Expression of superoxide dismutase in Listeria monocytogenes

Manganese uptake mediated by the NRAMP-type transporter MntH is required for acid tolerance in Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen that is characterized by its ability to withstand mild stresses (i.e. cold, acid, salt) often encountered in food products or food processing environments. In the previous phenotypic and genotypic characterization of a collection of L. monocytogenes strains, we have identified one strain 1381, originally obtained from EURL-lm, as acid sensitive (reduced survival at pH 2.3) and extremely acid intolerant (no growth at pH 4.9, which supports the growth of most strains). In this study, we investigated the cause of acid intolerance in strain 1381 by isolating and sequencing reversion mutants that were capable of growth at low pH (pH 4.8) to a similar extent as another strain (1380) from the same MLST clonal complex (CC2). Whole genome sequencing showed that a truncation in mntH, which encodes a homologue of an NRAMP (Natural Resistance-Associated Macrophage Protein) type Mn²⁺ transporter, is responsible for the acid intolerance phenotype observed in strain 1381. However, the mntH truncation alone was not sufficient to explain the acid sensitivity of strain 1381 at lethal pH values as strain 1381R1 (a mntH⁺ revertant) exhibited similar acid survival to its parental strain at pH 2.3. Further growth experiments demonstrated that Mn²⁺ (but not Fe²⁺, Zn²⁺, Cu²⁺, Ca²⁺, or Mg²⁺) supplementation fully rescues the growth of strain 1381 under low pH conditions, suggesting that a Mn²⁺ limitation is the likely cause of growth arrest in the mntH^- background. Consistent with the important role of Mn²⁺ in the acid stress response was the finding that mntH and mntB (both encoding Mn²⁺ transporters) had higher transcription levels following exposure to mild acid stress (pH 5). Taken together, these results provide evidence that MntH-mediated Mn²⁺ uptake is essential for the growth of L. monocytogenes under low pH conditions. Moreover, since strain 1381 was recommended for conducting food challenge studies by the European Union Reference Laboratory, the use of this strain in evaluating the growth of L. monocytogenes in low pH environments where Mn²⁺ is scarce should be reconsidered. Furthermore, since it is unknown when strain 1381 acquired the mntH frameshift mutation, the ability of the strains used for challenge studies to grow under food-related stresses needs to be routinely validated.

Investigating the Roles of Listeria monocytogenes Peroxidases in Growth and Virulence

Bacteria have necessarily evolved a protective arsenal of proteins to contend with peroxides and other reactive oxygen species generated in aerobic environments. Listeria monocytogenes encounters an onslaught of peroxide both in the environment and during infection of the mammalian host, where it is the causative agent of the foodborne illness listeriosis. Despite the importance of peroxide for the immune response to bacterial infection, the strategy by which L. monocytogenes protects against peroxide toxicity has yet to be illuminated. Here, we investigated the expression and essentiality of all the peroxidase-encoding genes during L. monocytogenes growth in vitro and during infection of murine cells in tissue culture. We found that chdC and kat were required for aerobic growth in vitro, and fri and ahpA were each required for L. monocytogenes to survive acute peroxide stress. Despite increased expression of fri, ahpA, and kat during infection of macrophages, only fri proved necessary for cytosolic growth. In contrast, the proteins encoded by lmo0367, lmo0983, tpx, lmo1609, and ohrA were dispensable for aerobic growth, acute peroxide detoxification, and infection. Together, our results provide insight into the multifaceted L. monocytogenes peroxide detoxification strategy and demonstrate that L. monocytogenes encodes a functionally diverse set of peroxidase enzymes. IMPORTANCE Listeria monocytogenes is a facultative intracellular pathogen and the causative agent of the foodborne illness listeriosis. L. monocytogenes must contend with reactive oxygen species generated extracellularly during aerobic growth and intracellularly by the host immune system. However, the mechanisms by which L. monocytogenes defends against peroxide toxicity have not yet been defined. Here, we investigated the roles of each of the peroxidase-encoding genes in L. monocytogenes growth, peroxide stress response, and virulence in mammalian cells.

Characterisation of the manganese superoxide dismutase of Enterococcus faecium

The manganese superoxide dismutase (SodA) of E. faecium strain AUS0004 has been characterised. It is most closely related to Enterococcus hirae, Enterococcus durans, Enterococcus villorium, and Enterococcus mundtii with 100%, 91,55%, 90,85%, and 90,58% homology, respectively, but more distant from SodA of E. faecalis (81.68%). A sodA deletion mutant has been constructed. Compared to the parental strain, the ΔsodA mutant was affected in aerobic growth and more sensitive to hydrogen peroxide (H₂O₂), cumene hydroperoxide (CuOOH), and the superoxide anion (O₂^•-) generator menadione. The E. faecium strain AUS0004 is part of those bacteria accumulating H₂O₂ to high concentrations (around 5 mM) starting from late exponential growth phase. Accumulation of the peroxide was around 25% less in the mutant suggesting that this part of H₂O₂ is due to the dismutation of O₂^•- by SodA. The sodA gene of E. faecium AUS0004 was induced by oxygen, peroxides and menadione but the corresponding regulator remains hitherto unknown. Finally, we showed that SodA activity is important for virulence in the Galleria mellonella model.

