Molecular control of bacterial death and lysis

TLR2 deficiency leads to increased Th17 infiltrates in experimental brain abscesses

TLR2 plays a pivotal role in recognizing Staphylococcus aureus, a common etiologic agent of CNS parenchymal infections, such as brain abscess. We previously reported that brain abscesses of TLR2 knockout (KO) mice exhibited elevated IL-17 levels, suggesting the presence of an alternative pathway available to respond to S. aureus infection that may involve Th17 cells. Both CD4(+) and CD8(+) T cell infiltrates were elevated in brain abscesses of TLR2 KO mice at days 3, 7, and 14 postinfection compared with wild-type animals. Intracellular cytokine staining revealed a significant increase in the frequency of IL-17-producing Th17 cells in TLR2 KO mice with relatively few IFN-gamma-positive cells. gammadelta T cells were also a source of IL-17 in brain abscesses. Microglia, astrocytes, and macrophages were shown to express both IL-17RA and IL-17RC. Despite receptor expression, IL-17 was relatively ineffective at eliciting glial activation, whereas the cytokine augmented the ability of TNF-alpha to induce CXCL2 and CCL2 expression by macrophages. Based on the ability of IL-17 to elicit the release of chemokines and other proinflammatory mediators, we propose that the exaggerated IL-17 response that occurs in TLR2 KO mice functions in a compensatory manner to control brain abscess pathogenesis, with cells other than glia as targets for IL-17 action. This is supported by our findings in which innate immune infiltrates were not significantly different between TLR2 KO and wild-type mice in conjunction with the lack of prolonged alterations in the synthesis of other proinflammatory molecules during the course of infection.

LytM-domain factors are required for daughter cell separation and rapid ampicillin-induced lysis in Escherichia coli

Bacterial cytokinesis is coupled to the localized synthesis of new peptidoglycan (PG) at the division site. This newly generated septal PG is initially shared by the daughter cells. In Escherichia coli and other gram-negative bacteria, it is split shortly after it is made to promote daughter cell separation and allow outer membrane constriction to closely follow that of the inner membrane. We have discovered that the LytM (lysostaphin)-domain containing factors of E. coli (EnvC, NlpD, YgeR, and YebA) are absolutely required for septal PG splitting and daughter cell separation. Mutants lacking all LytM factors form long cell chains with septa containing a layer of unsplit PG. Consistent with these factors playing a direct role in septal PG splitting, both EnvC-mCherry and NlpD-mCherry fusions were found to be specifically recruited to the division site. We also uncovered a role for the LytM-domain factors in the process of beta-lactam-induced cell lysis. Compared to wild-type cells, mutants lacking LytM-domain factors were delayed in the onset of cell lysis after treatment with ampicillin. Moreover, rather than lysing from midcell lesions like wild-type cells, LytM(-) cells appeared to lyse through a gradual loss of cell shape and integrity. Overall, the phenotypes of mutants lacking LytM-domain factors bear a striking resemblance to those of mutants defective for the N-acetylmuramyl-l-alanine amidases: AmiA, AmiB, and AmiC. E. coli thus appears to rely on two distinct sets of putative PG hydrolases to promote proper cell division.

Modulation of eDNA release and degradation affects Staphylococcus aureus biofilm maturation

Recent studies have demonstrated a role for Staphylococcus aureus cidA-mediated cell lysis and genomic DNA release in biofilm adherence. The current study extends these findings by examining both temporal and additional genetic factors involved in the control of genomic DNA release and degradation during biofilm maturation. Cell lysis and DNA release were found to be critical for biofilm attachment during the initial stages of development and the released DNA (eDNA) remained an important matrix component during biofilm maturation. This study also revealed that an lrgAB mutant exhibits increased biofilm adherence and matrix-associated eDNA consistent with its proposed role as an inhibitor of cidA-mediated lysis. In flow-cell assays, both cid and lrg mutations had dramatic effects on biofilm maturation and tower formation. Finally, staphylococcal thermonuclease was shown to be involved in biofilm development as a nuc mutant formed a thicker biofilm containing increased levels of matrix-associated eDNA. Together, these findings suggest a model in which the opposing activities of the cid and lrg gene products control cell lysis and genomic DNA release during biofilm development, while staphylococcal thermonuclease functions to degrade the eDNA, possibly as a means to promote biofilm dispersal.

