The autolytic ('suicidase') system of Enterococcus hirae: From lysine depletion autolysis to biochemical and molecular studies of the two muramidases of Enterococcus hirae ATCC 9790

Lysozyme and Its Application as Antibacterial Agent in Food Industry

Lysozymes are hydrolytic enzymes characterized by their ability to cleave the β-(1,4)-glycosidic bonds in peptidoglycan, a major structural component of the bacterial cell wall. This hydrolysis action compromises the integrity of the cell wall, causing the lysis of bacteria. For more than 80 years, its role of antibacterial defense in animals has been renowned, and it is also used as a preservative in foods and pharmaceuticals. In order to improve the antimicrobial efficacy of lysozyme, extensive research has been intended for its modifications. This manuscript reviews the natural antibiotic compound lysozyme with reference to its catalytic and non-catalytic mode of antibacterial action, lysozyme types, susceptibility and resistance of bacteria, modification of lysozyme molecules, and its applications in the food industry.

Membrane Activity of LL-37 Derived Antimicrobial Peptides against Enterococcus hirae: Superiority of SAAP-148 over OP-145

The development of antimicrobial agents against multidrug-resistant bacteria is an important medical challenge. Antimicrobial peptides (AMPs), human cathelicidin LL-37 and its derivative OP-145, possess a potent antimicrobial activity and were under consideration for clinical trials. In order to overcome some of the challenges to their therapeutic potential, a very promising AMP, SAAP-148 was designed. Here, we studied the mode of action of highly cationic SAAP-148 in comparison with OP-145 on membranes of Enterococcus hirae at both cellular and molecular levels using model membranes composed of major constituents of enterococcal membranes, that is, anionic phosphatidylglycerol (PG) and cardiolipin (CL). In all assays used, SAAP-148 was consistently more efficient than OP-145, but both peptides displayed pronounced time and concentration dependences in killing bacteria and performing at the membrane. At cellular level, Nile Red-staining of enterococcal membranes showed abnormalities and cell shrinkage, which is also reflected in depolarization and permeabilization of E. hirae membranes. At the molecular level, both peptides abolished the thermotropic phase transition and induced disruption of PG/CL. Interestingly, the membrane was disrupted before the peptides neutralized the negative surface charge of PG/CL. Our results demonstrate that SAAP-148, which kills bacteria at a significantly lower concentration than OP-145, shows stronger effects on membranes at the cellular and molecular levels.

Bacterial peptidoglycan (murein) hydrolases

Most bacteria have multiple peptidoglycan hydrolases capable of cleaving covalent bonds in peptidoglycan sacculi or its fragments. An overview of the different classes of peptidoglycan hydrolases and their cleavage sites is provided. The physiological functions of these enzymes include the regulation of cell wall growth, the turnover of peptidoglycan during growth, the separation of daughter cells during cell division and autolysis. Specialized hydrolases enlarge the pores in the peptidoglycan for the assembly of large trans-envelope complexes (pili, flagella, secretion systems), or they specifically cleave peptidoglycan during sporulation or spore germination. Moreover, peptidoglycan hydrolases are involved in lysis phenomena such as fratricide or developmental lysis occurring in bacterial populations. We will also review the current view on the regulation of autolysins and on the role of cytoplasm hydrolases in peptidoglycan recycling and induction of beta-lactamase.