title: Nisin/β-lactam adjunct therapy against Salmonella enterica serovar Typhimurium: a mechanistic approach.
abstract: OBJECTIVE
  Multidrug resistance exhibited by Salmonella strains has proved to be a big hurdle in the development of an effective anti-Salmonella therapy. In this context, we had previously demonstrated strong synergism of nisin/ceftriaxone and nisin/cefotaxime combinations against Salmonella enterica serovar Typhimurium. However, the mechanism remained unexplored. The present study was therefore planned in order to evaluate the underlying mechanisms responsible for the synergistic effect of nisin in combination with these β-lactam antibiotics against serovar Typhimurium.
  
  
  METHODS
  A membrane permeabilization assay along with pulse labelling studies were performed to confirm the ability of the combinations to permeabilize the bacterial membrane and to verify their effects on macromolecule synthesis. Additionally, analysis of peroxidative liver damage was performed and levels of nitric oxide, antioxidant enzymes, tumour necrosis factor-α and nuclear factor-κB were also measured.
  
  
  RESULTS
  1-N-phenylnapthylamine (NPN) uptake assay results confirmed a permeabilization-dependent mechanism, as NPN was taken up by treated cells in a time- and concentration-dependent manner, indicating that the combination influenced membrane permeability. Likewise, dose- and time-dependent inhibition of DNA, RNA and protein synthesis in the presence of both the combinations was observed. Interestingly, synergistic results inferred from in vivo assays confirmed the immuno-modulatory effects of the combinations in the treated mice.
  
  
  CONCLUSIONS
  Nisin/ceftriaxone and nisin/cefotaxime combinations exert their antibacterial activity against Salmonella by multiple modes of action that involve membrane permeabilization, inhibition of DNA, RNA and protein synthesis and direct immuno-modulatory activity.
authors.affiliation: Department of Microbiology, Basic Medical Sciences Block, Panjab University, Sector-14, Chandigarh 160014, India.
authors.forename: Aman Preet
authors.initials: AP
authors.lastname: Singh
authors.affiliation: Department of Biophysics, Basic Medical Sciences Block, Panjab University, Sector-14, Chandigarh 160014, India.
authors.forename: Simran
authors.initials: S
authors.lastname: Preet
authors.affiliation: Department of Microbiology, Basic Medical Sciences Block, Panjab University, Sector-14, Chandigarh 160014, India rishipraveen@yahoo.com.
authors.forename: Praveen
authors.initials: P
authors.lastname: Rishi
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology* / Q000627:therapeutic use; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D004195:Disease Models, Animal; D004359:Drug Therapy, Combination / Q000379:methods; D064420:Drug-Related Side Effects and Adverse Reactions; D005260:Female; D007155:Immunologic Factors / Q000494:pharmacology / Q000627:therapeutic use; D008099:Liver / Q000187:drug effects; D053858:Metabolic Networks and Pathways / Q000187:drug effects; D008807:Mice, Inbred BALB C; D009561:Nisin / Q000494:pharmacology* / Q000627:therapeutic use; D010539:Permeability / Q000187:drug effects; D012481:Salmonella Infections, Animal / Q000188:drug therapy* / Q000382:microbiology; D012486:Salmonella typhimurium / Q000187:drug effects* / Q000302:isolation & purification; D047090:beta-Lactams / Q000494:pharmacology* / Q000627:therapeutic use
chemical_list: D000900:Anti-Bacterial Agents; D007155:Immunologic Factors; D047090:beta-Lactams; D009561:Nisin
keywords: TNF-α; antimicrobial peptides; immuno-modulation; in vivo; synergy