Before revision:  Nonetheless, it is still a controversial topic why cells secrete even essential metabolites so often , in fact, even under isolation conditions, microbial cells secrete a variety of metabolites, including those essential for their growth.
After revision:  Nonetheless, it is still a controversial topic why cells secrete even essential metabolites so often , in fact, even under isolation conditions, microbial cells secrete various metabolites, including those essential for their growth.