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Journal of Dental & Oro-facial Research Vol 10 Issue 1 Jan-Jun 2014
ORIGINAL ARTICLE
Dental Pumice as a Source of Cross Contamination in Laboratories:
A Microbiologic Study
1
Associate
Professor,
Department
of
Prosthodontics, Faculty of Dental Sciences,
MS Ramaiah University of Applied Sciences,
Bangalore, Karnataka, India.
2
Prosthodontist & Implantologist, Private
Practitioner, Udupi, Karnataka, India.
Correspondence: Dr. Sivaranjani Gali. Associate
Professor, Department of Prosthodontics,
Faculty of Dental Sciences, MS Ramaiah
University of Applied Sciences, Bangalore,
Karnataka, India. Email: nature79gali@gmail.
com
How to Cite:
Gali S, D Souza M. Dental pumice as a
source of cross contamination in laboratories:
A microbiologic study. J Dent Orofac Res
2014;10(1):12-5.
1
JDOR
Sivaranjani Gali, 2Mariette D Souza
ABSTRACT
Aim: Cross contamination from the patient to dental personnel and again to the patient
is a matter of grave concern in dental operatory. In an evaluation of procedures that may
be a source of cross contamination, dental laboratory pumice used in the processing of
dental prosthetic appliances has been shown to be contaminated with microorganisms.
It may result in the inoculation of microorganisms into the patient’s mouths, thereby
posing a potential hazard to patients and dental professionals. Therefore, this study
was undertaken to assess the presence and extent of contamination in the polishing and
finishing procedures of dentures in the dental laboratories. Methods: Samples of used
pumice were collected randomly from dental laboratories in a sterile container. In the
microbiology laboratory, 1 g of each sample was weighed and placed in 1ml of sterile
saline solution, placed in centrifuged at 3000 rpm for 5 min. 1 ml of the supernatant
was then diluted to 10-fold and 0.5 ml of solution was spread in blood and McConkeys
agar at 37°C for 24 h. Surface counting method and identification of bacteria was done.
Results and Conclusion: It was found that pumice slurry was contaminated with
Acinetobacter, Klebsiella, Pseudomonas, and Staphylococcus; thus, recommending a strong
and strict follow-up of the infection control procedures even in the dental laboratories.
Key Words: Cross contamination, dental laboratories, dental pumice, infection control.
Introduction
influenza. With the large-volume workload of patients, exposure
to oral cavity by dental personnel has increased the probability of
exposure to pathogenic organisms. Survey shows that 15.9% of
general dentists, 17.2% of prosthodontists and 14.2% of dental
technicians have previous exposure to hepatitis B; their serums
remain positive for the antigen or antibody. When compared to
the frequency of 2-5% in general population, it is apparent that
dentists are at higher risk of acquiring hepatitis B. The frequency
of other, apparently practice related infectious diseases among
dentists are not as clearly defined as it is for hepatitis B. Infection
control in commercial laboratories has attracted increasing
interest as evidence by new laboratory control programs that have
been initiated. The National Association of Dental Laboratories
in United States has mandated inclusion of the formal infection
control program as a requirement for certification of its members
in Certified Dental Laboratories. Furthermore, a group of
concerned laboratories established the Dental Laboratory
Infection Control Council, which requires the member
Certain procedures of dental practice may potentially contribute
to the transmission of infections in the dental office. Considering
that dentists see a large number of patients and operatories
are used repeatedly during a typical practice day with dental
personnel moving from one patient to another during various
stages of treatment, certain office equipment, instruments, or
work surfaces may not be sterilized or disinfected. This results
in a cycle of cross contamination from the patient to dental
personnel and again to the patient. With a renewed awareness
brought about by the recognition of the most devastating disease,
acquired immunodeficiency syndrome, there is a greater outcry
from the general public who are more concerned about the
consequences that may occur if guidelines of infection control
are not followed.
Dental health care personnel are also aware of transmissible
diseases such as hepatitis B, herpes simplex II, mononucleosis, and
Received: 7 January ‘14 Accepted: 14 March ‘14 Conflict of Interest: None
Dental pumice and cross contamination … Gali S et al
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Journal of Dental & Oro-facial Research Vol 10 Issue 1 Jan-Jun 2014
laboratories complete comprehensive infection control training.
