ThedltOperon ofBacillus cereusIs Required for Resistance to Cationic Antimicrobial Peptides and for Virulence in Insects

Abstract

The dlt operon encodes proteins that alanylate teichoic acids, the major components of cell walls of gram-positive bacteria. This generates a net positive charge on bacterial cell walls, repulsing positively charged molecules and conferring resistance to animal and human cationic antimicrobial peptides (AMPs) in grampositive pathogenic bacteria. AMPs damage the bacterial membrane and are the most effective components of the humoral immune response against bacteria. We investigated the role of the dlt operon in insect virulence by inactivating this operon in Bacillus cereus, which is both an opportunistic human pathogen and an insect pathogen. The ⌬dlt Bc mutant displayed several morphological alterations but grew at a rate similar to that for the wild-type strain. This mutant was less resistant to protamine and several bacterial cationic AMPs, such as nisin, polymyxin B, and colistin, in vitro. It was also less resistant to molecules from the insect humoral immune system, lysozyme, and cationic AMP cecropin B from Spodoptera frugiperda. ⌬dlt Bc was as pathogenic as the wild-type strain in oral infections of Galleria mellonella but much less virulent when injected into the hemocoels of G. mellonella and Spodoptera littoralis. We detected the dlt operon in three gram-negative genera: Erwinia (Erwinia carotovora), Bordetella (Bordetella pertussis, Bordetella parapertussis, and Bordetella bronchiseptica), and Photorhabdus (the entomopathogenic bacterium Photorhabdus luminescens TT01, the dlt operon of which did not restore cationic AMP resistance in ⌬dlt Bc ). We suggest that the dlt operon protects B. cereus against insect humoral immune mediators, including hemolymph cationic AMPs, and may be critical for the establishment of lethal septicemia in insects and in nosocomial infections in humans.

Lantibiotic Resistance

“…Hence, cationic antimicrobial peptides (CAMPs), such as nisin and gallidermin, are repelled from the cell envelope of target microorganisms, such as L. monocytogenes, Bacillus cereus, Clostridium difficile, Streptococcus pneumoniae, and Staphylococcus aureus (45,(47)(48)(49)(50). This is a form of innate lantibiotic resistance and becomes particularly evident in the presence of antimicrobial peptides that trigger a signaling pathway that upregulates the process (45).…”

Regulation of bacterial virulence gene expression by cell envelope stress responses

“…Consistent with this, dlt null mutants of Gram-positive pathogens have increased sensitivity to CAMPs, increased killing by human neutrophils and reduced virulence in animal models of infection. [156][157][158][159][160][161][162][163][164] The dlt operon is also one of the best examples of virulence genes controlled by ESRs in Gram-positive bacteria. Even though many Gram-positive ESRs are widely conserved, their target genes can vary significantly between species.…”

Anti-Listeria Activities of Galleria mellonella Hemolymph Proteins

“…3B). It has been shown that that deletion of the dlt operon from Bacillus cereus resulted in reduced survival of Galleria larvae upon oral infection (2). The varied in vitro and in vivo survivability of EGD-e⌬dltB in Galleria hemolymph and a whole-animal model possibly indicates the specialized role of other virulence-associated proteins, like MprF, in providing resistance against antimicrobial host defense, such as phagocytosis and cationic antimicrobial peptides.…”

“…Identification of the two-component system virR/virS in Listeria revealed its dual role in virulence and resistance against CAMPs (13,20). It is known that the transcriptional regulator VirR independently regulates expression of mprF and the genes comprising the dlt operon, which are well known for providing resistance against CAMPs of both animal and bacterial origin (2,3,20,22). MprF synthesizes the lysylphosphatidyl glycerol membrane phospholipids (22), and the dlt operon (comprising of the genes dltA, dltB, dltC, and dltD)-encoded proteins are mainly responsible for adding Dalanine residues to the cell wall-associated lipotechoic acids (LTAs) (1).…”