Dental units: an environmental study of sources of potentially pathogenic mycobacteria

Bacterial biofilms: an emerging link to disease pathogenesis

The role of biofilms in the pathogenesis of some chronic human infections is now widely accepted. However, the criteria used to determine whether a given infection is caused by biofilms remain unclear. In this chapter we discuss three infections that are caused by biofilms--infectious kidney stones, bacterial endocarditis, and cystic fibrosis lung infections--and focus on the role of the biofilm in disease pathogenesis. Biofilms are also important as environmental reservoirs for pathogens, and the biofilm growth mode may provide organisms with survival advantages in natural environments and increase their virulence. The consequences of pathogens living in environmental biofilms and an analysis of some specific environmental biofilm systems are presented.

The effect of Sterilex Ultra for disinfection of dental unit waterlines

AIM

To evaluate the effect of a disinfectant agent based on hydrogen peroxide (Sterilex Ultra, Sterilex Corporation, Maryland, USA) on the microbiological water quality in dental unit waterlines (DUWL).

SETTING

Six older dental units were disinfected with Sterilex Ultra and another six units with sodium hypochlorite. The results were compared to six non-disinfected units examined during a period without patient treatment.

OUTCOME MEASURE

The number of colony forming units of bacteria (cfu)/ml of water was determined according to European Standards on water quality.

DESIGN

The water quality was initially determined during a six week period, and subsequently in more detail for two weeks. Finally, the effect of prolonged administration of Sterilex Ultra was investigated for six weeks.

RESULTS

Instillation of Sterilex Ultra according to the recommendations of the manufacturer initially reduced the number of bacteria in DUWL to <10(2) cfu/ml. However, following daily, as well as prolonged administration of Sterilex Ultra a gradual recolonisation was observed resulting in bacterial numbers >10(4) cfu/ml in a number of units. Major differences between the number of cfu/ml in individual units were observed.

CONCLUSION

Neither daily nor extended administration of Sterilex Ultra was capable of maintaining an acceptable water quality in these older dental units.

Waterborne pathogens and dental waterlines

Humans, like every other living thing on Earth, have evolved in a world dominated by many billions of microscopic life forms. Most of the time, we live in a state of harmony (or even mutualism) with our invisible coinhabitants. When this balance becomes disturbed however, the consequences can be devastating. Infectious diseases including malaria, tuberculosis, and AIDS remain the world's greatest mass murderers. Dental workers strive to reduce infection risks for their patients through infection control measures that reduce or eliminate potentially pathogenic agents in the clinical environment. As increasing numbers of patients with varying degrees of immune suppression present for dental treatment, the need to ensure an aseptic treatment environment will become a higher priority for the dental profession. The possibility that exposure to aerosols contaminated with endotoxin might exacerbate asthma or cause chronic respiratory problems in dental health care workers should be investigated. Although direct evidence of widespread complications among patients or occupationally acquired illness among dental workers is presently lacking, reducing the numbers of microorganisms present in dental treatment water is consistent with other empiric measures that form the basis of infection-control practice.

The genetics of nontuberculous mycobacterial infection

The molecular aetiology of familial susceptibility to disseminated mycobacterial disease, usually involving weakly pathogenic strains of mycobacteria, has now been elucidated in more than 30 families. Mutations have been identified in five genes in the interleukin-12-dependent interferon-gamma pathway, highlighting the importance of this pathway in human mycobacterial immunity. Knowledge derived from the study of these rare patients contributes to our understanding of the immune response to common mycobacterial pathogens such as Mycobacterium tuberculosis and Mycobacterium leprae, which remain major public health problems globally. This knowledge can be applied to the rational development of novel therapies and vaccines for these important mycobacterial diseases.

Risk assessment of dental unit waterline contamination

Biofilms form rapidly on dental unit waterlines. The majority of the organisms in the biofilm are harmless environmental species, but some dental units may harbour opportunistic respiratory pathogens. This paper describes a risk assessment approach to analysing the hazard from biofilm organisms contaminating dental unit waterlines on the respiratory health of both the dental team and patients. The health risk from the respiratory pathogens Legionella spp, Mycobacterium spp and Pseudomonads was found to be low. Nevertheless, in order to satisfy water regulations and comply with health and safety legislation dentists should institute infection-control measures to maintain the dental unit water at the standard of less than 200 colony-forming units per ml of aerobic bacteria.

