A gram-negative bacterium that occurs in the environment, primarily in the soil, water, and other moist places. It is an opportunistic pathogen that causes various diseases especially in people with weakened immune systems. These include urinary tract infections, gastrointestinal infections, respiratory system infections, soft tissue infections, bone and joint infections, dermatitis, bacteremia, and a range of systemic diseases, mainly in patients with severe burns and in immunosuppressed AIDS and cancer patients.