Catalase-peroxidases of Legionella pneumophila: cloning of the katA gene and studies of KatA function

Effector-mediated subversion of proteasome activator (PA)28αβ enhances host defense against Legionella pneumophila under inflammatory and oxidative stress conditions

Legionella pneumophila is a natural pathogen of amoebae that causes Legionnaires' Disease in immunocompromised individuals via replication within macrophages. L. pneumophila virulence and intracellular replication hinges on hundreds of Dot/Icm-translocated effector proteins, which are essential for biogenesis of the replication-permissive Legionella-containing vacuole (LCV). However, effector activity can also enhance mammalian host defense via effector-triggered immunity. The L. pneumophila effector LegC4 is important for virulence in amoebae but enhances host defense against L. pneumophila in the mouse lung and, uniquely, within macrophages activated with either tumor necrosis factor (TNF) or interferon (IFN)-γ. The mechanism by which LegC4 potentiates cytokine-mediated host defense in macrophages is unknown. Here, we found that LegC4 enhances cytokine-mediated phagolysosomal fusion with Legionella-containing vacuole (LCV) and binds host proteasome activator (PA)28α, which forms a heterooligomer with PA28β to facilitate ubiquitin-independent proteasomal degradation of oxidant-damaged (carbonylated) proteins. We found that oxidative stress was sustained in the presence of LegC4 and that the LegC4 restriction phenotype was relieved in PA28αβ-deficient macrophages and in the lungs of mice in vivo. Our data also show that oxidative stress is sufficient for LegC4-mediated restriction in macrophages producing PA28αβ. PA28αβ has been traditionally associated with antigen presentation; however, our data support a novel mechanism whereby effector-mediated subversion of PA28αβ enhances cell-autonomous host defense against L. pneumophila under inflammatory and oxidative stress conditions. This work provides a solid foundation to evaluate induced proteasome regulators as mediators of innate immunity.

GVPC Medium Manufactured without Oxygen Improves the Growth of Legionella spp. and Exhibits Enhanced Selectivity Properties

Glycine-vancomycin-polymyxin-cycloheximide agar (GVPC) is a recommended medium for the detection of Legionella spp. in water samples. However, its quality could be improved in terms of recovery of Legionella spp. and selectivity properties. Modifications were introduced in GVPC manufacture: autoclaving conditions (115°C, 15 min) and atmosphere during component-stirring (removal of oxygen and N₂ injection). The use of softer autoclaving conditions (115°C, 15 min) improved the growth of Legionella anisa by the spiral method and Legionella pneumophila after membrane filtration. The medium manufactured with O₂ removal and autoclaving for 15 min at 115°C allowed a faster growth of L. pneumophila (colonies visible at day 2) and a notable increase of L. anisa growth (colonies appearing at day 3, and statistically significant numbers of CFU at day 5). After 3 to 5 days of incubation, the improved media showed higher selectivity properties, particularly for Enterococcus faecalis ATCC 29212 and Pseudomonas aeruginosa ATCC 9027. A further improvement was achieved by the addition of N₂ during ingredient stirring, leading to a statistically significant faster growth of L. pneumophila at days 2 and 3 and L. anisa at day 3. Selectivity properties were also enhanced, resulting in the complete inhibition of both E. faecalis strains and Escherichia coli and complete-partial inhibition of P. aeruginosa. Oxygen removal during GVPC manufacture using a vacuum pump system promotes the growth of L. pneumophila and L. anisa, and markedly inhibits the growth of E. coli, P. aeruginosa, and E. faecalis. IMPORTANCE Currently, GVPC is a recommended medium for the detection of Legionella spp. in water samples. However, recovery of Legionella spp. and selectivity properties can be improved. GVPC medium manufactured without oxygen improved the growth of Legionella pneumophila and Legionella anisa. Oxygen removal during GVPC manufacture also improved selectivity properties. A further improvement was achieved by the addition of N₂ during ingredient stirring, leading to a faster growth of L. pneumophila at days 2 and 3 and L. anisa at day 3 and enhancement of selectivity properties. The introduction of the modified GVPC medium in routine practice can allow a better detection of Legionella spp. in water samples.

Genomic analysis reveals high intra-species diversity of Shewanella algae

Shewanella algae is widely distributed in marine and freshwater habitats, and has been proved to be an emerging marine zoonotic and human pathogen. However, the genomic characteristics and pathogenicity of Shewanella algae are unclear. Here, the whole-genome features of 55 S. algae strains isolated from different sources were described. Pan-genome analysis yielded 2863 (19.4 %) genes shared among all strains. Functional annotation of the core genome showed that the main functions are focused on basic lifestyle such as metabolism and energy production. Meanwhile, the phylogenetic tree of the single nucleotide polymorphisms (SNPs) of core genome divided the 55 strains into three clades, with the majority of strains from China falling into the first two clades. As for the accessory genome, 167 genomic islands (GIs) and 65 phage-related elements were detected. The CRISPR-Cas system with a high degree of confidence was predicted in 23 strains. The GIs carried a suite of virulence genes and mobile genetic elements, while prophages contained several transposases and integrases. Horizontal genes transfer based on homology analysis indicated that these GIs and prophages were parts of major drivers for the evolution and the environmental adaptation of S. algae. In addition, a rich putative virulence-associated gene pool was found. Eight classes of antibiotic-associated resistance genes were detected, and the carriage rate of β-lactam resistance genes was 100 %. In conclusion, S. algae exhibits a high intra-species diversity in the aspects of population structure, virulence-associated genes and potential drug resistance, which is helpful for its evolution in pathogenesis and environmental adaptability.

Knowledge to Predict Pathogens: Legionella pneumophila Lifecycle Systematic Review Part II Growth within and Egress from a Host Cell

Legionella pneumophila (L. pneumophila) is a pathogenic bacterium of increasing concern, due to its ability to cause a severe pneumonia, Legionnaires' Disease (LD), and the challenges in controlling the bacteria within premise plumbing systems. L. pneumophila can thrive within the biofilm of premise plumbing systems, utilizing protozoan hosts for protection from environmental stressors and to increase its growth rate, which increases the bacteria's infectivity to human host cells. Typical disinfectant techniques have proven to be inadequate in controlling L. pneumophila in the premise plumbing system, exposing users to LD risks. As the bacteria have limited infectivity to human macrophages without replicating within a host protozoan cell, the replication within, and egress from, a protozoan host cell is an integral part of the bacteria's lifecycle. While there is a great deal of information regarding how L. pneumophila interacts with protozoa, the ability to use this data in a model to attempt to predict a concentration of L. pneumophila in a water system is not known. This systematic review summarizes the information in the literature regarding L. pneumophila's growth within and egress from the host cell, summarizes the genes which affect these processes, and calculates how oxidative stress can downregulate those genes.

