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A genus of common Gram-negative, facultatively-anaerobic, rod-shaped bacteria of the family Enterobacteriaceae. Several strains of the these bacteria are pathogenic and cause opportunistic infections in immunocompromised (usually hospitalized) hosts and in those who are on mechanical ventilation. The urinary and respiratory tract are the most common sites of infection. It is also a fecal coliform, along with Escherichia. Two clinically-important species from this genus are E. aerogenes and E. cloacae.