CcpA coordinates central metabolism and biofilm formation in Staphylococcus epidermidis

Staphylococcus epidermidis is an opportunistic bacterium whose infections often involve the formation of a biofilm on implanted biomaterials. In S. epidermidis, the exopolysaccharide facilitating bacterial adherence in a biofilm is polysaccharide intercellular adhesin (PIA), whose synthesis requires the enzymes encoded within the intercellular adhesin operon (icaADBC). In vitro, the formation of S. epidermidis biofilms is enhanced by conditions that repress tricarboxylic acid (TCA) cycle activity, such as growth in a medium containing glucose. In many Gram-positive bacteria, repression of TCA cycle genes in response to glucose is accomplished by catabolite control protein A (CcpA). CcpA is a member of the GalR-LacI repressor family that mediates carbon catabolite repression, leading us to hypothesize that catabolite control of S. epidermidis biofilm formation is indirectly regulated by CcpA-dependent repression of the TCA cycle. To test this hypothesis, ccpA deletion mutants were constructed in strain 1457 and 1457-acnA and the effects on TCA cycle activity, biofilm formation and virulence were assessed. As anticipated, deletion of ccpA derepressed TCA cycle activity and inhibited biofilm formation; however, ccpA deletion had only a modest effect on icaADBC transcription. Surprisingly, deletion of ccpA in strain 1457-acnA, a strain whose TCA cycle is inactive and where icaADBC transcription is derepressed, strongly inhibited icaADBC transcription. These observations demonstrate that CcpA is a positive effector of biofilm formation and icaADBC transcription and a repressor of TCA cycle activity.

Oligomeric coiled-coil adhesin YadA is a double-edged sword

Yersinia adhesin A (YadA) is an essential virulence factor for the food-borne pathogens Yersinia enterocolitica and Yersinia pseudotuberculosis. Surprisingly, it is a pseudogene in Yersinia pestis. Even more intriguing, the introduction of a functional yadA gene in Y. pestis EV76 was shown to correlate with a decrease in virulence in a mouse model. Here, we report that wild type (wt) Y. enterocolitica E40, as well as YadA-deprived E40 induced the synthesis of neutrophil extracellular traps (NETs) upon contact with neutrophils, but only YadA-expressing Y. enterocolitica adhered to NETs and were killed. As binding seemed to be a prerequisite for killing, we searched for YadA-binding substrates and detected the presence of collagen within NETs. E40 bacteria expressing V98D,N99A mutant YadA with a severely reduced ability to bind collagen were found to be more resistant to killing, suggesting that collagen binding contributes significantly to sensitivity to NETs. Wt Y. pestis EV76 were resistant to killing by NETs, while recombinant EV76 expressing YadA from either Y. pseudotuberculosis or Y. enterocolitica were sensitive to killing by NETs, outlining the importance of YadA for susceptibility to NET-dependent killing. Recombinant EV76 endowed with YadA from Y. enterocolitica were also less virulent for the mouse than wt EV76, as shown before. In addition, EV76 carrying wt YadA were less virulent for the mouse than EV76 expressing YadA(V₉₈D,N₉₉A). The observation that YadA makes Yersinia sensitive to NETs provides an explanation as for why evolution selected for the inactivation of yadA in the flea-borne Y. pestis and clarifies an old enigma. Since YadA imposes the same cost to the food-borne Yersinia but was nevertheless conserved by evolution, this observation also illustrates the duality of some virulence functions.

TLR2 enhances NADPH oxidase activity and killing of Staphylococcus aureus by PMN

Toll-like receptors play an essential role in the detection of invading pathogens. TLR2 is expressed in high concentrations on neutrophils and has been implicated as a critical mediator inducing host antimicrobial defenses against Gram-positive bacteria. Neutrophil responses induced via TLR2 are likely to have important clinical consequences, since Gram-positive organisms, such as Staphylococcus aureus, are an increasingly important source of severe infections. In the present study, we report that TLR2 has a central role in killing of S. aureus by murine PMN via enhancement of NADPH oxidase activity. PMN from TLR2-deficient mice showed a similar inability to kill S. aureus in vitro and under in vivo-like conditions as PMN with a non-functional NADPH oxidase. This defect in killing by TLR2-deficient PMN was not related to phagocytosis but caused by delayed and reduced NADPH oxidase-mediated production of superoxide anion in response to S. aureus and other Gram-positive bacteria. The cause of this was independent of PI3K- and p38 signaling. The TLR2-enhanced induction of superoxide was a defect in proper NADPH oxidase assembly. We hypothesize that early activation of TLR2-signaling may enhance p47(phox) phosphorylation subsequent to phagocytosis-mediated phosphorylation. Summarized, these data demonstrate a novel role of TLR2 in the killing of S. aureus by ensuring a rapid activation of the NADPH oxidase complex.

