Diverse effects of bacterial cell wall components on mast cell degranulation,cysteinyl leukotriene generation and migration

Nowadays there is more and more evidence that mast cells take part in antibacterial defence. Mast cells have the ability to kill bacteria via phagocytose‐dependent or phagocytose‐independent ways and express antimicrobial peptides that can directly kill pathogens at their site of entry. What is more, mast cells are capable of processing bacterial antigens for presentation through class I and II MHC molecules. Some data indicate that these cells can release various proinflammatory mediators in response to activation with bacteria and/or their products, however this information is still far from complete. Therefore, in this study we examined the ability of PGN from Staphylococcus aureus, LPS from Eschericha coli and LAM from Mycobacterium smegmatis to stimulate mature rat mast cell degranulation as well as cysteinyl LT generation. We also studied the influence of these bacterial components on mast cell migration. We found that PGN, LPS and LAM all failed to induce mast cell degranulation and histamine release. At the same time, activation of mast cells with these bacterial antigens resulted in generation and release of significant amounts of LT. Moreover, we documented that, even in the presence of laminin, none of the bacterial antigens used stimulated mast cell migration. However, PGN did induce migration of RANTES‐primed mast cells, and LPS did stimulate mast cell migratory response after priming with IL‐6. Our results show that PGN, LPS and LAM might be among the important bacterial antigens involved in mast cell activation during bacterial infection.     

Internalization of Clostridium perfringens α‐toxin leads to ERK activation and is involved on its cytotoxic effect

Clostridium perfringens phospholipase C (CpPLC), also called α‐toxin, plays a key role in the pathogenesis of gas gangrene. CpPLC may lead to cell lysis at concentrations that cause extensive degradation of plasma membrane phospholipids. However, at sublytic concentrations it induces cytotoxicity without inducing evident membrane damage. The results of this work demonstrate that CpPLC becomes internalized in cells by a dynamin‐dependent mechanism and in a time progressive process: first, CpPLC colocalizes with caveolin both at the plasma membrane and in vesicles, and later it colocalizes with early and late endosomes and lysosomes. Lysosomal damage in the target cells is evident 9 h after CpPLC exposure. Our previous work demonstrated that CpPLCinduces ERK1/2 activation, which is involved in its cytotoxic effect. In this work we found that cholesterol sequestration, dynamin inhibition, as well as inhibition of actin polymerization, prevent CpPLC internalization and ERK1/2 activation, involving endocytosis in the signalling events required for CpPLC cytotoxic effect at sublytic concentrations. These results provide new insights about the mode of action of this bacterial phospholipase C, previously considered to act only locally on cell membrane.     

Porphyromonas gingivalis induces the production of interleukin‐31 by human mast cells,resulting in dysfunction of the gingival epithelial barrier

Interleukin (IL)‐31 is important for innate immunity in mucosal tissues and skin, and increased IL‐31 expression participates in the pathogenesis of chronic inflammatory diseases affecting the skin, airways, lungs, and intestines. We investigated the contribution of mast cells to the induction of IL‐31 production following infection with the periodontal pathogen, Porphyromonas gingivalis. We found that oral infection with P. gingivalis increased IL‐31 expression in the gingival tissues of wild‐type mice but not in those of mast cell‐deficient mice. The P. gingivalis‐induced IL‐31 production by human mast cells occurred through the activation of the JNK and NF‐κB signalling pathways and was dependent on the P. gingivalis lysine‐specific protease gingipain‐K. P. gingivalis infection induced IL‐31 receptor α and oncostatin M receptor β expression in human gingival epithelial cells. Notably, the P. gingivalis‐induced IL‐31 production by mast cells led to the downregulation of claudin‐1, a tight junction molecule, in gingival epithelial cells, resulting in an IL‐31‐dependent increase in the paracellular permeability of the gingival epithelial barrier. These findings suggest that IL‐31 produced by mast cells in response to P. gingivalis infection causes gingival epithelial barrier dysfunction, which may contribute to the chronic inflammation observed in periodontitis.     

