Antibiotic Resistance in Environmental Citrobacter freundii

Antibiotic resistance is an emerging concern in the clinical and environmental settings. Citrobacter freundii is a clinically relevant opportunistic gram-negative bacterium from the family Enterobacteriaceae that causes a variety of illnesses in immunocompromised patients. C. freundii related illnesses range from respiratory and urinary tract infection to diarrhea and sepsis. This species is commonly found in the gastrointestinal tract of humans but can also be found in water, soil, and food. The aim of this study is to determine the antibiotic resistance profiles of C. freundii isolated from environmental waters and to assess the emergence of any multi-drug resistant C. freundii obtained. Samples were collected from multiple water locations in Orange County, California. The water samples were plated on selective media having antibiotics. The bacterial isolates were identified using matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF). Antimicrobial susceptibility testing was determined using the disc diffusion method according to the Clinical and Laboratory Standards Institute (CLSI). Of the 22 Citrobacter freundii isolates that were collected, 23% were resistant to cefotaxime, 86% to cefoxitin, 23% to ceftazidime, 43% to chloramphenicol, 23% to ciprofloxacin, 32% to gentamicin, 59% to sulfisoxazole, 45% to tetracycline, 41% were resistant to trimethoprim, and 27% tested positive for extended spectrum β-Lactamases according to the Phenotypic Confirmatory Test. 12 of the C. freundii isolates were considered to be multidrug resistant based on having resistance to three or more antibiotics of different classes. This is the first reported study of antibiotic resistance profiles of C. freundii collected from environmental waters in the United States. The waters where the C. freundii were collected from may serve as a reservoir for antibiotic resistance. The emergence of multi-drug resistant C. freundii in the environment is an increasing threat to the treatment of bacterial infections. Further research needs to be conducted to elucidate the genetic basis of antibiotic resistance.