Neutrophil antimicrobial defense against Staphylococcus aureus is mediated by phagolysosomal but not extracellular trap-associated cathelicidin

Neutrophils kill invading pathogens by AMPs, including cathelicidins, ROS, and NETs. The human pathogen Staphylococcus aureus exhibits enhanced resistance to neutrophil AMPs, including the murine cathelicidin CRAMP, in part, as a result of alanylation of teichoic acids by the dlt operon. In this study, we took advantage of the hypersusceptible phenotype of S. aureus DeltadltA against cationic AMPs to study the impact of the murine cathelicidin CRAMP on staphylococcal killing and to identify its key site of action in murine neutrophils. We demonstrate that CRAMP remained intracellular during PMN exudation from blood and was secreted upon PMA stimulation. We show first evidence that CRAMP was recruited to phagolysosomes in infected neutrophils and exhibited intracellular activity against S. aureus. Later in infection, neutrophils produced NETs, and immunofluorescence revealed association of CRAMP with S. aureus in NETs, which similarly killed S. aureus wt and DeltadltA, indicating that CRAMP activity was reduced when associated with NETs. Indeed, the presence of DNA reduced the antimicrobial activity of CRAMP, and CRAMP localization in response to S. aureus was independent of the NADPH oxidase, whereas killing was partially dependent on a functional NADPH oxidase. Our study indicates that neutrophils use CRAMP in a timed and locally coordinated manner in defense against S. aureus.

Lipoproteins in Staphylococcus aureus mediate inflammation by TLR2 and iron-dependent growth in vivo

Lipoproteins (Lpp) are ligands of TLR2 and signal by the adaptor MyD88. As part of the bacterial cell envelope, Lpp are mainly involved in nutrient acquisition for Staphylococcus aureus. The impact of Lpp on TLR2-MyD88 activation for S. aureus in systemic infection is unknown. S. aureus strain SA113 deficient in the enzyme encoded by the prolipoprotein diacylglyceryl transferase gene (Deltalgt), which attaches the lipid anchor to pro-Lpp, was used to study benefits and costs of Lpp maturation. Lpp in S. aureus induced early and strong cytokines by TLR2-MyD88 signaling in murine peritoneal macrophages. Lpp contributed via TLR2 to pathogenesis of sepsis in C57BL/6 mice with IL-1beta, chemokine-mediated inflammation, and high bacterial numbers. In the absence of MyD88-mediated inflammation, Lpp allowed bacterial clearing from liver devoid of infiltrating cells, but still conferred a strong growth advantage in mice, which was shown to rely on iron uptake and storage in vitro and in vivo. With iron-restricted bacteria, the Lpp-related growth advantage was evident in infection of MyD88(-/-), but not of C57BL/6, mice. On the other hand, iron overload of the host restored the growth deficit of Deltalgt in MyD88(-/-), but not in immunocompetent C57BL/6 mice. These results indicate that iron acquisition is improved by Lpp of S. aureus but is counteracted by inflammation. Thus, lipid anchoring is an evolutionary advantage for S. aureus to retain essential proteins for better survival in infection.

Intracellular type III secretion by cytoplasmic Shigella flexneri promotes caspase-1-dependent macrophage cell death

The Gram-negative bacterium Shigella flexneri triggers pro-inflammatory apoptotic cell death in macrophages, which is crucial for the onset of an acute inflammatory diarrhoea termed bacillary dysentery. The Mxi-Spa type III secretion system promotes bacterial uptake and escape into the cytoplasm, where, dependent on the translocator/effector protein IpaB, caspase-1 [interleukin (IL)-1beta-converting enzyme] and its substrate IL-1beta are activated. Here, we show that in the course of a macrophage infection, IpaB is secreted intracellularly for more than 1 h post-infection and progressively accumulates in aggregates on the bacterial surface. Concomitantly, the bacterial pool of IpaB is gradually depleted. The protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) dose-dependently inhibited the Mxi-Spa-dependent secretion of IpaB triggered by the dye Congo red in vitro and abolished translocation of IpaB into the host-cell cytoplasm of S. flexneri-infected macrophages. CCCP specifically inhibited S. flexneri-triggered macrophage death in a dose-dependent manner, even if added up to 60 min post-infection. Addition of CCCP 15 min after infection blocked macrophage cell death, the activation of caspase-1 and the maturation of IL-1beta, without affecting uptake or escape of S. flexneri from the phagosome. By contrast, CCCP used at the same concentration had no effect on ATP-induced caspase-1 activation or staurosporine-induced apoptosis. Our results indicate that under the conditions used, CCCP rapidly and specifically blocks bacterial type III secretion, and thus, intracellular type III secretion promotes cytotoxicity of S. flexneri.