*3.6. Antibacterial Activity of Coated CT*

From a medical perspective, *Escherichia coli* (*E. coli*) and *Staphylococcus aureus* (*S. aureus*) are the most common representatives for Gram-negative and Gram-positive bacteria, respectively, that cause infections in the human body. Gram-negative bacteria (*E.coli*) tend

to be more resistant to antimicrobial agents that Gram-positive bacteria (*S. aureus*) because of the presence of the additional protection afforded by the outer membrane (peptidoglycan layer). However, in this study, *E. coli* growth was inhibited due to the presence of CA, which weakened the outer membrane of Gram-negative bacteria [59]. The result is supported by that of a previous study, where CA exhibited more inhibition to *E. coli* compared with *S. aureus* [60]. *S. aureus* was more resistant to the acidic environment since *S. aureus* is better at tolerating and adaptive to stress [61]. to be more resistant to antimicrobial agents that Gram-positive bacteria (*S. aureus*) because of the presence of the additional protection afforded by the outer membrane (peptidoglycan layer). However, in this study, *E. coli* growth was inhibited due to the presence of CA, which weakened the outer membrane of Gram-negative bacteria [59]. The result is supported by that of a previous study, where CA exhibited more inhibition to *E. coli* compared with *S. aureus* [60]. *S. aureus* was more resistant to the acidic environment since *S. aureus*  is better at tolerating and adaptive to stress [61].

From a medical perspective, *Escherichia coli* (*E. coli*) and *Staphylococcus aureus* (*S. aureus*) are the most common representatives for Gram-negative and Gram-positive bacteria, respectively, that cause infections in the human body. Gram-negative bacteria (*E.coli*) tend

*Polymers* **2022**, *14*, x 19 of 23

As shown in Figure 9, the zone of inhibition increases with the increase in coating layer and CA concentration for OD and OIR, which is attributed to the increase in reactive oxygen species (ROS). ROS is responsible for the inhibition and causes interruption of the bacterial cell wall synthesis process, growth biosynthesis inhibition, DNA transcription process interference, and metabolic pathway chain reaction disruption occurring in the bacteria cell [62]. However, IR samples showed lower CA concentrations able to inhibit *E.coli* growth. At 2M CA, both IR-1 and IR-2 samples showed the highest inhibition for *E.coli*. It is due to CA, which acts as a permeabilization agent able to inhibit *E. coli* growth by causing cell aggregation [59] and bacterial toxicity by blocking the permeability of the outer membrane [63,64]. Both bacterial species are resistant to CMC except for the IR-2 samples, while the IR-1 CMC sample only showed inhibition to *S. aureus* bacteria. The effectiveness of IR drying might also be the reason why a lower CA concentration is sufficient to inhibit bacterial growths. This finding might be due to the effective drying of CMC through IR drying, which permits its antimicrobial property to be preserved. It is achieved due to the uniform heating provided by the IR drying technique, as mentioned in Section 3.3. These findings indicate that CMC with CA as a cross-linker coating dried by IR improved the antibacterial property of cotton threads. As shown in Figure 9, the zone of inhibition increases with the increase in coating layer and CA concentration for OD and OIR, which is attributed to the increase in reactive oxygen species (ROS). ROS is responsible for the inhibition and causes interruption of the bacterial cell wall synthesis process, growth biosynthesis inhibition, DNA transcription process interference, and metabolic pathway chain reaction disruption occurring in the bacteria cell [62]. However, IR samples showed lower CA concentrations able to inhibit *E.coli* growth. At 2M CA, both IR-1 and IR-2 samples showed the highest inhibition for *E.coli*. It is due to CA, which acts as a permeabilization agent able to inhibit *E. coli* growth by causing cell aggregation [59] and bacterial toxicity by blocking the permeability of the outer membrane [63,64]. Both bacterial species are resistant to CMC except for the IR-2 samples, while the IR-1 CMC sample only showed inhibition to *S. aureus* bacteria. The effectiveness of IR drying might also be the reason why a lower CA concentration is sufficient to inhibit bacterial growths. This finding might be due to the effective drying of CMC through IR drying, which permits its antimicrobial property to be preserved. It is achieved due to the uniform heating provided by the IR drying technique, as mentioned in Section 3.3. These findings indicate that CMC with CA as a cross-linker coating dried by IR improved the antibacterial property of cotton threads.

**Figure 9.** Antibacterial activity of fabricated cotton thread with different coating layers of CT/CMC cross-linked with different CA concentrations. **Figure 9.** Antibacterial activity of fabricated cotton thread with different coating layers of CT/CMC cross-linked with different CA concentrations.

#### **4. Conclusions 4. Conclusions**

Based on these findings, the mechanical properties of coated samples are significantly affected by the drying method used at *p* < 0.05. However, the IR drying method is the best-method approach. A single coat of CT/CMC + CA showed the best properties among all samples, with single-coated samples dried via IR drying, exhibiting a less brittle and less dense structure with a slightly smoother surface as well as enhanced water absorption and tensile strength in contrast to double-coated samples. The ATR–FTIR analysis spectra corresponding to C=O from COOH of CA indicated that cross-linking between CMC and CA interaction occurred between CT/CMC-coated samples. Single-coated CMC cross-linked with 2M CA cotton thread dried with IR proved to be a successful candidate for the fabrication of cotton thread with beneficial properties for wound dressings and biomedical applications.

**Author Contributions:** Conceptualization, M.K.K.-A., N.S.N.M. and A.H.A.H.; methodology, M.K.K.-A.; software, S.Z.; validation, M.K.K.-A., A.H.A.H. and M.M.; formal analysis, M.K.K.-A., N.S.N.M. and A.H.A.H.; investigation, M.K.K.-A., N.S.N.M. and A.H.A.H.; data curation, M.K.K.-A.; writing original draft preparation, M.K.K.-A.; writing—review and editing, M.K.K.-A. and K.M.S.; supervision, K.M.S. and M.M.; project administration, S.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by TRGS/1/2019/UKM/02/1/2 of the Ministry of Higher Education Malaysia and 600-RMC/MYRA 5/3/LESTARI (041/2020) of Universiti Teknologi MARA.

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** We thank the Ministry of Higher Education Malaysia for research grant no. TRGS/1/2019/UKM/02/1/2 and Universiti Teknologi MARA for research grant no. 600-RMC/MYRA 5/3/LESTARI (041/2020), which funded this project, and Universiti Kebangsaan Malaysia (UKM) for providing the facilities for conducting this study. We also thank the Institute of Biology System, UKM, for providing the facility for conducting the antibacterial analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.
