Research Update

Coastal Beaches as Environmental Reservoirs of Virulence and Antibiotic Resistance Genes

When officials issue beach advisories or close recreational beaches for swimming, these decisions are based on high bacteria levels measured in water. However, another important aspect of beach health is shoreline sand.

Recent studies at several Great Lakes beaches have confirmed that high densities of the bacteria E. coli persist in sand throughout the beach season. Preliminary tests suggest that the E. coli community of Lake Huron beaches is genetically diverse and that the rate of genetic exchange among E. coli is high.

This reservoir of fecal bacteria may be a potential contact point between bacteria and the public. With Sea Grant funding, investigators at Central Michigan University are using a combination of laboratory techniques to develop detection methods for pathogenic genes in beach sand.

“When high cell densities bring bacteria in close physical proximity to one another, the bacteria can exchange genetic information readily,” says biologist Elizabeth Alm, leading to the emergence of new disease causing or antibiotic-resistant strains of bacteria.

Researchers collected samples of beach sand at several Lake Huron beaches in St. Clair and Macomb counties in 2005. In lab analyses, researchers have detected genes associated with pathogenic strains of E. coli and another bacteria, Shigella, from this sand.

In previous studies, researchers found that approximately one third of E. coli isolated from Lake Huron beaches are resistant to commonly used antibiotics; in sand microcosms incubated under simulated beach conditions, the E. coli were able to exchange genes encoding antibiotic resistance.