM 43672^T, D. natronolimnaea CBS 107.95^T, D. papillomatosis N 1280^T, and D. psychralcaliphila ILA-1^T were able to hydrolyze starch (Table 2). The performed studies on some of the Dietzia strains revealed that these bacteria were able to degrade Tween 80. The VP and MR tests were negative for some of the investigated strains, such as D. cercidiphylli YIM 65002^T (Li et al. 2008), D. kunjamensis K30-10^T (Mayilraj et al. 2006), D. lutea DSM 45074^T (Li et al. 2009), D. psychralcaliphila ILA-1^T (Yumoto et al. 2002), and D. schimae YIM 65001^T (Li et al. 2008).

Table 2 Some phenotypic properties of type strains of Dietzia species

Growth characteristics

D. schimae YIM 65001^T, D. kunjamensis K30-10^T, D. lutea DSM 45074^T, and D. maris DSM 43672^T had the highest tolerance to NaCl concentration (15 %). D. timorensis ID05-A0528^T was the least tolerant to NaCl concentration (tolerant to 7 %) among all of the Dietzia strains (Table 3). D. psychralcaliphila ILA-1^T, D. natronolimnaea CBS 107.95^T, D. cercidiphylli YIM 65002^T, and D. alimentaria 72^T tolerated up to 10 % NaCl. The maximum NaCl tolerance for the culturing of D. aurantiaca CCUG 35676^T, D. papillomatosis N 1280^T, and D. cinnamea IMMIB RIV-399^T was 12, 8, and 12 %, respectively.

Table 3 Growth conditions of type strains of Dietzia species

D. cercidiphylli YIM 65002^T, D. kunjamensis K30-10 ^T, D. natronolimnaea CBS 107.95^T, D. papillomatosis N 1280^T, D. psychralcaliphila ILA-1^T, and D. timorensis ID05-A0528^T had a similar range in growth temperature—10–37 °C (Table 3). A growth temperature range of 10–45 °C has also been reported for D. lutea DSM 45074^T, D. maris DSM 43672^T, and D. schimae YIM 65001^T (Table 3). These strains had the widest range of temperature for the cell growth. The growth temperature range for D. aerolata Sj14a^T, D. alimentaria 72^T, D. aurantiaca CCUG 35676^T, and D. cinnamea IMMIB RIV-399^T was 10–30 °C, 15–37 °C, 4–37 °C, and 22–45 °C, respectively (Table 3). In general, the optimum growth temperature for the various strains was between 25 and 30 °C.

The bacterial growth of D. aurantiaca CCUG 35676^T took place at a pH range of 5.5–12.5; the approximate optimum was at pH 7.0–8.0 (Kämpfer et al. 2012). Of all Dietzia strains, D. aurantiaca CCUG 35676^T grows in the largest pH range. Therefore, these researchers pointed out that growth at higher pH values of 10 must be interpreted with care, because even if the medium pH is adjusted to 12.5, CO₂ production will lead to a fairly rapid drop in the pH. Li et al. (2008) reported that two strains of D. cercidiphylli YIM 65002^T and D. schimae YIM 65001^T had a growth pH range of about 6.0–9.0. D. alimentaria 72^T, D. kunjamensis K30-10 ^T, and D. psychralcaliphila ILA-1^T had a similar pH growth range of 7.0–10.0 (Table 3).

Table 4 also shows sole carbon source utilization profiles of the type strains of Dietzia species. Among the strains, only D. timorensis ID05-A0528^T was able to utilize all of the mentioned sole carbon sources, including D-adonitol, aesculin, D-arabitol, L-arabinose, arbutin, cellobiose, D-fucose, D-fructose, gentiobiose, D-galactose, D-glucose, glycerol, inositol, inulin, lactose, D-lyxose, maltose, D-mannose, melezitose, melibiose, N-acetylglucosamine, raffinose, L-rhamnose, D-ribose, salicin, sucrose, D-tagatose, trehalose, turanose, xylitol, and D-xylose (Yamamura et al. 2010).

