TABLE 3.

Periplasmic elements other than penicillin binding proteins related to PGN biology

Target	Role(s)	Species	Effect(s) of target disruption	Reference(s)
Targets related to cell septation/division
AmiA, AmiB, AmiC	Periplasmic N-acetylmuramyl-l-alanine amidases that cleave stem peptides from glycan chains on PGN	E. coli	Formation of abnormal septa, causing growth in long chains of unseparated cells	64, 65
YebA, EnvC, and NlpD	LytM factors participating through their peptidase activity in cell elongation and division	H. influenzae	Alterations in division with irregular cell architecture and massive membrane blebbing for YebA and NlpD knockout mutants; a reduction in the amount of periplasmic proteins and an impaired capacity to adhere to epithelial cells, to form biofilms, and to resist the bactericidal power of serum with EnvC deletion	76
AmiC	A periplasmic N-acetyl-muramyl-l-alanine amidase that cleaves stem peptides from glycan chains on PGN	Burkholderia spp.	In daughter cells, an inability to separate, growing in filaments and losing motility, all of which leads to an inability to survive in the rat model and colonize the insect gut	68, 69
AmiC	A periplasmic N-acetylmuramyl-l-alanine amidase that cleaves the peptide side chains linked to the glycan strands on PGN and whose activity is potentiated by the presence of the regulator NlpD	X. campestris	Impaired daughter cell separation, aberrant cell and colony morphology, and impaired T3SS performance	70
AmiA	A periplasmic N-acetylmuramoyl-l-alanyl amidase essential for daughter cell separation	H. pylori	Appearance of long chains of unseparated cells and impaired motility; a reduced capacity for colonization of the stomach in a mouse model	66
AmiB	A periplasmic N-acetylmuramyl–l-alanine amidase involved in stem peptide cleavage from PGN chains	P. aeruginosa	Filamentous growth with a marked deficiency in the invagination of the inner membrane and increased permeability of the outer membrane	67
PBP3SAL	A specialized PGN synthase enabling formation of the division septum and promoting cell division in the acidic intraphagosomal environment	S. enterica serovar Typhimurium	Induction of PBP3SAL during infection favoring higher bacterial loads in murine models of infection when a wild type was compared with a knockout mutant	71
AmiA, AmiC, and SufI and the pathways for their correct expression (Cpx system) and export to the periplasm (Tat)	Periplasmic N-acetylmuramoyl-l-alanyl amidases that cleave stem peptides from glycan chains on PGN (AmiA and AmiC) and a divisomal transpeptidase (SufI)	S. Typhimurium (also E. coli)	For AmiA and AmiC, impairment in septation and separation of daughter cells (filamentation) and virulence attenuation in a BALB/c mouse infection model; for SufI, increased sensitivity to detergents and cationic antimicrobial peptides and impaired motility	72–75
Modification of PGN related to morphogenesis
PgdA	An N-deacetylase for PGN modification	H. pylori	Decreased resistance of PGN to lytic activity of lysozyme; double mutant (PgdA-PatA) impaired for mouse colonization	81
PatA	A putative O-acetyltransferase for PGN modification	H. pylori	Decreased resistance of the PGN to lytic activity of lysozyme; double mutant (PgdA-PatA) impaired for mouse colonization	81
Csd1 to Csd3 and CcmA	Periplasmic endopeptidase homologues essential to allow a decrease in PGN cross-linking levels	H. pylori	In single mutants, a curved instead of a helical shape, which was related to a decrease in colonization capacity in mouse stomach model competitions	80
Ape1	An O-acetylpeptidoglycan esterase responsible for de-O-acetylation of PGN	C. jejuni	Changes in PGN biochemistry; defects in virulence-associated features, including motility, biofilm formation, sodium deoxycholate resistance, adhesion, invasion, intracellular survival, induction of IL-8 release, and impairment of chick colonization	79
Pgp1	A d,l-carboxypeptidase involved in maintenance of cell shape, cleaving monomeric tripeptides to dipeptides	C. jejuni	Loss of helical shape; increase in the level of stimulation of NOD-1 receptors and derived induction of IL-8 release; decreased motility and biofilm formation; deficient for chick colonization	77
Pgp2	An l,d-carboxypeptidase involved in maintenance of cell shape, converting PGN tetrapeptides into tripeptides, which in turn are substrates for Pgp1	C. jejuni	Loss of helical shape; defective in motility on semisolid agar and biofilm formation; reduced fitness in chick colonization model	78
LtgA and LtgD	Nonessential, nonredundant lytic transglycosylases	N. gonorrhoeae	Decreased envelope integrity, leading to increased susceptibility to lysozyme and neutrophil killing	82
PGN-degrading enzymes related to sacculus remodeling to allow elongation and recycling
MltB	A membrane-bound lytic murein transglycosylase participating in PGN degradation	N. meningitidis	Inability to cause systemic infection in an infant rat model	84
MtgA	A biosynthetic transglycosylase	Brucella spp.	Upregulation during infection; in a knockout mutant, lower virulence in a mouse infection model	85, 86
MltE	A membrane-bound lytic transglycosylase	Erwinia amylovora	Upregulation during infection; in a knockout mutant, reduced virulence and growth in a pear model	85, 87
90_A18ORF1	A soluble lytic murein transglycosylase	Haemophilus influenzae	Upregulation of the 90_A18ORF1 gene during infection (a KO mutant is not available)	85, 88
Ipx10.11	A lytic murein transglycosylase	Pseudomonas syringae pv. tomato	Upregulation during infection; in a knockout mutant, impaired virulence in an Arabidopsis thaliana infection model	85, 89
Ddc	A d-alanine–d-alanine carboxypeptidase	A. baumannii	Hypersusceptibility to serum and polymyxin B; defective in intramacrophage survival; a drastically reduced capacity for survival in a mouse bloodstream infection model	83
ShyA, ShyB, and ShyC	Periplasmic proteins containing M23 family peptidase domains; for ShyA, a d,d-endopeptidase preferentially cleaving cross-links between tetrapeptides and located in the lateral cell wall; for ShyC, preferential location in the septum	V. cholerae	In a ShyA and ShyB double mutant, a significant growth deficiency; in a shyC mutant with depletion of ShyA, dramatic impairment of cell elongation rates and a significant increase in cell width, which would presumably affect attachment to host tissue	90