CHARACTERIZATION OF THE PERVASIVENESS OF MERCURY AND ANTIBIOTIC RESISTANCE DUE TO CO-SELECTION USING POPULATION STUDIES FROM HOLOCENE SPHAGNUM CORES IN MAINE

Increasing incidence of antibiotic resistance in bacteria has highlighted the importance of understanding the factors underlying microbial horizontal gene transfer. The extreme pervasiveness of antibiotic resistance in bacteria indigenous to Maine has led to the hypothesis of co-selection in which selection for mercury resistance indirectly selects for antibiotic resistance.

To establish a pre-antibiotic-era bacterial control group relevant to Maine that does not exhibit these resistances, ~1-m cores were extracted from sphagnum mats at three sites on an east-west transect across the center of the state: Round Pond Bog in Franklin Co., Hamilton Pond Bog in Kennebec Co., and Kanokolus Bog in Waldo Co. ¹⁴C dating indicates that basal samples are up to 2000 ybp; pollen analysis confirms that European disturbance of the landscape is recorded at depths ≤ 35 cm. Population studies utilized samples taken at 5-cm intervals from each of the cores, including modern sphagnum mats. Isolates were identified using 16S rRNA gene sequencing and characterized in terms of mercury-, zinc- and antibiotic-resistance.

Population studies of Round Pond Bog revealed that bacteria taken from core samples dating back 2000 years showed no significant difference in mercury resistance compared to more recent samples (p=0.17). In support of the co-selection hypothesis, these mercury-resistant isolates were also found to be multiply antibiotic resistant, despite originating in core samples dating to the pre-antibiotic era. Due to the geochemical characteristics of sphagnum peat bogs, including heavy metal sequestration and seasonally fluctuating water levels, this experimental system did not yield the expected negative control. What this study did show, however, was that indigenous bacteria acquired antibiotic resistance in environments devoid of antibiotics, thereby supporting the co-selection hypothesis.

Studies on this site and at Hamilton Pond Bog and Kanokolus Bog are continuing.