Occurrence of mycobacteria in water treatment lines and in water distribution systems

The frequency of recovery of atypical mycobacteria was estimated in two treatment plants providing drinking water to Paris, France, at some intermediate stages of treatment. The two plants use two different filtration processes, rapid and slow sand filtration. Our results suggest that slow sand filtration is more efficient for removing mycobacteria than rapid sand filtration. In addition, our results show that mycobacteria can colonize and grow on granular activated carbon and are able to enter distribution systems. We also investigated the frequency of recovery of mycobacteria in the water distribution system of Paris (outside buildings). The mycobacterial species isolated from the Paris drinking water distribution system are different from those isolated from the water leaving the treatment plants. Saprophytic mycobacteria (present in 41.3% of positive samples), potentially pathogenic mycobacteria (16.3%), and unidentifiable mycobacteria (54.8%) were isolated from 12 sites within the Paris water distribution system. Mycobacterium gordonae was preferentially recovered from treated surface water, whereas Mycobacterium nonchromogenicum was preferentially recovered from groundwater. No significant correlations were found among the presence of mycobacteria, the origin of water, and water temperature.

Effective control of dental chair unit waterline biofilm and marked reduction of bacterial contamination of output water using two peroxide-based disinfectants

Bacterial biofilm in dental unit waterlines (DUWs) is a widespread problem, and poses a potentially significant risk of infection to dental staff and patients, particularly those who are medically compromised or immunocompromised. The purpose of the present study was to investigate the level of bacterial contamination of dental chair unit output water in the Dublin Dental Hospital, and to investigate the efficacy of two hydrogen peroxide-based disinfectants in reducing bacterial loads to < or =200 cfu/mL as recommended by the American Dental Association. The chemical quality of dental chair unit input and output water was well within the limits recommended for potable water. Water supplied to the units yielded an average aerobic heterotrophic bacterial cell density of 184 cfu/mL. However, the corresponding density in output water was considerably higher; the average cell density in water from the three-in-one air/water syringes and cup fillers in 12 chairs was 8200 and 4300 cfu/mL, respectively. Dental unit water obtained from 18 separate reservoir-supplied units in general practices in the Dublin area yielded an average of 66000 cfu/mL. The bacterial species found were predominantly environmental organisms, which were also present at low levels in the input water. Some of the species identified (e.g., Burkholderia cepacia and Pseudomonas fluorescens) are known opportunistic pathogens. The capacity of two disinfectants, Sterilex Ultra and Sanosil, to reduce bacterial contamination to safe levels was compared. In a controlled study, once weekly overnight (15 h) disinfection using either agent reduced the bacterial density to below the American Dental Association recommended level of 200 cfu/mL. However, once disinfection ceased the bacterial loads increased to unacceptably high levels within three weeks. Electron microscopic analysis showed that both disinfectants markedly reduced biofilm in the DUWs, but the biofilm rapidly became extensive again when once weekly disinfection ceased. While both disinfectants were equally effective in lowering the bacterial counts to acceptable levels, Sterilex Ultra was associated with clogging of DUWs in some dental chair units after repeated usage, suggesting that Sanosil is a more suitable agent for routine use.

Nontuberculous mycobacteria in the environment

It is likely that the incidence of infection by environmental opportunistic mycobacteria will continue to rise. Part of the rise will be caused by the increased awareness of these microbes as human pathogens and improvements in methods of detection and culture. Clinicians and microbiologists will continue to be challenged by the introduction of new species to the already long list of mycobacterial opportunists (see Table 3). The incidence of infection will also rise because an increasing proportion of the population is aging or subject to some type of immunosuppression. A second reason for an increase in the incidence of environmental mycobacterial infection is that these microbes are everywhere. They are present in water, biofilms, soil, and aerosols. They are natural inhabitants of the human environment, especially drinking water distribution systems. Thus, it is likely that everyone is exposed on a daily basis. It is likely that certain human activities can lead to selection of mycobacteria. Important lessons have been taught by study of cases of hypersensitivity pneumonitis associated with exposure to metalworking fluid. First, the implicated metalworking fluids contained water, the likely source of the mycobacteria. Second, the metalworking fluids contain hydrocarbons (e.g., pine oils) and biocides (e.g., morpholine) both of which are substrates for the growth of mycobacteria [53,193]. Third, outbreak of disease followed disinfection of the metalworking fluid [136,137]. Although the metalworking fluid was contaminated with microorganisms, it was only after disinfection that symptoms developed in the workers. Because mycobacteria are resistant to disinfectants, it is likely that the recovery of the mycobacteria from the metalworking fluid [137] was caused by their selection. Disinfection may also contribute, in part, to the persistence of M avium and M intracellulare in drinking water distribution systems [33,89,240]. M avium and M intracellulare are many times more resistant to chlorine, chloramine, chlorine dioxide, and ozone than are other water-borne microorganisms [141,236]. Consequently, disinfection of drinking water results in selection of mycobacteria. In the absence of competitors, even the slowly growing mycobacteria can grow in the distribution system [33]. It is likely that hypersensitivity pneumonitis in lifeguards and therapy pool attendants [139] is caused by a similar scenario.