Principle and potential applications of the non-classical protein secretory pathway in bacteria

In addition to the extracellular proteins secreted by known secretory pathways, a number of cytoplasmic proteins without predicable or known signal sequences or secretory motifs have been found in the extracellular milieu, and were consequently classified as non-classically secreted proteins. Non-classical protein secretion is considered to be a general, conserved cellular phenomenon in both eukaryotes and prokaryotes. There are several research hotspots on the non-classical protein secretory pathway, and the most important two of them are the recognition principle of substrate proteins and possible secretory mechanisms. To date, researchers have made some progress in understanding the characteristics of these proteins. For example, it was discovered that many non-classically secreted proteins exist and are secreted in multimeric form. Some of these proteins prefer to be clustered and exported at the poles and the septum of the cell. The majority of these proteins play different functions when they are in the intra- and extracellular environments, and several of their functions are related to survival and pathogenicity. Furthermore, non-classically secreted proteins can be used as leading proteins to guide a POI (protein of interest) out of the cells, which provides a novel strategy for protein secretion with potential applications in the industry. Summarizing these findings, this review emphasizes the hot spots related to non-classically secreted proteins in bacteria, lists the most important hypotheses on the selection and secretion mechanisms of non-classically secreted proteins, and put forward their potential applications.

Pathogenomics of Virulence Traits of Plesiomonas shigelloides That Were Deemed Inconclusive by Traditional Experimental Approaches

One of the major challenges of modern medicine includes the failure of conventional protocols to characterize the pathogenicity of emerging pathogens. This is particularly apparent in the case of Plesiomonas shigelloides. Although a number of infections have been linked to this microorganism, experimental evidence of its virulence factors (VFs), obtained by traditional approaches, is somewhat inconclusive. Hence, it remains unclear whether P. shigelloides is a true or opportunistic one. In the current study, four publicly available whole-genome sequences of P. shigelloides (GN7, NCTC10360, 302-73, and LS1) were profiled using bioinformatics platforms to determine the putative candidate VFs to characterize the bacterial pathogenicity. Overall, 134 unique open reading frames (ORFs) were identified that were homologous or orthologous to virulence genes identified in other pathogens. Of these, 52.24% (70/134) were jointly shared by the strains. The numbers of strain-specific virulence traits were 4 in LS1; 7 in NCTC10360; 10 in 302-73; and 15 in GN7. The pathogenicity islands (PAIs) common to all the strains accounted for 24.07% ORFs. The numbers of PAIs exclusive to each strain were 8 in 302-73; 11 in NCTC10360; 14 in GN7; and 18 in LS1. A PAI encoding Vibrio cholerae ToxR-activated gene d protein was specific to 302-73, GN7, and NCTC10360 strains. Out of 33 antibiotic multi-resistance genes identified, 16 (48.48%) genes were intrinsic to all strains. Further, 17 (22.08%) of 77 antibiotic resistance islands were found in all the strains. Out of 23 identified distinct insertion sequences, 13 were only harbored by strain LS1. The number of intact prophages identified in the strains was 1 in GN7; 2 in 302-73; and 2 in NCTC10360. Further, 1 CRISPR element was identified in LS1; 2 in NCTC10360; and 8 in 302-73. Fifteen (78.95%) of 19 secretion systems and secretion effector variants were identified in all the strains. In conclusion, certain P. shigelloides strains might possess VFs associated with gastroenteritis and extraintestinal infections. However, the role of host factors in the onset of infections should not be undermined.

Deletion of oxyR in Legionella pneumophila causes growth defect on agar

The intracellular pathogen Legionella pneumophila (Lp) is a strict aerobe, surviving and replicating in environments where it frequently encounters reactive oxygen species (ROS), such as the nutrient-poor water environment and its replicative niche inside host cells. In many proteobacteria, the LysR-type regulator OxyR controls the oxidative stress response; however, the importance of the OxyR homologue in Lp is still unclear. Therefore, we undertook the characterization of phenotypes associated with the deletion of oxyR in Lp. Contrary to the wild type, the oxyR deletion mutant exhibits a severe growth defect on charcoal - yeast extract (CYE) agar lacking α-ketoglutarate supplementation. Growth in AYE broth (CYE without agar and charcoal), in amoeba and in human cultured macrophages, and survival in water is unaffected by the deletion. Supplementing CYE agar with antioxidants that neutralize ROS or introducing the oxyR gene in trans rescues the observed growth defect. Moreover, the mutant grows as well as the wild type on CYE plates made with agarose instead of agar, suggesting that a compound present in the latter is responsible for the growth defect phenotype.

Transcriptome analysis of Corynebacterium pseudotuberculosis biovar Equi in two conditions of the environmental stress

Corynebacterium pseudotuberculosis has been widely studied in an effort to understand its biological evolution. Transcriptomics has revealed possible candidates for virulence and pathogenicity factors of strain 1002 (biovar Ovis). Because C. pseudotuberculosis is classified into two biovars, Ovis and Equi, it was interesting to assess the transcriptional profile of biovar Equi strain 258, the causative agent of ulcerative lymphangitis. The genome of this strain was re-sequenced; the reassembly was completed using optical mapping technology, and the sequence was subsequently re-annotated. Two growth conditions that occur during the host infection process were simulated for the transcriptome: the osmotic and acid medium. Genes that may be associated with the microorganism's resilience under unfavorable conditions were identified through RNAseq, including genes present in pathogenicity islands. The RT-qPCR was performed to confirm the results in biological triplicate for each condition for some genes. The results extend our knowledge of the factors associated with the spread and persistence of C. pseudotuberculosis during the infection process and suggest possible avenues for studies related to the development of vaccines, diagnosis, and therapies that might help minimize damage to agribusinesses.