Lacticaseibacillus rhamnosus Impedes Growth of Listeria spp. in Cottage Cheese through Manganese Limitation

Acidification and nutrient depletion by dairy starter cultures is often sufficient to prevent outgrowth of pathogens during post-processing of cultured dairy products. In the case of cottage cheese, however, the addition of cream dressing to the curd and subsequent cooling procedures can create environments that may be hospitable for the growth of Listeria monocytogenes. We report on a non-bacterio-cinogenic Lacticaseibacillus rhamnosus strain that severely limits the growth potential of L. monocytogenes in creamed cottage cheese. The main mechanism underlying Listeria spp. inhibition was found to be caused by depletion of manganese (Mn), thus through competitive exclusion of a trace element essential for the growth of many microorganisms. Growth of Streptococcus thermophilus and Lactococcus lactis that constitute the starter culture, on the other hand, were not influenced by reduced Mn levels. Addition of L. rhamnosus with Mn-based bioprotective properties during cottage cheese production therefore offers a solution to inhibit undesired bacteria in a bacteriocin-independent fashion.

Εvaluation of oxygen availability on growth and inter-strain interactions of L. monocytogenes in/on liquid, semi-solid and solid laboratory media

The coexistence and interactions among Listeria monocytogenes strains in combination with the structural characteristics of foods, may influence their growth capacity and thus, the final levels at the time of consumption. In the present study, we aimed to evaluate the effect of oxygen availability in combination with substrate micro-structure on growth and inter-strain interactions of L. monocytogenes. L. monocytogenes strains, selected for resistance to different antibiotics (to enable distinct enumeration), belonging to serotypes 4b (C5, ScottA), 1/2a (6179) and 1/2b (PL25) and were inoculated in liquid (Tryptic Soy Broth supplemented with Yeast Extract - TSB-YE) and solid (TSB-YE supplemented with 0.6% and 1.2% agar) media (2-3 log CFU/mL, g or cm²), single or as two-strain cultures (1:1 strain-ratio). Aerobic conditions (A) were achieved with constant shaking or surface inoculation for liquid and solid media respectively, while static incubation or pour plated media corresponded to hypoxic environment (H). Anoxic conditions (An) were attained by adding 0.1% w/v sodium thioglycolate and paraffin overlay (for solid media). Growth was assessed during storage at 7 °C (n = 3 × 2). Inter-strain interactions were manifested by the difference in the final population between singly and co-cultured strains. Τhe extent of suppression increased with reduction in agar concentration, while the impact of oxygen availability was dependent on strain combination. During co-culture, in liquid and solid media, 6179 was suppressed by C5 by 4.0 (in TSB-YE under H) to 1.8 log units (in solid medium under An), compared to the single culture, which attained population of ca. 9.4 log CFU/mL or g. The growth of 6179 was also inhibited by ScottA by 2.7 and 1.9 log units, in liquid culture under H and An, respectively. Interestingly, in liquid medium under A, H and An, ScottA was suppressed by C5, by 3.3, 2.4 and 2.3 log units, respectively, while in solid media, growth inhibition was less pronounced. Investigating growth interactions in different environments could assist in explaining the dominance of L. monocytogenes certain serotypes.

Initial Transcriptomic Response and Adaption of Listeria monocytogenes to Desiccation on Food Grade Stainless Steel