It is evident that some infectious diseases can be transmitted to
individuals not in direct patient contact, and persons can contract
infectious diseases by handling contaminated materials.1
Verran et al.2 investigated the microbiological status of certain
high-risk areas in the dental technology laboratory namely
pumice slurry, impression agar, and curing water baths. They
concluded that pumice slurry freshly made using disinfectant was
free from contamination, but colony counts increased after 3 days
use. Verran and Winder compared the microbiologic status of
pumice slurry in clinical and non-clinical dental laboratories and
concluded that non-clinical laboratories are not immune from the
presence of potentially pathogenic microbes in pumice slurry.3
In an evaluation of procedures that may be a source of cross
contamination, dental laboratory pumice used in the processing
of dental prosthetic appliances has been shown by several
investigators to become contaminated with microorganisms
unless the appliance, pumice is treated with a disinfectant or
sterilizing agent.
Appliances may result in the direct inoculation of microorganisms
into the mouths of patients. Further, the processing of polishing
dental appliances with contaminated pumice produces aerosols
and splatter, which contain high concentrations of contaminating
microorganisms. These microorganisms are spread not only
within the dental laboratory that affects the dental personnel,
but also throughout the dental suite posing a potential hazard to
patients and dental professionals.
Thus, focusing on the potentiality of cross contamination during
dental procedures, this study was done to assess the presence
and amount of contamination during the finishing and polishing
procedures of dentures using pumice slurry.
supernatant was removed and was diluted to 10-fold 10-1 dilution
in saline solution. 0.5 ml of a supernatant was placed and spread
using glass spreaders in blood and McConkey’s agar. The agar
plates were then incubated at 37°C for 24 h (Figure 1).
After 24 h, colonies were counted by surface counting method
with the colony counter, and the numbers of colonies were
multiplied by the dilution factor. The colonies were identified by
Gram-stain and the biochemical reaction by the standard method.
The Gram-positive and Gram-negative bacteria were identified
by biochemical tests such as coagulase test and indole, citrate,
oxidase, catalase, urease, and H2S tests, respectively (Figure 2).
Results
The bacterial colonies recovered from pumice were found to
be Acinetobacter, Klebsiella, Pseudomonas, Staphylococcus citreus,
Staphylococcus aureus, coagulase-negative staphylococci, and
aerobic spore bearers (Table 1).
Discussion
Pumice slurry is usually used for polishing both the new and the
repaired prosthesis in the laboratories. The used prosthesis worn by
Aims and objectives
1. To assess the presence and extent of bacterial contamination
in the laboratory pumice in dental laboratories.
Figure 1: Bacterial colonies on blood agar.
Methods
Dental laboratory pumice in the dental laboratory was collected.
Samples of used pumice along with unused pumice, tap water
were taken as control. From the laboratory, the pumice collected
from the pan was mixed thoroughly, and a sample of used pumice
was collected using a sterile spatula in a sterile container. Unused
pumice with tap water was also collected. The samples were
reported to the microbiology laboratory for processing.
The microbiologic laboratory, 1 g of pumice from each of the
samples of used and unused pumice was weighed aseptically
using an electronic balance, placed in 1 ml of sterile saline
solution, centrifuged at 3000 rpm for about 5 min, then placed
upright on a test tube rack and left undisturbed. 1 ml of the
Dental pumice and cross contamination … Gali S et al
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Figure 2: Colonies of Gram-negative bacteria.
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Journal of Dental & Oro-facial Research Vol 10 Issue 1 Jan-Jun 2014
Acinetobacter, Pseudomonas, and Klebsiella were found in the range
of 4000 to >105 cfu/ml. These bacteria are widely distributed in
nature. Acinetobacter survives on moist surfaces, dry surfaces
such as human skin. Part of oropharyngeal flora found in the
small number of healthy people can proliferate to large numbers
in immunologically compromised patients. These opportunistic
pathogens cause infections in respiratory tract, wounds, and cause
septicemia. Cordes et al.6 in 1981 found that chronic exposure in
metallic dust may predispose workers to Acinetobacter infections.
Williams et al.7 in 1983 found Acinetobacter contamination of
laboratory pumice of counts more than 106/g.
Pseudomonas and Klebsiella are opportunistic pathogens and can
cause pneumonia in hospitalized patients. S. aureus and coagulasenegative Staphylococcus epidermis found in the laboratories
probably originated from skin of personnel handling the
prosthesis as they are normal commensals of skin. Their presence
in significant amounts of >105 is important because in case of
minor breaks in skin, superficial infections like boils, abscess can
occur. Williams et al.5 in 1986 recovered from two laboratories
fungi such as Aspergillus, Fusarium cephalosporium, which are
opportunistic pathogens subjected to predisposing factors. For
the immediate denture patients, the possibility of implanting
infected material in an open wound is more. Immediate dentures
and dentures for debilitated, elderly patients are more susceptible
to infections and should be processed with special attention.