Population structure and minimum core genome typing of Legionella pneumophila

Legionella pneumophila is an important human pathogen causing Legionnaires’ disease. In this study, whole genome sequencing (WGS) was used to study the characteristics and population structure of L. pneumophila strains. We sequenced and compared 53 isolates of L. pneumophila covering different serogroups and sequence-based typing (SBT) types (STs). We found that 1,896 single-copy orthologous genes were shared by all isolates and were defined as the minimum core genome (MCG) of L. pneumophila. A total of 323,224 single-nucleotide polymorphisms (SNPs) were identified among the 53 strains. After excluding 314,059 SNPs which were likely to be results of recombination, the remaining 9,165 SNPs were referred to as MCG SNPs. Population Structure analysis based on MCG divided the 53 L. pneumophila into nine MCG groups. The within-group distances were much smaller than the between-group distances, indicating considerable divergence between MCG groups. MCG groups were also supplied by phylogenetic analysis and may be considered as robust taxonomic units within L. pneumophila. Among the nine MCG groups, eight showed high intracellular growth ability while one showed low intracellular growth ability. Furthermore, MCG typing also showed high resolution in subtyping ST1 strains. The results obtained in this study provided significant insights into the evolution, population structure and pathogenicity of L. pneumophila.

The Legionella pneumophila kai operon is implicated in stress response and confers fitness in competitive environments

Legionella pneumophila uses aquatic protozoa as replication niche and protection from harsh environments. Although L. pneumophila is not known to have a circadian clock, it encodes homologues of the KaiBC proteins of Cyanobacteria that regulate circadian gene expression. We show that L. pneumophila kaiB, kaiC and the downstream gene lpp1114, are transcribed as a unit under the control of the stress sigma factor RpoS. KaiC and KaiB of L. pneumophila do not interact as evidenced by yeast and bacterial two-hybrid analyses. Fusion of the C-terminal residues of cyanobacterial KaiB to Legionella KaiB restores their interaction. In contrast, KaiC of L. pneumophila conserved autophosphorylation activity, but KaiB does not trigger the dephosphorylation of KaiC like in Cyanobacteria. The crystal structure of L. pneumophila KaiB suggests that it is an oxidoreductase-like protein with a typical thioredoxin fold. Indeed, mutant analyses revealed that the kai operon-encoded proteins increase fitness of L. pneumophila in competitive environments, and confer higher resistance to oxidative and sodium stress. The phylogenetic analysis indicates that L. pneumophila KaiBC resemble Synechosystis KaiC2B2 and not circadian KaiB1C1. Thus, the L. pneumophila Kai proteins do not encode a circadian clock, but enhance stress resistance and adaption to changes in the environments.

Implication of the VirD4 coupling protein of the Lvh type 4 secretion system in virulence phenotypes of Legionella pneumophila

The genome of the Philadelphia-1 strain of Legionella pneumophila, the causative organism of Legionnaires' disease, encodes two virulence-associated type 4 secretion systems (T4SSs), the Dot/Icm type 4B (T4BSS) and the Lvh type 4A (T4ASS). Broth stationary-phase cultures of most dot/icm mutants are defective in entry and evasion of phagosome acidification. However, those virulence defects can be reversed by incubating broth cultures of dot/icm mutants in water, termed water stress (WS). WS reversal requires the lvh T4ASS locus, suggesting an interaction between the two T4SSs in producing Legionella virulence phenotypes. In the current work, the loss of WS reversal in a dotA Δlvh mutant of strain JR32 was shown to be attributable to loss of the lvh virD4 gene, encoding the putative coupling protein of the T4ASS. Transformation of a dotA Δlvh mutant with virD4 also reversed entry and phagosome acidification defects in broth cultures. In addition, broth cultures of Δlvh and ΔvirD4 mutants, which were dot/icm(+), showed 5-fold and >6-fold increases in translocation of the Dot/Icm translocation substrates, proteins RalF and SidD, respectively. These data demonstrate that the Lvh T4ASS functions in both broth stationary-phase cultures conventionally used for infection and cultures exposed to WS treatment. Our studies in a dotA Δlvh mutant and in a dot/icm(+) background establish that VirD4 and the Lvh T4ASS contribute to virulence phenotypes and are consistent with independent functioning of Dot/Icm and Lvh T4SSs or functional substitution of the Lvh VirD4 protein for a component(s) of the Dot/Icm T4BSS.

Molecular diversity and high virulence of Legionella pneumophila strains isolated from biofilms developed within a warm spring of a thermal spa

Background

Several cases of legionellosis have been diagnosed in the same French thermal spa in 1986, 1994 and 1997. L. pneumophila serogroup 1 (Lp1) strains have been isolated from several patients, but the source of contamination was not identified despite the presence of different Lp1 in water samples of the three natural springs feeding the spa at this period.

Results

Our strategy was to investigate L. pneumophila (Lp) strains from natural biofilms developed in a sulphur-rich warm spring of this contaminated site. Biofilm analysis revealed the presence of three Lp serogroups (Lp1, Lp10 and Lp12). Surprisingly, Lp10 and Lp12 were not reported in the previous described studies from water samples. Besides, the new seven Lp1 we isolated exhibit a high molecular diversity and have been differentiated in five classes according to their DNA genome patterns obtained by PFGE and mip sequences. It must be noted that these DNA patterns are original and unknown in databases. Interestingly, the 27 Lp environmental strains we isolated display a higher cytotoxicity and virulence towards the amoeba Acanthamoeba castellanii than those of known Lp1 epidemic strains.

Conclusion

The characteristics of Legionella pneumophila Lp1 strains isolated from the warm spring are in agreement with their presence in biofilms and their probable long-term persistence in this ecosystem.

Virulence phenotypes of Legionella pneumophila associated with noncoding RNA lpr0035

The Philadelphia-1 strain of Legionella pneumophila, the causative organism of Legionnaires' disease, contains a recently discovered noncoding RNA, lpr0035. lpr0035 straddles the 5' chromosomal junction of a 45-kbp mobile genetic element, pLP45, which can exist as an episome or integrated in the bacterial chromosome. A 121-bp deletion was introduced in strain JR32, a Philadelphia-1 derivative. The deletion inactivated lpr0035, removed the 49-bp direct repeat at the 5' junction of pLP45, and locked pLP45 in the chromosome. Intracellular multiplication of the deletion mutant was decreased by nearly 3 orders of magnitude in Acanthamoeba castellanii amoebae and nearly 2 orders of magnitude in J774 mouse macrophages. Entry of the deletion mutant into amoebae and macrophages was decreased by >70%. The level of entry in both hosts was restored to that in strain JR32 by plasmid copies of two open reading frames immediately downstream of the 5' junction and plasmid lpr0035 driven by its endogenous promoter. When induced from a tac promoter, plasmid lpr0035 completely reversed the intracellular multiplication defect in macrophages but was without effect in amoebae. These data are the first evidence of a role for noncoding RNA lpr0035, which has homologs in six other Legionella genomes, in entry of L. pneumophila into amoebae and macrophages and in host-specific intracellular multiplication. The data also demonstrate that deletion of a direct-repeat sequence restricts the mobility of pLP45 and is a means of studying the role of pLP45 mobility in Legionella virulence phenotypes.

