Penicillins Summeries

Penicillin induced lysis is enzymatically mediated and not just due to cell wall disruption in actively growing cells. This was shown in S. pnuemoniae and S. aureus where murein-hydrolase defective bacteria failed to lyse in the presence of penicillin.

Although penicillin-induced lysis has been shown to be murein-hydrolase dependant, penicillin still has a potent bactericidal effect in the absence of murein-hydrolase, strongly suggesting that penicillin-induced lethality does not require murein-hydrolase activity and that lethality and lysis are two seperate events.Induced lysis is likely to be a secondary event that occurs only after loss of viability.

Signalling involves the secretion of a "death peptide" providing evidence that stationary-phase autolysis in S. pnuemoniae may be controlled by quorum sensing.

Based on the similarities of lrgAB and holin/antiholin and the fact that the lrgAB operon is part of a regulon involved in the control of murein-hydrolase activity, it was proposed that LrgA has a holin-/antiholin-like function, regulating bacterial murein hydrolase export.

The results of a study indicated that the lrgAB mutation causes increased murein hydrolase activity in the extracellular fraction.The lrgAB-expressing plasmid also reduces the extracellular murein hydrolase activity of the wildtype strain, indicating that the level of lrgAB expression is an important element of this phenotypic effect.

The lrgAB mutation decreased the tolerance of late-log phase cells to the killing effects of penicillin. This suggests that the lrgAB genes are not expressed during this stage of growth. In agreement with this is the observation that expression of the lrgAB operon during this growth stage using a constitutive promoter results in inreased tolerance to penicillin. It has been indicated that this operon is minimally expressed during the early exponential phase of growth.

Agr and Sar are regulators of staph virulence and a Agr-defective strain exhibits a reduced rate of autolysis compared with the parental strain, whereas a mutation in the Sar gene results in an increased autolysis rate. In addition to regulating virulence factor expression, the Agr and Sar regulators could play a significant role in the in vivo suseptibility of S. aureus to penicillin.It has been conclusively demonstated that Agr and Sar do indeed regulate lrgAB expression.

Cont.The expression of the lrgAB genes in the Agr-defective strain was reduced approximately 6 times whereas lrgAB expression in the Sar-defective strain was undetectable. The observation that Agr positively regulates lrgAB (which encodes an inhitor of murein hydrolase activity) suggests that the positive impact of this regulatory system on the autolysis rate might be mediated by additional, as of yet unidentified Agr-regulated factors.

Hypothesis that LrgA could inhibit murein hydrolase export, much like the bacteriophage Lambda S107 antiholin. Recent evidence suggests that an effector of this system, analogous to the lambda S105 holin, is encoded by a separate gene located in an unlinked region of the S. aureus genome. The relative activities of these protein could provide a means by which the activity of murein hydrolases, potentailly lethal enzymes, is regulated.

It is envisioned that in the presence of penicillin, these holin-like proteins cause membrane lesions, leading to the collapse of the membrane potentail, effectively causing the cells to bleed to death.

As inhibitors of a putative bacterial holin, the lrgAB gene products could be a key component of the observed negative regulation of murein hydrolase activity. It has been demonstarted that many murein hydrolases are physiologically active only durin the stationary phase, even though they are constitutively expressed throughout the growth cycle.

It is clear that peptidoglycan assembaly involves a complex system of proteins, each with highly specialised functions. A delecate balance exists between the PBP's, which are required for peptidoglycan polymerisation, and the murein hydrolases, which provide for cell wall expansion, septum formation and daughter cell separation.

Bacterial action of penicillin