The Staphylococcus aureus LytSR two-component regulatory system affects biofilm formation

Studies of the Staphylococcus aureus LytSR two-component regulatory system have led to the identification of the cid and lrg operons, which affect murein hydrolase activity, stationary-phase survival, antibiotic tolerance, and biofilm formation. The cid gene products enhance murein hydrolase activity and antibiotic tolerance whereas the lrg gene products inhibit these processes in a manner believed to be analogous to bacteriophage-encoded holins and antiholins, respectively. Importantly, these operons have been shown to play significant roles in biofilm development by controlling the release of genomic DNA, which then becomes an important structural component of the biofilm matrix. To determine the role of LytSR in biofilm development, a lytS knockout mutant was generated from a clinical S. aureus isolate (UAMS-1) and the effects on gene expression and biofilm formation were examined. As observed in laboratory isolates, LytSR was found to be required for lrgAB expression. Furthermore, the lytS mutant formed a more adherent biofilm than the wild-type and complemented strains. Consistent with previous findings, the increased adherence of the mutant was attributed to the increased prevalence of matrix-associated eDNA. Transcription profiling studies indicated that the lrgAB operon is the primary target of LytSR-mediated regulation but that this regulatory system also impacts expression of a wide variety of genes involved in basic metabolism. Overall, the results of these studies demonstrate that the LytSR two-component regulatory system plays an important role in S. aureus biofilm development, likely as a result of its direct influence on lrgAB expression.

An overlap between the control of programmed cell death in Bacillus anthracis and sporulation

The Staphylococcus aureus cid and lrg operons have been shown to control cell death and lysis in a manner thought to be analogous to programmed cell death (apoptosis) in eukaryotic organisms. Although orthologous operons are present in a wide variety of bacterial species, members of the Bacillus cereus group are unique in that they have a total of four cid-/lrg-like operons. Two of these operons are similar to the S. aureus cid and lrg operons, while the other two (designated clhAB(1) and clhAB(2)) are unique to this group. In the present study, the functions and regulation of these loci were examined. Interestingly, the Bacillus anthracis lrgAB mutant displayed decreased stationary-phase survival, whereas the clhAB(2) mutant exhibited increased stationary-phase survival compared to the parental and complementation strains. However, neither mutation had a dramatic effect on murein hydrolase activity or autolysis. Furthermore, a quantitative analysis of the sporulation efficiency revealed that both mutants formed fewer spores than did the parental strain. Similar to S. aureus, B. anthracis lrgAB transcription was shown to be induced by gramicidin and CCCP, agents known to dissipate the proton motive force, in a lytSR-dependent manner. Northern blot analyses also demonstrated a positive role for lytSR in the clhAB(2) transcription. Taken together, the results of the present study demonstrate that B. anthracis lrgAB and clhAB(2) play important roles in the control of cell death and lysis and reveal a previously unrecognized role of this system in sporulation.

