Bacteriophage therapy
In the lysogenic cycle, phages can be secreted from the cell without lysing the host, though the cell is actively producing new phage particles. 30 However, in this process, the lysogenic phages can sometimes leave parts of their genetic material behind or carry portions of the host DNA with them when assembled, thereby transporting bacterial genes from one strain to another via transduction. 31 For this reason, temperate phages are not suitable for strict therapeutic use as AMR genes, toxins and other genes that code for undesired virulence traits may be transferred throughout a bacterial population during an infection, which may give rise to a deteriorating situation of the current AMR crisis. Bacterial immunity against phage infection is often present. This may be done by altering the membrane protein receptors or by directly degrading the injected phage genome. Bacterial cells contain short DNA repeats known as CRISPR (clustered regularly interspaced short palindromic repeats). CRISPR spacers are found to be homologous (highly similar) to viral (such as those of phages) DNA. When certain bacteria are infected by a newly encountered phage, new CRISPR spacers are added as a ‘record’ of the encountered phage and its specific genome (usually DNA fragments). Consequently, during secondary infection by the same phage, the ‘familiar’ phage genome that is homologous to the recorded CRISPR sequence will be recognized, targeted, and destroyed with the help of Cas protein. These ‘adaptive and heritable’ immune systems are referred to as CRISPR -Cas systems. 32 Of interest, in terms of the temperate phages that infect the Bacillus species, the transition between lytic and lysogenic cycles is dependent upon the relative abundance of phages in an area measured by the concentration of a small-six-amino-acids-long communication peptide released by other individual phages. 33 In addition, phages have also been demonstrated to regularly interact with cells in our innate immune system. 34
Current clinical uses and limitations of phages
The unique biological characteristics of phages, in particular virulent phages, make them become potentially viable and valuable tools for the clinical management of bacterial infections, as well as a number of other diseases, such as cancers. 35 For bacterial infections, it is generally agreed by phage researchers that only virulent phages are suitable for this task ( the reason has been outlined in the above section). Phages’ high target specificity necessitates a delicate procedure in order to ensure that the correct, active phages against the bacterial target are selected for the final therapeutic application. The process begins with identifying the responsible strain(s) of bacteria. Provided that the techniques used are correct and optimized, the results of phage typing (i.e., a phagogram) can be made available very quickly within 24 hours at maximum. The time it takes to produce plaques in the petri dish is usually within a few hours. Alternative methods include DNA sequencing (relatively time-consuming), RiboPrinter ® analysis (within 9 hours). 36 An antibiogram is also taken to determine the responsible pathogenic ba cteria’s sensitivity to antibiotics. After this process, the preparation of phages begins. From scratch, there are four main stages in this typically 16- to 21-day procedure: isolation (e.g., from sewage or against a library of phages), amplification (litre-scale and small-scale), concentration, and purification (via
30 Bacteriophages 2016. 31 Steward 2018. 32 Mojica et al. 2005. 33 Erez et al. 2017.
34 Carroll-Portillo & Lin 2019. 35 Thiel 2004; Bar et al. 2008. 36 Astudillo Skliarova 2021.
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