**5. Photodynamic antimicrobial chemotherapy (PACT/aPDT)**

PDT is not only used in cancer therapy; as previously stated, it can also be used to treat a variety of other diseases. Nowadays, the most common and commercially available photodynamic agent is 5-amino laevulinic acid for acne treatment, which is gaining popularity among millennials [30, 54]. Acne vulgaris is caused due to the clogging of hair follicles by bacteria, dirt, dust, and sebum. The main bacteria causing acne is *Propionibacterium acne*. PDT is a very effective tool in destroying this kind of antimicrobial-resistant microbes and thus can reduce the severity of acne vulgaris.

In the past itself, it is proved that porphyrins can improve perifollicular inflammatory reaction and trigger the aspect of keratinocyte-derived IL-8 by producing cytotoxic singlet oxygen [20].

Photodynamic antimicrobial chemotherapy (PACT) is more adventitious than regular microbial treatment as it can effectively destroy every infection causing micro-organisms. Even antimicrobial-resistive microbes can be destroyed using PACT [77]. The mechanism of action of PACT is mainly similar to that of the PDT mechanism. PACT also acts in the presence of photosensitizers such as hematoporphyrin derivative monomers, dimers, and oligomers, as well as cationic PS acting in the wavelength range of 610–630 nm, the light of appropriate dose and wavelength, and molecular oxygen. These three react to produce singlet oxygen, which is capable of causing cytotoxicity in microbial cells. The mode of cell death may vary from necrosis to apoptosis depending upon the intensity of applied photons.

PACT has various applications including treating oral bacterial infections and to that treating COVID-19. The use of PACT in various diseases includes:

