the amount of microorganisms in the oral fluid ( saliva ) ranges from 4 millions to 5 billions per 1 milliliter , and in dental plaque between 10 and 1000 billions per 1 gram .
their resistance to non - specific oral protective factors and environmental factors ( e.g. , antiseptic solutions ) depends on the cellular membrane , a thick layer of mucus , a capsule , or other biological characteristics . for this reason ,
standard bacterial cultures with differing biological characteristics were selected for this study : staphylococcus aureus are gram - positive cocci found in the nostrils of up to 60% of humans ; they form microcapsules , and are resistant to high temperatures ( up to 70c ) , disinfectant , and antiseptic solutions , and cause suppurative inflammatory processes .
staphylococcus aureus is an important pathogen because of its virulence , antimicrobial resistance , and association with various diseases , including fatal systemic infections , intoxication , cutaneous infectious , and opportunistic diseases .
klebsiella pneumoniae are capsule - forming bacteria found in the respiratory tract and are capable of causing atypical pneumonia .
candida albicans is a fungus and is part of the normal microflora of the oral cavity , but it can cause an opportunistic infection in immuno - depressed subjects .
escherichia coli is part of the normal microflora of the large intestine ; however , due to poor hygiene it can enter the oral cavity and cause suppurative inflammatory processes .
the procedures that a prosthodontist performs may cause damage to the patient s oral mucosa and gingiva .
the saliva mixes with blood . during the manufacturing of the dental impressions , blood and saliva
enter the setting impression material , and it becomes contaminated with bacteria and viruses [ 47 ] .
for this reason , according to general hygiene norms , such impressions should be disinfected ; yet disinfection does not always yield the desirable results . when producing gypsum casts using non - disinfected or incompletely disinfected dental impressions , the microorganisms travel from the surface of the impression material into the cast .
the infected material then is taken to the manufacturing facilities and may pose threat to dental technicians .
dental casts are subject to several forms of contamination , ranging from direct contact with a patient s saliva , to procedures involving measurement , planning , manufacture , laboratory shipping , and storage .
dental technicians touch the impressions or the casts with their hands , and their skin is frequently damaged due to their work .
when cutting the dental casts , bacteria , fungi , or particles of the disinfectant may travel with the plaster dust from the material to the respiratory tract of personnel , causing infections , and may also accumulate on the clothing or open surfaces in the room .
the aim of study was to evaluate the survival of staphylococcus aureus , klebsiella pneumoniae , and escherichia coli bacteria , and candida albicans fungus in non - disinfected dental casts .
the results of the study could be used to determine how long dental casts may pose threat to the health of the personnel and the patients .
perhaps disinfection of dental impressions would be sufficient , skipping additional disinfection of the dental casts in order to reduce exposure of the personnel to possible allergens  the residues of disinfectant solutions in the plaster .
although extensive research has been carried out , there are no published studies in which similar issues were addressed .
during this experimental study we investigated the survival of the selected bacteria and fungi in dental casts .
the study was conducted using standard bacterial ( staphylococcus aureus atcc 25928 , klebsiella pneumoniae atcc 13883 , and escherichia coli atcc 25922 ) and fungal ( candida albicans atcc 60193 ) cultures . saline suspensions ( mcfarland turbidity standard 0.5 )
were prepared using standard bacterial cultures cultivated in trypticase soy agar for 24 hours at 3537c , and candida albicans culture cultivated in sabouraud agar for 3 days .
we evaluated and compared changes in the number of the microorganisms in gypsum casts at different time intervals .
gypsum casts ( an equivalent to dental cast ) were manufactured in a sterile laminar - flow hood under sterile conditions , using standard alabaster plaster class 2  moldabaster 
the plaster was sterilized ; therefore , there was no need to form a control group .
the dental casts were prepared by pouring 65 g sterile plaster powder , 5 ml sterile distilled water , and 1 ml of a specific bacterial or fungal suspension ( mcfarland turbidity standard 0.5 ) into a sterile mixing container .
a sterile spatula was used to stir the contents until an integral mass was formed , which was subsequently placed into a sterile closed container . during the test period
the closed containers with the samples were placed on a dental lab shelf to simulate casual cast storage conditions .
the number of the aforementioned standard microorganisms per 1 g of the substance was evaluated after 1 , 2 , 24 , 48 , 72 , 96 , and 120 hours .
sterile clamps were used in the making of each sample . the sample  a piece of the gypsum cast of a known weight ( ca .