The focal complex of epithelial cells provides a signalling platform for interleukin‐8 induction in response to bacterial pathogens

Bacterial pathogens can induce an inflammatory response from epithelial tissues due to secretion of the pro‐inflammatory chemokine interleukin‐8 (IL‐8). Many bacterial pathogens manipulate components of the focal complex (FC) to induce signalling events in host cells. We examined the interaction of several bacterial pathogens with host cells, including Campylobacter jejuni, to determine if the FC is required for induction of chemokine signalling in response to bacterial pathogens. Our data indicate that secretion of IL‐8 is triggered by C. jejuni, Helicobacter pylori and Salmonella enterica serovar Typhimurium in response to engagement of β₁ integrins. Additionally, we found that the secretion of IL‐8 from C. jejuni infected epithelial cells requires FAK, Src and paxillin, which in turn are necessary for Erk 1/2 recruitment and activation. Targeting the FC component paxillin with siRNA prevented IL‐8 secretion from cells infected with several bacterial pathogens, including C. jejuni, Helicobacter pylori, Salmonella enterica serovar Typhimurium, Staphylococcus aureus, Pseudomonas aeruginosa, and Vibrio parahaemolyticus. Our findings indicate that maximal IL‐8 secretion from epithelial cells in response to bacterial infection is dependent on the FC. Based on the commonality of the host response to bacterial pathogens, we propose that the FC is a signalling platform for an epithelial cell response to pathogenic organisms.     

A critical role for CARD9 in pneumocystis pneumonia host defence

Caspase recruitment domains‐containing protein 9 (CARD9) is an adaptor molecule critical for key signalling pathways initiated through C‐type lectin receptors (CLRs). Previous studies demonstrated that Pneumocystis organisms are recognised through a variety of CLRs. However, the role of the downstream CARD9 adaptor signalling protein in host defence against Pneumocystis infection remains to be elucidated. Herein, we analysed the role of CARD9 in host defence against Pneumocystis both in CD4‐depleted CARD9^?/? and immunocompetent hosts. Card9 gene‐disrupted (CARD9^?/?) mice were more susceptible to Pneumocystis, as evidenced by reduced fungal clearance in infected lungs compared to wild‐type (WT) infected mice. Our data suggests that this defect was due to impaired proinflammatory responses. Furthermore, CARD9^?/? macrophages were severely compromised in their ability to differentiate and express M1 and M2 macrophage polarisation markers, to enhanced mRNA expression for Dectin‐1 and Mincle, and most importantly, to kill Pneumocystis in vitro. Remarkably, compared to WT mice, and despite markedly increased organism burdens, CARD9^?/? animals did not exhibit worsened survival during pneumocystis pneumonia (PCP), perhaps related to decreased lung injury due to altered influx of inflammatory cells and decreased levels of proinflammatory cytokines in response to the organism. Finally, although innate phase cytokines were impaired in the CARD9^?/? animals during PCP, T‐helper cell cytokines were normal in immunocompetent CARD9^?/? animals infected with Pneumocystis. Taken together, our data demonstrate that CARD9 has a critical function in innate immune responses against Pneumocystis.     

Repression of bacterial lipoprotein production by Francisella novicida facilitates evasion of innate immune recognition

Innate recognition systems, including the Toll‐like receptors (TLRs), play a critical role in activating host defences and proinflammatory pathways in response to infection. Pathogens have developed strategies to subvert TLRs in order to survive and replicate within the host. The model intracellular pathogen, Francisella novicida, modulates host defences to promote survival and replication in macrophages. TLR2, which recognizes bacterial lipoproteins (BLPs), is critical for activating host defences and proinflammatory cytokine production in response to Francisella infection. Here we show that the F. novicida protein FTN_0757 acts to repress BLP production, dampening TLR2 activation. The ΔFTN_0757 mutant strain induced robust TLR2‐dependent cytokine production in macrophages compared with wild‐type bacteria, and produced increased amounts of BLPs. The deletion of one BLP (FTN_1103) from ΔFTN_0757 decreased the total BLP concentration to near wild‐type level sand correlated with a decrease in the inductionof TLR2 signalling. The overproduction of BLPs also contributed to the in vivo attenuation of the ΔFTN_0757 mutant, which was significantly rescued when FTN_1103 was deleted. Taken together, these data reveal a novel mechanism of immune evasion by the downregulation of BLP expression to subvert TLR2 activation, which is likely used by numerous other intracellular bacterial pathogens.     

