From the FDA Drug Label
In vitro serial dilution tests have shown that the spectrum of antibacterial activity of sulfamethoxazole and trimethoprim includes the common urinary tract pathogens with the exception of Pseudomonas aeruginosa The following organisms are usually susceptible: Escherichia coli, Klebsiella species, Enterobacter species, Morganella morganii, Proteus mirabilis and indole-positive Proteus species including Proteus vulgaris The usual spectrum of antimicrobial activity of sulfamethoxazole and trimethoprim includes the following bacterial pathogens isolated from middle ear exudate and from bronchial secretions: Haemophilus influenzae, including ampicillin-resistant strains, and Streptococcus pneumoniae. Shigella flexneri and Shigella sonnei are usually susceptible
The FDA drug label does not mention Citrobacter koseri as a susceptible organism to Bactrim (trimethoprim-sulfamethoxazole) 1.
From the Research
Bactrim (trimethoprim-sulfamethoxazole) is not the first-line treatment for Citrobacter koseri infections, and its effectiveness is not well-established in recent studies. The most recent and highest quality study available is from 2014 2, which discusses the optimum management of Citrobacter koseri infection, but does not specifically recommend Bactrim as a treatment option. Instead, it mentions that various types of antibiotics, including aminoglycosides, carbapenems, cephalosporins, chloramphenicol, and quinolones, are used for the treatment of C. koseri infections.
Some older studies, such as the one from 1988 3, suggest that trimethoprim-sulfamethoxazole has a broad spectrum of antibacterial activity against both gram-positive and gram-negative organisms, including Citrobacter. However, these studies are outdated, and more recent evidence is needed to support the use of Bactrim for Citrobacter koseri infections.
Other studies, such as the one from 2001 4, focus on the treatment of neonatal Citrobacter koseri meningitis with carbapenems, and do not mention Bactrim as a treatment option. Another study from 2016 5 discusses the in vitro bactericidal activity of trimethoprim-sulfamethoxazole against carbapenem-resistant Acinetobacter baumannii, but this is not directly relevant to the treatment of Citrobacter koseri infections.
In summary, while Bactrim may have some activity against Citrobacter koseri, its effectiveness is not well-established in recent studies, and it is not recommended as a first-line treatment for these infections. The choice of antibiotic should be based on antimicrobial susceptibility testing and the severity of the infection, and other antibiotics such as aminoglycosides, carbapenems, cephalosporins, chloramphenicol, and quinolones may be more effective. Patients should be monitored for potential side effects, and dosage adjustments may be necessary in cases of impaired kidney function.