Approaches

Examples

Advantages and limitations

Biological

Gene engineering

«+»:

• Stability of genetically engineered structures; good biocompatibility; • preservation of bacterial cell viability and function;

«-»:

• technical complexity of designing genetically engineered strains; high cost;

• lack of flexibility in the attachment of therapeutic agents;

• the effectiveness of therapeutic drug release is often low.

Reducing the virulence of the bacterial strain

Salmonella typhimurium strains auxotrophic with respect to some amino acids [44]

Increasing of the bacterial tropism to tumor tissue

E. coli strains express tumor cell-specific ligands [45]

Enzymatic activation of pro-drugs inside the tumor

The modified E. coli strain specifically binds to heparan sulfate proteoglycan on the surface of colorectal cancer cells and produces the enzyme myrosinase, which converts the pro-drug glucosinolate into sulforaphane, a compound with antitumor properties [46]

Production of cytotoxic and/or immunostimulating agents inside the tumor area

The thermosensitive genetically engineered E. coli strain Nissle 1917 produces a therapeutic protein, tumor necrosis factor α (TNF-α), inside the tumor [47];

S. typhimurium strain with the reduced virulence expresses the pro-apoptotic cytokine FasL [48]

Chemical

Carbodiimide method of forming an amide (peptide) bond between the amino group of the bacterial wall and the carboxyl group of the ligand

Nanoliposomes are covalently attached to the surface of M. marinus by creating an amide bond between activated carboxyl groups of nanoliposomes and amino groups of the bacterial wall [49]

«+»:

• High stability of biohybrids;

significant load capacity

«-»:

• complex chemistry, which often requires preliminary modification of bacteria and attached components;

• the potential impact on the viability and function of bacteria, or even a change in their properties.

Biotin-streptavidin complex

Liposomes loaded with paclitaxel are attached to the surface of Salmonella typhimurium due to binding of streptavidin covering the surface of liposomes with biotin on the outer membrane proteins of the bacterium [50]

Azide-cyclooctine "click chemistry"

Cerium oxide nanoparticles are anchored on the surface of E. coli MG1655 through a reaction between the azo-dibenzocyclooctin fragment of the cerium complex and the azide group on the bacterial cell wall [51]

Biomineralization

Magnetotactic bacteria are able to synthesize magnetosomes - specific cellular organelles that are products of biomineralization of iron or other metals  [52]

Metalloorganic imidazolate frameworks

The ZIF-8 imidazolate framework loaded with doxorubicin is formed on the surface of E. coli strain MG1655 as a result of the reaction of 2-methylimidazole with ZnSO₄ in the presence of the bacterium [53]

Physical

Electrostatic interaction between the elements of biohybrid structures and the bacterial wall

The "sandwich" encapsulating the probiotic Bacillus coagulans strain is formed by alternating layers of cationic chitosan polysaccharide and anionic alginate polysaccharide due to electrostatic interaction forces [54]

«+»:

• Simplicity of the modification

• versatility in attaching various components

• potentially negligible effect on bacterial viability

«-»:

• potentially low stability of the biohybrid

• limited load capacity.