1.0 g )  was ground to powder in a sterile mortar , and then placed into a sterile glass with 5 ml of sterile saline solution .
the suspension of plaster powder was diluted at the ratio of 1:5 , 1:25 , 1:125 , and 1:625 .
one ml of the suspension from each test tube with different dilution ratios was poured into 10 ml of 45c trypticase soy agar ( or sabouraud agar , in the case of candida albicans ) in 2 petri dishes that were then stored at 3537c ( in case of candida albicans , sabouraud agar was stored at 2025c ) .
after the cultivation of the indicated duration , colonies of the respective bacterium or fungus in the 2 petri dishes were evaluated , calculating the mean number of the colonies ; multiplying this number by the degree of dilution revealed the number of bacteria or fungus per 1 g of plaster .
the data were considered to be statistically significant if the probability of an error was p<0.05 .
the study showed that k. pneumoniae survived the longest in gypsum casts  up to 4 days  compared to the other studied microorganisms ( figure 1 ) .
a statistically significant reduction in the number of k. pneumoniae ( from 837911935 to 44120845 ) in gypsum casts was observed after 1 day ( p<0.05 ) .
the number of k. pneumoniae colonies was also the highest , compared to other bacteria .
after the first and the second hour in the gypsum cast , the number of bacteria remained nearly unchanged .
it was only after 2 days that a statistically significant difference was detected ( a reduction from 59154676 to 9285102 , p<0.05 ) .
gram - positive bacteria ( staphylococci ) remained viable for up to 3 days in the gypsum casts , presumably due to their cell membrane composed of a thick peptidoglycan layer , a microcapsule ( a thin layer of mucus over the cell membrane ) , and high resistance to environmental factors ( figure 2 ) .
no gram - negative bacteria ( e.g. , e. coli ) were found in gypsum casts after 2 days , and a statistically significant reduction in their numbers was observed just after 2 hours ( from 19483336 to 470574 , p<0.05 ) ( figure 3 ) .
microorganisms with a eukaryote structure ( e.g. , the fungus c. albicans ) died within 2 days , and a statistically significant reduction was observed after 1 day ( from 31 3662759 to 12 2911234 , p<0.05 ) ( figure 4 ) .
figure 5 presents the comparison of the standard microorganisms amounts ( the bacteria and the fungi ) at different time intervals , and the quantitative reduction in their numbers .
the quantitative evaluation of the viability of standard microorganisms in gypsum casts showed that physical - chemical characteristics of gypsum casts have different effects on the survival of the standard microorganisms .
structural and physiological characteristics of k. pneumoniae and s. aureus affect their survival ( their quantity ) with respect to that of e. coli and c. albicans .
the findings of the study showed that the microorganisms did not multiply in the gypsum casts ; instead , their numbers significantly dropped .
the study was conducted using the most common oral bacteria and fungus : staphylococcus aureus , klebsiella pneumoniae , candida albicans , and escherichia coli .
studies performed in japan showed that all the microorganisms used in our study  staphylococcus aureus , klebsiella pneumoniae , and escherichia coli  were detected in the oral cavities of 13.6% of patients with no health complaints . according to other authors ,
staphylococcus aureus is detected in the oral cavities of 15.8% of patients , and candida albicans in 63.0% of patients .
the findings of our study showed that the duration of the survival of microorganisms in gypsum casts varied depending on the types of the studied cultures .
these bacteria have a thick layer of peptidoglycan and a microcapsule , which is a mucous layer of polysaccharide origin .
the gram - negative bacteria klebsiella pneumoniae and escherichia coli differ from staphylococci in their cell membrane structure
. however , klebsiella bacteria have a thick mucous layer of polysaccharide origin  a capsule  that ensured their longest survival in gypsum casts compared to other microorganisms .
escherichia coli have no capsule and thus are more susceptible to unfavorable environmental conditions , thus these bacteria survived in gypsum casts for only 12 days .
the fungus candida albicans , although it has a eukaryote cell structure , survived in gypsum casts longer than escherichia coli did because fungi have a chitin membrane .
the oral cavity may contain pathogenic as well as relatively pathogenic microorganisms that can be transferred into the environment through impressions . in patients with damaged oral mucosa , staphylococcus aureus , klebsiella pneumoniae , and escherichia coli may cause suppurative inflammatory processes .
