In 1993, a study from the Universities of Calgary and Georgia reported a high prevalence of mercury resistance in human fecal flora with resistance to two or more antibiotics . The authors also studied mercury- and antibiotic-resistance in six adult monkeys before amalgam fillings were placed, while they were in place and after replacement with glass ionomer fillings in four of the monkeys . For five weeks after the placement of amalgams and five weeks after replacement with glass ionomers, the monkeys showed an increased proportion of mercury- and ampicillin-resistant organisms in the oral and intestinal floras. There was no increase in tetracycline-resistant organisms. The authors conclude that amalgam fillings may be a factor in worldwide antibiotic resistance and that human studies should be conducted. But it was impossible to correlate the presence or number of amalgam fillings in the humans with mercury- or antibiotic-resistance since the amalgam status was not reported [3,4] Shearer called the reported resistance to ampicillin in the monkeys as "inconsistent (and unexplained)." 
Other studies have failed to show a significant correlation between mercury in amalgam fillings and antibiotic resistance. In a 1996 human study comparing ten patients with a mean number of 19 amalgam filling surfaces had their amalgam fillings removed . Ten patients without amalgam fillings served as a control group. There were no significant differences in mercury or antibiotic resistance between the amalgam and nonamalgam group detected in the oral microflora, but patients in the amalgam group showed a small but statistically insignificant increase in mercury- and antibiotic resistant organisms in the intestinal flora. There was a significant correlation between the prevalence of mercury- and multiple antibiotic-resistance in the intestinal flora. The authors conclude that amalgam fillings do not seem to be a significant factor in the development of mercury- or antibiotic-resistant organisms in the oral and intestinal flora, but do not rule out significant effects in certain groups of patients, e.g., immunocompromised patients.
Mercury and multiple antibioticresistance was studied in 1995 in 191 people in three groups: those with dental amalgam, those that had their amalgam fillings removed, and those with no known exposure to amalgams . There were no significant differences between the groups in mercury- or antibiotic-resistance in the intestinal gram-negative aerobic microflora. In a similar study published in the year 2000 by the same group of investigators in 209 people, the minimal inhibitory concentrations of cefuroxime, penicillin, tetracycline, or mercury to 839 strains of Streptococcus mutans were not different in any of the groups. They concluded, "mercury derived from dental amalgam fillings did not select resistant S. mutans strains." 
Dr. Wahl practices dentistry in Wilmington, Delaware. This article was originally published in Quintessence International 32:696-710, 2001, and is reproduced here with the kind permission permission of Quintessence Publishing Co. The author thanks Drs. J. Rodway Mackert, Ivar A. Mjör, and Fred Eichmiller for reading the manuscript and offering several helpful suggestions.