The foodborne pathogen Listeria monocytogenes survives exposure to a variety of stresses including desiccation in the food industry. Strand-specific RNA sequencing was applied to analyze changes in the transcriptomes of two strains of L. monocytogenes (Lm 568 and Lm 08-5578) during desiccation [15°C, 43% relative humidity (RH)] on food grade stainless steel surfaces over 48 h to simulate a weekend with no food production. Both strains showed similar survival during desiccation with a 1.8-2 Log CFU/cm2 reduction after 48 h. Analysis of differentially expressed (DE) genes (>twofold, adjusted p-value <0.05) revealed that the initial response to desiccation was established after 6 h and remained constant with few new genes being DE after 12, 24, and 48 h. A core of 81 up- and 73 down-regulated DE genes were identified as a shared, strain independent response to desiccation. Among common upregulated genes were energy and oxidative stress related genes e.g., qoxABCD (cytochrome aa3) pdhABC (pyruvate dehydrogenase complex) and mntABCH (manganese transporter). Common downregulated genes related to anaerobic growth, proteolysis and the two component systems lmo1172/lmo1173 and cheA/cheY, which are involved in cold growth and flagellin production, respectively. Both strains upregulated additional genes involved in combatting oxidative stress and reactive oxygen species (ROS), including sod (superoxide dismutase), kat (catalase), tpx (thiol peroxidase) and several thioredoxins including trxAB, lmo2390 and lmo2830. Osmotic stress related genes were also upregulated in both strains, including gbuABC (glycine betaine transporter) and several chaperones clpC, cspA, and groE. Significant strain differences were also detected with the food outbreak strain Lm 08-5578 differentially expressing 1.9 × more genes (726) compared to Lm 568 (410). Unique to Lm 08-5578 was a significant upregulation of the expression of the alternative transcription factor σB and its regulon. A number of long antisense transcripts (lasRNA) were upregulated during desiccation including anti0605, anti0936, anti1846, and anti0777, with the latter controlling flagellum biosynthesis and possibly the downregulation of motility genes observed in both strains. This exploration of the transcriptomes of desiccated L. monocytogenes provides further understanding of how this bacterium encounters and survives the stress faced when exposed to dry conditions in the food industry.

The role of sodA and sodB in Aeromonas hydrophila resisting oxidative damage to survive in fish macrophages and escape for further infection

Several bacteria have been defined as extracellular pathogens; however, in recent years, it has been confirmed that they have the ability to survive and escape the attack of host phagocytes, thus causing further infection. Previous studies have shown that Aeromonas hydrophila could survive in fish macrophages; however, the mechanism remains unknown. In this study, sodA and sodB of the strain A. hydrophila B11 were stable silenced by shRNA. The survival rates of intracellular sodA-RNAi and sodB-RNAi decreased by 91.8% and 74.9% and the immune escape rates decreased by about 32% and 92% respectively. At the same time, reactive oxygen species (ROS) in fish macrophages that phagocytosed sodA-RNAi and sodB-RNAi increased by 40% and 32.6%, respectively, compared to those of macrophages that phagocytosed the wild-type strain. Compared to sodA, the expression of sodB predominates in A. hydrophila without oxidative stress; however, when exposed to oxidative stress, the magnitude of up-regulation of sodA expression is significantly higher than that of sodB. With increased of methyl viologen concentration, the survival rates of sodA-RNAi and sodB-RNAi were significantly decreased. The expressions of sodA and sodB did not affect the growth of A. hydrophila without oxidative stress, but the inhibition of sodA and sodB expression led to a slight decrease in bacterial growth under oxidative stress. These results indicated that (1) sodA and sodB play an important role in the process of bacterial resistance to ROS damage in host phagocytic cells, allowing them to survive or even escape fish macrophages; (2) the sodB expression was dominant in A. hydrophila without oxidative stress, the sodA expression was up-regulated more significantly under oxidative stress, and sodA and sodB contributed equally to the process of bacterial resistance to ROS; (3) sodA and sodB complement each other and cooperate in the process of intracellular survival of bacteria to protect against ROS damage.

Food-Associated Stress Primes Foodborne Pathogens for the Gastrointestinal Phase of Infection

The incidence of foodborne outbreaks and product recalls is on the rise. The ability of the pathogen to adapt and survive under stressful environments of food processing and the host gastrointestinal tract may contribute to increasing foodborne illnesses. In the host, multiple factors such as bacteriolytic enzymes, acidic pH, bile, resident microflora, antimicrobial peptides, and innate and adaptive immune responses are essential in eliminating pathogens. Likewise, food processing and preservation techniques are employed to eliminate or reduce human pathogens load in food. However, sub-lethal processing or preservation treatments may evoke bacterial coping mechanisms that alter gene expression, specifically and broadly, resulting in resistance to the bactericidal insults. Furthermore, environmentally cued changes in gene expression can lead to changes in bacterial adhesion, colonization, invasion, and toxin production that contribute to pathogen virulence. The shared microenvironment between the food preservation techniques and the host gastrointestinal tract drives microbes to adapt to the stressful environment, resulting in enhanced virulence and infectivity during a foodborne illness episode.

