**7. sRNA expressions in infections**

In *S.aureus*, some of the known sRNAs with their targets involve regulating major biochemical pathways that are further responsible for producing virulence factors [56]. *Staphylococcus aureus* sRNAs were identified using different techniques in various strains, and their expression profiling during the course of infections in humans was studied. Functions of around 250 sRNAs expressed under different conditions are yet unknown. But the expression profiling of *RNAIII* in clinical isolates from nasal cystic fibrosis patients was studied. In most of the clinical isolates of acute infections, *RNAIII* has been expressed in in-vivo conditions [57, 58]. Therefore these data infers that *RNAIII* majorly involves in the regulation of virulence factors and production of *agr*-defective mutants [59]. However, there has been a difference in the variation of agr-defective mutants in healthy and infected patients. Thus, agr regulation occurs during acute infections, whereas the agr mutants expression can only be observed during the stages of the chronic or dormant infection [60].

Expression profiling of five different small RNAs like *RNAIII*, *RsaE*, *RsaH*, *RsaG* and *RsaA* in *S. aureus* strains isolated from three conditions, including cutaneous infections, chronic cystic fibrosis and commensal nasal colonization [61]. Expression patterns of five small RNAs were strain-specific and do not have any correlations with respect to the variations of the infectivity pattern or colonization. However, it has been observed that there was a uniform expression pattern among the commensal strains in comparison to the infectious strains. Therefore, these results show that *S.aureus* was mainly a commensal strain and became an opportunistic pathogen [62, 63].

*S. aureus* regulatory RNA, SSR42, which modulates the expression of approximately 80 mRNA species, including several virulence factors, in *S.aureus* strains UAMS-1 and USA300 (LAC) during stationary-phase growth. Mutagenesis studies revealed that SSR42 codes for an 891-nucleotide RNA molecule and that the full-length transcript mediates the molecule's regulatory effects. Western blotting and functional assays indicated that the regulatory effects of SSR42 correlate with biologically significant changes in corresponding protein abundances. Further, in *S.aureus* strain LAC, SSR42 is required for wild-type levels of erythrocyte lysis, resistance to human polymorphonuclear leukocyte killing, and pathogenesis in a murine model of skin and soft tissue infection (**Figure 3**) [65].

#### **Figure 3.**

S. aureus *sRNAs from the RNome implicated in bacterial virulence. Multitasking RNAIII is the effector of quorum sensing to perceive population density and regulates multiple targets involved in peptidoglycan synthesis, adhesions, exotoxins production and virulence. RNAIII internally encodes hemolysin represented in blue color. It contains three repressor domains which are represented in red color, containing accessible UCCC motifs that interact with antisense pairings, with the ribosome binding sites of numerous target mRNAs for translational repression (Tr.R), some triggering endoribonuclease III (RNase III) cleavages to induce target mRNA degradations and irreversible gene expression decay. Translation of at least two exotoxins is activated by RNAIII, one encoded (hld), and another (hla) by translation activation (Tr.A). SprD is expressed from the genome of a converting phage and interacts, by antisense pairings, with the 59 part of the sbi mRNA encoding an immune evasion molecule. SprD possesses an important role in* S. aureus *virulence, but the mechanism of its control is yet to be elucidated, with Sbi being only one player among others. The 891-nucleotide long SSR42 affects extracellular virulence expression, hemolysis, neutrophil virulence, and pathogenesis and contains a putative internal ORF. The mechanisms of target regulation remain to be elucidated. The SCCmec-encoded psm-mec RNA suppresses agrA translation and attenuates MRSA virulence, acting as a dual-function RNA regulator [64].*

## **8. Pathogenicity Island encoded RNAs**

SCCmec (Staphylococcal Cassette Chromosome mec) is responsible for the regulation of antibiotic resistance genes, particularly for the methicillin resistance genes in facultative *S.aureus*. Thus it helps the pathogen to adapt under different stress conditions for survival in the hosts. Elements involved in these processes are genomic islands, transposons, plasmids, and the pathogenicity islands (PIs) acquired horizontally and encode various virulence factors like toxins and cell attachment factors, superantigens factors, invasion factors and two-component system [66]. Apart from the protein-coding genes, it pathogenicity islands also codes for phage-related genes and involves sRNA [67]. Several sRNAs are found in

#### Staphylococcus aureus *and Virulence-Related Small RNA DOI: http://dx.doi.org/10.5772/intechopen.98520*

numerous copies distributed encircling the *S.aureus* genome and also some additional copies are present in the plasmids. Multiple copies are present due to either repeated events of gene duplication or horizontally gene transfer [68].

However, the sRNAs expressed from *S.aureus* Pathogenicity Islands (SaPIs) were involved in the regulation of gene expression present on the regions of cognate PIs (**Table 1**). Therefore it forms the functional linkage between the PIs and the genome of the organisms. Expression of SprD (Small Pathogenicity islands D) by PIw involves repression of sbi mRNAs during the initiation process of translation, which encodes an immune evasion molecule [69]. A central hairpin of SprD binds with the sbi mRNA RBS and thus prevents the initiation of the translation process. SprD sRNA has a prominent effect on virulence factors, it involves different pathways for regulating staphylococcal infectivity by altering the expression patterns of SprD. Several other sRNAs are also responsible for pathogenicity through regulatory networks by either direct or indirect way and other translational process regulatory networks. However, from the recently determined sRNAs, 4 are present in PIs and other 6 are in the SCCmec mobile element, with 54 to 400 nucleotides long in size [70].

Teg152 and SprF are two sRNAs that are completely complementary to other two sRNAs SprA1 and SprG. In type I TA (Toxins-Antitoxins) modules, the pairing of SprA1 with Teg152 and SprG with SprF sRNAs takes place. SprA and SprG encodes smaller hydrophobic peptides [53]. SprA1 is a multifunctional sRNA with pressumed antisense function. It's 3-end pairs with 39-UTRs region of three different mRNA targets. The independent transcriptional regulation is responsible for synthesizing appropriate expression levels of sRNAs for effective functional regulation [71].


#### **Table 1.**

*Groups of virulence factors encoded by PAI (pathogenicity islands).*
