Pike, R.; (2005) Isolation and characterisation of mercury and antibiotic-resistant oral bacteria. Doctoral thesis , University of London.

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Abstract

As very little information was available on mercury sensitivity testing, the first aim was to determine the most suitable agar and concentration of mercuric chloride to use in this project. The primary objective of the study was to determine whether mercury released from amalgam fillings could increase the prevalence of mercury-resistant bacteria in the oral flora of children. This was achieved through cross-sectional and longitudinal studies. The second aim was to determine whether changes in mercury resistance correlated with changes in the incidence of antibiotic resistance. The final aim was to determine whether individual mercury-resistant isolates contained the merA gene. In the cross-sectional study, saliva and plaque samples were collected from patients with and without amalgam fillings. No significant differences in the proportion of mercury-resistant bacteria were detected between the two groups. One hundred and thirty nine mercury-resistant bacteria were isolated and 41% (with amalgam) and 33% (without amalgam) of these were also resistant to one or more antibiotics. Resistance to tetracycline was most common. Sixteen patients were enrolled into the longitudinal study. The proportions of mercury-and antibiotic-resistant bacteria were determined on 3 separate occasions (2 pre-amalgam and 1 post-amalgam). There was not a statistically significant change in the incidence of mercury- or antibiotic-resistant bacteria during the month after the installation of the amalgam fillings. However, a linear association between the number of surfaces and proportion of mercury-resistant bacteria was observed. Eighty eight mercury-resistant bacteria were isolated and 27% (pre-amalgam) and 40% (post- amalgam) of these were resistant to one or more antibiotics. Resistance to erythromycin was most common. One hundred and thirty two mercury-resistant bacteria were screened for the merA gene using PCR and 2 sets of primers. Sixty three percent of the streptococci were found to contain the merA gene. Coagulase-negative staphlyococci, Rothia dentocariosa and Neisseria species contained the merA gene, while Staphylococcus aureus and Pseudomonas stutzeri did not. All sequenced amplicons were found to be up to 95% identical to the Bacillus cereus RC607 merA gene. The results of this study have failed to demonstrate any definitive link between the presence of mercury amalgam in teeth and the presence, or proportion, of mercury- or antibiotic-resistant bacteria in the oral cavity of children.

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