Impact of Sod on the expression of stress-related genes in Listeria monocytogenes 4b G with/without paraquat treatment

Listeria monocytogenes is a foodborne pathogen that causes listeriosis. Paraquat can generate reactive oxygen species (ROS) in cells, which results in oxidative stress. It was first shown that 1 mM of paraquat inhibited the growth rate of a superoxide dismutase (sod)-deletion mutant (∆sod) generated from L. monocytogenes 4b G but not in the wild-type, and induced the expression of other resistance genes (kat, fri, perR, sigB, and recA) as well as sod in the wild type. Interestingly, without paraquat treatment the expression of all the 5 genes were repressed in ∆sod compared to the wild type, while the expression of recA triggering SOS response, a global response to DNA damage, was increased in ∆sod in the presence of 1 mM paraquat. Taken together, these results suggest that SOD plays a central role in oxidant defense of L. monocytogenes 4b G, and SOS probably significantly impacts ∆sod survival under oxidative stress.

The expression of superoxide dismutase (SOD) and a putative ABC transporter permease is inversely correlated during biofilm formation in Listeria monocytogenes 4b G

Little is known about the molecular basis of biofilm formation in Listeria monocytogenes. The superoxide dismutase (SOD) of the deletion mutant of lm.G_1771 gene, which encodes for a putative ABC transporter permease, is highly expressed in biofilm. In this study, the sod gene deletion mutant Δsod, and double deletion mutant of the sod and lm. G_1771 genes Δ1771Δsod were used to investigate the role of SOD and its relationship to the expression of the putative ABC transporter permease in biofilm formation. Our results showed that the ability to form a biofilm was significantly reduced in the Δsod mutant and the Δ1771Δsod double mutant. Both Δsod and Δ1771Δsod mutants exhibited slow growth phenotypes and produced more reactive oxygen species (ROS). The growth was inhibited in the mutants by methyl viologen (MV, internal oxygen radical generator) treatment. In addition, the expression of one oxidation resistance gene (kat), two stress regulators encoding genes (perR and sigB), and one DNA repair gene (recA) were analyzed in both the wild-type L. monocytogenes 4b G and the deletion mutants by RT-qPCR. The expression levels of the four genes were increased in the deletion mutants when biofilms were formed. Taken together, our data indicated that SOD played an important role in biofilm formation through coping with the oxidant burden in deficient antioxidant defenses.

Bactericidal antibiotics do not appear to cause oxidative stress in Listeria monocytogenes

Oxidative stress can be an important contributor to the lethal effect of bactericidal antibiotics in some bacteria, such as Escherichia coli and Staphylococcus aureus. Thus, despite the different target-specific actions of bactericidal antibiotics, they have a common mechanism leading to bacterial self-destruction by internal production of hydroxyl radicals. The purpose of the present study was to determine if a similar mechanism is involved in antibiotic killing of the infectious human pathogen, Listeria monocytogenes. We treated wild-type L. monocytogenes and oxidative stress mutants (Δsod and Δfri) with three different bactericidal antibiotics and found no difference in killing kinetics. In contrast, wild-type E. coli and an oxidative stress mutant (ΔsodA ΔsodB) differed significantly in their sensitivity to bactericidal antibiotics. We conclude that bactericidal antibiotics did not appear to cause oxidative stress in L. monocytogenes and propose that this is caused by its noncyclic tricarboxylic acid (TCA) pathway. Hence, in this noncyclic metabolism, there is a decoupling between the antibiotic-mediated cellular requirement for NADH and the induction of TCA enzyme activity, which is believed to mediate the oxidative stress reaction.

Listeria monocytogenes PrsA2 is required for virulence factor secretion and bacterial viability within the host cell cytosol

In the course of establishing its replication niche within the cytosol of infected host cells, the facultative intracellular bacterial pathogen Listeria monocytogenes must efficiently regulate the secretion and activity of multiple virulence factors. L. monocytogenes encodes two predicted posttranslocation secretion chaperones, PrsA1 and PrsA2, and evidence suggests that PrsA2 has been specifically adapted for bacterial pathogenesis. PrsA-like chaperones have been identified in a number of Gram-positive bacteria, where they are reported to function at the bacterial membrane-cell wall interface to assist in the folding of proteins translocated across the membrane; in some cases, these proteins have been found to be essential for bacterial viability. In this study, the contributions of PrsA2 and PrsA1 to L. monocytogenes growth and protein secretion were investigated in vitro and in vivo. Neither PrsA2 nor PrsA1 was found to be essential for L. monocytogenes growth in broth culture; however, optimal bacterial viability was found to be dependent upon PrsA2 for L. monocytogenes located within the cytosol of host cells. Proteomic analyses of prsA2 mutant strains in the presence of a mutationally activated allele of the virulence regulator PrfA revealed a critical requirement for PrsA2 activity under conditions of PrfA activation, an event which normally takes place within the host cell cytosol. Despite a high degree of amino acid similarity, no detectable degree of functional overlap was observed between PrsA2 and PrsA1. Our results indicate a critical requirement for PrsA2 under conditions relevant to host cell infection.

