FIGURE 1.

Representation of main mechanisms of bacterial resistance against phage infection (from left in a clockwise sense). 1. (A) Phage recognizes bacterial membrane receptor and can carry out infection; (B) alterations in receptors are produced by mutations and prevent phage from recognizing receptor, so it Will not infect bacteria; (C) bacteria can block recognition by producing inhibitors that bind to receptors. 2. Production of exopolysaccharide or extracellular matrix. 3. OMVs are composed of membrane lipids, membrane proteins, and periplasmic components. Some bacterial species use them as a decoy against phages as a defense mechanism. 4. (A) Bacteria block injection of DNA from other phages, acquiring Sie systems through prophages with this type of protein; (B) once bacterium has prophage in its genome with proteins that code for Sie system, it will be able to block entry of DNA from other phages. 5. (A) R–M system distinguishes between methylated and unmethylated DNA. Restriction enzymes cannot cut methylated DNA, which is also a protection system for bacterial DNA; (B) if phage DNA is not methylated, this system can cut injected DNA. 6. CRISPR-Cas system is an adaptative defense system, which recognizes phage DNA sequences, incorporates them into system, and then produces enzymes that are capable of recognizing these sequences to cut them. 7. PICI system is found in bacterial genome and induced by helper prophages to produce mature phage particles that assemble PICI system itself to kill infected cells and to be able to spread this system to adjacent cells. 8. (A) One of most characterized Abi systems is toxin–antitoxin system. Under normal conditions, bacterium expresses both proteins equally, so cell death does not occur; (B) When organism is subjected to stress situations, such as phage infection, toxin is highly expressed in comparison with antitoxin, causing cell death, which consequently produces a reduction in phage replication.