Rifaximin, a poorly absorbed rifamycin derivative, exhibited time-dependent bactericidal activity and at concentrations as low as 1/32 of the minimum inhibitory concentration (MIC) caused morphological alterations in both susceptible and resistant bacterial strains. Spontaneous rifaximin-resistant clones appeared with an incidence of 2.6×10⁻⁷. The percentage of Escherichia coli cells cured of various plasmids ranged from: 4.5-70% (Flac), 0-18% (pBP507), 7.7-43.8% (plasmid carrying ESBL genes) and 22.4-41.6% (plasmid encoding toxin from ETEC mex264). 8.4-18.2 and <0.1- 18% of Staphylococcus aureus cells were cured (plasmid-mediated penicillinase), 9.5-58.6% of Morganella morganii (ESBL), 10.6-47.1% Citrobacter freundii(ESBL), 2.3-38.7% of Proteus mirabilis (ESBL) and 14.3-66.6% of Klebsiella pneumoniae (ESBL). Rifaximin reduced plasmid transfer from donor to recipient strains by >99%. The MIC of ceftazidime was reduced (2-4 dilutions) in the presence of rifaximin (0.5xMIC) in ESBL producing strains. Rifaximin lowered the viability and virulence of the bacteria even though they developed resistance to the compound.

In conclusion, the present findings add new features to the microbiological characteristics of rifaximin and suggest that if in vivo pathogens are exposed to sub-MICs of the drug, not only are their physiological functions compromised, but gene virulence and antibiotic resistance are not fully expressed.