Rhodococcus equi's extreme resistance to hydrogen peroxide is mainly conferred by one of its four catalase genes

Rhodococcus equi is one of the most widespread causes of disease in foals aged from 1 to 6 months. R. equi possesses antioxidant defense mechanisms to protect it from reactive oxygen metabolites such as hydrogen peroxide (H(2)O(2)) generated during the respiratory burst of phagocytic cells. These defense mechanisms include enzymes such as catalase, which detoxify hydrogen peroxide. Recently, an analysis of the R. equi 103 genome sequence revealed the presence of four potential catalase genes. We first constructed ΔkatA-, ΔkatB-, ΔkatC-and ΔkatD-deficient mutants to study the ability of R. equi to survive exposure to H(2)O(2)in vitro and within mouse peritoneal macrophages. Results showed that ΔkatA and, to a lesser extent ΔkatC, were affected by 80 mM H(2)O(2). Moreover, katA deletion seems to significantly affect the ability of R. equi to survive within murine macrophages. We finally investigated the expression of the four catalases in response to H(2)O(2) assays with a real time PCR technique. Results showed that katA is overexpressed 367.9 times (± 122.6) in response to exposure to 50 mM of H(2)O(2) added in the stationary phase, and 3.11 times (± 0.59) when treatment was administered in the exponential phase. In untreated bacteria, katB, katC and katD were overexpressed from 4.3 to 17.5 times in the stationary compared to the exponential phase. Taken together, our results show that KatA is the major catalase involved in the extreme H(2)O(2) resistance capability of R. equi.

Implication of proteins containing tetratricopeptide repeats in conditional virulence phenotypes of Legionella pneumophila

Legionella pneumophila, the causative agent of Legionnaires' disease, is a ubiquitous freshwater bacterium whose virulence phenotypes require a type IV secretion system (T4SS). L. pneumophila strain JR32 contains two virulence-associated T4SSs, the Dot/Icm and Lvh T4SSs. Defective entry and phagosome acidification phenotypes of dot/icm mutants are conditional and reversed by incubating broth-grown stationary-phase cultures in water (WS treatment) prior to infection, as a mimic of the aquatic environment of Legionella. Reversal of dot/icm virulence defects requires the Lvh T4SS and is associated with a >10-fold induction of LpnE, a tetratricopeptide repeat (TPR)-containing protein. In the current study, we demonstrated that defective entry and phagosome acidification phenotypes of mutants with changes in LpnE and EnhC, another TPR-containing protein, were similarly reversed by WS treatment. In contrast to dot/icm mutants for which the Lvh T4SS was required, reversal for the ΔlpnE or the ΔenhC mutant required that the other TPR-containing protein be present. The single and double ΔlpnE and ΔenhC mutants showed a hypersensitivity to sodium ion, a phenotype associated with dysfunction of the Dot/Icm T4SS. The ΔlpnE single and the ΔlpnE ΔenhC double mutant showed 3- to 9-fold increases in translocation of Dot/Icm T4SS substrates, LegS2/SplY and LepB. Taken together, these data identify TPR-containing proteins in a second mechanism by which the WS mimic of a Legionella environmental niche can reverse virulence defects of broth-grown cultures and implicate LpnE and EnhC directly or indirectly in translocation of Dot/Icm T4SS protein substrates.

Legionella pneumophila transcriptional response to chlorine treatment

Legionella pneumophila is a ubiquitous environmental microorganism found in freshwater that can cause an acute form of pneumonia known as Legionnaires' disease. Despite widespread use of chlorine to ensure drinking water quality and awareness that L. pneumophila may escape these treatments, little is known about its effects on L. pneumophila. The aim of this study was to investigate the L. pneumophila transcriptional response induced by chlorine treatment. Transcriptome analysis, using DNA arrays, showed that a sublethal dose of chlorine induces a differential expression of 391 genes involved in stress response, virulence, general metabolism, information pathways and transport. Many of the stress response genes were significantly upregulated, whereas a significant number of virulence genes were repressed. In particular, exposure of L. pneumophila to chlorine induced the expression of cellular antioxidant proteins, stress proteins and transcriptional regulators. In addition, glutathione S-transferase specific activity was enhanced following chlorine treatment. Our results clearly indicate that chlorine induces expression of proteins involved in cellular defence mechanisms against oxidative stress that might be involved in adaptation or resistance to chlorine treatment.

The kinetic properties producing the perfunctory pH profiles of catalase-peroxidases

Many structure-function relationship studies performed on the catalase-peroxidase enzymes are based on limited kinetic data. To provide a more substantive understanding of catalase-peroxidase function, we undertook a more exhaustive evaluation of catalase-peroxidase catalysis as a function of pH. Kinetic parameters across a broad pH range for the catalase and peroxidase activities of E. coli catalase peroxidase (KatG) were obtained, including the separate analysis of the oxidizing and reducing substrates of the peroxidase catalytic cycle. This investigation identified ABTS-dependent inhibition of peroxidase activity, particularly at low pH, unveiling that previously reported pH optima are clearly skewed. We show that turnover and efficiency of peroxidase activity increases with decreasing pH until the protein unfolds. The data also suggest that the catalase pH optimum is more complex than it is often assumed to be. The apparent optimum is in fact the intersection of the optimum for binding (7.00) and the optimum for activity (5.75). We also report the apparent pK(a)s for binding and catalysis of catalase activity as well as approximate values for certain peroxidatic and catalatic steps.

