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Nurul Asyiqin Zulkifli Bench D 22th September 2016 ^[1]

Classification

Higher order taxa

Bacteria; Firmicutes; Negativicutes; Veillonellales; Veillonellaceae; Veillonella. ^[1]

Species

Veillonella parvula

Type strain: ATCC 10790 T, ATCC 17742 T, CCUG 5123 T, DSM 2008 T, JCM 12972 T, KCTC 5019 T, NCTC 11810 T, Prevot Te 3 T, Prévot Te3 T, strain ATCC 10790 T, Te 3 T, VTT E-001737 T. ^[2]

Description and significance

Veilonella parvula is a small, strictly anaerobic, gram negative coccus bacterium that lack flagella, spores and capsules ^[3]. The genus Veillonella was first isolated by Veillon and Zuber in 1898 and currently subdivided into 13 species: V. atypica, V. caviae, V. criceti, V. denticariosi, V. dispar, V. magna, V. montpellierensis, V. parvula, V. ratti, V. rodentium, V. rogosae, V. seminalis, and V. tobetsuensis ^[4] ^[5]. V. parvula form part of the normal flora of the oral, respiratory, gastrointestinal and genitourinary tracts in humans and animals ^[6]. V. parvula have been isolated from clinical specimens ^[7], typically using selective agar based on vancomycin resistance ^[8] ^[9].

The identification of Veillonella isolates to the species level is difficult because of a lack of conventional phenotypic and biochemical testing in providing sufficient discrimination between species. Thus, molecular methods based on 16S ribosomal DNA gene sequencing such as PCR random fragment-length polymorphism is used to identify Veillonella strains at the species level ^[10] ^[11]. In addition, sequence analysis of housekeeping genes, including rpoB, dnaK and gyrB have also been used to identify Veillonella species ^[5].

V. parvula is often regarded as contaminant or commensal in the oral, gastrointestinal, and genitourinary tracts microflora ^[12]. However it has also been reported to be pathogenically involved in infections including meningitis ^[7], osteomyelitis ^[13], endocarditis ^[14], periodontitis, periodontal abscess and various acute oral conditions ^[31] ^[32]. In addition, V. parvula has been associated with severe early childhood caries ^[16]. Rather than being a sole pathogen, V. parvula is more often involved in multispecies infections ^[17].

V. parvula play a central role in establishing multispecies oral biofilms with the early, middle and late colonizers ^[18] ^[19] ^[20]. This is due to its ability to form intergenic coaggregates with other bacteria, such as Streptococcus mutans ^[21]. Studies have showed that dual-species biofilms such as V. parvula and S. mutans biofilms are more resistant to antimicrobial treatments than single-species biofilms ^[22].

The understanding of interactions of V. parvula with other bacteria such as Streptococcus in oral biofilm formation is important to help preventing oral infectious diseases ^[4].

Genome structure

Referring to the complete genome sequence of V. parvula type strain Te3T, the genome consists of one main circular chromosome comprised of 2,132,142 base pairs with a GC content of 38.6%. 1,920 genes identified in the genome, with 1,859 were protein coding genes, 61 were RNAs and 15 pseudogenes ^[17]. 73.6% of the genes identified were assigned a putative function whereas the remainders were annotated as hypothetical proteins ^[17].

Cell structure and metabolism

Cell structure. V. parvula cell is small, non-motile, cocci in shape (approximately 0.3 to 0.5 µm in diameter) that are normally observed in pairs or short chains and are non-sporulating ^[17]. Like other gram-negative bacteria, the cell wall of V. parvula comprises of an outer membrane that consist of lipopolysaccharides (LPS) which serve as virulence factor in V. parvula ^[23]. The peptidoglycan of V. parvula is made up of A1γ-type with glutamic acid in D configuration, diaminopimelic acid in meso configuration and covalently bound Putrescine or cadaverine to the alpha-carboxyl group of the D-glutamic acid residue ^[24]. Plasmenylethanolamine and plasmenylserine are major constituents of cytoplasmic membrane of V. parvula that play an important role in the regulation of membrane fluidity ^[25].

Biofilm formation. Dental plaque is a multispecies biofilm in which its development requires the adherence of pioneer species to the salivary proteins and glycoproteins absorbance on tooth enamel ^[26]. V. parvula is frequently found in dental plaque in which Veillonellae and streptococci are known to be the early colonizers of dental biofilm ^[27]. Bacteria within multispecies biofilm interact with each other in many ways including using the metabolic end products of other species for growth, in which V. parvula metabolise lactic acid that is produced as a waste product by streptococci, into weaker acids (propionic acid and acetic acid) that are less acidic to solubilise enamel. The ability of V. parvula to utilise lactic acid that is produced by other bacteria makes V. parvula a central player in establishing multispecies oral biofilms ^[18] ^[19] ^[20]. Like other Veillonella species, V. parvula is able to form intergenic co-aggregates with other bacteria that are in the same ecological niche ^[28]. Thus, even though V. parvula cannot adhere to the surface itself, the bacterium is able to attach to specific surface structures present on other bacteria cells and this attachment is mediated by lectin-carbohydrate interactions ^[17]. Previous study have shown that V. parvula are able to cause lactic acid concentration to remain constant over time in a dual-species biofilm making the dual-species biofilms to be more resistant to antimicrobial treatments than single –species biofilms ^[22].

