Staphylococcus aureus is a bacterium which has gathered much attention over the past decade due to the emergence of both antibiotic-resistant and hyper-aggressive strains. These pose a significant threat to human health, particularly to individuals already weakened through other illnesses.
When S. aureus enters the human bloodstream, it must adapt in order to survive the challenging conditions faced. In particular, it must respond to the nutritional environment of the blood which is depleted in essential cofactors such as free iron. Additionally, S. aureus must survive attacks from the host immune system which will attempt to kill the invader through phagocytes and the production of antibodies.
Theworkcarriedouthereaimedtounderstandhowsmall,non-codingRNAs(sRNAs) regulate S. aureus’ adaptation to the host bloodstream. These sRNAs are typically associated with regulating the translational efficiency and stability of mRNAs. Through use of a technique called “UV cross-linking, ligation and sequencing of hybrids” (CLASH), I identified novel targets of many sRNAs. In particular, I studied how RsaA, an sRNA involved in membrane homeostasis, regulates the translation of a transmembrane transporter involved in antiseptic and antibiotic resistance. Additionally, I identified interactions between RsaE, an sRNA involved in metabolism, and several toxin mRNAs from the phenol-soluble modulin class. This is a novel example of the direct link between cellular metabolism and virulence.
However, the most striking finding was that not only do sRNAs target mRNAs, but that they also target each other. I focused on two distinct sRNA – sRNA interactions; one between RsaA and RNAIII, and another between RsaE and RsaOG. The interaction between RsaE and RsaOG is an example of a so-called ’sponging interaction’, where RsaOG is able to antagonise the activity of RsaE. This has the effect of freeing RsaE’s targets from their regulation. I hypothesise that this ultimately induces the necessary metabolic changes required in order to survive the immediate nutritional stresses incurred after entering the bloodstream. Regarding RsaA and RNAIII, I hypothesise that this interaction is responsible for balancing virulent versus dormant behaviour. I suggest that RsaA is able to induce the destruction of RNAIII in order to steer the cell away from aggressive behaviour and into a more latent state. Additionally, the interaction between these two sRNAs also appears to operate on exquisitely short timescales, demonstrating how capable bacteria are at adapting to stresses.
Ultimately, this work suggests that interactions between sRNAs are likely to be widespread and form a crucial aspect of stress responses in general. The experiments detailed herein have certainly not exhausted the produced data and I suspect that it will be utilised further in the future