Van Reenen8 reported Gram-positive cocci and Gram-negative
bacilli as the causative agent of denture stomatitis. Katberg, Kahn
et al.9 reported the presence of oral organisms such as streptococci,
lactobacillus, and diphtheroids. Such contamination of dentures
polished from pumice could be a source of cross contamination
to the patients at an unacceptable risk. Denture wearers are older
persons who as a group are considered at a higher risk with respect
to infections. The process of aging brings with it a reduction in
the level of respiratory function. This loss of efficiency is shown
in the elderly with limited muscular activity and the increased
incidence of respiratory diseases. Transmission of common
epidemic diseases such as influenza may cause significant
morbidity and mortality in this group.
Dental technicians spend a considerable amount of time in
pumicing and polishing dentures. Therefore, they are more
exposed to aerosol and splatter. Aerosol dissemination of
Acinetobacter should be of concern, as the organisms are associated
with eyes, ears, and respiratory tract. Infection of the skin may
be a hazard for those whose hands are in constant contact with
contaminated pumice. Dermatitis has been shown to increase the
carriage rate and number of Acinetobacter recovered from skin.
Runnells10 in an overview of infection control in dental practice
recommended that the working pumice be discarded after each
use or disinfected at least daily after use and can be replaced often
preferably daily in busy clinics and laboratories; at least weekly in
Table 1: Bacterial colony forming units per milliliter of the
samples of contaminated pumice.
Used pumice
Colony count (cfu/ml)
Acinetobacter
≤105
Staphylococci citreus
5000‑10,000
Pseudomonas
3000
Klebsiella
≤105
Staphylococci aureus
3000
Aerobic spore bearers
3000
different patients polished on the same rag wheel and with the same
undisposed pumice holds every possibility of cross contamination.
Small surface scratches and pits in the denture bases accompanied
with porosity may harbor microorganisms in mucinous films,
which cannot be easily removed by brushing. Subsequently,
the abrading action of pumice dislodges the organisms from the
denture base causing them to adhere to the particles of pumice
and the polishing buff. This poses a risk of contamination from the
repaired prosthesis to the new prosthesis that is polished on the
same polishing lathe with the same pumice slurry. Moreover, the
pumice slurry if undisposed and left exposed to air not only could
be contaminated from air borne organisms, water borne microbes
from the tap water used for mixing the pumice and also from the
skin of the dental personnel handling the prosthesis.
The results show the potential risk of cross contamination in
the laboratory, which can be transmitted to the patient and the
health personnel. The frequency of disposing the used pumice
also determines the contamination levels. The laboratory was
found to be disposing pumice only weekly.
The counts of Gram-negative bacteria such as Acinetobacter,
Pseudomonas, Klebsiella of >105 are found to be opportunistic
pathogenicity. They have been implicated as etiologic agents or
as secondary invaders in human infections. The prevalence of
organisms is of particular concern not only due to their potential
risks to dental patients, but also because of their involvement in
various infectious diseases to which dental professionals may be
at higher risk.
Dentures contaminated with Gram-negative bacilli may serve
as forms to introduce the organisms into the oral cavity where
they could colonize the oropharyngeal region and increase
the risks of pneumonia. Dentures from hospitalized patients
sent to the dental laboratory for repair are a possible source
of Gram-negative contaminant organisms. Milagres Reyes4
suggested that pneumonia caused by Gram-negative bacteria
may be initiated as a result of endogenous aspiration of
oropharyngeal flora or inhalation of bacteria laden aerosols.
This may be an especially significant factor for hospitalized,
debilitated patients, and elderly patients. Williams5 in 1970
found same species recovered in pumice in association with
respiratory infections of hospitalized patients.
Dental pumice and cross contamination … Gali S et al
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Journal of Dental & Oro-facial Research Vol 10 Issue 1 Jan-Jun 2014
less busy clinics and laboratories. Contaminated pumice should
never be left over a weekend to potentially incubate and growing
organisms. Lathe attachments such as rag wheels, stones, and burs
be removed from the lathe after each use and stored in a disinfectant.
Polishing wheels items may be damaged in a liquid chemical
sterilizer.
Lathe shields, air infiltration be used to contain contaminated
splashes, air borne contamination. Care needs to be taken to
clean, disinfect touch and splash surfaces in the laboratory.
Clothing worn during the procedures be covered with disposable
apron when prosthesis or impressions are handled.