An ortholog of OxyR in Legionella pneumophila is expressed postexponentially and negatively regulates the alkyl hydroperoxide reductase (ahpC2D) operon

Legionella pneumophila expresses two peroxide-scavenging alkyl hydroperoxide reductase systems (AhpC1 and AhpC2D) that are expressed differentially during the bacterial growth cycle. Functional loss of the postexponentially expressed AhpC1 system is compensated for by increased expression of the exponentially expressed AhpC2D system. In this study, we used an acrylamide capture of DNA-bound complexes (ACDC) technique and mass spectrometry to identify proteins that bind to the promoter region of the ahpC2D operon. The major protein captured was an ortholog of OxyR (OxyR(Lp)). Genetic studies indicated that oxyR(Lp) was an essential gene expressed postexponentially and only partially complemented an Escherichia coli oxyR mutant (GS077). Gel shift assays confirmed specific binding of OxyR(Lp) to ahpC2D promoter sequences, but not to promoters of ahpC1 or oxyR(Lp); however, OxyR(Lp) weakly bound to E. coli OxyR-regulated promoters (katG, oxyR, and ahpCF). DNase I protection studies showed that the OxyR(Lp) binding motif spanned the promoter and transcriptional start sequences of ahpC2 and that the protected region was unchanged by treatments with reducing agents or hydrogen peroxide (H(2)O(2)). Moreover, the OxyR(Lp) (pBADLpoxyR)-mediated repression of an ahpC2-gfp reporter construct in E. coli GS077 (the oxyR mutant) was not reversed by H(2)O(2) challenge. Alignments with other OxyR proteins revealed several amino acid substitutions predicted to ablate thiol oxidation or conformational changes required for activation. We suggest these mutations have locked OxyR(Lp) in an active DNA-binding conformation, which has permitted a divergence of function from a regulator of oxidative stress to a cell cycle regulator, perhaps controlling gene expression during postexponential differentiation.

Phylogenetic distribution of catalase-peroxidases: are there patches of order in chaos?

Hydrogen peroxide features in many biological oxidative processes and must be continuously degraded enzymatically either via a catalatic or a peroxidatic mechanism. For this purpose ancestral bacteria evolved a battery of different heme and non-heme enzymes, among which heme-containing catalase-peroxidases (CP) are one of the most widespread representatives. They are unique since they can follow both H(2)O(2)-degrading mechanisms, the catalase activity being clearly dominant. With the fast increasing amount of genomic data available, we were able to perform an extensive search for CP and found almost 300 sequences covering a large range of microorganisms. Most of them were encoded by bacterial genomes, but we could also find some in eukaryotic organisms other than fungi, which has never been shown until now. Our screen also reveals that approximately 60% of the bacteria do not possess CP genes. Chaotic distribution among species and incongruous phylogenetic reconstruction indicated existence of numerous lateral gene transfers in addition to duplication events and regular speciation. The results obtained show an impressively complex gene transmission pattern, and give some new insights about the role of CP and the origin of life on earth. Finally, we propose for the first time bacterial candidates that may have participated in the transfer of CP from bacteria to eukaryotes.

Environmental mimics and the Lvh type IVA secretion system contribute to virulence-related phenotypes of Legionella pneumophila

Legionella pneumophila, the causative organism of Legionnaires' disease, is a fresh-water bacterium and intracellular parasite of amoebae. This study examined the effects of incubation in water and amoeba encystment on L. pneumophila strain JR32 and null mutants in dot/icm genes encoding a type IVB secretion system required for entry, delayed acidification of L. pneumophila-containing phagosomes, and intracellular multiplication when stationary-phase bacteria infect amoebae and macrophages. Following incubation of stationary-phase cultures in water, mutants in dotA and dotB, essential for function of the type IVB secretion system, exhibited entry and delay of phagosome acidification comparable to that of strain JR32. Following encystment in Acanthamoeba castellanii and reversion of cysts to amoeba trophozoites, dotA and dotB mutants exhibited intracellular multiplication in amoebae. The L. pneumophila Lvh locus, encoding a type IVA secretion system homologous to that in Agrobacterium tumefaciens, was required for restoration of entry and intracellular multiplication in dot/icm mutants following incubation in water and amoeba encystment and was required for delay of phagosome acidification in strain JR32. These data support a model in which the Dot/Icm type IVB secretion system is conditionally rather than absolutely required for L. pneumophila virulence-related phenotypes. The data suggest that the Lvh type IVA secretion system, previously thought to be dispensable, is involved in virulence-related phenotypes under conditions mimicking the spread of Legionnaires' disease from environmental niches. Since environmental amoebae are implicated as reservoirs for an increasing number of environmental pathogens and for drug-resistant bacteria, the environmental mimics developed here may be useful in virulence studies of other pathogens.

Integration of curated databases to identify genotype-phenotype associations

BACKGROUND

The ability to rapidly characterize an unknown microorganism is critical in both responding to infectious disease and biodefense. To do this, we need some way of anticipating an organism's phenotype based on the molecules encoded by its genome. However, the link between molecular composition (i.e. genotype) and phenotype for microbes is not obvious. While there have been several studies that address this challenge, none have yet proposed a large-scale method integrating curated biological information. Here we utilize a systematic approach to discover genotype-phenotype associations that combines phenotypic information from a biomedical informatics database, GIDEON, with the molecular information contained in National Center for Biotechnology Information's Clusters of Orthologous Groups database (NCBI COGs).

RESULTS

Integrating the information in the two databases, we are able to correlate the presence or absence of a given protein in a microbe with its phenotype as measured by certain morphological characteristics or survival in a particular growth media. With a 0.8 correlation score threshold, 66% of the associations found were confirmed by the literature and at a 0.9 correlation threshold, 86% were positively verified.

CONCLUSION

Our results suggest possible phenotypic manifestations for proteins biochemically associated with sugar metabolism and electron transport. Moreover, we believe our approach can be extended to linking pathogenic phenotypes with functionally related proteins.

Compensatory functions of two alkyl hydroperoxide reductases in the oxidative defense system of Legionella pneumophila

