In vitro models for oral malodor

Investigating the association between nitrate dosing and nitrite generation by the human oral microbiota in continuous culture

The generation of nitrite by the oral microbiota is believed to contribute to healthy cardiovascular function, with oral nitrate reduction to nitrite associated with systemic blood pressure regulation. There is the potential to manipulate the composition or activities of the oral microbiota to a higher nitrate-reducing state through nitrate supplementation. The current study examined microbial community composition and enzymatic responses to nitrate supplementation in sessile oral microbiota grown in continuous culture. Nitrate reductase (NaR) activity and nitrite concentrations were not significantly different to tongue-derived inocula in model biofilms. These were generally dominated by Streptococcus spp., initially, and a single nitrate supplementation resulted in the increased relative abundance of the nitrate-reducing genera Veillonella, Neisseria, and Proteus spp. Nitrite concentrations increased concomitantly and continued to increase throughout oral microbiota development. Continuous nitrate supplementation, over a 7-day period, was similarly associated with an elevated abundance of nitrate-reducing taxa and increased nitrite concentration in the perfusate. In experiments in which the models were established in continuous low or high nitrate environments, there was an initial elevation in nitrate reductase, and nitrite concentrations reached a relatively constant concentration over time similar to the acute nitrate challenge with a similar expansion of Veillonella and Neisseria. In summary, we have investigated nitrate metabolism in continuous culture oral biofilms, showing that nitrate addition increases nitrate reductase activity and nitrite concentrations in oral microbiota with the expansion of putatively NaR-producing taxa.IMPORTANCEClinical evidence suggests that blood pressure regulation can be promoted by nitrite generated through the reduction of supplemental dietary nitrate by the oral microbiota. We have utilized oral microbiota models to investigate the mechanisms responsible, demonstrating that nitrate addition increases nitrate reductase activity and nitrite concentrations in oral microbiota with the expansion of nitrate-reducing taxa.

Microbial fuel cell compared to a chemostat

Microbial Fuel Cells (MFCs) represent a green and sustainable energy conversion system that integrate bacterial biofilms within an electrochemical two-electrode set-up to produce electricity from organic waste. In this review, we focus on a novel exploratory model, regarding "thin" biofilms forming on highly perfusable (non-diffusible) anodes in small-scale, continuous flow MFCs due to the unique properties of the electroactive biofilm. We discuss how this type of MFC can behave as a chemostat in fulfilling common properties including steady state growth and multiple steady states within the limit of biological physicochemical conditions imposed by the external environment. With continuous steady state growth, there is also continuous metabolic rate and continuous electrical power production, which like the chemostat can be controlled. The model suggests that in addition to controlling growth rate and power output by changing the external resistive load, it will be possible instead to change the flow rate/dilution rate.

Discovery, development and exploitation of steady-state biofilms

Early in vitro biofilm models go back even beyond the invention of the word 'biofilm'. In the dental field, biofilms were simply known as dental plaque and many of the first in vitro models were termed 'artificial mouth microcosm plaques'. The purpose of this review is to highlight important elements of research from over the years regarding in vitro biofilm models, including data from our own laboratories. This helps us to interpret the models and point the way to the future development of biofilm testing. Many hypotheses regarding biofilm phenomena, particularly ecology, metabolism and physiology of volatile sulphur compounds (VSCs) and volatile organic compound (VOC) production could potentially be supported or disproved. In this way, the methods we use for screening biologically active agents including inhibitors, biocides and antimicrobial compounds in general can be improved. Hopefully, any lessons learnt in the past may be of value for the future. In this review, we focus around the need for growth rate controlled long-term biofilms; being continuously monitored using recent technical advances in bioluminescence, selective real-time electrodes, pH electrodes and continuous on-line analysis of the gas phase (both qualitatively and quantitatively). These features allow for accurate determination of growth rate and/or metabolic rate as well as pave the way towards automated assays and fine control of metabolism; impossible to achieve according to conventional biofilm theory. We also attempt to address the questions; can biofilm systems be improved to maintain long term 'real' or 'true' steady states over weeks or months, or are we limited to quasi-steady state systems for a limited period of time.

Relationship of a turbidity of an oral rinse with oral health and malodor in Vietnamese patients

AIM

In the present study, the relationship between the turbidity of mouth-rinse water and oral health conditions, including oral malodor, in patients with (n = 148) and without (n = 231) periodontitis was examined.