Deep RNA sequencing of L. monocytogenes reveals overlapping and extensive stationary phase and sigma B-dependent transcriptomes, including multiple highly transcribed noncoding RNAs

Background

Identification of specific genes and gene expression patterns important for bacterial survival, transmission and pathogenesis is critically needed to enable development of more effective pathogen control strategies. The stationary phase stress response transcriptome, including many σ^B-dependent genes, was defined for the human bacterial pathogen Listeria monocytogenes using RNA sequencing (RNA-Seq) with the Illumina Genome Analyzer. Specifically, bacterial transcriptomes were compared between stationary phase cells of L. monocytogenes 10403S and an otherwise isogenic ΔsigB mutant, which does not express the alternative σ factor σ^B, a major regulator of genes contributing to stress response, including stresses encountered upon entry into stationary phase.

Results

Overall, 83% of all L. monocytogenes genes were transcribed in stationary phase cells; 42% of currently annotated L. monocytogenes genes showed medium to high transcript levels under these conditions. A total of 96 genes had significantly higher transcript levels in 10403S than in ΔsigB, indicating σ^B-dependent transcription of these genes. RNA-Seq analyses indicate that a total of 67 noncoding RNA molecules (ncRNAs) are transcribed in stationary phase L. monocytogenes, including 7 previously unrecognized putative ncRNAs. Application of a dynamically trained Hidden Markov Model, in combination with RNA-Seq data, identified 65 putative σ^B promoters upstream of 82 of the 96 σ^B-dependent genes and upstream of the one σ^B-dependent ncRNA. The RNA-Seq data also enabled annotation of putative operons as well as visualization of 5'- and 3'-UTR regions.

Conclusions

The results from these studies provide powerful evidence that RNA-Seq data combined with appropriate bioinformatics tools allow quantitative characterization of prokaryotic transcriptomes, thus providing exciting new strategies for exploring transcriptional regulatory networks in bacteria.

See minireivew http://jbiol.com/content/8/12/107.

Role of growth temperature in freeze-thaw tolerance of Listeria spp

The food-borne pathogen Listeria monocytogenes can grow in a wide range of temperatures, and several key virulence determinants of the organism are expressed at 37 degrees C but are strongly repressed below 30 degrees C. However, the impact of growth temperature on the ability of the bacteria to tolerate environmental stresses remains poorly understood. In other microorganisms, cold acclimation resulted in enhanced tolerance against freezing and thawing (cryotolerance). In this study, we investigated the impact of growth temperature (4, 25, and 37 degrees C) on the cryotolerance of 14 strains of L. monocytogenes from outbreaks and from food processing plant environments and four strains of nonpathogenic Listeria spp. (L. welshimeri and L. innocua). After growth at different temperatures, cells were frozen at -20 degrees C, and repeated freeze-thaw cycles were applied every 24 h. Pronounced cryotolerance was exhibited by cells grown at 37 degrees C, with a <1-log decrease after 18 cycles of freezing and thawing. In contrast, freeze-thaw tolerance was significantly reduced (P < 0.05) when bacteria were grown at either 4 or 25 degrees C, with log decreases after 18 freeze-thaw cycles ranging from 2 to >4, depending on the strain. These findings suggest that growth at 37 degrees C, a temperature required for expression of virulence determinants of L. monocytogenes, is also required for protection against freeze-thaw stress. The negative impact of growth at low temperature on freeze-thaw stress was unexpected and has not been reported before with this or other psychrotrophic microorganisms.

Comparison of the stress response of Listeria monocytogenes strains with sprout colonization

Twenty-nine strains of the foodborne pathogen Listeria monocytogenes were tested for their ability to colonize alfalfa, radish, and broccoli sprouts and their capacity to withstand acid and oxidative stress, two stresses common to the sprouting environment. Wide variation in the ability of different strains to colonize alfalfa sprouts were confirmed, but the variations among radish and broccoli sprouts were not as large. With a few exceptions, strains that were poor colonizers of alfalfa tended to be among the poorer colonizers of radish and broccoli and vice versa. The strains also were variable in their resistance to both acid and oxidative stress. Statistical analysis revealed no correlation between acid stress and sprout colonization, but there was a positive correlation between resistance to oxidative stress and colonization of all three sprout types. Although the response to oxidative stress is important for L. monocytogenes virulence, it also may be important for life outside of a host.

