Abstract: Syngas fermentation is a leading microbial process for the conversion of carbon monoxide, carbon dioxide, and hydrogen to valuable biochemicals. Clostridium autoethanogenum stands as a model organism for this process, showcasing its ability to convert syngas into ethanol industrially with simultaneous fixation of carbon and reduction of greenhouse gas emissions. A deep understanding on the metabolism of this microorganism and the influence of operational conditions on fermentation performance is key to advance the technology and enhancement of production yields. In this work, we studied the individual impact of acetic acid concentration, growth rate, and mass transfer rate on metabolic shifts, product titres, and rates in CO fermentation by C. autoethanogenum. Through continuous fermentations performed at a low mass transfer rate, we measured the production of formate in addition to acetate and ethanol. We hypothesise that low mass transfer results in low CO concentrations, leading to reduced activity of the Wood–Ljungdahl pathway and a bottleneck in formate conversion, thereby resulting in the accumulation of formate. The supplementation of the medium with exogenous acetate revealed that undissociated acetic acid concentration increases and governs ethanol yield and production rates, assumedly to counteract the inhibition by undissociated acetic acid. Since acetic acid concentration is determined by growth rate (via dilution rate), mass transfer rate, and working pH, these variables jointly determine ethanol production rates. These findings have significant implications for process optimisation as targeting an optimal undissociated acetic acid concentration can shift metabolism towards ethanol production. Key points: • Very low CO mass transfer rate leads to leaking of intermediate metabolite formate. • Undissociated acetic acid concentration governs ethanol yield on CO and productivity. • Impact of growth rate, mass transfer rate, and pH were considered jointly. Graphical abstract: [Figure not available: see fulltext.]

Syngas fermentation to biofuels and chemicals is an emerging technology in the biobased economy. Mass transfer is usually limiting the syngas fermentation rate, due to the low aqueous solubilities of the gaseous substrates. Membrane bioreactors, as efficient gas–liquid contactors, are a promising configuration for overcoming this gas-to-liquid mass transfer limitation, so that sufficient productivity can be achieved. We summarize the published performances of these reactors. Moreover, we highlight numerous parameters settings that need to be used for the enhancement of membrane bioreactor performance. To facilitate this enhancement, we relate mass transfer and other performance indicators to the type of membrane material, module, and flow configuration. Hollow fiber modules with dense or asymmetric membranes on which biofilm might form seem suitable. A model-based approach is advocated to optimize their performance.

Implant-associated infections (IAI) are often recurrent, expensive to treat, and associated with high rates of morbidity, if not mortality. We biofunctionalized the surface of additively manufactured volume-porous titanium implants using electrophoretic deposition (EPD) as a way to eliminate the peri-operative bacterial load and prevent IAI. Chitosan-based (Ch) coatings were incorporated with different concentrations of silver (Ag) nanoparticles or vancomycin. A full-scale in vitro and in vivo study was then performed to evaluate the antibacterial, immunogenic, and osteogenic activity of the developed implants. In vitro, Ch + vancomycin or Ch + Ag coatings completely eliminated, or reduced the number of planktonic and adherent Staphylococcus aureus by up to 4 orders of magnitude, respectively. In an in vivo tibia intramedullary implant model, Ch + Ag coatings caused no adverse immune or bone response under aseptic conditions. Following Staphylococcus aureus inoculation, Ch + vancomycin coatings reduced the implant infection rate as compared to chitosan-only coatings. Ch + Ag implants did not demonstrate antibacterial effects in vivo and even aggravated infection-mediated bone remodeling including increased osteoclast formation and inflammation-induced new bone formation. As an explanation for the poor antibacterial activity of Ch + Ag implants, it was found that antibacterial Ag concentrations were cytotoxic for neutrophils, and that non-toxic Ag concentrations diminished their phagocytic activity. This study shows the potential of EPD coating to biofunctionalize porous titanium implants with different antibacterial agents. Using this method, Ag-based coatings seem inferior to antibiotic coatings, as their adverse effects on the normal immune response could cancel the direct antibacterial effects of Ag nanoparticles. Statement of Significance: Implant-associated infections (IAI) are a clinical, societal, and economical burden. Surface biofunctionalization approaches can render complex metal implants with strong local antibacterial action. The antibacterial effects of inorganic materials such as silver nanoparticles (Ag NPs) are often highlighted under very confined conditions in vitro. As a novelty, this study also reports the antibacterial, immunogenic, and osteogenic activity of Ag NP-coated additively-manufactured titanium in vivo. Importantly, it was found that the developed coatings could impair the normal function of neutrophils, the most important phagocytic cells protecting us from IAI. Not surprisingly, the Ag NP-based coatings were outperformed by an antibiotic-based coating. This emphasizes the importance of also targeting implant immune-modulatory functions in future coating strategies against IAI.

The aim of this work was to investigate the productivity of continuous lactate fermentations at increased cell densities on simulated lignocellulosic hydrolyzate and wheat straw hydrolyzates using Ca(OH)₂suspensions as base. To this end, a semi-automated membrane bioreactor prototype with external tubular membranes was designed and constructed. Long-term continuous fermentations were run in membrane bioreactor mode and compared to batch and continuous mode. Fouling of the membranes proved to be the major bottleneck in the process. The semi-automated membrane module switch elegantly circumvented this problem, allowing constant operating conditions throughout extended periods of time. Maximal productivities of 30.5 g kg⁻¹.h⁻¹could be reached in the prototype during 30.5 h combined with an average lactate concentration of 85.4 g kg⁻¹and complete consumption of C5 and C6 carbohydrates. In this fermentation zone, the (liquid) residence time was 2.8 h. These numbers demonstrate significant performance improvements in comparison with batch and (regular) continuous conditions.