Breaking down the microbiology world one bite at a time
Cut Off the Head, and the Bacteria Will Follow
Staphylococcus aureus is a major human pathogen responsible for a wide variety of infections, including bacteremia, infective endocarditis, skin and soft tissue infections, and osteomyelitis.1 A master of adaptation, strains of S. aureus have developed resistance to antibiotics such as methicillin and vancomycin; in addition, S. aureus has an arsenal of virulence factor “swords” that allow it to expertly outmaneuver and evade host immune responses.2,3 Thus, there is a dire need for new strategies to combat S. aureus infections.
S. aureus uses regulatory networks to sense environmental conditions and adjust the production of the virulence factors needed for survival, acting as the “brains” of the “swords”.4 Two such networks are the accessory gene regulator (agr) and the staphylococcal accessory regulator (sarA), both promising targets for novel antibiotics as they are the most thoroughly studied S. aureus regulatory networks. agr coordinates communication between S. aureus cells to sense their population size and controls the production of toxins during an infection.4 sarA regulates many genes, including agr, and has been previously shown to regulate the expression of extracellular proteases, which are also considered virulence factors that can contribute to evasion of host defenses, invasion of host tissue, or acquisition of nutrients. Overexpression of these proteases, however, can decrease S. aureus virulence. In this study, Beenken and her team looked at the potential for agr or sarA to be used as targets against S. aureus infection, specifically in osteomyelitis. Osteomyelitis involves the bone becoming inflamed or swollen. This condition can happen if, following an injury, an infection begins in the bone or spreads to the bone from elsewhere in the body.5
The researchers found that sarA affects the presence of virulence factors by the production of extracellular proteases. α-toxin, responsible for forming pores in the host cell’s membranes and causing cell death, was decreased in the sarA mutant, but its presence was restored and increased in the protease-deficient sarA mutant. LukS and LukF, components of the Panton-Valentine leukocidin (PVL), another pore-forming toxin, were only detected in truncated forms in the sarA mutant, while full-length forms were detected in the protease-deficient sarA mutant. These results suggested that agr activated the expression of these virulence factors, but they would not be phenotypically seen as they were rendered inactive by the extracellular proteases as a result of mutated sarA.
Mutating sarA, agr, or both reduced the bacteria’s ability to damage bone cells (both osteoblasts, bone-forming cells, and osteoclasts, bone-destroying cells). This effect was reversed when the bacteria’s proteases were also mutated. This further supports that the virulence factors detected, such as LukS and LukF, were not cytotoxic in their truncated form. Staphylococcal protein A (Spa) contributes to biofilm formation and osteoclast activation, which causes increased bone destruction during osteomyelitis. Spa was not detected in sarA mutants or sarA/agr mutants, but it was increased in agr mutants. Its presence was again restored in protease-deficient sarA and sarA/agr mutants. Mouse models of osteomyelitis showed less bone damage and fewer signs of osteomyelitis in sarA and sarA/agr mutants compared to regular strains, but this was not seen in agr mutants. Once again, this decrease was able to be reversed in protease-deficient mutants. These results clearly show how sarA regulates virulence factor expression by controlling the expression of proteases; mutation of sarA, therefore, leads to overexpression of proteases that destroy or truncate virulence factors, decreasing S. aureus virulence in osteomyelitis models.
Beenken and her team make three major arguments for sarA as a more potential target against S. aureus osteomyelitis:
- agr is not associated with biofilm formation, while sarA is. Biofilm formation is a known and crucial step in osteomyelitis, and mutation of sarA showed decreased or truncated production of biofilm virulence factors like Spa.5
- Toxins activated by agr are rendered nonfunctional by the increased production of extracellular proteases as a result of sarA mutation. The truncation of virulence factors LukS and LukF, for example, argue this conclusion, for even though these toxins were detected in the sarA mutant, they did not show an effect on cytotoxicity.
- agr mutants have been isolated from osteomyelitis patients, indicating that this may not be an ideal target. The mouse model did not show a significant difference in virulence between agr mutants and typical S. aureus strain osteomyelitis.
Continued research on potential novel antibiotic targets for S. aureus infections are necessary, especially as antibiotic resistance continues to rise. While S. aureus has a wide variety of virulence factors to harm and evade the host, these double-edged swords are believed to provide ample targets against S. aureus. This study shows that it may be best to aim not at the weapons of the enemy, but at the head, the regulatory networks like sarA, to put an end to the war against S. aureus.
Additional sources:
Urish KL, Cassat JE. Staphylococcus aureus osteomyelitis: Bone, bugs, and surgery. Infect Immun. 2020 Jun 22;88(7):e00932-19. https://doi.org/10.1128/iai.00932-19
Tong SYC, Davis JS, Eichenberger E, Holland TL, Fowler VG. Staphylococcus aureus infections: Epidemiology, pathophysiology, clinical manifestations, and management. Clin Microbiol Rev. 2015 May 27;28(3). https://doi.org/10.1128/cmr.00134-14
Vestergaard M, Frees D, Ingmer H. Antibiotic resistance and the MRSA problem. Microbiol Spectr. 2019 Mar 22;7(2). https://doi.org/10.1128/microbiolspec.gpp3-0057-2018
de Jong NWM, van Kessel KPM, van Strijp JAG. Immune evasion by Staphylococcus aureus. Microbiol Spectr. 2019 Mar 29;7(2). https://doi.org/10.1128/microbiolspec.gpp3-0061-2019
Jenul C, Horswill AR. Regulation of Staphylococcus aureus virulence. Microbiol Spectr. 2019 Apr 5;7(2). https://doi.org/10.1128/microbiolspec.gpp3-0031-2018
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