Dysregulated haemolysin promotes bacterial outer membrane vesicles‐induced pyroptotic‐like cell death in zebrafish

Inflammasomes are important innate immune components in mammals. However, the bacterial factors modulating inflammasome activation in fish, and the mechanisms by which they alter fish immune defences, remain to be investigated. In this work, a mutant of the fish pathogen Edwardsiella piscicida (E. piscicida), called 0909I, was shown to overexpress haemolysin, which could induce a robust pyroptotic‐like cell death dependent on caspase‐5‐like activity during infection in fish nonphagocyte cells. E. piscicida haemolysin was found to mainly associate with bacterial outer membrane vesicles (OMVs), which were internalised into the fish cells via a dynamin‐dependent endocytosis and induced pyroptotic‐like cell death. Importantly, bacterial immersion infection of both larvae and adult zebrafish suggested that dysregulated expression of haemolysin alerts the innate immune system and induces intestinal inflammation to restrict bacterial colonisation in vivo. Taken together, these results suggest a critical role of zebrafish innate immunity in monitoring invaded pathogens via detecting the bacterial haemolysin‐associated OMVs and initiating pyroptotic‐like cell death. These new additions to the understanding of haemolysin‐mediated pathogenesis in vivo provide evidence for the existence of noncanonical inflammasome signalling in lower vertebrates.     

Effect of a transient perturbation on marine bacterial communities with contrasting history

Aims: To evaluate the importance of the bacterial composition on the resilience of the organic matter assimilation in the sea. Methods and Results: Chemostats were inoculated with coastal and offshore bacterial communities. Bacterial density and protein synthesis increased before stabilizing, and this response to confinement was more marked in the offshore chemostats. Before the toluene perturbation the community structure in the coastal chemostats remained complex whereas the offshore chemostats became dominated by Alteromonas sp. After the perturbation, bacterial protein synthesis was inhibited before peaking briefly at a level fivefold to that observed before the perturbation and then stabilizing at a level comparable to that before the perturbation. Alteromonas dominated both the coastal and the offshore communities immediately after the perturbation and the coastal communities did not recover their initial complexity. Conclusions: Cell lysis induced by the toluene perturbation favoured the growth of Alteromonas which could initiate growth rapidly in response to the nutrient pulse. Despite their different community structure in situ, the resilience of protein synthesis of coastal and offshore bacterial communities was dependent on Alteromonas, which dominated in the chemostats. Significance and Impact of the Study: Here we show that although Alteromonas sp. dominated in artificial offshore and coastal communities in chemostats, their response time to the shock was different. This suggests that future perturbation studies on resilience in the marine environment should take account of ecosystem history.     

Neutral sphingomyelinase 2 is a key factor for PorB‐dependent invasion of Neisseria gonorrhoeae

Disseminated gonococcal infection (DGI) is a rare but serious complication caused by the spread of Neisseria gonorrhoeae in the human host. Gonococci associated with DGI mainly express the outer membrane protein PorB_IA that binds to the scavenger receptor expressed on endothelial cells (SREC‐I) and mediates bacterial uptake. We recently demonstrated that this interaction relies on intact membrane rafts that acquire SREC‐I upon attachment of gonococci and initiates the signalling cascade that finally leads to the uptake of gonococci in epithelial cells. In this study, we analysed the role of sphingomyelinases and their breakdown product ceramide. Gonococcal infection induced increased levels of ceramide that was enriched at bacterial attachment sites. Interestingly, neutral but not acid sphingomyelinase was mandatory for PorB_IA‐mediated invasion into host cells. Neutral sphingomyelinase was required to recruit the PI3 kinase to caveolin and thereby activates the PI3 kinase‐dependent downstream signalling leading to bacterial uptake. Thus, this study elucidates the initial signalling processes of bacterial invasion during DGI and demonstrates a novel role for neutral sphingomyelinase in the course of bacterial infections.     

Protective role of mouse mast cell tryptase Mcpt6 in melanoma

Tryptase‐positive mast cells populate melanomas, but it is not known whether tryptase impacts on melanoma progression. Here we addressed this and show that melanoma growth is significantly higher in tryptase‐deficient (Mcpt6^?/?) versus wild‐type mice. Histochemical analysis showed that mast cells were frequent in the tumor stroma of both wild‐type and Mcpt6^?/? mice, and also revealed their presence within the tumor parenchyma. Confocal microscopy analysis revealed that tryptase was taken up by the tumor cells. Further, tryptase‐positive granules were released from mast cells and were widely distributed within the tumor tissue, suggesting that tryptase could impact on the tumor microenvironment. Indeed, gene expression analysis showed that the absence of Mcpt6 caused decreased expression of numerous genes, including Cxcl9, Tgtp2, and Gbp10, while the expression of 5p‐miR3098 was enhanced. The levels of CXCL9 were lower in serum from Mcpt6^?/? versus wild‐type mice. In further support of a functional impact of tryptase on melanoma, recombinant tryptase (Mcpt6) was taken up by cultured melanoma cells and caused reduced proliferation. Altogether, our results indicate a protective role of mast cell tryptase in melanoma growth.     