Assembly and development of the Pseudomonas aeruginosa biofilm matrix

Virtually all cells living in multicellular structures such as tissues and organs are encased in an extracellular matrix. One of the most important features of a biofilm is the extracellular polymeric substance that functions as a matrix, holding bacterial cells together. Yet very little is known about how the matrix forms or how matrix components encase bacteria during biofilm development. Pseudomonas aeruginosa forms environmentally and clinically relevant biofilms and is a paradigm organism for the study of biofilms. The extracellular polymeric substance of P. aeruginosa biofilms is an ill-defined mix of polysaccharides, nucleic acids, and proteins. Here, we directly visualize the product of the polysaccharide synthesis locus (Psl exopolysaccharide) at different stages of biofilm development. During attachment, Psl is anchored on the cell surface in a helical pattern. This promotes cell–cell interactions and assembly of a matrix, which holds bacteria in the biofilm and on the surface. Chemical dissociation of Psl from the bacterial surface disrupted the Psl matrix as well as the biofilm structure. During biofilm maturation, Psl accumulates on the periphery of 3-D-structured microcolonies, resulting in a Psl matrix-free cavity in the microcolony center. At the dispersion stage, swimming cells appear in this matrix cavity. Dead cells and extracellular DNA (eDNA) are also concentrated in the Psl matrix-free area. Deletion of genes that control cell death and autolysis affects the formation of the matrix cavity and microcolony dispersion. These data provide a mechanism for how P. aeruginosa builds a matrix and subsequently a cavity to free a portion of cells for seeding dispersal. Direct visualization reveals that Psl is a key scaffolding matrix component and opens up avenues for therapeutics of biofilm-related complications.

Pseudomonas aeruginosa causes life-threatening, persistent infections in cystic fibrosis patients, despite highly aggressive antimicrobial therapy. Persistence is due, in part, to the ability of these bacteria to form surface-associated communities (biofilms) enmeshed in an extracellular matrix. This matrix is a poorly defined mixture of protein, polysaccharide, and DNA. An understanding of the organization and composition of the biofilm matrix will assist in the development of therapeutics aimed at disrupting biofilms. Using reagents that specifically recognize the P. aeruginosa Psl exopolysaccharide, we visualized matrix formation in real time during a biofilm development cycle. This revealed a highly organized and coordinated assembly of both polysaccharide and DNA components of the matrix. At late stages of biofilm morphogenesis, a Psl-free matrix cavity, occupied with numerous motile cells, developed. Mutants with reduced cell lysis were unable to form the Psl matrix cavity, whereas those with elevated cell death and lysis formed a larger matrix cavity, leading to accelerated dispersion. We propose that programmed cell death and autolysis are critical for the proper timing of biofilm development and dispersion. The data indicate that Psl is a key scaffolding component of the biofilm matrix, a property that likely plays a critical role in P. aeruginosa persistence.

The chromosomal toxin gene yafQ is a determinant of multidrug tolerance for Escherichia coli growing in a biofilm

Escherichia coli is refractory to elevated doses of antibiotics when it is growing in a biofilm, and this is potentially due to high numbers of multidrug-tolerant persister cells in the surface-adherent population. Previously, the chromosomal toxin-antitoxin loci hipBA and relBE have been linked to the frequency at which persister cells occur in E. coli populations. In the present study, we focused on the dinJ-yafQ-encoded toxin-antitoxin system and hypothesized that deletion of the toxin gene yafQ might influence cell survival in antibiotic-exposed biofilms. By using confocal laser scanning microscopy and viable cell counting, it was determined that a Delta yafQ mutant produced biofilms with a structure and a cell density equivalent to those of the parental strain. In-depth susceptibility testing identified that relative to wild-type E. coli, the Delta yafQ strain had up to a approximately 2,400-fold decrease in cell survival after the biofilms were exposed to bactericidal concentrations of cefazolin or tobramycin. Corresponding to these data, controlled overexpression of yafQ from a high-copy-number plasmid resulted in up to a approximately 10,000-fold increase in the number of biofilm cells surviving exposure to these bactericidal drugs. In contrast, neither the inactivation nor the overexpression of yafQ affected the tolerance of biofilms to doxycycline or rifampin (rifampicin). Furthermore, deletion of yafQ did not affect the tolerance of stationary-phase planktonic cells to any of the antibacterials tested. These results suggest that yafQ mediates the tolerance of E. coli biofilms to multiple but specific antibiotics; moreover, our data imply that this cellular pathway for persistence is likely different from that of multidrug-tolerant cells in stationary-phase planktonic cell cultures.