METHODS

The turbidity of 20 mL distilled water that the patients rinsed in their mouths 10 times was measured using a turbidimeter. Oral malodor was evaluated using an organoleptic test and Oral Chroma. Oral health conditions, including decayed teeth, periodontal status, oral hygiene status, proteolytic activity of the N-benzoyl-dl-arginine-2-napthilamide (BANA) test on the tongue coating, and salivary flow rate, were assessed.

RESULTS

Turbidity showed significant correlations with oral malodor and all oral health parameters in the periodontitis group. In the non-periodontitis group, turbidity showed significant correlations with oral malodor and oral health parameters, including dental plaque, tongue coating, BANA test, and salivary flow rate. The regression analysis indicated that turbidity was significantly associated with methyl mercaptan and the BANA test in the periodontitis group, and with hydrogen sulfide, dental plaque, tongue coating, and salivary flow rate in the non-periodontitis group.

CONCLUSION

The findings of the present study indicate that the turbidity of mouth-rinse water could be used as an indicator of oral health conditions, including oral malodor.

Microbial volatile compounds in health and disease conditions

Microbial cultures and/or microbial associated diseases often have a characteristic smell. Volatile organic compounds (VOCs) are produced by all microorganisms as part of their normal metabolism. The types and classes of VOC produced is wide, including fatty acids and their derivatives (e.g. hydrocarbons, aliphatic alcohols and ketones), aromatic compounds, nitrogen containing compounds, and volatile sulfur compounds. A diversity of ecological niches exist in the human body which can support a polymicrobial community, with the exact VOC profile of a given anatomical site being dependent on that produced by both the host component and the microbial species present. The detection of VOCs is of interest to various disciplines, hence numerous analytical approaches have been developed to accurately characterize and measure VOCs in the laboratory, often from patient derived samples. Using these technological advancements it is evident that VOCs are indicative of both health and disease states. Many of these techniques are still largely confined to the research laboratory, but it is envisaged that in future bedside 'VOC profiling' will enable rapid characterization of microbial associated disease, providing vital information to healthcare practitioners.

Effects of chlorhexidine on a tongue-flora microcosm and VSC production using an in vitro biofilm perfusion model

An in vitro perfusion biofilm model, derived from tongue-scrape microflora removed from one individual, was employed to study sulfide biogenesis and the effects of repeated exposure to chlorhexidine (CHX). Volatile sulfur compounds (VSC) were measured using a carbon veil electrode within the biofilm and a halimeter for liquid and gas phase levels, respectively. The microflora of the perfusate and the biofilm were assessed by microbiological techniques and polymerase chain reaction (PCR) to estimate diversity. Biofilms treated with a 1 mL pulse of 0.1% CHX twice a day for three days showed (1) a large reduction in viable count (>90% kill), (2) a (slow) reduction in the VSC production rate, consistent with the reduction in microbes rather than direct inhibitory effects on the biotransformation steps, and (3) a preferential reduction of strict anaerobes. Treated biofilms allowed to recover over 3-5 days showed a nominal amount of regrowth in some experiments, although population numbers were still well below those found in untreated controls. The microbiological composition of biofilms treated but allowed to recover was markedly different from the controls, with proportionally fewer strict anaerobes. Thus, CHX treatment caused detectable ecological shifts with consequent long-term effects on the response of the biofilm in terms of VSC generation, consistent with clinical observations. The model appears highly suited for testing the efficacy of putative anti-malodour or antimicrobial agents.

In vitrobiofilm model for studying tongue flora and malodour

AIMS

To develop a perfusion biofilm system to model tongue biofilm microflora and their physiological response to sulfur-containing substrates (S-substrates) in terms of volatile sulfide compound (VSC) production.

METHODS AND RESULTS

Tongue-scrape inocula were used to establish in vitro perfusion biofilms which were examined in terms of ecological composition using culture-dependent and independent (PCR-DGGE) approaches. VSC-specific activity of cells was measured by a cell suspension assay, using a portable industrial sulfide monitor which was also used to monitor VSC production from biofilms in situ. Quasi steady states were achieved by 48 h and continued to 96 h. The mean (+/-SEM) growth rate for 72-h biofilms (n=4) was micro=0.014 h(-1) (+/-0.005 h(-1)). Comparison of biofilms, perfusate and original inoculum showed their ecological composition to be similar (Pearson coefficient>0.64). Perfusate and biofilm cells derived from the same condition (co-sampled) were equivalent with regard to VSC-specific activities which were up-regulated in the presence of S-substrates.

CONCLUSIONS

The model maintained a stable tongue microcosm suitable for studying VSC production; biofilm growth in the presence of S-substrates up-regulated VSC activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

The method is apt for studying ecological and physiological aspects of oral biofilms and could be useful for screening inhibitory agents.