Implication of (Mn)superoxide dismutase of Enterococcus faecalis in oxidative stress responses and survival inside macrophages

The gene encoding the manganese-containing superoxide dismutase (MnSOD) of Enterococcus faecalis was characterized. It is transcribed monocistronically from an upstream promoter identified by rapid amplification of cDNA ends (RACE)-PCR. A sodA mutant was constructed and characterized. Growth of the mutant strain was not significantly different from that of its wild-type counterpart in standing and aerated cultures. However, the mutant was more sensitive towards menadione and hydroperoxide stresses. The response to H(2)O(2) stress was analysed in more detail, and the mode of killing of this oxidant was different under anaerobic and aerobic conditions. Cultures grown and challenged under anaerobic conditions were highly sensitive to treatment with 35 mM H(2)O(2). They were largely protected by the iron chelator deferoxamine, which suggested that killing was mainly due to an enhanced Fenton reaction. In contrast, neither strain was protected by the iron chelators deferoxamine and diethylenetriaminepentaacteic acid when grown and challenged under aerobic conditions, which suggested that inactivation of the cells by H(2)O(2) was due to another killing mode. The sodA mutant was more sensitive under these conditions, showing that MnSOD is also important for protecting the cells from damage under aerobic conditions. Finally, the MnSOD of Ent. faecalis may be considered to be a virulence factor, since survival of the corresponding mutant strain was highly affected inside mouse peritoneal macrophages.

Control ofListeriaSuperoxide Dismutase by Phosphorylation

Superoxide dismutases (SODs) are enzymes that protect organisms against superoxides and reactive oxygen species (ROS) produced during their active metabolism. ROS are major mediators of phagocytes microbicidal activity. Here we show that the cytoplasmic Listeria monocytogenes MnSOD is phosphorylated on serine and threonine residues and less active when bacteria reach the stationary phase. We also provide evidence that the most active nonphosphorylated form of MnSOD can be secreted via the SecA2 pathway in culture supernatants and in infected cells, where it becomes phosphorylated. A Deltasod deletion mutant is impaired in survival within macrophages and is dramatically attenuated in mice. Together, our results demonstrate that the capacity to counteract ROS is an essential component of L. monocytogenes virulence. This is the first example of a bacterial SOD post-translationally controlled by phosphorylation, suggesting a possible new host innate mechanism to counteract a virulence factor.

Global changes in gene expression observed at the transition from growth to stationary phase in Listeria monocytogenes ScottA batch culture

Listeria monocytogenes is a food-borne Gram-positive bacterium that is responsible for a variety of infections (worldwide) annually. The organism is able to survive a variety of environmental conditions and stresses, however, the mechanisms by which L. monocytogenes adapts to environmental change are yet to be fully elucidated. An understanding of the mechanism(s) by which L. monocytogenes survives unfavourable environmental conditions will aid in developing new food processing methods to control the organism in foodstuffs. We have utilized a proteomic approach to investigate the response of L. monocytogenes batch cultures to the transition from exponential to stationary growth phase. Proteomic analysis showed that batch cultures of L. monocytogenes perceived stress and began preparations for stationary phase much earlier (approximately A(600) = 0.75, mid-exponential) than predicted by growth characteristics alone. Global analysis of the proteome revealed that the expression levels of more than 50% of all proteins observed changed significantly over a 7-9 h period during this transition phase. We have highlighted ten proteins in particular whose expression levels appear to be important in the early onset of the stationary phase. The significance of these findings in terms of functionality and the mechanistic picture are discussed.

Effect of acid adaptation on the fate of Listeria monocytogenes in THP-1 human macrophages activated by gamma interferon

In Listeria monocytogenes the acid tolerance response (ATR) takes place through a programmed molecular response which ensures cell survival under unfavorable conditions. Much evidence links ATR with virulence, but the molecular determinants involved in the reactivity to low pHs and the behavior of acid-exposed bacteria within host cells are still poorly understood. We have investigated the effect of acid adaptation on the fate of L. monocytogenes in human macrophages. Expression of genes encoding determinants for cell invasion and intracellular survival was tested for acid-exposed bacteria, and invasive behavior in the human myelomonocytic cell line THP-1 activated with gamma interferon was assessed. Functional approaches demonstrated that preexposure to an acidic pH enhances the survival of L. monocytogenes in activated human macrophages and that this effect is associated with an altered pattern of expression of genes involved in acid resistance and cell invasion. Significantly decreased transcription of the plcA gene, encoding a phospholipase C involved in vacuolar escape and cell-to-cell spread, was observed in acid-adapted bacteria. This effect was due to a reduction in the quantity of the bicistronic plcA-prfA transcript, concomitant with an increase in the level(s) of the monocistronic prfA mRNA(s). The transcriptional shift from distal to proximal prfA promoters resulted in equal levels of the prfA transcript (and, as a consequence, of the inlA, hly, and actA transcripts) under neutral and acidic conditions. In contrast, the sodC and gad genes, encoding a cytoplasmic superoxide dismutase and the glutamate-based acid resistance system, respectively, were positively regulated at a low pH. Morphological approaches confirmed the increased intracellular survival and growth of acid-adapted L. monocytogenes cells both in vacuoles and in the cytoplasm of interferon gamma-activated THP-1 macrophages. Our data indicate that preexposure to a low pH has a positive impact on subsequent challenge of L. monocytogenes with macrophagic cells.

