ObjectiveIdentification and analysis, animal regression test, and drug susceptibility study were conducted on a pathogenic strain causing diarrhea in cynomolgus monkeys, aiming to provide a practical basis for the clinical treatment of Citrobacter freundii. MethodsFresh fecal samples were collected from nine diarrheal cynomolgus monkeys and streaked on Salmonella-Shigella (SS) medium, LB medium, and Columbia blood agar medium, and incubated at 37 ℃ for 24 h. Subsequently, the isolated strain was identified by colony morphology observation, Gram staining, biochemical tests and 16S rRNA gene sequencing, and PCR was used to detect its major virulence genes. After propagation, the isolate was intragastrically administered to healthy cynomolgus monkeys and C57BL/6 mice for animal regression tests to evaluate its pathogenicity. Finally, the disk-diffusion method was used to detect the antimicrobial susceptibility of the isolate. ResultsA single bacterial strain, designated MF071743, was isolated from 9 fecal samples collected from diarrheal cynomolgus monkeys. The isolate formed pink, smooth, round colonies on SS medium; translucent, smooth, moist, shiny colonies with neat edges on LB medium; and smooth, moist, off-white colonies on Columbia blood agar medium. Gram staining revealed that the isolate was a Gram-negative, spore-free short bacillus. Biochemical tests showed that the isolate was positive for motility test, mannitol test, hydrogen sulfide test, methyl red test, citrate utilization test, gas production from glucose test, raffinose test, sorbitol test, and D-xylose test, but negative for phenylalanine test, gluconate test, indole test, Voges Proskauer (VP) test, urease test, lysine test, ornithine test, and adonitol test. The results of 16S rRNA gene sequencing showed that the gene sequence similarity between strain MF071743 and Citrobacter freundii was 99.0%. PCR results showed that this strain carries genes encoding the urease accessory proteins UreD, UreE, and UreF. Animal intragastric administration assays demonstrated that the strain caused loose stools in cynomolgus monkeys, and all C57BL/6 mice died within 72 h. Drug susceptibility test results indicated that the isolate was sensitive to 16 antibiotics, including ceftriaxone, amikacin, gentamicin, ceftazidime, and levofloxacin, while it exhibited resistance to 5 antibiotics, including ampicillin, cefazolin, vancomycin, erythromycin, and cephalexin. ConclusionA strain of Citrobacter freundii with certain drug resistance was isolated from the feces of diarrheal cynomolgus monkeys. The results of this study provide a reference for the diagnosis and treatment of gastrointestinal diseases in captive laboratory monkeys.