Legionella pneumophila expresses two catalase-peroxidase enzymes that exhibit strong peroxidatic but weak catalatic activities, suggesting that other enzymes participate in decomposition of hydrogen peroxide (H2O2). Comparative genomics revealed that L. pneumophila and its close relative Coxiella burnetii each contain two peroxide-scavenging alkyl hydroperoxide reductase (AhpC) systems: AhpC1, which is similar to the Helicobacter pylori AhpC system, and AhpC2 AhpD (AhpC2D), which is similar to the AhpC AhpD system of Mycobacterium tuberculosis. To establish a catalatic function for these two systems, we expressed L. pneumophila ahpC1 or ahpC2 in a catalase/peroxidase mutant of Escherichia coli and demonstrated restoration of H2O2 resistance by a disk diffusion assay. ahpC1::Km and ahpC2D::Km chromosomal deletion mutants were two- to eightfold more sensitive to H2O2, tert-butyl hydroperoxide, cumene hydroperoxide, and paraquat than the wild-type L. pneumophila, a phenotype that could be restored by trans-complementation. Reciprocal strategies to construct double mutants were unsuccessful. Mutant strains were not enfeebled for growth in vitro or in a U937 cell infection model. Green fluorescence protein reporter assays revealed expression to be dependent on the stage of growth, with ahpC1 appearing after the exponential phase and ahpC2 appearing during early exponential phase. Quantitative real-time PCR showed that ahpC1 mRNA levels were approximately 7- to 10-fold higher than ahpC2D mRNA levels. However, expression of ahpC2D was significantly increased in the ahpC1 mutant, whereas ahpC1 expression was unchanged in the ahpC2D mutant. These results indicate that AhpC1 or AhpC2D (or both) provide an essential hydrogen peroxide-scavenging function to L. pneumophila and that the compensatory activity of the ahpC2D system is most likely induced in response to oxidative stress.

Identification and characterization of novel ColE1-type, high-copy number plasmid mutants in Legionella pneumophila

ColE1-type plasmids are commonly used in bacterial genetics research, and replication of these plasmids is regulated by interaction of RNA I and RNA II. Although these plasmids are narrow-host-range, they can be maintained in Legionella pneumophila under antibiotic selection, with low-copy number and instability. Here, we have described the isolation of two novel spontaneous mutants of pBC(gfp)Pmip, pBG307 and pBG309, which are able to mark the L. pneumophila with strong green fluorescence when exposed to visible light. One of the mutants, pBG307, has a single CG-->TA mutation in RNA II promoter located 2-bases upstream the - 10 region. Another one, pBG309, has the same mutation, as well as an additional CG-->AT mutation in the 76th nucleotide of RNA I, or in the 6th nucleotide of RNA II. A plasmid with the single mutation in RNA I, pBG308, was also constructed. Characterization of these plasmids carrying the enhanced green fluorescent protein (gfpmut2) gene revealed that the green fluorescence intensities of these plasmids were 2- to 30-fold higher than that of the wild type and both of the mutations contribute to increase the plasmid copy number and/or plasmid stability. The mutation located in RNA II promoter played a more dominant role in elevating the copy number, compared to the mutation in RNA I. We also tested the mutant plasmids for replication in Escherichia coli, and found that their copy number and stability were dramatically decreased, except pBG307. Our data suggest that these plasmids might be useful and convenient in genetic studies in L. pneumophila.

A minor catalase/peroxidase from Burkholderia cenocepacia is required for normal aconitase activity

The opportunistic bacterium Burkholderia cenocepacia C5424 contains two catalase/peroxidase genes, katA and katB. To investigate the functions of these genes, katA and katB mutants were generated by targeted integration of suicide plasmids into the katA and katB genes. The catalase/peroxidase activity of the katA mutant was not affected as compared with that of the parental strain, while no catalase/peroxidase activity was detected in the katB mutant. However, the katA mutant displayed reduced resistance to hydrogen peroxide under iron limitation, while the katB mutant showed hypersensitivity to hydrogen peroxide, and reduced growth under all conditions tested. The katA mutant displayed reduced growth only in the presence of carbon sources that are metabolized through the tricarboxylic acid (TCA) cycle, as the growth defect was abrogated in cultures supplemented with glucose or glycerol. This phenotype was also correlated with a marked reduction in aconitase activity. In contrast, aconitase activity was not reduced in the katB mutant and parental strains. The authors conclude that the KatA protein is a specialized catalase/peroxidase that has a novel function by contributing to maintain the normal activity of the TCA cycle, while KatB is a classical catalase/peroxidase that plays a global role in cellular protection against oxidative stress.

Non-classical protein secretion in bacteria

BACKGROUND

We present an overview of bacterial non-classical secretion and a prediction method for identification of proteins following signal peptide independent secretion pathways. We have compiled a list of proteins found extracellularly despite the absence of a signal peptide. Some of these proteins also have known roles in the cytoplasm, which means they could be so-called "moon-lightning" proteins having more than one function.

RESULTS

A thorough literature search was conducted to compile a list of currently known bacterial non-classically secreted proteins. Pattern finding methods were applied to the sequences in order to identify putative signal sequences or motifs responsible for their secretion. We have found no signal or motif characteristic to any majority of the proteins in the compiled list of non-classically secreted proteins, and conclude that these proteins, indeed, seem to be secreted in a novel fashion. However, we also show that the apparently non-classically secreted proteins are still distinguished from cellular proteins by properties such as amino acid composition, secondary structure and disordered regions. Specifically, prediction of disorder reveals that bacterial secretory proteins are more structurally disordered than their cytoplasmic counterparts. Finally, artificial neural networks were used to construct protein feature based methods for identification of non-classically secreted proteins in both Gram-positive and Gram-negative bacteria.

CONCLUSION

We present a publicly available prediction method capable of discriminating between this group of proteins and other proteins, thus allowing for the identification of novel non-classically secreted proteins. We suggest candidates for non-classically secreted proteins in Escherichia coli and Bacillus subtilis. The prediction method is available online.

Surface polysaccharides enable bacteria to evade plant immunity

Plants have an immune system to perceive pathogenic or potentially beneficial bacteria. Aspects of perception, signal transduction and the responses that the plant produces resemble features of innate immunity observed in animals. Plant reactions are various and include the production of antimicrobial compounds. Bacteria that are successful in establishing pathogenic or symbiotic interactions have developed multiple ways to protect themselves. We review the general importance of bacterial surface polysaccharides in the evasion of plant immune responses and elaborate on their role in protecting symbiotic bacteria against toxic reactive oxygen species during invasion of the host plant.