Smad3 deficiency in mast cells provides efficient host protection against acute septic peritonitis

Mast cells play an important role in innate immunity as well as in allergic reaction. However, regulatory mechanisms underlying mast cell-mediated innate immune responses remain largely unknown. Here we determined whether Smad3, a major signal transducer of TGF-beta, regulates innate immune response by mast cells against Gram-negative bacteria. Bone marrow-derived mast cells (BMMC) obtained from Smad3 null mutant mice showed augmented capacity to produce proinflammatory cytokines upon stimulation with a Gram-negative bacteria-associated product, LPS. In acute septic peritonitis model induced by cecal ligation and puncture, mast cell-deficient W/W(v) mice reconstituted with Smad3 null BMMC had significantly higher survival rate than W/W(v) mice reconstituted with wild-type BMMC, which was associated with higher production of proinflammatory cytokines in the peritoneal cavity. These in vitro and in vivo results suggest that Smad3 in mast cells functions as inhibitory for mast cell-mediated innate immune response against Gram-negative bacteria. Suppression of Smad3 expression in mast cells may thus have therapeutic potential for Gram-negative bacterial infection such as acute septic peritonitis by augmenting innate immune responses of mast cells.     

SiaA and SiaD are essential for inducing autoaggregation as a specific response to detergent stress in Pseudomonas aeruginosa

Cell aggregation is a stress response and serves as a survival strategy for Pseudomonas aeruginosa strain PAO1 during growth with the toxic detergent Na‐dodecylsulfate (SDS). This process involves the psl operon and is linked to c‐di‐GMP signalling. The induction of cell aggregation in response to SDS was studied. Transposon and site‐directed mutagenesis revealed that the cupA‐operon and the co‐transcribed genes siaA (PA0172) and siaD (PA0169) were essential for SDS‐induced aggregation. While siaA encodes a putative membrane protein with a HAMP and a PP2C‐like phosphatase domain, siaD encodes a putative diguanylate cyclase involved in the biosynthesis of c‐di‐GMP. Complementation studies uncovered that the loss of SDS‐induced aggregation in the formerly isolated spontaneous mutant strain N was caused by a non‐functional siaA allele. DNA‐microarray analysis of SDS‐grown cells revealed consistent activation of eight genes, including cupA1, with known or presumptive important functions in cell aggregation in the parent strain compared with non‐aggregating siaA and siaD mutants. A siaAD‐dependent increase of cupA1 mRNA levels in SDS‐grown cells was also shown by Northern blots. These results clearly demonstrate that SiaAD are essential for inducing cell aggregation as a specific response to SDS and suggest that they are responsible for perceiving and transducing SDS‐related stress.     

RIG‐I detects infection with live Listeria by sensing secreted bacterial nucleic acids

Immunity against infection with Listeria monocytogenes is not achieved from innate immune stimulation by contact with killed but requires viable Listeria gaining access to the cytosol of infected cells. It has remained ill‐defined how such immune sensing of live Listeria occurs. Here, we report that efficient cytosolic immune sensing requires access of nucleic acids derived from live Listeria to the cytoplasm of infected cells. We found that Listeria released nucleic acids and that such secreted bacterial RNA/DNA was recognized by the cytosolic sensors RIG‐I, MDA5 and STING thereby triggering interferon β production. Secreted Listeria nucleic acids also caused RIG‐I‐dependent IL‐1β‐production and inflammasome activation. The signalling molecule CARD9 contributed to IL‐1β production in response to secreted nucleic acids. In conclusion, cytosolic recognition of secreted bacterial nucleic acids by RIG‐I provides a mechanistic explanation for efficient induction of immunity by live bacteria.     

