title: The potent bactericidal activity of streptomycin in the guinea pig model of tuberculosis ceases due to the presence of persisters.
abstract: OBJECTIVE
  The biphasic kill curve of isoniazid against Mycobacterium tuberculosis in guinea pigs is due to the presence of persisters rather than selection of isoniazid-resistant mutants. To determine whether this phenomenon is common to other bactericidal drugs, we studied the activity of streptomycin and its ability to select for streptomycin-resistant mutants in the guinea pig model of tuberculosis.
  
  
  METHODS
  Pharmacokinetic studies were performed to establish the human-equivalent dose of streptomycin. Guinea pigs were aerosol-infected with M. tuberculosis and 2 weeks later streptomycin was given for 5 days/week via intramuscular injection. Bactericidal activity was assessed by homogenizing and plating lungs for cfu until 10 weeks of treatment. At each timepoint, cfu were isolated, suspended in normal saline and re-plated on plates containing 0.5, 1.0, 2.0 or 10.0 mg/L streptomycin.
  
  
  RESULTS
  The human-equivalent dose of streptomycin was determined to be 70 mg/kg. Streptomycin showed potent activity during the first 14 days of treatment, rescuing all animals from acute tuberculosis-related death and reducing lung cfu by ∼4 log(10). However, streptomycin activity was dramatically reduced thereafter, as lung cfu declined by only ∼1 log(10) over the next 56 days of treatment. Although streptomycin-resistant mutants were detectable, their frequency of isolation was identical at treatment initiation and after 70 days of treatment.
  
  
  CONCLUSIONS
  The reduced activity of streptomycin during the second phase of monotherapy is not associated with the selection of streptomycin-resistant mutants but, rather, with the presence of phenotypically tolerant 'persisters'.
authors.affiliation: Center for Tuberculosis Research, Department of Medicine, Johns Hopkins University School of Medicine, Baltimore, MD, USA.
authors.forename: Zahoor
authors.initials: Z
authors.lastname: Ahmad
authors.forename: Michael L
authors.initials: ML
authors.lastname: Pinn
authors.forename: Eric L
authors.initials: EL
authors.lastname: Nuermberger
authors.forename: Charles A
authors.initials: CA
authors.lastname: Peloquin
authors.forename: Jacques H
authors.initials: JH
authors.lastname: Grosset
authors.forename: Petros C
authors.initials: PC
authors.lastname: Karakousis
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000008:administration & dosage / Q000494:pharmacology*; D015169:Colony Count, Microbial; D004195:Disease Models, Animal; D024881:Drug Resistance, Bacterial; D004361:Drug Tolerance; D005260:Female; D006168:Guinea Pigs; D007273:Injections, Intramuscular; D008168:Lung / Q000382:microbiology; D050296:Microbial Viability / Q000187:drug effects*; D009169:Mycobacterium tuberculosis / Q000187:drug effects*; D013307:Streptomycin / Q000008:administration & dosage / Q000494:pharmacology*; D014376:Tuberculosis / Q000188:drug therapy*
chemical_list: D000900:Anti-Bacterial Agents; D013307:Streptomycin