World climate change causes global higher temperatures and increasingly extreme weather phenomena to become more and more frequent. Thus, measures have to be taken to avoid reaching a point of no return. One possible step is to reduce emissions of greenhouse gases (e.g. CO₂) from the production of platform chemicals by switching from crude oil to microbial gas fermentation based production (Sanchez et al., 2023, https://www.iea.org/energy-system/industry/chemicals). The acetogenic bacterium Acetobacterium woodii has already been genetically engineered to produce lactate as a secondary product from H₂ and CO₂ next to acetate (Mook et al., 2022, doi: 10.1007/s00253-022-11770-z). Since A. woodii lacks the ability to produce higher value products through chain elongation, the addition of the bacterium Clostridium drakei in a synthetic co-culture allows the valorisation of lactate in combination with H₂ and CO₂ to more thought after chemicals such as butyrate or hexanoate. The inoculation strategy for this co-culture needs to be optimised and cell numbers from both species have to be quantifiable. Quantification of cells was done using fluorescence in situ hybridization (FISH) via distinguishable probes targeting both species individually. Two probes were constructed that hybridise to specific fragments of the V4 region of the 16S rDNA in A. woodii and C. drakei and carry differing fluorescent dyes (Pacific Blue for C. drakei, ATTO495 for A. woodii). These probes were then tested to find the optimal hybridisation conditions. Different inoculation strategies were tested for the co-culture by inoculating the strains both simultaneously and sequentially with different cell concentration ratios (A. woodii/C. drakei: 70/30, 50/50, 30/70). Additionally, carriers were added to test for possible improvements by allowing the cells to form stationary biofilms. Cell counts of both strains were quantified by hybridising the previously mentioned probes under the optimised conditions and observing them by both flow cytometry and fluorescent microscopy. The fermentation characteristics were determined by measuring gas consumption through loss in pressure, product spectra and production rates through gas chromatography and high pressure liquid chromatography, and capturing scanning and transmission electron microscopy images.