Both inducible nitric oxide synthase and NADPH oxidase contribute to the control of virulent phase I Coxiella burnetii infections

Host control of Coxiella burnetii infections is believed to be mediated primarily by activated monocytes/macrophages. The activation of macrophages by cytokines leads to the production of reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI) that have potent antimicrobial activities. The contributions of ROI and RNI to the inhibition of C. burnetii replication were examined in vitro by the use of murine macrophage-like cell lines and primary mouse macrophages. A gamma interferon (IFN-gamma) treatment of infected cell lines and primary macrophages resulted in an increased production of nitric oxide (NO) and hydrogen peroxide (H2O2) and a significant inhibition of C. burnetii replication. The inhibition of replication was reversed in the murine cell line J774.16 upon the addition of either the inducible nitric oxide synthase (iNOS) inhibitor NG-monomethyl-L-arginine (NGMMLA) or the H2O2 scavenger catalase. IFN-gamma-treated primary macrophages from iNOS-/- and p47phox-/- mice significantly inhibited replication but were less efficient at controlling infection than IFN-gamma-treated wild-type macrophages. To investigate the contributions of ROI and RNI to resistance to infection, we performed in vivo studies, using C57BL/6 wild-type mice and knockout mice lacking iNOS or p47phox. Both iNOS-/- and p47phox-/- mice were attenuated in the ability to control C. burnetii infection compared to wild-type mice. Together, these results strongly support a role for both RNI and ROI in the host control of C. burnetii infection.

Properties of catalase-peroxidase lacking its C-terminal domain

Catalase-peroxidases have a two-domain structure. The N-terminal domain contains the bifunctional active site, but the function of the C-terminal domain is unknown. We produced catalase-peroxidase containing only its N-terminal domain (KatG(Nterm)). Removal of the C-terminal domain did not result in unexpected changes in secondary structure as evaluated by CD, but KatG(Nterm) had neither catalase nor peroxidase activity. Partial recovery of both activities was achieved by incubating KatG(Nterm) with the separately expressed and isolated KatG C-terminal domain. Spectroscopic measurements revealed a shift in heme environment from a mixture of high-spin species (wtKatG) to exclusively hexacoordinate, low-spin (KatG(Nterm)). Moreover, a > 1000-fold lower kon for CN- binding was observed for KatG(Nterm). EPR spectra for KatG(Nterm) and the results of site-specific substitution of active site histidines suggested that the distal histidine was the sixth ligand. Thus, one important role for the C-terminal domain may be to support the architecture of the active site, preventing heme ligation by this catalytically essential residue.

Icm/dot-independent entry of Legionella pneumophila into amoeba and macrophage hosts

Legionella pneumophila, the causative agent of Legionnaires' disease, expresses a type IVB secretion apparatus that translocates bacterial proteins into amoeba and macrophage hosts. When stationary-phase cultures are used to infect hosts, the type IVB apparatus encoded by the icm/dot genes is required for entry, delay of phagosome-lysosome fusion, and intracellular multiplication within host cells. Null mutants with mutations in icm/dot genes are defective in these phenotypes. Here a new model is described in which hosts are infected with stationary-phase cultures that have been incubated overnight in pH 6.5 buffer. This model is called Ers treatment because it enhances the resistance to acid, hydrogen peroxide, and antibiotic stress beyond that of stationary-phase cultures. Following Ers treatment entry into amoeba and macrophage hosts does not require dotA, which is essential for Legionella virulence phenotypes when hosts are infected with stationary-phase cultures, dotB, icmF, icmV, or icmX. Defective host entry is also suppressed for null mutants with mutations in the KatA and KatB catalase-peroxidase enzymes, which are required for proper intracellular growth in amoeba and macrophage hosts. Ers treatment-induced suppression of defective entry is not associated with increased bacterial adhesion to host cells or with morphological changes in the bacterial envelope but is dependent on protein expression during Ers treatment. By using proteomic analysis, Ers treatment was shown to induce a protein predicted to contain eight tetratricopeptide repeats, a motif previously implicated in enhanced entry of L. pneumophila. Characterization of Ers treatment-dependent changes in expression is proposed as an avenue for identifying icm/dot-independent factors that function in the entry of Legionella into amoeba and macrophage hosts.

Vital roles of an interhelical insertion in catalase–peroxidase bifunctionality

The loop connecting the F and G helices of catalase-peroxidases contains a approximately 35 amino acid structure (the FG insertion) that is absent from monofunctional peroxidases. These two groups of enzymes share highly similar active sites, yet the monofunctional peroxidases lack appreciable catalase activity. Thus, the FG insertion may serve a role in catalase-peroxidase bifunctionality, despite its peripheral location relative to the active site. We produced a variant of Escherichia coli catalase-peroxidase (KatG) lacking its FG insertion (KatG(DeltaFG)). Absorption spectra indicated the heme environment of KatG(DeltaFG) was highly similar to wild-type KatG, but the variant retained only 0.2% catalase activity. In contrast, the deletion reduced peroxidase activity by only 50%. Kinetic parameters for the peroxidase and residual catalase activities of KatG(DeltaFG) as well as pH dependence studies suggested that the FG insertion supports hydrogen-bonded networks critical for reactions involving H2O2. The structure also appears to regulate access of electron donors to the active site.

Properties of a novel periplasmic catalase–peroxidase from Escherichia coli O157:H7

A subset of catalase-peroxidases are distinguished by their periplasmic location and their expression by pathogens. Kinetic and spectral properties have not been reported for any of these enzymes. We report the cloning, expression, isolation, and characterization of KatP, a periplasmic catalase-peroxidase from Escherichia coli O157:H7. Absorption spectra indicated a mixture of heme states dominated by the pentacoordinate and hexacoordinate high-spin forms. Apparent k(cat) values for catalase (1.8x10(4) s(-1)) and peroxidase (77 s(-1)) activities were greater than those of other catalase-peroxidases. However, apparent K(M) values for H2O2 were also higher (27 mM for catalase and 3 mM for peroxidase). Ferric KatP reacted with peracetic acid to form compound I (8.8x10(3) M(-1) s(-1)) and with CN(-) to form a ferri-cyano complex (3.9x10(5) M(-1) s(-1)) consistent with other catalase-peroxidases. The isolation and characterization of KatP opens new avenues to explore mechanisms by which the periplasmic catalase-peroxidases may contribute to bacterial virulence.

