Citrobacter koseri: A species of gram-negative enterobacteria found in WATER; SEWAGE; SOIL; and FOOD. It can be present in any clinical specimen as an opportunistic pathogen.Citrobacter: A genus of gram-negative, rod-shaped enterobacteria that can use citrate as the sole source of carbon.Escherichia: A genus of gram-negative, facultatively anaerobic, rod-shaped bacteria whose organisms occur in the lower part of the intestine of warm-blooded animals. The species are either nonpathogenic or opportunistic pathogens.Enterobacteriaceae Infections: Infections with bacteria of the family ENTEROBACTERIACEAE.Psoas Abscess: Abscess of the PSOAS MUSCLES resulting usually from disease of the lumbar vertebrae, with the pus descending into the muscle sheath. The infection is most commonly tuberculous or staphylococcal.Citrobacter freundii: A species of gram-negative, facultatively anaerobic, rod-shaped bacteria found in humans and other animals including MAMMALS; BIRDS; REPTILES; and AMPHIBIANS. It has also been isolated from SOIL and WATER as well as from clinical specimens such as URINE; THROAT; SPUTUM; BLOOD; and wound swabs as an opportunistic pathogen.Citrobacter rodentium: A species of gram-negative bacteria in the genus CITROBACTER, family ENTEROBACTERIACEAE. As an important pathogen of laboratory mice, it serves as a model for investigating epithelial hyperproliferation and tumor promotion. It was previously considered a strain of CITROBACTER FREUNDII.