Metabolic functions. Like other Veillonella species, V. parvula possess an unusual metabolism in which they use methylmalonyl-CoA decarboxylase to convert free energy derived from decarboxylation reactions into an electrochemical gradient of sodium ions ^[17]. V. parvula is unable to ferment on sugars. They utilise metabolic end products of co-existing carbohydrate-fermenting bacteria such as lactic acid for its metabolism ^[17]. V. parvula is well known for its ability to use lactic acid as a carbon and energy source for growth ^[29].

Ecology

V. parvula is a strict anaerobic gram-negative bacterium that is predominantly found in the oral cavity, respiratory, gastrointestinal and genitourinary flora of humans and animal. In the oral cavity, it is most frequently found in dental plaque ^[17].

In the oral cavity, V. parvula is able to form biofilms with other bacteria with similar niches via its ability to form intergenic co-aggregates with other bacteria. Metabolic interactions have been suggested between S. mutans and V. parvula in multispecies biofilm, in which the presence of V. parvula leads to a higher resistance of S. mutans against antimicrobial treatments ^[22] ^[30]. V. parvula is highly associated with lactic-acid producing species such as streptococci and this is due to its reliance on lactate as a nutrient source ^[5].

Pathology

V. parvula is rarely considered as pathogen as it is part of the normal flora of the oral, respiratory, gastrointestinal and genitourinary tracts in humans. However, it have been reported as pathogen in periodontitis, periodontal abscess and various acute oral conditions ^[31] ^[32]. It is also one of the most common anaerobic pathogens in chronic maxillary sinusitis and deep neck infections ^[33] ^[34]. V. parvula is also frequently reported in osteomyelitis infections ^[13]. In addition, V parvula is associated with severe childhood caries ^[16] and intraradicular infections ^[35]^[36].

V. parvula has also been implicated in rare cases of meningitis ^[7], endocarditis ^[14], discitis, abscessed orchiepididymitis with sepsis ^[38], prosthetic joint infection ^[37] and bacterimia ^[39]. However, it is noted that more often V. parvula is involved in multispecies infections rather than being a sole pathogen of an infection ^[17].

Application to biotechnology

Currently, for V. parvula type strain SHI-1, gene tuf that is associated with product of translocation elongation factor are of particular interest to the infectious disease researches as the gene is shown to exhibit the property of antibiotic resistance ^[41]. The genome sequence of V. parvula SHI-1 isolate are currently being worked on and the full genome sequence will aid in the future analysis of this bacterium and the potential of tuf gene and other genes as drug targets could be further investigated ^[42].

Another current research on V. parvula focused on the ability to genetically transform V. parvula ^[40]. The study used V. parvula strain PK1910 and obtained spontaneous mutations conferring streptomycin resistance which carry a K43N substitution in the RpsL protein. Using the mutated rpsL gene as a selection marker, the study examined the possibility of using electroporation to introduce DNA into V. parvula. The transformation was successful however the transformation efficiency is too low to be used routinely as a tool for generating mutations. As a further study, the research group are currently testing the possibility of using non-antibiotic selection markers or auxotrophic mutants as recipient strains for genetic transformation. In addition, they are also testing the potential of V. parvula to be used as a shuttle vector.

Current research

One of the recent discoveries on V. parvula include the study on the effect of V. parvula on the antimicrobial resistance and gene expression of S. mutans grown in a dual-species biofilm ^[22]. Previous studies have shown that S. mutans and V. parvula dual-species biofilms have different properties from single-species biofilms in which dual-species biofilms have a different acid production profile and a higher resistance to chlorhexidine. The current study aimed to test whether the susceptibility of S. mutans grown in the presence of V. parvula is also decreased when it is exposed to various other antimicrobial. The results suggest that V. parvula changes the physiology of S. mutans. This finding showed that the presence of another bacterium can change the phenotype of a pathogen and can increase its resistance to antimicrobial treatment. This study show that the study of pathogens in poly-microbial diseases, such as caries and periodontists, should be focused more on multispecies biofilms.

Another recent discovery on V. parvula from the study of 'Identification of Veillonella species in the tongue biofilm using a novel-one step PCR method' indicates that V. parvula along with other Veillonella species can be potentially used as an index of oral hygiene status ^[5]. The study found that as Veillonella species are highly associated with lactic-acid-producing species, this property may be exploited to be used as a sensitive biological indicator and early warning sign of acid production. This would help to predict the development of future caries, especially among children.