1. Antimicrob Agents Chemother. 2019 Jun 24;63(7). pii: e00602-19. doi:
10.1128/AAC.00602-19. Print 2019 Jul.

Activity of Antibiotics against Staphylococcus aureus in an In Vitro Model of
Biofilms in the Context of Cystic Fibrosis: Influence of the Culture Medium.

Diaz Iglesias Y(1), Wilms T(2), Vanbever R(2), Van Bambeke F(3).

Author information: 
(1)Pharmacologie cellulaire et moléculaire, Louvain Drug Research Institute,
Université catholique de Louvain (UCLouvain), Brussels, Belgium.
(2)Advanced Drug Delivery and Biomaterials, Louvain Drug Research Institute,
Université catholique de Louvain (UCLouvain), Brussels, Belgium.
(3)Pharmacologie cellulaire et moléculaire, Louvain Drug Research Institute,
Université catholique de Louvain (UCLouvain), Brussels, Belgium
francoise.vanbambeke@uclouvain.be.

Staphylococcus aureus is a highly prevalent pathogen in the respiratory tract of 
young patients with cystic fibrosis (CF) and causes biofilm-related infections.
Here, we set up an in vitro model of a biofilm grown in Trypticase soy broth
supplemented with glucose and NaCl (TGN) or in artificial sputum medium (ASM) and
used it to evaluate on a pharmacodynamic basis the activity of antibiotics used
in CF patients and active on staphylococci (meropenem, vancomycin, azithromycin, 
linezolid, rifampin, ciprofloxacin, tobramycin). Rheological studies showed that 
ASM was more elastic than viscous, as was also observed for sputa from CF
patients, with elastic and viscous moduli being, respectively, similar to and
slightly lower than those of CF sputa. Biofilms formed by methicillin-sensitive
S. aureus strain ATCC 25923 and methicillin-resistant S. aureus strain ATCC 33591
reached maturity after 24 h, with biomass (measured by crystal violet staining)
and metabolic activity (assessed by following resazurin metabolization) being
lower in ASM than in TGN and viability (assessed by bacterial counts) being
similar in both media. Full concentration-response curves of antibiotics obtained
after 24 h of incubation of biofilms showed that all antibiotics were drastically
less potent and less efficient in ASM than in TGN toward viability, metabolic
activity, and biomass. Tobramycin selected for small-colony variants,
specifically in biofilms grown in ASM; the auxotrophism of these variants could
not be established. These data highlight the major influence exerted by the
culture medium on S. aureus responsiveness to antibiotics in biofilms. The use of
ASM may help to determine effective drug concentrations or to evaluate new
therapeutic options against biofilms in CF patients.

Copyright © 2019 American Society for Microbiology.

DOI: 10.1128/AAC.00602-19 
PMID: 31036685