Norfloxacin has in vitro activity against a broad range of gram-positive and gram-negative aerobic bacteria. The fluorine atom at the 6 position provides increased potency against gram-negative organisms, and the piperazine moiety at the 7 position is responsible for antipseudomonal activity.

Norfloxacin inhibits bacterial deoxyribonucleic acid synthesis and is bactericidal. At the molecular level, three specific events are attributed to norfloxacin in E. coli cells:

1. inhibition of the ATP-dependent DNA supercoiling reaction catalyzed by DNA gyrase,
2. inhibition of the relaxation of supercoiled DNA,
3. promotion of double-stranded DNA breakage.

Resistance to norfloxacin due to spontaneous mutationin vitro is a rare occurrence (range: 10^-9 to 10^-12 cells). Resistant organisms have emerged during therapy with norfloxacin in less than 1% of patients treated. Organisms in which development of resistance is greatest are the following:

  Pseudomonas aeruginosa
  Klebsiella pneumoniae
  Acinetobacter spp.
  Enterococcus spp.

For this reason, when there is a lack of satisfactory clinical response, repeat culture and susceptibility testing should be done. Nalidixic acid-resistant organisms are generally susceptible to norfloxacin in vitro; however, these organisms may have higher minimum inhibitory concentrations (MICs) to norfloxacin than nalidixic acid-susceptible strains. There is generally no cross-resistance between norfloxacin and other classes of antibacterial agents. Therefore, norfloxacin may demonstrate activity against indicated organisms resistant to some other antimicrobial agents including the aminoglycosides, penicillins, cephalosporins, tetracyclines, macrolides, and sulfonamides, including combinations of sulfamethoxazole and trimethoprim. Antagonism has been demonstrated in vitro between norfloxacin and nitrofurantoin.

Norfloxacin has been shown to be active against most strains of the following microorganisms both in vitro and in clinical infections as described in the INDICATIONS AND USAGE section.

Gram-positive aerobes

  Enterococcus faecalis
  Staphylococcus aureus
  Staphylococcus epidermidis
  Staphylococcus saprophyticus
  Streptococcus agalactiae

Gram-negative aerobes

  Citrobacter freundii
  Enterobacter aerogenes
  Enterobacter cloacae
  Escherichia coli
  Klebsiella pneumoniae
  Neisseria gonorrhoeae
  Proteus mirabilis
  Proteus vulgaris
  Pseudomonas aeruginosa
  Serratia marcescens

The following in vitro data are available, but their clinical significance is unknown.

Norfloxacin exhibits in vitro MICs of ≤ 4 μg/mL against most ( ≥ 90%) strains of the following microorganisms; however, the safety and effectiveness of norfloxacin in treating clinical infections due to these microorganisms have not been established in adequate and well-controlled clinical trials.

Gram-negative aerobes

  Citrobacter diversus
  Edwardsiella tarda
  Enterobacter agglomerans
  Haemophilus ducreyi
  Klebsiella oxytoca
  Morganella morganii
  Providencia alcalifaciens
  Providencia rettgeri
  Providencia stuartii
  Pseudomonas fluorescens
  Pseudomonas stutzeri

Other

  Ureaplasma urealyticum

  NOROXIN is not generally active against obligate anaerobes.

  Norfloxacin has not been shown to be active against Treponema pallidum. (See WARNINGS.)

Susceptibility Tests

Dilution Techniques

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