Reduction of Helicobacter Infection in IL-10−/– Mice is Dependent On CD4+ T Cells but not on Mast Cells

Background: In contrast to wild type, interleukin‐10‐deficient (IL‐10^?/–) mice are able to clear Helicobacter infection. In this study, we investigated the immune response of IL‐10^?/– mice leading to the reduction of Helicobacter infection. Materials and Methods: We characterized the immune responses of Helicobacter felis‐infected IL‐10^?/– mice by studying the systemic antibody and cellular responses toward Helicobacter. We investigated the role of CD4⁺ T cells in the Helicobacter clearance by injecting H. felis‐infected IL‐10^?/– mice with anti‐CD4 depleting antibodies. To examine the role of mast cells in Helicobacter clearance, we constructed and infected mast cells and IL‐10 double‐deficient mice. Results: Reduction of Helicobacter infection in IL‐10^?/– mice is associated with strong humoral (fivefold higher serum antiurease antibody titers were measured in IL‐10^?/– in comparison to wild‐type mice, p < .008) and cellular (urease‐stimulated splenic CD4⁺ T cells isolated from infected IL‐10^?/– mice produce 150‐fold more interferon‐γ in comparison to wild‐type counterparts, p < .008) immune responses directed toward Helicobacter. Depletion of CD4⁺ cells from Helicobacter‐infected IL‐10^?/– mice lead to the loss of bacterial clearance (rapid urease tests are threefold higher in CD4⁺ depleted IL‐10^?/– in comparison to nondepleted IL‐10^?/– mice, p < .02). Mast cell IL‐10^?/– double‐deficient mice clear H. felis infection, indicating that mast cells are unnecessary for the bacterial eradication in IL‐10^?/– mice. Conclusion: Taken together, these results suggest that CD4⁺ cells are required for Helicobacter clearance in IL‐10^?/– mice. This reduction of Helicobacter infection is, however, not dependent on the mast cell population.     

The lipoprotein HP1454 of Helicobacter pylori regulates T‐cell response by shaping T‐cell receptor signalling

Helicobacter pylori (HP) is a Gram‐negative bacterium that chronically infects the stomach of more than 50% of human population and represents a major cause of gastric cancer, gastric lymphoma, gastric autoimmunity, and peptic ulcer. It still remains to be elucidated, which HP virulence factors are important in the development of gastric disorders. Here, we analysed the role of the HP protein HP1454 in the host–pathogen interaction. We found that a significant proportion of T cells isolated from HP patients with chronic gastritis and gastric adenocarcinoma proliferated in response to HP1454. Moreover, we demonstrated in vivo that HP1454 protein drives Th1/Th17 inflammatory responses. We further analysed the in vitro response of human T cells exposed either to an HP wild‐type strain or to a strain with a deletion of the hp1454 gene, and we revealed that HP1454 triggers the T‐cell antigen receptor‐dependent signalling and lymphocyte proliferation, as well as the CXCL12‐dependent cell adhesion and migration. Our study findings prove that HP1454 is a crucial bacterial factor that exerts its proinflammatory activity by directly modulating the T‐cell response. The relevance of these results can be appreciated by considering that compelling evidence suggest that chronic gastric inflammation, a condition that paves the way to HP‐associated diseases, is dependent on T cells.     

Bacteria-Host Cell Interaction Mediated by Cellular Cholesterol/Glycolipid-Enriched Microdomains

Gram negative bacterial infection is a leading cause of fatality and is attributed, at least in part, to the bacteria's capacity to persist in the host in spite of appropriate antibiotic therapy. It has been suggested that bacteria evade antibiotics by hiding within host cells. We sought to investigate this important aspect of infections in mast cells, which are inflammatory cells found in close proximity to the host-environment interface and which have recently been reported to play a crucial role in the early innate immune response to bacteria. We examined mast cell interactions with FimH-expressing E. coli, one of the major opportunistic pathogens of humans. We determined that in serum free conditions, these bacteria were able to trigger mast cell uptake without loss of bacterial viability. CD48, a mannose containing GPI (glycosylphosphatidylinositol)-linked molecule was found to be the receptor of FimH-expressing E. coli in mouse mast cells. We found that the internalization via CD48 was blocked by filipin, a cholesterol binding drug known to disrupt cholesterol/glycolipid-enriched microdomains and the bacteria-encasing vacuoles were rich in cholesterol inside cells. Interestingly, we found that mast cells subsequently expelled majority of the intracellular bacteria in 24 hours. This expulsion process was blocked by lovastatin/cyclodextrin treatment, which is known to inhibit cellular trafficking of cholesterol/glycolipid-enriched microdomains. Thus, the bacterial entry into and expulsion from mast cells were critically dependent on cholesterol/glycolipid-enriched microdomains, which represents a novel mode of tussle between the pathogen and the mast cell occurring in opsonin deficient sites in the body or even at other sites in naive or immunocompromised hosts which have low systemic levels of E. coli specific antibody.     

