title: The effect of growth rate on pyrazinamide activity in Mycobacterium tuberculosis - insights for early bactericidal activity?
abstract: BACKGROUND
  Pyrazinamide (PZA) plays an essential part in the shortened six-month tuberculosis (TB) treatment course due to its activity against slow-growing and non-replicating organisms. We tested whether PZA preferentially targets slow growing cells of Mycobacterium tuberculosis that could be representative of bacteria that remain after the initial kill with isoniazid (INH), by observing the response of either slow growing or fast growing bacilli to differing concentrations of PZA.
  
  
  METHODS
  M. tuberculosis H37Rv was grown in continuous culture at either a constant fast growth rate (Mean Generation Time (MGT) of 23.1 h) or slow growth rate (69.3 h MGT) at a controlled dissolved oxygen tension of 10 % and a controlled acidity at pH 6.3 ± 0.1. Cultures were exposed to step-wise increases in the concentration of PZA (25 to 500 μgml(-1)) every two MGTs, and bacterial survival was measured. PZA-induced global gene expression was explored for each increase in PZA-concentration, using DNA microarray.
  
  
  RESULTS
  At a constant pH 6.3, actively dividing mycobacteria were susceptible to PZA, with similar responses to increasing concentrations of PZA at both growth rates. Three distinct phases of drug response could be distingished for both slow growing (69.3 h MGT) and fast growing (23.1 h MGT) bacilli. A bacteriostatic phase at a low concentration of PZA was followed by a recovery period in which the culture adapted to the presence of PZA and bacteria were actively dividing in steady-state. In contrast, there was a rapid loss of viability at bactericidal concentrations. There was a notable delay in the onset of the recovery period in quickly dividing cells compared with those dividing more slowly. Fast growers and slow growers adapted to PZA-exposure via very similar mechanisms; through reduced gene expression of tRNA, 50S, and 30S ribosomal proteins.
  
  
  CONCLUSIONS
  PZA had an equivalent level of activity against fast growing and slow growing M. tuberculosis. At both growth rates drug-tolerance to sub-lethal concentrations may have been due to reduced expression of tRNA, 50S, and 30S ribosomal proteins. The findings from this study show that PZA has utility against more than one phenotypic sub-population of bacilli and could be re-assessed for its early bactericidal activity, in combination with other drugs, during TB treatment.
authors.affiliation: Public Health England, National Infection Service, Porton Down, Salisbury, Wiltshire, SP4 0JG, UK.
authors.forename: Steven T
authors.initials: ST
authors.lastname: Pullan
authors.affiliation: Public Health England, National Infection Service, Porton Down, Salisbury, Wiltshire, SP4 0JG, UK.
authors.forename: Jon C
authors.initials: JC
authors.lastname: Allnutt
authors.affiliation: School of Biological Sciences, University of East Anglia, Norwich Research Park, NR4 7TJ, UK.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Devine
authors.affiliation: Public Health England, National Infection Service, Porton Down, Salisbury, Wiltshire, SP4 0JG, UK.
authors.forename: Kim A
authors.initials: KA
authors.lastname: Hatch
authors.affiliation: Public Health England, National Infection Service, Porton Down, Salisbury, Wiltshire, SP4 0JG, UK.
authors.forename: Rose E
authors.initials: RE
authors.lastname: Jeeves
authors.affiliation: Public Health England, National Infection Service, Porton Down, Salisbury, Wiltshire, SP4 0JG, UK.
authors.forename: Charlotte L
authors.initials: CL
authors.lastname: Hendon-Dunn
authors.affiliation: Public Health England, National Infection Service, Porton Down, Salisbury, Wiltshire, SP4 0JG, UK.
authors.forename: Philip D
authors.initials: PD
authors.lastname: Marsh
authors.affiliation: Public Health England, National Infection Service, Porton Down, Salisbury, Wiltshire, SP4 0JG, UK. Joanna.bacon@phe.gov.uk.
authors.forename: Joanna
authors.initials: J
authors.lastname: Bacon
mesh_terms: D000995:Antitubercular Agents / Q000494:pharmacology*; D020869:Gene Expression Profiling; D015964:Gene Expression Regulation, Bacterial / Q000187:drug effects; D007538:Isoniazid / Q000494:pharmacology; D009169:Mycobacterium tuberculosis / Q000187:drug effects* / Q000235:genetics / Q000254:growth & development*; D011718:Pyrazinamide / Q000494:pharmacology*; D012343:RNA, Transfer / Q000235:genetics; D012269:Ribosomal Proteins / Q000235:genetics
chemical_list: D000995:Antitubercular Agents; D012269:Ribosomal Proteins; D011718:Pyrazinamide; D012343:RNA, Transfer; D007538:Isoniazid
keywords: Chemostats; Gene expression; Growth rate; Mycobacterium tuberculosis; Pyrazinamide; trans-translation