Microbial contamination in four brands of irreversible hydrocolloid impression materials

A previous investigation showed unopened irreversible hydrocolloid impression material to be contaminated with viable microorganisms. This study tested and compared four brands of commercial irreversible hydrocolloid impression material in factory-sealed containers for the presence of viable microorganisms. Twenty-four measured samples of each brand were taken from previously unopened containers using a sterile technique. The samples were placed on chocolate agar plates or in thioglycolate broth tubes and were incubated along with appropriate parallel controls. After incubation, colonies were enumerated, stained with Gram's stain and identified using standard microbiologic methods. The four brands contained viable organisms in 50% to 100% of the samples incubated of agar media, and in 12% to 67% of the samples incubated in thioglycolate media. Samples from the top and middle portions of the containers had approximately equal contamination frequencies. The concentration of organisms varied from 12 to 82 colony-formed units per gram of contaminated sample. Most organisms isolated were common environmental contaminants. These samples contained viable microorganisms which, during routine use, may present a hazard to immunocompromised patients.