A major catalase (KatB) that is required for resistance to H2O2 and phagocyte-mediated killing in Edwardsiella tarda

Edwardsiella tarda causes haemorrhagic septicaemia in fish and gastro- and extra-intestinal infections in animals including humans. Resistance to phagocyte-mediated killing is one of the virulence factors of Ed. tarda. The authors' previous studies using TnphoA transposon mutagenesis indicated that katB mutants derived from the strain PPD130/91 are at least 1.6 log higher in LD50 values than the wild-type strain. These findings suggest the involvement of catalase (KatB) in Ed. tarda pathogenesis. In this study, experiments were conducted to characterize the contribution of KatB to Ed. tarda infection. Zymographic analyses indicated that the 22 Ed. tarda strains examined expressed three different types of catalase-peroxidases (Kat1-3) based on their mobility in non-denaturing polyacrylamide gels. KatB (Kat1), the major catalase enzyme, was expressed in eight out of 22 Ed. tarda strains, and was commonly found in virulent strains except AL9379. AL9379 has a mutated katB, which has a base substitution and a deletion that translate into stop codons in the catalase gene. KatB produced by PPD130/91 was located in both periplasmic and cytoplasmic fractions and was constitutively expressed in various growth phases. Kinetics studies indicated that the catalase provided resistance to H2O2- and phagocyte-mediated killing. Infection kinetics studies of katB mutant 34 in gourami fish demonstrated its inability to survive and replicate in phagocyte-rich organs and this prevented the dissemination of infections when compared to the wild-type. Complementation of catalase mutants restored the production of catalase, and led to an increase in the resistance to H2O2- and phagocyte-mediated killing, and a decrease in LD50 values. This study has identified and characterized a major catalase gene (katB) that is required for resistance to H2O2- and phagocyte-mediated killing in Ed. tarda. The results also suggest that catalase may play a role as a virulence factor in Ed. tarda pathogenesis.

Legionella pneumophila catalase-peroxidases are required for proper trafficking and growth in primary macrophages

Legionella pneumophila, a parasite of aquatic amoebae and pathogen of pulmonary macrophages, replicates intracellularly, utilizing a type IV secretion system to subvert the trafficking of Legionella-containing phagosomes. Defense against host-derived reactive oxygen species has been proposed as critical for intracellular replication. Virulence traits of null mutants in katA and katB, encoding the two Legionella catalase-peroxidases, were analyzed to evaluate the hypothesis that L. pneumophila must decompose hydrogen peroxide to establish a replication niche in macrophages. Phagosomes containing katA or katB mutant Legionella colocalize with LAMP-1, a late endosomal-lysosomal marker, at twice the frequency of those of wild-type strain JR32 and show a decreased frequency of bacterial replication, in similarity to phenotypes of mutants with mutations in dotA and dotB, encoding components of the Type IV secretion system. Quantitative similarity of the katA/B phenotypes indicates that each contributes to virulence traits largely independently of intracellular compartmentalization (KatA in the periplasm and KatB in the cytosol). These data support a model in which KatA and KatB maintain a critically low level of H(2)O(2) compatible with proper phagosome trafficking mediated by the type IV secretion apparatus. During these studies, we observed that dotA and dotB mutations in wild-type strain Lp02 had no effect on intracellular multiplication in the amoeba Acanthamoeba castellanii, indicating that certain dotA/B functions in Lp02 are dispensable in that experimental model. We also observed that wild-type JR32, unlike Lp02, shows minimal contact-dependent cytotoxicity, suggesting that cytotoxicity of JR32 is not a prerequisite for formation of replication-competent Legionella phagosomes in macrophages.

Lateral gene transfer (LGT) between Archaea and Escherichia coli is a contributor to the emergence of novel infectious disease

BACKGROUND

Lateral gene transfer is the major mechanism for acquisition of new virulence genes in pathogens. Recent whole genome analyses have suggested massive gene transfer between widely divergent organisms.

PRESENTATION OF THE HYPOTHESIS

Archeal-like genes acting as virulence genes are present in several pathogens and genomes contain a number of archaeal-like genes of unknown function. Archaea, by virtue of their very different evolutionary history and different environment, provide a pool of potential virulence genes to bacterial pathogens.

TESTING THE HYPOTHESIS

We can test this hypothesis by 1)identifying genes likely to have been transferred (directly or indirectly) to E. coli O157:H7 from archaea; 2)investigating the distribution of similar genes in pathogens and non-pathogens and 3)performing rigorous phylogenetic analyses on putative transfers.

IMPLICATIONS OF THE HYPOTHESIS

Although this hypothesis focuses on archaea and E. coli, it will serve as a model having broad applicability to a number of pathogenic systems. Since no archaea are known vertebrate pathogens, archaeal-like transferred genes that are associated with virulence in bacteria represent a clear model for the emergence of virulence genes.

The response regulator LetA regulates the stationary-phase stress response in Legionella pneumophila and is required for efficient infection of Acanthamoeba castellanii

In order to identify a potential regulator of virulence gene expression in Legionella pneumophila, the L. pneumophila homologue of the response regulator GacA, LetA, was identified and cloned, facilitating the generation of a L. pneumophila letA insertion mutant. The L. pneumophila letA insertion mutant was more sensitive to oxidative and acid stress than the wild-type. The letA mutant exhibited reduced infectivity and was defective for intracellular growth within Acanthamoeba castellanii. Transcription of the rpoS and dotA genes was reduced in the letA mutant. Our data indicate that the response regulator LetA functions as a regulator of the stationary-phase stress response in L. pneumophila and is required for efficient replication within A. castellanii.

Hydrogen peroxide fluxes and compartmentalization inside growing Escherichia coli

Escherichia coli generates about 14 microM hydrogen peroxide (H(2)O(2)) per s when it grows exponentially in glucose medium. The steady-state intracellular concentration of H(2)O(2) depends on the rates at which this H(2)O(2) is dissipated by scavenging enzymes and by efflux from the cell. The rates of H(2)O(2) degradation by the two major scavenging enzymes, alkyl hydroperoxide reductase and catalase, were quantified. In order to estimate the rate of efflux, the permeability coefficient of membranes for H(2)O(2) was determined. The coefficient is 1.6 x 10(-3) cm/s, indicating that permeability is substantial but not unlimited. These data allowed internal H(2)O(2) fluxes and concentrations to be calculated. Under these growth conditions, Ahp scavenges the majority of the endogenous H(2)O(2), with a small fraction degraded by catalase and virtually none persisting long enough to penetrate the membrane and exit the cell. The robust scavenging activity maintains the H(2)O(2) concentration inside glucose-grown cells at <10(-7) M, substantially below the level (10(-6) M) at which toxicity is evident. When extracellular H(2)O(2) is present, its flux into the cell can be rapid, but the internal concentration may still be an order of magnitude lower than that outside. The presence of such gradients was confirmed in experiments that revealed different degrees of oxidative stress in cocultured scavenger-deficient mutants. The limited permeability of membranes to H(2)O(2) rationalizes the compartmentalization of scavenging systems and predicts that bacteria that excrete redox-cycling drugs do not experience the same H(2)O(2) dose that they impose on their competitors.