# Removal of Antibiotics by Biochars: A Critical Review
## 3 Antibiotics and Bacterial Cytology
### Characteristics of Bacterial Cytology  
Bacterial cytology is the study of bacterial cellular structure and organization. It is a useful tool to identify pathogenic bacteria, understand the disease mechanisms, and target antibiotics [57]. The identification of bacteria is the first step in bacterial cytology. Bacterial cells can be visualized by light, electron, or fluorescence microscopy. Usually, light microscopy observations are performed. Before the light microscopy observations, bacterial cells are stained with crystal violet (C\({}_{25}\)H\({}_{30}\)ClN\({}_{3}\)). The Gram-positive bacteria retain the stain and exhibit a purple color when observed under the microscope, while the Gram-negative bacteria do not retain the stain and appear pink. After the staining, the bacterial cell morphology is determined. Spherical bacteria are cocci, rod-shaped bacteria are bacilli, and spiral-shaped bacteria are spirella. This information, together with staining, is helpful in the determination of bacterial genus and species. However, Gram staining cannot be applied to all bacteria, because not all bacteria contain peptidoglycan cell wall layers that retain the stain, such as mycoplasma. Mycobacteria contain a complex cell wall with high contents of mycolic acids and are thus identified by acid-fast staining. Certain morphological properties of bacteria give clues to their pathogenicity. These properties include the presence of capsules (helping bacteria to evade the immune system), pili and flagella (helping with adhesion to hosts), endospores, intracellular inclusions, etc. [58].  
Protein synthesis and DNA replication are the fundamental processes in all living organisms. Thus, bacteria synthesize proteins and replicate DNA. The protein synthesis of bacteria is similar to human and animal protein synthesis, but bacterial ribosomes (70 s ribosome), composed of 50 s and 30 s subunits, are smaller than eukaryotic ribosomes (80 s ribosome). Certain antibiotics, such as streptomycin, erythromycincin, and tetracycline, inhibit protein synthesis by selectively binding bacterial ribosome subunits. Fluoroquinolones such as ciprofloxacin inhibit DNA replication directly by binding topoisomerase IV and DNA gyrase enzymes. Another way to inhibit DNA replication is to inhibit folate synthesis by binding the dihydrofolate synthase (DHPS) enzyme, which is necessary for DNA replication. Sulfonamide antibiotics inhibit DNA replication by blocking folate synthesis. The most commonly used antibiotics, \(\beta\)-lactams such as penicillin, target peptidoglycan synthesis by inhibiting the transpeptidase enzyme, which is used for crosslinking reactions of the peptidoglycan layers [59]. Thus, it is not surprising that \(\beta\)-lactams are highly successful against Gram-positive bacteria.  
This mode of action of antibiotics provides insights into the engineering of adsorbents. Successful adsorbents can be produced by generating pore structures in biochar that allow for the diffusion of antibiotics and create similar surface functional groups to those contained by antibiotic-binding enzymes. The modification of the biochars for the removal of antibiotics is reviewed in the next section.