Cytology of terminally differentiated Epulopiscium mother cells

Epulopiscium sp. type B, a member of the Firmicutes, is a large (up to 300 microm), cigar-shaped bacterial symbiont of surgeonfish that propagates itself by forming multiple intracellular offspring. This unusual form of reproduction is an apparent modification of a developmental program used by some Firmicutes to produce an endospore. At the onset of offspring formation, the Epulopiscium cell divides at both poles. The polar cells are engulfed by the larger mother cell and grow within the mother cell. At the final stages of development, the Epulopiscium mother cell lyses. Here we describe changes in Epulopiscium cell structure, focusing on mother cell DNA replication and cell death. DNA replication was examined by labeling cells with the nucleotide analog bromodeoxyuridine. As expected, DNA replication occurs in the developing offspring. However, well after passage of genetic information from parent to offspring is complete, DNA within the mother cell continues to replicate. Using fluorescence microscopy, we found that near the end of the offspring growth cycle, mother cell DNA disintegrates. The mother cell membrane and wall deteriorate as well. DNA replication within this terminally differentiated cell indicates the importance of mother cell nucleoids in cell maintenance and the development of offspring. The synchronized timing of mother cell deterioration within a population suggests that the Epulopiscium mother cell undergoes a programmed cell death. The programmed death of the mother cell may allow for the timely release of resources accumulated in the mother cell to provide nutrients to populations of these intestinal microbes and their host.

Survival of bacterial biofilms within neutrophil extracellular traps promotes nontypeable Haemophilus influenzae persistence in the chinchilla model for otitis media

Nontypeable Haemophilus influenzae (NTHi) is a leading cause of acute and chronic otitis media, which are a major public health problem worldwide. The persistence of NTHi during chronic and recurrent otitis media infections involves multicellular biofilm communities formed within the middle-ear chamber. Bacterial biofilms resist immune clearance and antibiotic therapy due in part to encasement within a polymeric matrix. In this study, the contribution of biofilms to bacterial persistence in vivo and composition of the NTHi biofilm matrix during experimental otitis media were investigated. The presence of biofilms within the chinchilla middle-ear chamber was significantly correlated with increased bacterial load in middle-ear effusions and tissue. Examination of thin sections revealed polymorphonuclear cells within a DNA lattice containing elastase and histones, which is consistent with the definition of neutrophil extracellular traps. Viable multicellular biofilm communities with biofilm phenotypes were found within the DNA lattice throughout the biofilm. Further, NTHi was resistant to both phagocytic and extracellular neutrophil killing in vitro by means of lipooligosaccharide moieties that promote biofilm formation. These data support the conclusion that NTHi subverts neutrophil extracellular traps to persist in vivo. These data also indicate that a more inclusive definition for biofilms may be warranted.

Kinetic analysis of growth rate, ATP, and pigmentation suggests an energy-spilling function for the pigment prodigiosin of Serratia marcescens

Serratia marcescens is a gram-negative environmental bacterium and opportunistic pathogen. S. marcescens expresses prodigiosin, a bright red and cell-associated pigment which has no known biological function for producing cells. We present here a kinetic model relating cell, ATP, and prodigiosin concentration changes for S. marcescens during cultivation in batch culture. Cells were grown in a variety of complex broth media at temperatures which either promoted or essentially prevented pigmentation. High growth rates were accompanied by large decreases in cellular prodigiosin concentration; low growth rates were associated with rapid pigmentation. Prodigiosin was induced most strongly during limited growth as the population transitioned to stationary phase, suggesting a negative effect of this pigment on biomass production. Mathematically, the combined rate of formation of biomass and bioenergy (as ATP) was shown to be equivalent to the rate of prodigiosin production. Studies with cyanide inhibition of both oxidative phosphorylation and pigment production indicated that rates of biomass and net ATP synthesis were actually higher in the presence of cyanide, further suggesting a negative regulatory role for prodigiosin in cell and energy production under aerobic growth conditions. Considered in the context of the literature, these results suggest that prodigiosin reduces ATP production by a process termed energy spilling. This process may protect the cell by limiting production of reactive oxygen compounds. Other possible functions for prodigiosin as a mediator of cell death at population stationary phase are discussed.