Three species—P. vulgaris, P. mirabilis, and P. penneri—are opportunistic human pathogens. This set includes frequencies for P. vulgaris only. Proteus includes pathogens responsible for many human urinary tract infections. Proteus mirabilis causes wound and urinary tract infections. Most strains of Proteus mirabilis are sensitive to ampicillin and cephalosporins. Proteus vulgaris is not sensitive to these antibiotics. However, this organism is isolated less often in the laboratory and usually only targets immunosuppressed individuals. Proteus vulgaris occurs naturally in the intestines of humans and a wide variety of animals; also manure, soil and polluted waters. Proteus mirabilis, once attached to urinary tract, infects the kidney more commonly than E. coli. Proteus mirabilis are often found as free-living organisms in soil and water.