The effect of drying dental unit waterline biofilms on the bacterial load of dental unit water

AIM

To evaluate drying of the dental unit waterlines (DUWL) as a new method of controlling the bacterial biofilm therein and thereby to reduce the number of living bacteria in dental unit water.

SETTING

18 dental units were incorporated into the study. Six units constituted the experimental units, which were emptied for stagnant water in the DUWL every night; six units were chlorinated every night (positive controls), and six units were left untreated (negative controls).

OUTCOME MEASURE

Water samples from the ultrasonic scaler were examined microbiologically according to the guidelines from the Danish Standard Association and the number of colony forming units (cfu) per ml of water was determined.

RESULT

Drying of DUWL did not reduce the number of cfu per ml in dental unit water below the levels found in DUWL left untreated.

CONCLUSION

The drying of the waterlines for about 16 hours per day during a 19-day period did not result in reduced counts of bacteria in water samples from the experimental units.

The effect of a disinfectant/coolant irrigant on microbes isolated from dental unit water lines

The purpose of this study was to assess water samples from a hospital dental clinic to determine whether a disinfectant/coolant irrigant containing chlorhexidine (Lines, Micrylium Laboratories) affects the presence of microbial organisms in dental unit waterlines. Water samples from three hospital dental operatories were collected at baseline and after overnight treatment with a disinfectant-containing irrigant followed by sterile water irrigation. Saliva of treated patients and sterile water rinse specimens were collected from the waterlines of these operatories for three consecutive days, then weekly for eight weeks after treatment. Specimens were cultured to identify total heterotrophic plate counts as well as presence of Pseudomonas aeruginosa and Candida species. Baseline organism counts varied from 10(3) to 10(5) colony-forming units per milliliter. After treatment, no organisms were detected in waterline discharge. Decontamination of dental unit waterlines is possible using a disinfectant/irrigant followed by sterile water irrigation. The potential for contamination of the lines from patients' saliva may have been reduced due to use of anti-retraction valves and the disinfectant/sterile water irrigation, as conducted in this study.

Identification of nontuberculous mycobacteria existing in tap water by PCR-restriction fragment length polymorphism

This paper presents the finding of the possible cause of the high false-positive rate in acid-fast staining in histological examinations. Using acid-fast staining, culture, and PCR, acid-fast bacilli were detected in 83.7% of 49 hospital tap water samples and nontuberculous mycobacteria (NTM) were detected in 20.4% of the same 49 samples. The 10 NTM isolates were also identified to the species level using PCR-restriction fragment length polymorphism. Our findings indicate that NTM in hospital tap water are the possible cause of false positives in acid-fast staining and of nosocomial infection in immunocompromised patients.

Chlorine disinfection of atypical mycobacteria isolated from a water distribution system

We studied the resistance of various mycobacteria isolated from a water distribution system to chlorine. Chlorine disinfection efficiency is expressed as the coefficient of lethality (liters per minute per milligram) as follows: Mycobacterium fortuitum (0.02) > M. chelonae (0.03) > M. gordonae (0.09) > M. aurum (0.19). For a C.t value (product of the disinfectant concentration and contact time) of 60 mg.min.liter(-1), frequently used in water treatment lines, chlorine disinfection inactivates over 4 log units of M. gordonae and 1.5 log units of M. fortuitum or M. chelonae. C.t values determined under similar conditions show that even the most susceptible species, M. aurum and M. gordonae, are 100 and 330 times more resistant to chlorine than Escherichia coli. We also investigated the effects of different parameters (medium, pH, and temperature) on chlorine disinfection in a chlorine-resistant M. gordonae model. Our experimental results follow the Arrhenius equation, allowing the inactivation rate to be predicted at different temperatures. Our results show that M. gordonae is more resistant to chlorine in low-nutrient media, such as those encountered in water, and that an increase in temperature (from 4 degrees C to 25 degrees C) and a decrease in pH result in better inactivation.

