Focusing on the human pathogen Staphylococcus aureus, new research led by scientists from the University of Sheffield in the U.K., examines how bacteria maintain their unique cell walls and how antibiotics can disrupt these maintenance mechanisms to kill.
The cell wall—a strong and dynamic polymeric net of sugars and amino acids (peptidoglycans)—envelops most bacteria and is essential for bacterial growth, division, and survival. Antibiotics such as beta-lactams—penicillin, methicillin, cephalosporin—and non-beta-lactams such as vancomycin, attack the peptidoglycan cell wall to quell bacterial infections. But how exactly antibiotics accomplish this has been unclear.
The research team including scientists from Xiamen University in China, Masaryk University in the Czech Republic, and McMaster University in Canada, revealed two bactericidal mechanisms triggered by antibiotics—one linked to bacterial growth and the other to their division. These mechanisms depend on two peptidoglycan disrupting enzymes (peptidoglycan hydrolases) both of which create holes that span the entire thickness of the cell wall. These perforations enlarge as the bacteria grow, eventually killing them.
The study, which presented a model of how bacteria balance peptidoglycan production and hydrolysis and shed new light on how antibiotics target the cell wall to kill bacteria, was published in an article in the Proceedings of the National Academy of Sciences, titled “Demonstration of the role of cell wall homeostasis in Staphylococcus aureus growth and the action of bactericidal antibiotics.” The scientists plan to exploit the insights revealed in the study to create new therapeutics for antibiotic-resistant superbugs.
The first documented penicillin treatment was carried out in Sheffield in 1930 by Cecil George Paine, a member of the University’s pathology department. Paine treated an eye infection with a crude filtrate from a penicillin-producing mold supplied by his lecturer, Alexander Fleming.
“Penicillin and other antibiotics in its class have been a centerpiece of human healthcare for over 80 years and have saved over 200 million lives. However, their use is severely threatened by the global spread of antimicrobial resistance,” said Simon Foster, PhD, from the University of Sheffield’s School of Biosciences. “Concentrating on the superbug MRSA [methicillin resistant S. aureus], our research revealed that the antibiotics lead to the formation of small holes that span the cell wall that gradually enlarge as part of growth-associated processes, eventually killing the bacteria. We also identified some of the enzymes that are involved in making the holes.”
In their earlier studies, the team described the molecular architecture of the peptidoglycan mesh using atomic force microscopy, as an expanded hydrogel whose external surface is a porous open network but whose internal surface is smoother and denser. Based on this architecture the scientists determined the role of coordinated peptidoglycan synthesis and hydrolysis in S. aureus growth, division, and antibiotics-induced bactericidal action, in their current study.
“Both methicillin and vancomycin treatment leads to the appearance of perforating holes throughout the cell wall due to peptidoglycan hydrolases,” the authors noted.
Methicillin alone also results in plasmolysis—a process where the protoplasm shrinks away from the cell wall. During bacterial cell division, an extension of the cell wall called the septum forms down the middle of the cell with each new partition resulting in an independent cell. Methicillin also causes swollen and misshapen septa—a process that is inhibited by vancomycin.
The authors showed that while the inhibition of peptidoglycan hydrolase activity in the presence of cell wall antibiotics reduces killing, the deregulation of hydrolase activity brought about by the loss of teichoic acids—a copolymer that fortifies bacterial cell walls—increases the death rate.
Based on these findings and how the enzymes involved in making holes in the cell wall are controlled, the scientists also showed the efficacy of a novel combination therapy against S. aureus.
Foster added, “Our findings get to the heart of understanding how existing antibiotics work and give us new avenues for further treatment developments in the face of the global pandemic of antimicrobial resistance.”
The study showed that the independent regulation of cell wall synthesis and hydrolysis in S. aureus results in growth, equilibrium, or death, and points to new means of controlling this deadly pathogen.