Microbiology, 2021

456 11 • Mechanisms of Microbial Genetics
Clinical Focus
Resolution
Although Mark survived his bout with necrotizing fasciitis, he would now have to undergo a skin-grafting
surgery, followed by long-term physical therapy. Based on the amount of muscle mass he lost, it is unlikely
that his leg will return to full strength, but his physical therapist is optimistic that he will regain some use
of his leg.
Laboratory testing revealed the causative agent of Mark’s infection was a strain of Staphylococcus aureus.
As required by law, Mark’s case was reported to the state health department and ultimately to the Centers
for Disease Control and Prevention (CDC). At the CDC, the strain of Staphylococcus aureus strep isolated
from Mark was analyzed more thoroughly for methicillin resistance.
Methicillin resistance is genetically coded and is increasing among strains of S. aureus through horizontal
gene transfer. Strains of S. aureus that are resistant to methicillin are typically resistant to virtually all betalactam
antibiotics and other classes of antibiotics as well. In necrotizing fasciitis, blood flow to the infected
area is typically limited because of the action of various genetically encoded bacterial toxins. This is why
there is typically little to no bleeding as a result of the incision test. Unfortunately, these bacterial toxins
limit the effectiveness of intravenous antibiotics in clearing infection from the skin and underlying tissue,
meaning that antibiotic resistance alone does not explain the ineffectiveness of Mark’s treatment.
Nevertheless, intravenous antibiotic therapy was warranted to help minimize the possible outcome of
sepsis, which is a common outcome of necrotizing fasciitis. Through genomic analysis by the CDC of the
strain isolated from Mark, several of the important virulence genes were shown to be encoded within
pathogenicity islands that were associated with prophages. Horizontal transfer of pathogenicity islandencoded
virulence factors between strains of S. aureus has been shown to occur through induction of
prophage and can be induced by treatment with antibiotics.
Go back to the previous Clinical Focus box.
Access for free at openstax.org.