Pseudomonas fragi biofilm on stainless steel (at low temperatures) affects the survival of Campylobacter jejuni and Listeria monocytogenes and their control by a polymer molybdenum oxide nanocomposite coating.

Pseudomonas spp. are widely distributed bacteria on surfaces in the food production and processing environment, where they form extracellular polymeric substance rich biofilms that interact with other bacteria. In this study, the influence of biofilm of Pseudomonas fragi ATCC 4973 on Listeria monocytogenes ATCC 19115 and Campylobacter jejuni NCTC 11168 was investigated at 5 °C and 15 °C on stainless steel in broth and food homogenates (fish or chicken meat). Stainless steel was then coated with PVDF-HFP/PVP/MoO3 nanocomposite and examined for surface changes (scanning electron microscope, static contact angle, Vickers hardness and elastic modulus). The effect of the prepared nanocomposite coating on P. fragi and on L. monocytogenes and C. jejuni was evaluated in mono- and co-culture. P. fragi produced more biofilm at 15 °C than at 5 °C, especially when food homogenates were used as growth media. Co-cultivation with pathogens did not affect biofilm production by P. fragi, but significant changes were observed in L. monocytogenes and C. jejuni, resulting in a decrease and increase, respectively, in the determined number of culturable biofilm cells. The first change was probably due to competition for the surface, and the second to the oxygen gradient. Stainless steel was then coated with a PVDF-HFP/PVP/MoO3 nanocomposite, which was characterised by lower roughness and higher wettability, but lower hardness compared to uncoated stainless steel. The prepared nanocoating showed bactericidal activity when tested in phosphate buffered saline. When used in food homogenates, a reduction of over 95 % in bacterial counts was observed. An abundant biofilm of P. fragi proved protective to L. monocytogenes and C. jejuni against the functionalised nanocomposite surface when tested in food homogenates. The control of spoilage Pseudomonas spp., which are common in the food production and processing environment, is important for reducing the contamination of food with spoilage bacteria and with pathogens such as L. monocytogenes and C. jejuni, which may be present in the same environment. The PVDF-HFP/PVP/MoO3 nanocomposite showed good potential for use as a coating for food contact surfaces, but possible migration of nanoparticles from the nanocomposite coating to food should be evaluated before its commercial use.

Ultrasonic photoacoustic emitter of graphene-nanocomposites film on a flexible substrate.

Photoacoustic devices generating high-amplitude and high-frequency ultrasounds are attractive candidates for medical therapies and on-chip bio-applications. Here, we report the photoacoustic response of graphene nanoflakes - Polydimethylsiloxane composite. A protocol was developed to obtain well-dispersed graphene into the polymer, without the need for surface functionalization, at different weight percentages successively spin-coated onto a Polydimethylsiloxane substrate. We found that the photoacoustic amplitude scales up with optical absorption reaching 11 MPa at ∼ 228 mJ/cm2 laser fluence. We observed a deviation of the pressure amplitude from the linearity increasing the laser fluence, which indicates a decrease of the Grüneisen parameter. Spatial confinement of high amplitude (> 40 MPa, laser fluence > 55 mJ/cm2) and high frequency (Bw-6db ∼ 21.5 MHz) ultrasound was achieved by embedding the freestanding film in an optical lens. The acoustic gain promotes the formation of cavitation microbubbles for moderate fluence in water and in tissue-mimicking material. Our results pave the way for novel photoacoustic medical devices and integrated components.