A pilot study of three methods for the reduction of bacterial contamination of dental unit water systems in routine use

Three different methods for minimizing the bacterial contamination of the water system in a SIRONA C2 type dental unit were investigated sequentially. Without any decontamination method, water from the hand piece, air-water-jet and mouthwash were continuously contaminated by 10(3) to 10(5) colony forming units (cfu) of aerobic mesophilic bacteria per milliliter. A reduction to below 100 cfu/ml was achieved by continuous adding of a chemical microbicide based on hydrogen peroxide and silver ions. However, this was only possible after rinsing the system thoroughly for at least two minutes after interruptions of the treatment. Long-lasting low counts of below 100/ml were obtained by means of an in-line bacteria filter, in connection with the provision of a thermo-chemical or thermal decontamination of the water pipes and hand pieces after the filter. The electrolyte release of chlorine from the dental unit tap water by anodic oxidation without addition of any chemical disinfectant also resulted in continuously low colony numbers of the water. In this case, regular decontamination of the end parts of the pipes and hand pieces was not necessary.

Nosocomial infections due to nontuberculous mycobacteria

Nontuberculous mycobacteria (NTM) are ubiquitous in the environment and cause colonization, infection, and pseudo-outbreaks in health care settings. Data suggest that the frequency of nosocomial outbreaks due to NTM may be increasing, and reduced hot water temperatures may be partly responsible for this phenomenon. Attention to adequate high-level disinfection of medical devices and the use of sterile reagents and biologicals will prevent most outbreaks. Because NTM cannot be eliminated from the hospital environment, and because they present an ongoing potential for infection, NTM should be considered in all cases of nosocomial infection, and careful surveillance must be used to identify potential outbreaks. Analysis of the species of NTM and the specimen source may assist in determining the significance of a cluster of isolates. Once an outbreak or pseudo-outbreak is suspected, molecular techniques should be applied promptly to determine the source and identify appropriate control measures.

Biofilms augment the number of free-living amoebae in dental unit waterlines

Freshwater amoebae are ubiquitous. Some species can cause infections in humans while others can ingest and protect opportunistic bacteria. Although the presence of free-living amoebae in various water sources has been reported, few studies have looked at their concentration, which may be clinically relevant, especially if they are present in healthcare devices. A simple technique was used to detect, observe, and evaluate the concentration of free-living amoebae in dental unit and tap water samples. Fifty-three water samples were collected from 35 dental units (air/water syringes) and 18 water taps. The technique was based on the ability of waterborne bacteria to create a biofilm and serve as substratum for the development of amoebae naturally present in the water samples. Laboratory-grown freshwater biofilms support the proliferation of a wide variety of free-living amoebae. All the dental unit water samples tested contained amoebae at concentrations up to 330/mL, or more than 300 times the concentration in tap water from the same source. Hartmanella, Vanella, and Vahlkampfia spp. were the most frequently encountered. Naegleria and Acanthamoeba spp. were also present in 40% of the samples. Four of the samples collected from dental units, but none from water taps, contained amoebae able to proliferate at 44 degrees C. Biofilms that form inside some dental instruments can considerably increase the concentration of free-living amoebae, some of which are potential human pathogens.

Characterization of biofilm growth and biocide susceptibility testing of Mycobacterium phlei using the MBEC assay system

The importance of non-tuberculosis mycobacterial biofilm species in medicine, industry and the environment has recently gained attention. Our objectives were to characterize biofilm growth of Mycobacterium phlei M4, as a model of rapidly growing mycobacteria using the minimal biofilm eradication concentration (MBEC) and to compare biocide susceptibility of planktonic and biofilm organisms. Scanning electron microscopy was also carried out to observe biofilm morphology. With the exception of Sporicidin and Virkon the minimum bactericidal concentration values for all biocides tested were lower than the MBEC values. The MBEC assay system was seen to produce multiple and reproducible biofilms of M. phlei and to be a useful tool for susceptibility studies.

Microbiological safety of drinking water

Emerging pathogens in drinking water have become increasingly important during the decade. These include newly-recognized pathogens from fecal sources such as Cryptosporidium parvum, Campylobacter spp., and rotavirus, as well as pathogens that are able to grow in water distribution systems, like Legionella spp., mycobacteria, and aeromonads. To perform a risk analysis for the pathogens in drinking water, it is necessary to understand the ecology of these organisms. The ecology of the drinking-water distribution system has to be evaluated in detail, especially the diversity and physiological properties of water bacteria. The interactions between water bacteria and (potential) pathogens in such diverse habitats as free water and biofilms are essential for the survival or growth of hygienically relevant organisms in drinking water. Results of epidemiological studies together with ecological data are the basis for effective resource protection, water treatment, and risk assessment.

