Heterotrophic plate count (HPC) bacteria are naturally present in all aqueous environments. These bacteria undergo multiplication cycles in drinking water, especially in closed containers (bottled water) or in tap water when chlorine levels are dissipated, such as in dead ends in water mains or household plumbing. A study was undertaken to estimate health risk from these naturally occurring bacteria by the determination of cytotoxicity and invasiveness in a human enterocyte cell line. HPC bacteria were isolated from bottled and tap water samples by enumerating them under physical and chemical conditions analogous to human physiology. All HPC bacteria were examined at both log and lag phase of their growth cycles. Bacterial broth supernatant fluids were also tested to serve as critical negative controls. Naturally occurring HPC bacteria demonstrated low invasiveness and cytotoxicity with more than 95% of isolates showing equivalency to broth supernatant fluid. When showing either invasiveness or cytotoxicity, only a small number of cells from the culture were positive. Of those that were positive, log phase HPC bacteria were significantly more cytotoxic and invasive than those from stationary phase. Bacterial broth controls demonstrated varied, but often marked, cytotoxicity.