**4.3 Tularemia**

*Francisella. tularensis* is an intracellular pathogen that causes a disease called Tularemia. The disease is considered a potential biological threat for humans due to its extreme infectivity and substantial capacity to cause severe illness and death. The hallmark of the bacterium is their capability to survive and replicate within macrophages [113] and other cell types [114, 115]. The bacterium's survival depends on its ability to combat the microbicidal activity of macrophages such as ROS and reactive nitrogen species*. F. tularensis* require oxygen for their growth and possess ROS-scavenging enzymes such as super oxide dismutases, peroxidases, and catalases [116, 117].

#### *Superoxide Dismutase: A Key Enzyme for the Survival of Intracellular Pathogens in Host DOI: http://dx.doi.org/10.5772/intechopen.100322*

Like other bacterial pathogens, *F. tularensis* contains two types of SOD gene: FeSOD (sod-B) and CuZnSOD (sod-C). SOD-B plays a dual role in protecting *F. tularensis* from the oxidative stress of the host. SOD-B binds to the iron with high affinity and limits the availability of iron requirement to produce the highly lethal OH·. Secondly, detoxification of superoxide prevents cellular damage of DNA, proteins, and lipids associated with O2− toxicity [53, 54]. SOD-B dismutation decreasing the reaction of O2 with NO to form peroxynitrite (ONOO) and protect bacteria from ONOO- toxicity [55]. ONOO- has been shown to have a significant role in the IFN-γ -induced killing of *F. tularensis* (live vaccine strain) LVS by murine macrophages [99, 118]. However, the genome sequence of *F. tulrensis* LVS has possessed a single functional copy of the sod-B gene [117]. Hence, sod-B gene alteration leads to reduced SOD-B enzyme expression might be associated with high sensitivity to oxidative stress suggesting that sod-B is essential for bacterial survival under oxidative stress conditions. Therefore, increased survival of mice infected with sod-B mutant *F. tularensis* suggesting that SOD-B plays a role in virulence [56].

A recent study suggests SOD-C (CuZnSOD) of *F. tularensis* also plays a vital role in virulence factors. SOD-C is localized in the periplasm to protect from superoxide radicals (O2−) derived from host cells. *F. tularensis* depleted sod-C (∆sodC) mutant and *F. tularensis* ∆sodC mutant with attenuated sod-B gene expression (sodB ∆sodC) exhibited attenuated intracellular survival in IFN-γ-activated macrophages compared to the wild-type *F. tularensis* LVS. Transcomplementation of the sod-C gene in ∆sodC mutant bacteria or checking the IFN-γ-dependent production of O2− or NO enhanced the survival of the sod mutant's bacteria in macrophage. The virulence capacity of the sodB ∆sodC mutant bacteria was significantly more attenuated as compared to ∆sodC mutant. Furthermore, lack of IFN-γ, iNOS, or PHOX restored the virulence of ∆sodC mutant strains, suggesting that the CuZnSOD of the bacterium is playing a critical role in restricting the bactericidal activities of ROS and RNS. The ∆sodC and sodB ∆sodC mutants were also significantly attenuated for virulence in intranasally challenged C57BL/6 mice compared to the wild-type *F. tularensis* LVS, indicating that SOD-C is required for resisting host-generated ROS and contribute to survival and virulence of *F. tularensis* in mice [119].

#### **4.4 Staphylococcus (boils and toxic shock syndrome)**

*Staphylococcus aureus* is a gram-positive bacterium, which causes a broad spectrum of diseases in humans. It is a facultative intracellular bacterium that invades and replicates within many types of phagocytic and non-professional phagocytes cells, such as endothelial cells, mammary cells, fibroblasts, and osteoclasts [120]. Bacterium commonly symptomatically colonizes in one-third of the population of the globe and is a leading cause of antibiotic-resistant [121]. Methicillin-resistant *S. aureus* (MRSA) strains are one of the utmost dangerous species and have shown resistance to all β-lactam antibiotics as well as other antimicrobials [122]. *S. aureus* is capable of subverting xenophagy and escaping from the cytosol of the host cell during intracellular infection [118, 122, 123]. During intracellular survival, *S. aureus* is capable to protects itself from the oxidative burst by numerous mechanisms, including enzymes such as SODs that detoxify the action of ROS activity [124, 125]. *S. aureus* serves two distinct SODs, SOD-A and SOD-M, both of which are cytoplasmic and reported as Mn-dependent [57, 126]. All Staphylococci species are contained SOD-A protein, while *S. aureus* also has a unique protein SOD-M [58]. The loss of either SOD-A or SOD-M in a skin model of infection or loss of both SODs in a systemic mouse model

of infection diminishes the ability of *S. aureus* to cause disease, highlighting the importance of SOD in the virulence [127, 128].

The lack of both SODs in *S. aureus* shown bacterium is more sensitive to host cells during manganese starvation, suggesting the importance of SOD in overcoming nutritional immunity. Mn starvation in host-mediated protein calprotectin reduces staphylococcal SOD activity during in vitro and in-vivo infection. Hence, Mn deficiency renders *S. aureus* more sensitive to oxidative stress and neutrophil-mediated killing [127, 129, 130]. SOD-A protein is essential for countering oxidative stress and disease progression when manganese is abundant. At the same time, SOD-M is important under manganese-deplete conditions. However, SOD-A is strictly manganese-dependent, whereas SOD-M contains either of two or more different metal atoms, having similar enzymatic activity when filled with manganese or iron. During host-dependent Mn starvation, *S. aureus* enables the ability of SOD-M to utilize Fe to retain its SOD activity. Subsequently, *S. aureus* enhances the ability to overcome nutritional immunity, resistance to oxidative stress, and ultimately induced virulence and infection [59].