The dental unit waterline controversy: defusing the myths, defining the solutions

BACKGROUND AND OVERVIEW

This article reviews the literature on the subject of dental unit waterline contamination. It has been expanded from the text of a lecture given at the Scientific Frontiers in Dentistry program sponsored by the National Institute for Dental and Craniofacial Research in Bethesda, Md., in July 1999. The author examines the underlying biological causes of waterline colonization by microorganisms, the evidence of potential health consequences and possible means of improving the quality of dental water. He also describes examples of devices currently marketed to improve and maintain the quality of dental treatment water.

CONCLUSIONS

Microorganisms colonize dental units and contaminate dental treatment water. While documented instances of related illness are few, water that does not meet potable-water standards is inappropriate for use in dentistry.

CLINICAL IMPLICATIONS

Exposure to water containing high numbers of bacteria violates basic principles of clinical infection control. Dentists should consider available options for improving the quality of water used in dental treatment.

Microbial biofilm formation and contamination of dental-unit water systems in general dental practice

Dental-unit water systems (DUWS) harbor bacterial biofilms, which may serve as a haven for pathogens. The aim of this study was to investigate the microbial load of water from DUWS in general dental practices and the biofouling of DUWS tubing. Water and tube samples were taken from 55 dental surgeries in southwestern England. Contamination was determined by viable counts on environmentally selective, clinically selective, and pathogen-selective media, and biofouling was determined by using microscopic and image analysis techniques. Microbial loading ranged from 500 to 10(5) CFU. ml(-1); in 95% of DUWS water samples, it exceeded European Union drinking water guidelines and in 83% it exceeded American Dental Association DUWS standards. Among visible bacteria, 68% were viable by BacLight staining, but only 5% of this "viable by BacLight" fraction produced colonies on agar plates. Legionella pneumophila, Mycobacterium spp., Candida spp., and Pseudomonas spp. were detected in one, five, two, and nine different surgeries, respectively. Presumptive oral streptococci and Fusobacterium spp. were detected in four and one surgeries, respectively, suggesting back siphonage and failure of antiretraction devices. Hepatitis B virus was never detected. Decontamination strategies (5 of 55 surgeries) significantly reduced biofilm coverage but significantly increased microbial numbers in the water phase (in both cases, P < 0.05). Microbial loads were not significantly different in DUWS fed with soft, hard, deionized, or distilled water or in different DUWS (main, tank, or bottle fed). Microbiologically, no DUWS can be considered "cleaner" than others. DUWS deliver water to patients with microbial levels exceeding those considered safe for drinking water.

The production of nitric oxide and tumor necrosis factor by murine macrophages infected with mycobacterial strains differing by hemolytic activity

In this study, we compared the secretion of nitric oxide (NO) and tumor necrosis factor (TNF-alpha) by murine macrophages infected in vitro with hemolytic or unhemolytic mycobacteria isolates. We observed that unhemolytic mycobacteria induced more intensive NO production by macrophages and were more susceptible to bactericidal effect of mononuclear phagocytes than hemolytic mycobacterial strains. In contrast, the high-virulence hemolytic isolates induced significantly stronger TNF-alpha production by infected macrophages than the low-virulence unhemolytic bacilli.

Primary mycobacterial infection of the uvula

Tuberculosis, and non-tuberculous mycobacterial infections are becoming more common thus it is more likely that otolaryngologists will encounter these conditions. We describe an otherwise well patient, with symptoms and signs from chronic uvular inflammation, who proved to have a primary mycobacterial infection. This is an unique presentation in the literature and reminds clinicians of the need, where uncertainty exists in diagnosis, to consider mycobacterial infections.

Microbial contamination of dental unit waterlines: the scientific argument

The quality of dental unit water is of considerable importance since patients and dental staff are regularly exposed to water and aerosols generated from the dental unit. The unique feature of dental chair water lines is the capacity for rapid development of a biofilm on the dental water supply lines combined with the generation of potentially contaminated aerosols. The biofilm, which is derived from bacteria in the incoming water and is intrinsically resistant to most biocides, then becomes the primary reservoir for continued contamination of the system. Dental water may become heavily contaminated with opportunistic respiratory pathogens such as Legionella and Mycobacterium spp. The significance of such exposure to patients and the dental team is discussed. There is at the present time, no evidence of a widespread public health problem from exposure to dental unit water. Nevertheless, the goal of infection control is to minimise the risk from exposure to potential pathogens and to create a safe working environment in which to treat patients. This paper evaluates the range of currently available infection control methods and prevention strategies which are designed to reduce the impact of the biofilm on dental water contamination, and are suitable for use in general practice. Bacterial load in dental unit water can be kept at or below recommended guidelines for drinking water (less than 200 colony forming units/ml) using a combination of readily available measures and strict adherence to maintenance protocols. Sterile water should be employed for all surgical treatments.