title: Tannic Acid as a Potential Modulator of Norfloxacin Resistance in Staphylococcus Aureus Overexpressing norA.
abstract: BACKGROUND
  Tannins have shown inhibitory effects against pathogenic bacteria, and these properties make tannins potential modifying agents in bacterial resistance.
  
  
  METHODS
  The minimum inhibitory concentration (MIC) of tannic acid (TA), gallic acid (GA) and norfloxacin (Nor) against Staphylococcus aureus SA-1119 (NorA-effluxing strain) was determined using broth microdilution tests. To assess the modulation of antibiotic resistance, the MIC of Nor was determined in growth media with or without TA or GA at a subinhibitory concentration (1/4 MIC). The checkerboard method was performed to obtain the fractional inhibitory concentration index (FICI) for the combined application of TA and Nor.
  
  
  RESULTS
  TA displayed a weak inhibitory effect (MIC 512 μg/ml) against S. aureus SA-1119, while no inhibitory effect was displayed by GA (MIC >512 μg/ml). However, when TA was tested at a subinhibitory concentration in combination with Nor, the MIC of Nor against S. aureus SA-1119 decreased from 128 to 4 μg/ml (32-fold); this effect was not observed for GA. In the checkerboard assay, the MIC of TA and Nor decreased from 512 to 128 μg/ml (4-fold) and from 128 to 8 μg/ml (16-fold), respectively. The combination of TA and Nor presented an FICI as low as 0.31, which indicates a synergistic interaction.
  
  
  CONCLUSIONS
  TA is a potential agent for increasing the clinical efficacy of Nor to control resistant S. aureus.
authors.forename: Helena Taina
authors.initials: HT
authors.lastname: Diniz-Silva
authors.forename: Isis Caroline da Silva
authors.initials: IC
authors.lastname: Cirino
authors.forename: Vivyanne Dos Santos
authors.initials: Vdos S
authors.lastname: Falcão-Silva
authors.forename: Marciane
authors.initials: M
authors.lastname: Magnani
authors.forename: Evandro Leite
authors.initials: EL
authors.lastname: de Souza
authors.forename: José P
authors.initials: JP
authors.lastname: Siqueira-Júnior
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism*; D024881:Drug Resistance, Bacterial / Q000187:drug effects* / Q000235:genetics*; D004357:Drug Synergism; D005707:Gallic Acid / Q000494:pharmacology; D015870:Gene Expression; D008826:Microbial Sensitivity Tests; D027425:ATP-Binding Cassette, Sub-Family C Proteins / Q000235:genetics* / Q000378:metabolism*; D009643:Norfloxacin / Q000494:pharmacology*; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000235:genetics; D013634:Tannins / Q000494:pharmacology*
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D005707:Gallic Acid; D027425:ATP-Binding Cassette, Sub-Family C Proteins; D009643:Norfloxacin; D013634:Tannins; C075040:NorA protein, Staphylococcus