Council on Dental Materials; on Dental Therapeutics11 in 1978
recommended that prosthetic devices be thoroughly cleaned
before grinding or polishing procedures that might generate
aerosols. Shields, blower evacuation systems be used when
prosthetic materials are polished or ground.
In order to reduce the aerosol splatter, in the pumice polishing area:
1. Use of air suction with a velocity of at least 200 ft/min at dental
lathe measured in the immediate area of pumice wheel.
2. Use of a plexiglass shield on lathe to intercept splatter particles
and keep away from operators head.
3. Placing pumice lathe such that splatter is directed toward wall.
of the polishing equipment, use of safety glasses, masks and gloves
by personnel using the lathe were recommended.
Summary and Conclusion
Cross contamination is prevalent at every procedure involving
patients and laboratories. The risk of infection from patients
to the professional and vice versa cannot be ruled out. This
study was done to estimate contamination in the pumice
slurry, which can act as a source of cross infection in the
laboratories. The pumice slurry was contaminated with Gramnegative bacilli such as Acinetobacter, Klebsiella, Pseudomonas,
and Gram-positive staphylococci. The results emphasize the
importance of strict infection control during dental polishing
procedures.
References
1. Runnells RR. An overview of infection control in dental
practice. J Prosthet Dent 1988;47:556-69.
2. Verran J, Kossar S, McCord JF. Microbiological study of
selected risk areas in dental technology laboratories. J Dent
1996;24(1-2):77-80.
3. Verran J, Winder C, McCord JF, Maryan CJ. Pumice
slurry as a crossinfection hazard in nonclinical (teaching)
dental technology laboratories. Int J Prosthodont
1997;10(3):283-6.
4. Reyes M. The aerobic Gram-negative bacillary pneumonias.
Med Clin North Am 1980;64:370-83.
5. Williams HN, Falkler WA Jr, Hasler JF, Libonati JP.
The recovery and significance of nonoral opportunistic
pathogenic bacteria in dental laboratory pumice. J Prosthet
Dent 1985;54(5):725-30.
6. Cordes LG, Brink EW, Checko PJ, Lentnek A, Lyons RW,
Hayes PS, et al. A cluster of Acinetobacter pneumonia in
foundry workers. Ann Intern Med 1981;95(6):688-93.
7. Williams HN, Falkler WA Jr, Smith AG, Hasler JF. The
isolation of fungi from laboratory dental pumice. J Prosthet
Dent 1986;56(6):737-40.
8. van Reenen JF. Microbiologic studies on denture stomatitis.
J Prosthet Dent 1973;30(4 Pt 2):493-505.
9. Kahn RC, Lancaster MV, Kate W Jr. The microbiologic
cross-contamination of dental prostheses. J Prosthet Dent
1982;47(5):556-9.
10.Runnells RR. An overview of infection control in dental
practice. J Prosthet Dent 1988;59(5):625-9.
11. Infection Control in the Dental Office. Council on dental
materials and devices. Council on dental therapeutics. J Am
Dent Assoc 1978;97(4):673-7.
12.Setz J, Heeg P. Disinfection of pumice. J Prosthet Dent
1996;76(4):448-50.
He then suggested specific information of the polishing hood
which included a metal enclosure adapted to the front of existing
lathe hoods, provision of ports for hands to enter the polishing
area and a large viewing window with transparent acetate sheets
that can be easily replaced affording a good vision.
Council on Prosthetic Devices and Dental Laboratory
recommended separate pumice pans for new and existing
prosthesis as well as separate polishing burs. A liquid disinfectant
(5 parts sodium hypochlorite to 100 parts distilled water) is used
as a mixing medium in pumice. Addition of 3 parts green soap in
disinfectant solution and suspension of pumice and changing of
pumice daily was recommended.
Setz and Heeg12 used combinations of pumice and disinfectant
comparing with the conventional mixture of pumice and water.
The results revealed use of steribim, which is pumice mixed with
benzoic acid and use of antiseptic octenidine were effective in
reducing the number of bacteria by 99.99% compared to the
conventional procedure.
Verran et al.2 used Virkon 1% solution, a disinfectant that is both
bactericidal and viricidal. Pumice slurry made with Virkon solution
was initially unable to resist the challenge of contaminating
organisms. In the absence of Virkon, they found aerobic Grampositive bacilli; members of the coli-aerogenes predominated with
streptococci viridans. The subsequent contamination originating
primarily from skin, air appears to indicate decreasing ability of
the disinfectant to cope with the microbial load. Therefore, use of
higher concentrations of Virkon, increased frequency of cleansing
Dental pumice and cross contamination … Gali S et al
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