Table 4 Sole carbon source utilization profiles of the type strains of Dietzia species

Chemotaxonomic characterization

Cellular lipid and fatty acid compositions

The polar lipid profiles present in the type strains of Dietzia species are given in Table 5. As shown in this table, diphosphatidylglycerol, phosphatidylglycerol, phosphatidylinositol mannoside, phosphatidylinositol, and phosphatidylethanolamine (PE) were most important phospholipids in these strains. Dietzia aerolata Sj14a^T and D. lutea DSM 45074^T contain all of the mentioned polar lipids. D. alimentaria 72 ^T and D. cercidiphylli YIM 65002^T had all of the phospholipids except PE. In contrast, each recognized Dietzia species only contained two or three of these five kinds of phospholipid components (Table 5). The polar lipid profiles of D. aurantiaca CCUG 35676^T and D. alimentaria 72^T also contained several unidentified glycolipids. Moreover, several unknown aminolipids were detected in D. aerolata Sj14a^T and D. alimentaria 72^T.

Table 5 Polar lipids and isoprenoid menaquinone present in the type strains of Dietzia species

Table 6 shows the gas chromatography (GC) analysis of the cellular fatty acid profiles of the type strains of all Dietzia species under the same conditions according to the standard Microbial Identification System (MIDI). Whole-cell fatty acids consist of predominantly straight-chain saturated and unsaturated components. C_13:0, C_13:0 2-OH, C_13:0 3-OH, and C_14:1ω5c were only present as cellular fatty acids of D. schimae. D. cercidiphylli YIM 65002^T also only had iso-C_15:1 (0.4 %) fatty acid. The highest amounts of C_14:0, C_15:0, C_16:0, C_17:0, and C_18:0 fatty acids were in D. schimae YIM 65001^T (2.9 %), D. cinnamea IMMIB RIV-399^T (8.3 %), D. timorensis ID05-A0528^T (48.0 %), D. aurantiaca CCUG 35676^T (25.9 %), and D. psychralcaliphila ILA-1^T (13.9 %). The highest levels of 10-methyl C_16:0, 10-methyl C_17:0, and 10-methyl C_18:0 were found in D. cinnamea IMMIB RIV-399^T (3.2 %), D. cinnamea IMMIB RIV-399^T (11.1 %), and D. natronolimnaea CBS 107.95^T (30.2 %), respectively (Table 6). However, D. cinnamea IMMIB RIV-399^T contained the maximum amount of methyl-saturated fatty acids (43.1 %). Iso-C_17:0 and anteiso-C_17:0 fatty acids were present only in D. cercidiphylli YIM 65002^T. Also, D. aerolata Sj14a^T was only the genus containing iso-C_19:0, anteiso-C_19:0, and iso-C_20:0 fatty acids. The fatty acid profile of all of the strains consisted of C_16:1ω7c except for D. timorensis ID05-A0528^T. D. timorensis ID05-A0528^T possessed the highest C_18:1ω9c amount (39.0 %). D. cercidiphylli YIM 65002^T had the second highest C_18:1ω9c level (27.9 %) followed by D. kunjamensis K30-10^T (27.3 %).

Table 6 Fatty acid profiles of the type strains of Dietzia species

MAs are complex hydroxylated branched-chain fatty acids with a long alkyl branch in two positions. Generally, the degree of unsaturation, the average carbon number (ACN), and the structure of the α-alkyl branch can be determined by characterizing the MAs (Nishiuchi et al. 1999). The ACN is calculated by summing the multiplication product of the number of overall carbons (N _c) and the composition (C _c, %) of each molecular species ($ACN={{{\left( {\sum {{N_C}\times {C_c}} } \right)}} \left/ {100 } \right.}$). A high-resolution procedure [capillary GC/mass spectroscopy (MS)] can be used to analyze MAs from Dietzia. Using this procedure, Nishiuchi et al. (1999) separated trimethylsilyl ether derivatives of MA methyl ester based on their number of total carbons and double bonds. These authors found that capillary GC/MS is able to detect minor components of molecular species and to reveal differences of the MA profiles among Nocardia species.