Two guard cell mitogen‐activated protein kinases,MPK9 and MPK12, function in methyl jasmonate‐induced stomatal closure in Arabidopsis thaliana

Methyl jasmonate (MeJA) and abscisic acid (ABA) signalling cascades share several signalling components in guard cells. We previously showed that two guard cell‐preferential mitogen‐activated protein kinases (MAPKs), MPK9 and MPK12, positively regulate ABA signalling in Arabidopsis thaliana. In this study, we examined whether these two MAP kinases function in MeJA signalling using genetic mutants for MPK9 and MPK12 combined with a pharmacological approach. MeJA induced stomatal closure in mpk9‐1 and mpk12‐1 single mutants as well as wild‐type plants, but not in mpk9‐1 mpk12‐1 double mutants. Consistently, the MAPKK inhibitor PD98059 inhibited the MeJA‐induced stomatal closure in wild‐type plants. MeJA elicited reactive oxygen species (ROS) production and cytosolic alkalisation in guard cells of the mpk9‐1, mpk12‐1 and mpk9‐1 mpk12‐1 mutants, as well in wild‐type plants. Furthermore, MeJA triggered elevation of cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt) in the mpk9‐1 mpk12‐1 double mutant as well as wild‐type plants. Activation of S‐type anion channels by MeJA was impaired in mpk9‐1 mpk12‐1. Together, these results indicate that MPK9 and MPK12 function upstream of S‐type anion channel activation and downstream of ROS production, cytosolic alkalisation and [Ca²⁺]_cyt elevation in guard cell MeJA signalling, suggesting that MPK9 and MPK12 are key regulators mediating both ABA and MeJA signalling in guard cells.     

Early sequence of events triggered by the interaction of Neisseria meningitidis with endothelial cells

Neisseria meningitidis is a bacterium responsible for severe sepsis and meningitis. Following type IV pilus‐mediated adhesion to endothelial cells, bacteria proliferating on the cellular surface trigger a potent cellular response that enhances the ability of adhering bacteria to resist the mechanical forces generated by the blood flow. This response is characterized by the formation of numerous 100 nm wide membrane protrusions morphologically related to filopodia. Here, a high‐resolution quantitative live‐cell fluorescence microscopy procedure was designed and used to study this process. A farnesylated plasma membrane marker was first detected only a few seconds after bacterial contact, rapidly followed by actin cytoskeleton reorganization and bulk cytoplasm accumulation. The bacterial type IV pili‐associated minor pilin PilV is necessary for the initiation of this cascade. Plasma membrane composition is a key factor as cholesterol depletion with methyl‐β‐cyclodextrin completely blocks the initiation of the cellular response. In contrast membrane deformation does not require the actin cytoskeleton. Strikingly, plasma membrane remodelling undermicrocolonies is also independent of common intracellular signalling pathways as cellular ATP depletion is not inhibitory. This study shows that bacteria‐induced plasma membrane reorganization is a rapid event driven by a direct cross‐talk between type IV pili and the plasma membrane rather than by the activation of an intracellular signalling pathway that would lead to actin remodelling.     

Identification of MltG as a potential terminase for peptidoglycan polymerization in bacteria

Bacterial cells are fortified against osmotic lysis by a cell wall made of peptidoglycan (PG). Synthases called penicillin‐binding proteins (PBPs), the targets of penicillin and related antibiotics, polymerize the glycan strands of PG and crosslink them into the cell wall meshwork via attached peptides. The average length of glycan chains inserted into the matrix by the PBPs is thought to play an important role in bacterial morphogenesis, but polymerization termination factors controlling this process have yet to be discovered. Here, we report the identification of Escherichia coli MltG (YceG) as a potential terminase for glycan polymerization that is broadly conserved in bacteria. A clone containing mltG was initially isolated in a screen for multicopy plasmids generating a lethal phenotype in cells defective for the PG synthase PBP1b. Biochemical studies revealed that MltG is an inner membrane enzyme with endolytic transglycosylase activity capable of cleaving at internal positions within a glycan polymer. Radiolabeling experiments further demonstrated MltG‐dependent nascent PG processing in vivo, and bacterial two‐hybrid analysis identified an MltG‐PBP1b interaction. Mutants lacking MltG were also shown to have longer glycans in their PG relative to wild‐type cells. Our combined results are thus consistent with a model in which MltG associates with PG synthetic complexes to cleave nascent polymers and terminate their elongation.