The Resistance and Adaptation of Selected Oral Bacteria to Mercury and Its Impact on Their Growth

Abstract

Selected strains of oral Streptococcus and Actinomyces have been tested for their ability to grow in the presence of mercury. Strains were tested for growth on a semi-defined medium with low mercury-binding characteristics. Sensitivities were initially measured on agar plates, and subsequently, selected strains were grown in broth so that the impact of mercury on the growth characteristics could be determined. Streptococci were more resistant to mercury (5 μg/mL to 40 μg/mL) than Actinomyces (< 5 μg/ mL to 30 μg/mL). The most resistant streptococci included S. mitis biovar 1, S. salivarius, S. sobrinus, and one strain of S. mutans, all of which grew on agar with 40 μg/mL of mercury. Two other S. mutans strains were more sensitive, being inhibited by 10 and 20 pg/mL mercury. The most resistant Actinomyces was A. naeslundii genospecies 1 (ATCC12104), which grew on medium with 30 ug/mL mercury; two strains of Actinomyces were completely inhibited by 5 ug/mL. Mercury caused increased lag times and reduced cell density in broth cultures. Enrichment cultures of samples of human dental plaque showed that streptococci were the most resistant organisms that could be cultured on the medium and that these strains could adapt to relatively high mercury concentrations. S. oralis and S. mitis biovar 1 were the most resistant organisms isolated from enriched cultures, growing in broth media with 65 ug/mL mercury. Mercury was bound to cell walls and cell cytoplasm of streptococci grown in the presence of mercury. The levels of resistance to mercury of most of the organisms tested indicate that they would be able to grow in the environment of human dental plaque containing mercury.

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