Genera with long-carbon-chain MAs, such as all of the Gordonia species, usually have an ACN in the upper 50s and 60s, none or only small quantities of saturated fatty acids (SFAs), and large amounts of polyunsaturated acids with up to five double bonds. Each species containing long-carbon-chain MAs also contained a predominant content of di-, tri- or tetraenoic acids, the α-alkyl branches of which varied in size from C₁₂ to C₁₈. However, genera containing short-carbon-chain MAs, such as D. maris 58001^T and Rhodococcus strains, possess an ACN in the 30s and large quantities of SFAs. Also, the main α-alkyl branch of this group is C_14:0 and C_16:0 (Nishiuchi et al. 1999, 2000). Thus, short-carbon-chain MAs are present in most Dietzia species. For example, the carbon-chain length of MAs for D. kunjamensis K30-10^T, D. psychralcaliphila ILA-1^T, D. maris DSM 43672^T, and D. natronolimnaea CBS 107.95^T is 33–40, 34–39, 33–38, and 34–38, respectively (Mayilraj et al. 2006). Therefore, due to the short chain length of their MAs (34–38), Dietzia strains can be differentiated readily from Rhodococcus (34–52), Nocardia (46–60), Gordonia (48–66), Tsukamurella (64–78), and Mycobacterium (60–90) (Nishiuchi et al. 2000; Goodfellow and Maldonado 2006). This short-carbon-chain length of probably indicates that Dietzia species possess a novel pathway for fatty acid biosynthesis. However, their carbon chain length is not a very excellent chemotaxonomic index because there is a degree of overlap among the four genera Rhodococcus, Nocardia, Dietzia and Gordonia.

The use of rapid identification methods, such as a combination from the morphological and chemotaxonomic characteristics (Koerner et al. 2009), can be applied to separate Dietzia from other amycelial MA-containing genera. Niwa et al. (2012) characterized 16 human clinical isolates of Dietzia species which had been previously misidentified as R. equi using phenotypic methods, including traditional and commercial (API Coryne) biochemical tests, antimicrobial susceptibility testing, and 16S rRNA gene and DNA gyrase β subunit (gyrB) gene sequencing. These authors concluded that the hydrolysis of adenine and the Christie–Atkins–Munch–Petersen (CAMP) reaction can suitably discriminate between Dietzia isolates and the type strain of R. equi. Traditional and commercial phenotypic profiles are inappropriate for identifying Dietzia species. Also, in a study by Niwa et al. (2012), the analysis of 16S rRNA gene and gyrB gene sequences were able to differentiate all Dietzia strains from the type strain of R. equi. Antimicrobial susceptibility testing and carbon utilization assays with the application of easy-to-perform procedures (Biotype-100 strips) also can be an effective approach to identify these microorganisms (Bizet et al. 1997)

The amino acids and sugars of whole-cell hydrolysates

Hydrolysis of the peptidoglycans of whole cells showed that meso-diaminopimelic acid as a diamino acid was the major amino acid present in Dietzia strains. However, this amino acid has been not reported in two strains of D. aerolata Sj14a^T and D. aurantiaca CCUG 35676^T. The major cell-wall sugars were arabinose and galactose. The glycan moiety of the cell wall contained N-acetyl residues (N-acetylmuramic acid) (Duckworth et al. 1998; Yumoto et al. 2002; Mayilraj et al. 2006; Yassin et al. 2006; Jones et al. 2008; Li et al. 2008, 2009; Kämpfer et al. 2010; Yamamura et al. 2010; Kim et al. 2011; Kämpfer et al. 2012).

