Extracellular Polymeric Matrix Production and Relaxation under Fluid Shear and Mechanical Pressure in Staphylococcus aureus Biofilms
Authors: Hou Jiapeng , Veeregowda Deepak H. , van de Belt-Gritter Betsy , Busscher Henk J. , van der Mei Henny C. 
Published: October 2017

The viscoelasticity of a biofilm's EPS (extracellular polymeric substance) matrix conveys protection against mechanical challenges, but adaptive responses of biofilm inhabitants to produce EPS are not well known. Here, we compare the responses of a biofilm of an EPS-producing (ATCC 12600) and a non-EPS producing (5298)
Staphylococcus aureus
strain to fluid shear and mechanical challenge. Confocal laser scanning microscopy confirmed absence of calcofluor-white-stainable EPS in biofilms of
S. aureus
5298. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy combined with tribometry indicated that polysaccharide production per bacterium in the initial adhering layer was higher during growth at high shear than at low shear and that this increased EPS production extended to entire biofilms, as indicated by tribometrically measured coefficients of friction (CoF). CoF of biofilms grown under high fluid shear were higher than those when grown under low shear, likely due to wash-off polysaccharides. Measurement of a biofilm's CoF implies application of mechanical pressure that yielded an immediate increase in the polysaccharide band area of
S. aureus
ATCC 12600 biofilms due to their compression. Compression decreased after relief of pressure to the level observed prior to mechanical pressure. For biofilms grown under high shear, this coincided with a higher percent whiteness in optical coherence tomography-images indicative of water outflow, returning back into the biofilm during stress relaxation. Biofilms grown under low shear, however, were stimulated during tribometry to produce EPS, also after relief of stress. Knowledge of factors that govern EPS production and water flow in biofilms will allow better control of biofilms under mechanical challenge and better understanding of the barrier properties of biofilms against antimicrobial penetration.

IMPORTANCE
Adaptive responses of biofilm inhabitants in nature to environmental challenges such as fluid shear and mechanical pressure often involve EPS production with the aim of protecting biofilm inhabitants. EPS can assist biofilm bacteria in remaining attached or can impede antimicrobial penetration. The TriboChemist is a recently introduced instrument, allowing the study of initially adhering bacteria to a germanium crystal using ATR-FTIR spectroscopy, while simultaneously allowing measurement of the coefficient of friction of a biofilm, which serves as an indicator of the EPS content of a biofilm. EPS production can be stimulated by both fluid shear during growth and mechanical pressure, while increased EPS production can continue after pressure relaxation of the biofilm. Since EPS is pivotal in the protection of biofilm inhabitants against mechanical and chemical challenges, knowledge of the factors that make biofilm inhabitants decide to produce EPS, as provided in this study, is important for the development of biofilm control measures.