The superoxide dismutase gene sodM is unique to Staphylococcus aureus: absence of sodM in coagulase-negative staphylococci

Superoxide dismutase (SOD) profiles of clinical isolates of Staphylococcus aureus and coagulase-negative staphylococci (CoNS) were determined by using whole-cell lysates and activity gels. All S. aureus clinical isolates exhibited three closely migrating bands of activity as previously determined for laboratory strains of S. aureus: SodM, SodA, and a hybrid composed of SodM and SodA (M. W. Valderas and M. E. Hart, J. Bacteriol. 183:3399-3407, 2001). In contrast, the CoNS produced only one SOD activity, which migrated similarly to SodA of S. aureus. Southern analysis of eight CoNS species identified only a single sod gene in each case. A full-length sod gene was cloned from Staphylococcus epidermidis and determined to be more similar to sodA than to sodM of S. aureus. Therefore, this gene was designated sodA. The deduced amino acid sequence of the S. epidermidis sodA was 92 and 76% identical to that of the SodA and SodM proteins of S. aureus, respectively. The S. epidermidis sodA gene expressed from a plasmid complemented a sodA mutation in S. aureus, and the protein formed a hybrid with SodM of S. aureus. Both hybrid SOD forms as well as the SodM and SodA proteins of S. aureus and the S. epidermidis SodA protein exist as dimers. These data indicate that sodM is found only in S. aureus and not in the CoNS, suggesting an important divergence in the evolution of this genus and a unique role for SodM in S. aureus.

Characterization of catalase and superoxide dismutase in Staphylococcus carnosus 833 strain

AIMS

Staphylococcus carnosus, used as starter culture in fermented meat products, decreases the level of volatiles arising from lipid oxidation. To analyse its antioxidant capacities, catalase and superoxide dismutase (SOD) were characterized.

METHODS AND RESULTS

Catalase and SOD activities were measured with spectrophotometric methods and visualized on non-denaturing polyacrylamide gels. The corresponding sod gene was identified by PCR. Southern hybridizations and enzymatic analyses showed that there was a single catalase and a single SOD in Staph. carnosus 833 strain. The gene encoding the Staph. carnosus SOD was found to encode a protein closely related to SOD requiring manganese. Catalase and SOD levels increased in mid-log phase. Only catalase was induced by oxygen, nitrate or nitrite while glucose induced neither enzyme. Metal ion limitation increased catalase and decreased SOD activities.

CONCLUSION

Staph. carnosus synthesizes both enzymes in conditions encountered in sausage manufacturing. These results could explain the antioxidant properties of Staph. carnosus starter culture.

SIGNIFICANCE AND IMPACT OF THE STUDY

The knowledge of the antioxidant properties of Staphylococci will allow a more rational use of these starters in meat fermented products.

Characterization of the single superoxide dismutase of Staphylococcus xylosus

Staphylococcus xylosus is a facultative anaerobic bacterium used as a starter culture for fermented meat products. In an attempt to analyze the antioxidant capacities of this organism, the superoxide dismutase (SOD) was characterized. S. xylosus contains a single cytoplasmic SOD, which was not inhibited by H2O2. The SOD activity in crude extracts was completely lost upon metal depletion, but it could be recovered by manganese and very weakly by iron. It is therefore suggested that the S. xylosus SOD is a manganese-preferring enzyme. The corresponding gene, sod, was isolated from a genomic library of S. xylosus DNA and complemented the growth defect of an Escherichia coli SOD-deficient mutant. As deduced from the nucleotide sequence, sod encodes a protein of 199 amino acids with a molecular mass of 22.5 kDa. Two transcriptional start sites 25 and 120 bp upstream of the sod start codon were identified. A terminator-like structure downstream of the gene suggested a monocistronic sod mRNA. Regulation of sod expression was studied using fusions of the sod promoters to a genomic promoterless beta-galactosidase gene. The sod expression was not affected by manganese and increased slightly with paraquat. It was induced during stationary phase in a complex medium but not in a chemically defined medium. To investigate the physiological role of SOD, a mutant devoid of SOD activity was constructed. Growth experiments showed that sod is not essential for aerobic growth in complex medium. However, in chemically defined medium without leucine, isoleucine, and valine, the sod mutant hardly grew, in contrast to the wild-type strain. In addition, the mutant was sensitive to hyperbaric oxygen and to paraquat. Therefore, sod plays an important role in the protection of S. xylosus from oxidative stress.

