# The Process of Natural and Styrene-Butadiene Rubbers Biodegradation by _Lactobacillus plantarum_
## 3 Results and Discussion  
The majority of the experiments showed that an addition of different types of butadiene-styrene and natural rubber mixtures to the culture medium allowed bacteria to produce bacterial cellulose, showing bacterial activity, and thus its carbon uptake from the rubber substrate. This demonstrated that _Lactobacillus plantarum_ could obtain the glucose necessary for its metabolic processes from butadiene-styrene and natural rubbers. When it comes to mean growth dynamics in the case of sulfuric vulcanizate of natural rubber (NR), 6 mm of cellulose was obtained after 14 days. Similar results were achieved for two mixtures: vulcanizate of natural rubber with an addition of carbon black (NR + CB) and sulfuric vulcanizate of natural rubber with an addition of cross-linking substances (NR + cross-linking substances). An addition of these two mixtures resulted in obtaining 5 mm-thick cellulose membranes in each case. The slowest growth was observed for the culture medium containing the mixture of sulfuric vulcanizate of natural rubber with an addition of carbon black and cross-linking substances (NR + CB + cross-linking substances). In this case, 4 mm of a cellulose membrane was produced by the bacterial strain after 14 days of the culture process. Regarding SBRs, the best results were observed for the mixture of styrene-butadiene rubber with an addition of carbon black (SBR + CB), Figure 1a. The growth dynamics for this substance was the highest, and 6 mm of bacterial cellulose was obtained after 14 days of culturing. An addition of vulcanizate of styrene-butadiene rubber (SBR) made it possible to obtain 5 mm of cellulose. The slowest growth was observed for the culture medium containing the mixture of sulfuric vulcanizate of styrene-butadiene rubber (SBR + cross-linking substances), Figure 1b. In this case, 3 mm of a cellulose membrane was produced by the bacterial strain after 14 days of the culture process. Positive results were also observed for sulfuric vulcanizate rubber EPDM (ethylene propylene diene monomer) filled with carbon black. In this case, 5 mm of a cellulose membrane was obtained after 14 days of the culture process. The same result was achieved for ground rubber from transmission tapes. The growth rate of cellulose on the bioreactor surface is correlated with an amount of carbon taken from the waste. For tests with the best efficiency (6 mm cellulose), further research is being carried out for the optimization of selected parameters (Figure 2). Figure 1: Thickness of bacterial cellulose (red arrows) cultured on: (**a**) SBR + CB; (**b**) SBR + cross-linking substances. Low activities of the investigated bacterial strains were observed in some bioreactors. This conclusion was withdrawn after analysis of bacterial cellulose thickness after the experiments. The lowest thickness of cellulose was observed in the case of three types of samples, i.e., SBR + cross linking substances, NR + CB + cross linking substances, and EPDM + CB. Some additives in the mixtures, which were used as the main carbon source may have been harmful for the bacteria and prevented them from growing. The authors analyzed these cases to obtain optimal results. To confirm the preliminary results, that the bacterial strain can gain glucose from styrene-butadiene and natural rubbers to use it for its metabolic processes, an analysis of the elemental composition was conducted. The average carbon content in the rubber material used as the carbon source for bacteria was 89.07--95.40%. After 14 days of culturing, the content ranged between 56.45 and 65.7%. Due to the characteristics of the distribution of the percentage of carbon content in the measurement before and after cultivation, it was decided that the non-parametric Mann-Whitney U test should be used. The independent test was used because the study failed to combine the pre- and post-culture measurements corresponding to one gum fragment.