# The Effect of Human Blood Plasma Conditioning Films on Platelet Tungsten Bag Surface Properties
## 4 Discussion
This is important since it would seem that both the physicochemical characteristics of the bacteria, the surface roughness and amount of time the conditioning film was retained influenced the surface properties. The most significant physical change observable for any of the CFs was the significant increase in cells present on all of the surfaces after a 24 h incubation of human plasma and _Ser. marcescens_ under SEM. This demonstrated that the _Ser. marcescens_ proliferated during the 24 h incubation period using nutrients present in the human plasma, as this same effect was not observed when _Ser. marcescens_ was incubated without the addition of human plasma. Further, when the roughness was determined for the three surfaces under these conditions it was apparent that human plasma combined with _Ser. marcescens_ demonstrated a reduction in surface roughness. However, both SEM and surface profile results demonstrated that _S. epidermidis_ either was not capable of using the nutrients as efficiently as _Ser. marcescens_ or that the human plasma was reducing the _S. epidermidis_ proliferation. This may be due to the specific strain of _S. epidermidis_ used being potentially sensitive to human plasma, whilst the strain of _Ser. marcescens_ used likely was less sensitive and could therefore demonstrate plentiful growth and freely use the human plasma as a nutrient source. In agreement with this result, work by others has demonstrated that bacterial growth can be influenced by the presence of plasma and serum [27]. Further, bacteria preferentially adhere within gaps or pits on a surface due to protection from unfavourable environmental factors and an increase in contact area between the cell wall of the organism and the substrate [28,29]. Higher bacterial adhesion on rough surfacescan contribute to missed detection of bacterial strains that tend to adhere on surfaces, and this can affect the quality and safety of platelet storage bags by promoting more bacterial adhesion and biofilm formation on surfaces [24]. The growth of _Ser. marcescens_ may have caused some pits to be filled in by a combination of bacterial cells, human plasma and products produced by the bacteria, such as extracellular polymeric substances (EPS). Overall, the effects of the conditioning films on the roughness of the surfaces demonstrated that the presence of material from the human plasma or from bacterial cells was able to fill in some of the gaps present on the rougher surface. When differences in the effect of the CFs were determined chemically, the EDX analysis demonstrated some small but insignificant changes. This demonstrated that whilst other methods of analysis determined changes in the surface properties, the elemental makeup of the surfaces were not significantly altered. Overall, the small changes appeared to be a reduction in the detection of carbon and subsequent increases in the oxygen and chlorine detected for EDX. The analysis of the surface chemistry via ATR-FTIR demonstrated that the addition of a CF resulted in new IR absorptions that were assignable to the proteins in the plasma and the organisms. The penetration depth of the evanescent wave into the sample was likely to be influenced by the differing refractive indices of the CF and the p-PVC substrate (the refractive index and thickness of the CF is also unknown). The latter is in addition to the usual relationship between the refractive indices of the sample and IRE, the wavelength of the IR radiation and the angle of incidence of the IR beam with the contact surface of the IRE (fixed at 45 degrees in the system used) thus making estimation of the absolute amounts of specific components in the spectrum difficult or impossible, though on a given sample at a given frequency, changes in the relative level of an adsorbed species can be observed and compared qualitatively. The minimum concentration of plasma and/or organism on the substrate required for detection by the IR method used is unknown. The most notable observations were related to the interaction of _S.