Identification and molecular analysis of superoxide dismutase isoforms in Helicobacter pylori

Three electromorphs of iron superoxide dismutase (FeSOD) were identified among 29 Helicobacter pylori isolates by native gel electrophoresis and activity staining. The electromorphs designated isoforms A, B, and C are characterized by slow, intermediate and fast electrophoretic migration, respectively, which was not observed under denaturing conditions. The isoforms were not associated with virulence determinants and with the outcome of disease. Sequence analysis of the sodB gene in strains producing different FeSOD isoforms and comparison of deduced protein sequences revealed that differences in the electric migration behavior are associated with exchange of charged amino acids, suggesting that faster migration is caused by a more negative total charge of the proteins. Electrophoretic migration of native FeSOD was not influenced by changes in the iron cofactor concentration, oxidative stress, and different media, indicating that FeSOD isoforms represent stable strain-specific markers.

Characterization of the major superoxide dismutase of Staphylococcus aureus and its role in starvation survival, stress resistance, and pathogenicity

A Staphylococcus aureus mutant (SPW1) which is unable to survive long-term starvation was shown to have a transposon insertion within a gene homologous to the sodA family of manganese-dependent superoxide dismutases (SOD). Whole-cell lysates of the parental 8325-4 strain demonstrated three zones of SOD activity by nondenaturing gel electrophoresis. The activities of two of these zones were dependent on manganese for activity and were absent in SPW1. The levels of SOD activity and sodA expression were growth-phase dependent, occurring most during postexponential phase. This response was also dependent on the level of aeration of the culture, with highest activity and expression occurring only under high aeration. Expression of sodA and, consequently, SOD activity could be induced by methyl viologen but only during the transition from exponential- to postexponential-phase growth. SPW1 was less able to survive amino acid limitation and acid stress but showed no alteration in pathogenicity in a mouse abscess model of infection compared to the parental strain 8325-4.

Protection against murine listeriosis by an attenuated recombinant Salmonella typhimurium vaccine strain that secretes the naturally somatic antigen superoxide dismutase

A recombinant (r)-Salmonella typhimurium aroA vaccine strain was constructed which secretes the naturally somatic protein of Listeria monocytogenes, superoxide dismutase (SOD), by the HlyB/HlyD/TolC export machinery. Vaccine efficacy of the SOD-bearing carrier strain was compared with that of the p60-secreting construct, S. typhimurium p60s (J. Hess, I. Gentschev, D. Miko, M. Welzel, C. Ladel, W. Goebel, and S. H. E. Kaufmann, Proc. Natl. Acad. Sci. USA 93:1458-1463, 1996). Vaccination of mice with both constructs induced protection against a lethal challenge with the intracellular pathogen, L. monocytogenes. While the somatic listerial antigen, SOD, is immunologically uncharacterized, the naturally secreted protein of L. monocytogenes, p60, is known to be highly immunogenic. Our data emphasize the high vaccine potential of r-Salmonella constructs secreting antigens of somatic or secreted origin. Moreover, they suggest that the HlyB/HlyD/TolC-based antigen delivery system with attenuated Salmonella spp. as the carrier is capable of potentiating the immune response against foreign proteins independent from their immunogenicity in and display by the natural host.

Reduction of exogenous ferric iron by a surface-associated ferric reductase of Listeria spp

The reduction of exogenous ferric iron by Listeria monocytogenes, a Gram-positive food-borne pathogen, was investigated. Using an assay incorporating the ferrous iron chelator ferrozine, we showed that intact cells of L. monocytogenes, when exposed to ferric iron, were able to rapidly reduce and solubilize the iron to the ferrous form. Reduction occurred only after direct contact between the bacteria and the iron source. A number of different ferric iron chelates, including transferrin and lactoferrin-bound iron, haemoglobin, ferritin, and iron complexed to siderophores, could be reduced. The ferric reductase activity was expressed by both reference strains and clinical isolates of L. monocytogenes and by all other species of Listeria, although significant quantitative differences were observed. In L. monocytogenes, the expression of ferric reductase was not affected by the growth phase of the bacteria nor by the presence or absence of iron in the growth medium. However, expression was greatly reduced in bacteria grown anaerobically and when cultured in media of reduced pH. In addition, bacteria grown at a cold temperature displayed greater ferric reductase activity than cells grown at higher temperatures. A surface-associated ferric reductase system may be one component of a general iron scavenging mechanism which can be used by Listeria growing in a variety of environments.