The respiratory quinone system and the DNA G + C content

The isoprenoid or respiratory quinones are a class of terpenoid lipids with a similar inherent potential in chemotaxonomy. They are constituents of the bacterial plasma membranes and play important roles in electron transport, oxidative phosphorylation, and possibly active transport (Goodfellow and Maldonado 2006). The different Dietzia species contained dehydrogenated menaquinones with eight isoprene units [MK-8(H₂)] as the predominant isoprenologue (Table 5). MK-8(H₂) is also the major isopronolog found in the genus Rhodococcus (Rainey et al. 1995). A minor amount of MK-7(H₂) was also observed in D. cinnamea IMMIB RIV-399^T, D. papillomatosis N 1280^T, D. aerolata Sj14a^T, and D. aurantiaca CCUG 35676^T. Moreover, the quinone system of D. aerolata Sj14a^T and D. aurantiaca CCUG 35676^T was composed of MK-9(H₂) (Table 5).

DNA base composition, especially guanine + cytosine (G + C) content as a bacterial taxonomic marker, was considered. For example, actinobacteria have a high content of G + C in their genomes, whereas clostridia have low GC-containing genomes. DNA with a high GC content is more stable than DNA with low a GC content (Sueoka 1961). The mol% G + C content of DNA generally applies as part of the description of the type strain of the type species of a new genus. As illustrated in Table 5, among the various Dietzia species, the mean G + C content of genomic DNA varied from approximately 64.7 (D. alimentaria 72^T) to 73 % (D. maris DSM 43672^T). However, the G + C contents of the DNA for different Dietzia species are similar to that of Corynebacterium (51–67 %), Mycobactenurn (70–72 %), Nocardia (64–72 %), Rhodococcus (63–73 %), Tsukamurella (67–68 %), and Gordonia (63–69 %) which phylogenetically were members of the MA-containing group (Rainey et al. 1995).

Concluding remarks

The present overview presents the results of a taxonomic study on the family Dietziaceae since the name Dietzia was formally introduced more than half a century ago. Over recent years, there has been a growing interest in species of the genus Dietzia, especially in D. maris, D. natronolimnaea, D. cinnamea, D. psychralcaliphila, and D. cerdiciphylli (Von der Weid et al. 2007; Alonso-Gutiérrez et al. 2011; Nakano et al. 2011; Gharibzahedi et al. 2012b). Apart from the well-established use of Dietzia strains in therapeutic biotreatments for adult paratuberculosis animals, the production of carotenoid pigments, and animal feed additives, various applications in biosurfactant and biodemulsifier production, pollutant bioremediation, biodegradation of petroleum hydrocarbons and crude oil, and in the production of extracellular polymeric substances have also been developed. The application of Dietzia strains as biotechnological tools could thus be used to improve the optimization and quality assurance of food ingredients and products, the capability of environmental pollution degradation and remediation methods, and the efficiency of bioconversion systems for energy recovery and bioprocessing of value-added products. In order to develop these bacteria for agro-environmental applications, specific and robust methods that are easy to apply, efficient in terms of time and cost, and characterized by a high sensitivity and reproducibility need to be developed to accumulate accurate information on their epidemiology and occurrence, whether in the environment and/or in food. The cellular micro-morphological, biochemical, growth and chemotaxonomic characteristics of a microorganism are frequently applied to isolate and identify that microorganism. However, these strategies are not always adequate for the classification and identification. Phylogenetic analysis using 16S rRNA gene sequences and the neighbor-joining method have been successfully applied as a molecular clock to estimate relationships among these bacteria, but more recently it has also become important as a means to identify an unknown bacterium to the genus or species level. Moreover, gyrB gene sequencing could be efficiently used to reliably identify Dietzia species. A nested-PCR amplification procedure combined with denaturing gradient gel electrophoresis enabled the detection of Gordonia populations from wastewater treatment plant foam samples (Shen et al. 2007); this approach could also improve the identification of Dietzia bacteria in different environments. The use of a considerable number of powerful molecular techniques, such as massive parallel sequencing, metagenomics, metatranscriptomics, and metaproteomics, will allow researchers to probe deeper into the characteristics of species of the genus Dietzia, providing more comprehensive results than conventional diagnosis and enhancing understanding of their epidemiology and natural history. The future perspective is to use these molecular procedures to overcome certain limitations, generate insights into the biology and molecular processes of Dietzia species, and also provide useful quantitative results.