Many important cellular processes are performed by molecular machines, composed of multiple
proteins that physically interact to execute
biological functions. An example is the bacterial
peptidoglycan (PG) synthesis machine, responsible for the synthesis of the main component of the cell wall and the target of many contemporary
antibiotics. One approach for the identification of essential components of a cellular machine involves the determination of its minimal
protein composition. Staphylococcus aureus is a Gram-positive pathogen, renowned for its resistance to many commonly used
antibiotics and prevalence in hospitals. Its genome encodes a low number of
proteins with PG synthesis activity (9
proteins), when compared to other model organisms, and is therefore a good model for the study of a minimal PG synthesis machine. We deleted seven of the nine genes encoding PG synthesis
enzymes from the S. aureus genome without affecting normal growth or cell morphology, generating a strain capable of PG biosynthesis catalyzed only by two
penicillin-binding proteins, PBP1 and the bi-functional PBP2. However, multiple PBPs are important in clinically relevant environments, as bacteria with a minimal PG synthesis machinery became highly susceptible to cell wall-targeting
antibiotics, host lytic
enzymes and displayed impaired virulence in a Drosophila
infection model which is dependent on the presence of specific
peptidoglycan receptor proteins, namely PGRP-SA. The fact that S. aureus can grow and divide with only two active PG synthesizing
enzymes shows that most of these
enzymes are redundant in vitro and identifies the minimal PG synthesis machinery of S. aureus. However a complex molecular machine is important in environments other than in vitro growth as the expendable PG synthesis
enzymes play an important role in the pathogenicity and antibiotic resistance of S. aureus.