Influence of the mechanical properties of gelled culture media on the behavior of S. epidermidis bacteria

Details

Supervisors

Jonas, Alain M. ; Glinel, Karine

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil biomédical, à finalité spécialisée

Abstract

In recent years, the microbial population residing on the human skin has attracted a great deal of interest in the scientific community. Indeed, this organ, serving as barrier between the inner part of the host organism and its environment, hosts a myriad of microorganisms to form a rich niche, referred to as the ”skin microbiota”. Amongst resident skin bacteria, Staphylococcus epidermis (S. epidermidis) is one of the most prevalent microbes of this environment. Despite the fact that this bacterium was initially extensively studied for its opportunistic pathogenicity, which represents a challenge for patients exhibiting weak immune defenses, especially in the hospital sector, many benefits have been associated to its ubiquitous presence on human skin. Among others, S. epidermidis possesses an antimicrobial action against Cutibacterium acnes overgrowth from which acne vulgaris is believed to originate. To enjoy S. epidermidis beneficial action to its full potential in novel therapeutic solutions, encapsulation is mandatory to protect the user against the possible adverse effects related to this microbe. In this view, the design of a patch containing wells where S. epidermidis would be confined has been considered. In this context, optimization of the gelled growth medium used in the wells is necessary to support the growth and viability of the bacteria as well as to promote as much as possible their beneficial behavior. With this aim, different agarose gels were prepared, aiming to cover a wide range of mechanical properties by means of concentration variation or agarose gel strength variation. In the first instance, the prepared gels were mechanically characterized either using shear dynamic mechanical analysis or rheology depending on the gel characteristics. The storage and loss moduli as well as the damping factor were measured during amplitude and frequency sweep tests and the steady viscosity was determined. An increase in either concentration or gel strength is shown to increase the moduli and the viscosity while the damping factor is globally the same in steady regime for all gels. In the second instance, these gels were used as culture media for S. epidermidis, which was embedded in these matrices. The growth was assessed by measuring the fluorescence intensity related to fluorophore expression by the considered strain. The growth mode was observed by means of epifluorescence and confocal microscopy while the metabolic activity was tested using an alamarBlue assay test. An increase in either agarose concentration or gel strength was suggested to decrease the growth rate as well as to promote the transition to aggregate growth mode. The reduction of alamarBlue seems to be slowed down by an increase in agarose gel strength, which could or not be related to the metabolic activity of the bacterial population. However, no clear influence of the concentration was observed on the metabolic activity. Since an increase in agarose concentration or gel strength causes an enhancement of the viscoelastic properties of the gel, the stiffer or more viscous the gel, the slower the growth and the more aggregates are favored over planktonic cells. Nevertheless, the change of viscoelastic properties does not seem to influence the metabolic activity of the bacteria. These results thus suggest that the mechanical properties of gelled media may be optimized to control bacterial growth, without significantly impacting metabolic activity, which offers an interesting possibility for the making of bacterial patches displaying long term activity.