## 4 Discussion
In order to investigate the possibility of using synthesised \(\gamma\)-PGA-based hydrogels as drug-delivery vehicles, studies on the antimicrobial activity of hydrogels loaded with antibiotics were carried out. During a disc-diffusion assay, pure hydrogel discs, hydrogel discs loaded with antibiotics, and a paper disc containing antibiotics were tested against selected pathogens. _S. aureus_ and _E. coli_ were chosen to represent Gram-positive and -negative bacteria. Both pathogens are the most common cause of healthcare-associated infections [30]. Chloramphenicol, an antibacterial agent with a broad spectrum of activity against Gram-positive and -negative bacteria [31], was used as a model drug. Paper discs containing antibiotics and hydrogel discs loaded with antibiotics exhibited antimicrobial activity against _E. coli_ and _S. aureus_. After 24 h of the experiment, higher antimicrobial activity against bothpathogens was observed for antibiotics released from the paper discs. It could be assumed that, during the first period of the experiment, only a small amount of antibiotics was released from the hydrogel discs, and that dose was too low to indicate a similar effect as the antibiotics release from the paper discs. However, after transferring hydrogel discs loaded with antibiotics and paper discs onto freshly seed TSA plates, the hydrogel discs with antibiotics showed noticeably higher antimicrobial activity against the tested pathogens. Along with extending the duration of the experiment, more drugs were released from the hydrogel discs, resulting in observed antibacterial activity up to 120 h. The performed experiment indicated prolonged activity of hydrogels loaded with antibiotics in comparison to paper discs containing antibiotics. Moreover, hydrogel discs dissolved during the experiment, which might be the result of the hydrolysis of ester bonds caused by humidity and/or enzymes produced by bacteria. Such hydrogel behaviour may be desirable for certain applications, such as wound dressing [32]. The antibacterial effect of pure hydrogel discs observed after 120 h could be explained just by the low pH generated by the hydrolysis of the hydrogel. The developed \(\gamma\)-PGA-PEG hydrogels were tested as probiotic-delivery systems. The protective effect of \(\gamma\)-PGA-based hydrogels on probiotic cells in low pH was determined. Various hydrogels are known as materials that might improve the survivability of probiotic bacteria under gastrointestinal-track conditions, as well as during storage at various temperatures [33]. Moreover, the \(\gamma\)-PGA biopolymer was already successfully tested as a delivery vehicle for probiotic bacteria [12; 34]. Those facts encourage further research, during which the suitability of \(\gamma\)-PGA-based hydrogels for applications as probiotic-delivery systems could be investigated. In the human stomach, probiotic bacteria are subjected to gastric acidic with low pH. Immediately after consuming certain food and drinks, gastric pH could be elevated to a value in the range of 3.0-4.0; however, this decreases over time to a value below 2.0 [29]. Therefore, the protective effect of the selected \(\gamma\)-PGA-based hydrogels on probiotic bacteria was tested under various pH values, 3.0 and 1.5, respectively. The viability of the tested probiotic bacteria entrapped in the hydrogels was mostly higher than that of free cells subjected to pH 3.0 and pH 1.5. Certain differences in the viability of bacteria protected by PEG400, PEG1000, and PEG1000-3 arm were observed. Those differences might be related to the different swelling behaviours of those hydrogels; the PEG1000-3 arm hydrogel had a lower SR under acidic conditions than that of the PEG400 and PEG1000 hydrogels. Less of the harmful medium could penetrate inside a less swollen PEG1000-3 arm hydrogel; thus, contact between entrapped probiotic bacteria and acid medium is more limited. This is in contrast to the hydrogel with the linear PEG1000 cross-linker, which had the highest swelling ratio between the tested hydrogels.