User:S4344138

Emily Mantilla 43441383 Bench B 23/09/2016 ^[1]

Classification

Higher order taxa

Prokaryote – Bacteria – Firmicutes – Negativicutes – Veillonellales – Veillonellaceae – Veillonella

Species

Veillonella parvula

Identified strains ^[2]:

AC2_8_11_AN_NA_2
ACS-068-V-Sch12
ATCC 17745
DSM 2008
HSIVP1

Description and significance

V. parvula is one of the six species belonging to the genus Veillonella which are gram-negative cocci and obligate anaerobes ^[1],^[2]. It is a nonfermentative organism that lives optimally at 37 °C and a PH range 6.5-8.0, thus, it is commonly found in the human oral, intestinal and vaginal microflora ^[1]. The NCBI genome record for strain DSM 2008 reports a collection date before 1898 in France isolated from the intestinal tract ^[2]. This organism has been identified as a causal pathogen of periodontitis, bacterimia, endocarditis and opportunistic infections ^[2]. Several studies have confirmed that it has an important role in the formation of dental plaque biofilm favouring the association of other periodontal pathogens and contributing as causal agents of systemic diseases ^[3]. The high prevalence of this pathogen in patients that have poor oral health has prompted to understand the role of V. parvula in health and disease as well as its contribution to the human microbiome.

Genome structure

There is complete genome available from the NCBI for "V. parvula DSM 2008" (RefSeq NC_013520.1). The genome has a size of 2.13 Mb with 1,893 genes, 9 pseudo-genes and 3 frameshift genes. The GC content is 38.6% and encodes 1,822 proteins. There are 12 rRNA, 48 tRNA and 2 other RNA ^[2].

Cell structure and metabolism

V. parvula is a gram-negative organism meaning that it has a peptidoglycan layer in-between the outer cell membrane, which contains lipopolysaccharide (LPS), and the inner cell membrane. ^[1],^[8]. This bacterial species is particularly good at colonizing and surviving the acid environment of the mouth in part due to its metabolism. Some RNA-sequencing studies have identified a histidine biosynthesis pathway particularly upregulated in this organism suggesting that the high levels of intra-cellular histidine assist with an increased intra-cellular buffering capacity, facilitating the adaptation to the acidic environment ^[4]. It is generally appreciated that V. parvula depends on the production of lactate by other bacterial species, such as Streptococcus, forming stable symbiotic relationships as in biofilms ^[3],^[4]. The increased gene expression for lactate degradation indicates that V. parvula prefers short-chain organic acids as its main energy source including pyruvate and oxaloacetate ^[4],^[5]. Additionally, it has been found that theVeillonella genus has a metabolic activity that produces hydrogen sulfide (H2S) from L-cysteine involving the cystathionine beta-synthase and cystathionine gamma-lyase enzymes ^[5]. An increase in the concentration of lactate, such as after consuming food, causes an increase in the production of H2S leading to characteristic malodors in the mouth and formation of caries lesions ^[4],^[5].

Ecology

V.parvula is a dominant species in the oral microbiome along other Veillonella species such as V. atypica and V.dispar ^[5]. This pathogen has been isolated from the periodontal pocket, tongue and saliva in the oral cavity ^[3]. However, it is also present in the gastrointestinal and female genital tracts ^[6]. In the oral cavity it has a role in dental plaque biofilm formation so it is commonly found in association with saccharolytic bacteria contributing to the cause of caries ^[5]. Additionally, it is found in the subgingival biofilm of patients with chronic periodontitis establishing as an opportunistic pathogen in subjects that have poor oral hygiene compared to those with moderate or good hygiene ^[5], ^[10]. Previous cases of infectious endocarditis have reported the isolation of different Veillonella species, including V. parvula ^[7]. Remarkably, most of the cases involved the patient having a chronic periodontal disease whilst presenting other clinical complications, indicating the possibility of migration and establishment in regions with the appropriate conditions for bacterimia ^[7],^[10].

Pathology

V.parvula has not only been associated with periodontal and endovascular infections but also with more serious cases including, osteomyelitis, meningitis, prosthetic joint infection, and pleuropulmonary infection ^[7],^[8]. These are the only cases in which reportedly V. parvula has been identified as the sole causal organism ^[7]. Its isolation and culture in the laboratory it is readily possible in the appropriate anaerobic conditions, however, little is known about the cellular and molecular mechanisms leading to pathogenesis and the correspondent immune response against V. parvula ^[8]. Several studies have identified the LPS endotoxin as the virulence factor responsible for triggering inflammatory response during infection by inducing cytokine production ^[8]. However, it was found that the cytokine levels produced by V. parvula LPS are lower than those induced by E. coli LPS ^[8]. These findings correspond with the observed clinical symptoms IN both types of infection. For instance, V. parvula infection has less complicated biological effects than E.coli infection which can cause acute inflammatory reaction and even death ^[8]. One study has discovered that oral V. parvula LPS is recognized by the toll-like receptor 4 (TLR4) and activates the p38 MAP kinase triggering an inflammatory response pathway in peripheral blood mononuclear cells ^[8]. In this way, the model indicates how periodontal lesionS cause TLR4 activation leading to an inflammatory response characteristic of periodontitis ^[8].

Application to biotechnology

A novel study dedicated to investigate the human oral metagenome as a source of antimicrobial agents found that glutamyl-tRNA reductase (GluTR)-encoding clones from the oral metagenome were likely to produce antibacterial porphyrins ^[9]. Thus, a novel antibacterial protein known as HOAb112C was isolated from a BAC library of saliva and dental plaque metagenomic DNA ^[8]. In this way, it was possible to identify an effective antibacterial activity against organisms including B. subtillis, S. epidermis and S. aureus, however, it did not show an effect on V. parvula nor other bacterial species ^[8]. In reported cases of V.parvula infection, such as infectious endocarditis, it was noted that this species is highly resistant to penicillin G and vancomycin but remains susceptible to amoxicillin-clavulanate, metronidazole and clindamycin which are used currently as preferred antibiotic treatments ^[7].

Current research

The is a current interest in the differential gene expression of Veillonella species in carious dentine compared to healthy dentine in order to explain the pathogenesis of dental biofilms ^[4]. A study found that among the three species analysed: V. parvula, V. atypica and V. dispar, the expression of genes differ depending on the PH of the oral environment and availability of substrates ^[4]. For instance, V. parvula was identified as the only organism capable of regulating intracellular PH control through an upregulated histidine metabolism and the presence of a potassium uptake system that is not present in the other 2 species ^[4]. Therefore, the role of V. parvula as an opportunistic pathogen is in part due to the presence of genes involved in pyruvate metabolism, transferases and membrane transport systems, but also due to the upregulation of genes that permit the adaptation of the organism to acid environments in order to cause caries lesions ^[4].

The analysis of Veilonella spp. in the oral cavity has been extensively characterized on different populations, however, the first study on tongue biofilms on Thai children was presented early this year ^[10]. To elucidate the frequency and distribution of the bacterial species in this population, the authors used the one-step PCR method with Species-specific primer sets designed from the conserved region of the rpoB gene ^[10]. The results indicated that the method was effective to isolate common strains along unknown ones. Thus, suggesting that further confirmation is required by the analysis of biofilms in different populations in order to correlate the prevalence of systemic diseases with oral hygiene practices ^[10].