title: In vitro susceptibility of Clostridium difficile to rifaximin and rifampin in 359 consecutive isolates at a university hospital in Houston, Texas.
abstract: OBJECTIVE
  This was an in vitro study to analyse the susceptibility of Clostridium difficile isolates to rifampin and rifaximin.
  
  
  METHODS
  Stool samples from patients who had nosocomial diarrhoea and C difficile toxin B at a university hospital between August 2006 and December 2007 were cultured for C difficile. Susceptibility of C difficile isolates to rifaximin and rifampin was determined by agar dilution and E strips, respectively. C difficile isolates were analysed via PCR for genes encoding toxins A and B, for binary toxin (BT), and for partial deletions of the tcdC gene (tcdC-del).
  
  
  RESULTS
  Rifaximin exhibited high-level activity against 359 C difficile isolates, with MIC(50) <0.01 microg/ml and MIC(90) 0.25 microg/ml; rifampin had MIC(50) <0.002 microg/ml and MIC(90) 4 microg/ml. Among isolates analysed, 55 (15%) were positive for BT and tcdC-del. 28 (8% of 359) isolates were resistant to rifampin (> or = 32 microg/ml), of which 6 (2% of 359) were resistant to rifaximin and rifampin with MIC values > or = 32 microg/ml. 2 of the 28 isolates resistant to rifampin were A(+)/B(+)/BT(+)/tcdC-del(+), 5 were A(+)/B(+)/BT(-)/tcdC-del(+), 4 were A(+)/B(+)/BT(+)/tcdC-del(-), 13 were A(+)/B(+)/BT(-)/tcdC-del(-), and 4 had no detectable toxin genes. Of the 11 isolates resistant to rifaximin alone, 1 was A(+)/B(+)/BT(-)/tcdC-del(+), 2 were A(+)/B(+)/BT(+)/tcdC-del(-), 6 were A(+)/B(+)/BT(-)/tcdC-del(-), and 2 had no detectable toxin genes.
  
  
  CONCLUSIONS
  The study demonstrates that rifaximin has high-level activity against C difficile in vitro. Determination of resistance to rifampin by E strip did not predict rifaximin resistance.
authors.affiliation: Center for Infectious Disease, University of Texas School of Public Health, Houston, Texas 77030, USA. zhi-dong.jiang@sph.uth.tmc.edu
authors.forename: Z-D
authors.initials: ZD
authors.lastname: Jiang
authors.forename: H L
authors.initials: HL
authors.lastname: DuPont
authors.forename: M
authors.initials: M
authors.lastname: La Rocco
authors.forename: K W
authors.initials: KW
authors.lastname: Garey
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics; D001427:Bacterial Toxins / Q000235:genetics; D016360:Clostridioides difficile / Q000737:chemistry / Q000187:drug effects* / Q000235:genetics; D003967:Diarrhea / Q000382:microbiology; D004761:Enterocolitis, Pseudomembranous / Q000382:microbiology; D004768:Enterotoxins / Q000235:genetics; D005798:Genes, Bacterial; D006785:Hospitals, University; D006801:Humans; D008826:Microbial Sensitivity Tests; D012293:Rifampin / Q000494:pharmacology*; D012294:Rifamycins / Q000494:pharmacology*; D000078262:Rifaximin; D013781:Texas
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D001427:Bacterial Toxins; D004768:Enterotoxins; D012294:Rifamycins; C041348:tcdA protein, Clostridium difficile; C031815:toxB protein, Clostridium difficile; D000078262:Rifaximin; D012293:Rifampin