escherichia coli may cause not only a local inflammatory process , but bacteremia as well .
candida albicans , as part of the normal oral microflora , is found in 4060% of individuals ; however , this fungus can cause candidiasis in immuno - compromised states or in cases of antibiotic overuse .
most commonly , alginate and silicone dental impressions are used in clinical practice . due to their composition , structure , and the hydrophilic setting mechanism ,
alginate impressions are easily infected by oral microorganisms . a more monolithic structure of the silicone impression mass and its non - hydrophilic setting mechanism significantly reduce the attachment of microorganisms to the impression surface .
selecting a suitable impression mass may reduce the spreading of the infective agents as early as the first stages of prosthesis manufacturing .
alginate impressions are more frequently contaminated with oral bacteria and fungi , compared to silicone impressions , and they are also more difficult to disinfect .
alginate impressions pose a significantly higher threat of contaminating dental casts with bacteria and fungi , compared to silicone impressions , thus silicone impressions are safer with respect to the transmission of the infective agents .
previous studies have shown that dental impressions delivered at dental laboratories contain bacteria irrespectively of whether they were disinfected or washed under running water .
this suggests that dental casts produced using well - disinfected impressions require no additional disinfection .
if the dental casts are stored under normal conditions of a dental technician s office , the number of microorganisms should change more rapidly than it did during our study due to frequently changing temperature and humidity , and because of the plaster ph of 6.1 .
an additional disinfection of dental casts would not yield significant results , compared to the risk that the dental technicians would face because of exposure to a potential allergen  the disinfectant on the surface of the dental cast .
although studies have shown that the effect of the disinfectant on the precision of the dental casts is not significant , the risk remains that the disinfectant may alter the physical characteristics of the dental cast surface , making it more porous , brittle , and more susceptible to wear and tear .
rinsing of the oral cavity with an antiseptic solution prior to odontological procedures may reduce the presence of microorganisms in the oral cavity .
the most popular oral antiseptic , chlorhexidine , is not always effective against fungi or bacterial spores .
thus , mouth rinsing with an oral antiseptic prior to taking impressions followed by full disinfection of the impressions would reliably reduce the contamination of dental casts with microorganisms . as our results show
if the impression is disinfected and pre - impression mouth rinsing is used consistently , the dental casts would pose less threat because of the poor survivability of bacteria and fungi in gypsum .
gram - positive bacteria staphylococcus aureus and gram - negative bacteria klebsiella pneumoniae demonstrated the longest survival in dental casts ( up to 4 days ) , compared to escherichia coli and candida albicans , which remained viable during the first 2 days ) .
dental casts are not a suitable medium for the multiplication of the colonizing microorganisms . individual protective measures ( gloves , protective goggles , and respiratory protection measures ) should be used in order to avoid possible contamination of dental impressions and casts with microorganisms , and to prevent their transmission and spread in environments such as dental laboratories .
 backgroundthis study evaluated the survival of the most prevalent oral bacteria and fungi ( staphylococcus aureus , klebsiella pneumoniae , escherichia coli , and candida albicans ) in dental casts , and compared changes in the amounts of these microorganisms at different time intervals to determine how long dental casts may pose threat to the health of dental personnel and patients.material/methodswhen manufacturing the casts , regular water was replaced with sterile distilled water , where suspensions of the studied bacteria or the fungus at certain concentrations were prepared . when the dental casts were fully set ( solidified ) , plaster shavings were examined immediately after the contact of the studied microorganism with the plaster , as well as after 1 , 2 , 24 , 48 , 72 , 96 , and 120 hours . following that 
 , we measured how the amount of the studied bacteria and fungi in 1 gram of the plaster changed within the studied period of time.resultsklebsiella pneumoniae survived in plaster for up to 4 days , and the reduction in the number of these bacteria became statistically significant after 1 day ( p<0.05 ) . 
 staphylococcus aureus remained viable in plaster for up to 4 days , and the number of these bacteria dropped after 1 day ( p<0.05 ) . 
 escherichia coli disappeared after 2 days , and a reduction was already observed after 2 hours ( p<0.05 ) . 
 candida albicans in plaster models died within 2 days , and a reduction in their number was observed after 1 day ( p<0.05).conclusionsthe microorganisms did not multiply in the gypsum casts and their number significantly dropped instead of increasing . 
Background
Material and Methods
Results
Discussion
Conclusions