How can streptococci tolerate oxygen

Gene assignments and gene numbers above the diagram are based on Oralgen annotation. Panel shows agarose gel electrophoresis of the PCR products. Inclusion of rRex resulted in mobility shift and such interaction was concentration-dependent. D Alignment of promoter regions of selected genes identified in TW that contain putative Rex binding sites.

Positions of these elements were shown in numbers relative to start codon ATG of the respective genes. Consensus sequence was the conserved Rex-binding sites identified in B.

E Alignment of the putative and proven Rex-binding sites of the up-regulated genes in TW Promoters were scanned and aligned to the consensus Rex-binding site from S. The indentified nucleotide sequences were then subjected to analysis using WebLogo University of California, Berkeley to generate the S. Results showed that the Rex-binding site in S. Computer-aided analysis of the rex promoter revealed two regions with high similarity to the consensus sequence of Rex-binding site that have been identified in Streptomyces coelicolor, Bacillus subtilis and S.

EMSA was then carried out using either the PCR-amplified promoter region of rex or oligonucleotides specific to the putative binding site Table 2.

As shown in Figure 1C , rRex was able to bind to the promoter sequence of rex , which further suggests that Rex is poised for auto-regulation. The guaA gene was up-regulated by approximately 5-fold in TW, which had the rex -coding sequence deleted and replaced by a non-polar kanamycin resistance element [25] , [29] , [34]. To verify the direct role of Rex in the regulation of the altered genes, we used EMSA to assess the interactions between rRex and the promoters of selected genes identified from DNA microarray analysis of TW [25].

We tested the promoters of two up-regulated genes in adhE SMU. As seen in Figure 2 , all of the promoters tested displayed a dose-dependent mobility shift to higher molecular mass of rRex-promoter complexes. As expected in a cold competition assay, mobility shift was decreased with increasing amount of unlabeled DNA probe included in the EMSA reactions Fig. In addition, mutations of nucleotides that are considered critical within the Rex-box abolished rRex binding Fig.

Consistently, no apparent mobility shift was observed when rRex was mixed with the promoter of levD , which is not regulated by Rex and whose promoter does not have an apparent Rex-box Fig. These results further suggest the binding of rRex to the Rex-box of adhE promoter is nucleotides-specific. Promoters of selected genes were mixed with rRex and the impact of the presence of Rex on mobility was visualized via SYBR Gold staining.

Data presented here show that rRex specifically binds the promoters of adhE A , frdC B , gshR C , tpn D , and ldh E , respectively, and when added at different concentrations, causes mobility shift as a result of formation of rRex-promoter complexes. Recent studies in B.

To evaluate how nicotinamide moieties may affect Rex activity in S. At low concentrations, rRex binds better to the repressor site of frdC when compared to gshR A. Studies in B. The reconstituted samples were used to carry out the EMSA reactions with the promoter of adhE without the addition of exogenous nicotinamide. It is apparent that Rex in S. As shown previously by DNA microarray analysis and further confirmed by RealTime-PCR in this study [25] Table 3 , at least 17 genes with putative functions in carbohydrates metabolism were differentially regulated in response to Rex-deficiency.

Of the genes up-regulated in TW, several are known to be involved in heterofermentative catabolism [4] , [6] , [26] , including pfl SMU. To this effect, we sought to determine the effects of Rex-deficiency on end-point pH and lactate production after 24 hours of growth and evaluate the glycolytic rate via pH drop experiments.

As seen in Figure 5A , we noticed significant increases in the end-point pH of the Rex-deficient mutant, TW, especially during growth under aerobic conditions. In addition, pH drop experiments showed that TW had a slow pH drop upon initiation by addition of glucose and maintained a baseline threshold of pH 4.

As expected, complementation of the mutant with rex in pDL shuttle vector was able to partly restore these phenotypes to the wildtype levels Fig. S2 , S3. The differences between TW and UA in end-point pH, lactate production, and glycolytic rates could be attributed at least in part to the increases in ethanol and acetoin production as a result of the increased expression of AdhE and the acetoin dehydrogenase complex AdhABCD in response to Rex deficiency [25] , [46].

Similar trends were also observed when the strains were grown in BMS, although the differences were not as pronounced as in BMGS data not shown. However, the exact metabolite pools await further investigation.

A Panel shows results of End-point pH analysis of S. Rex-deficiency increases the final resting pH, especially under aerobic conditions. Previously, Rex-deficiency in S. When assessed by acid-killing at pH 2. Unlike TW, the survival rate of the GuaA-deficient mutants, TW, was slightly increased, but again the differences were not statistically significant from UA Fig. Complementation of this double mutant with rex plus its cognate promoter in JB and with guaA and its cognate promoter in JB were able to partly restore the phenotype to a level similar to TW and TW, respectively data not shown.

Hydrogen peroxide killing assays were used to evaluate the impact of GuaA-deficiency on oxidative stress tolerance. The survival rate of JB, was further reduced by almost 2-logs, relative to UA, which suggests an additive effect of Rex- and GuaA-deficiency on the susceptibility of JB to hydrogen peroxide. Consistently, when the strains were grown in the presence of methyl viologen 10 mM, final concentration , both TW and JB displayed greater challenge to methyl viologen than UA Fig.

Again, possession of a wildtype copy of the deleted gene plus the cognate promoter region in shuttle vector was able to partly complement the Rex-deficiency in TW data not shown. A Panel shows survival rates of S. B Panel B shows growth curves of S. Strain labels are the same as in panel A. Previous studies showed that deficiency of Rex in S. To more fully evaluate the role of the rex operon in biofilm formation, biofilms of the Rex- and GuaA-deficient single mutants, TW and TW, and the double mutant, JB, were grown in BMS and BMGS on hydroxylapatite discs in well plates simulating a static growth model under both aerobic and anaerobic conditions.

When grown under anaerobic conditions on hydroxylapatite discs in BMS, wild-type UA formed robust biofilms after 48 hours Fig. Similar to our previous data, TW formed loose biofilms with porous structure, averaging a biovolume of Similar but less dramatic differences were observed between the different strains when grown under in the presence of glucose Fig.

A highly conserved transcriptional regulator, Rex has been implicated in regulation of key enzymes in carbon and energy metabolism [22] , [23] , [24] , [45]. Previously, we showed that deficiency of Rex in S. Here, we provided further evidences that similar to B. Like S. Competitive co-infection experiments with wild-type and perR -deficient bacteria demonstrated that the perR mutant was more rapidly cleared and attenuated for virulence in a baboon model of GAS pharyngitis. Comparative analysis of global gene expression in wild-type M3 GAS and the isogenic perR mutant cultured to late-exponential phase identified 42 genes regulated by the PerR regulon Gryllos et al.

These data indicate that PerR regulates the expression of a diverse set of genes, enhances GAS resistance to phagocytic clearance and contributes to pharyngeal colonization in a non-human primate model of GAS pharyngeal colonization Gryllos et al.

Grifantini and coworkers Grifantini et al. Analysis of wild-type and perR mutant transcriptomes revealed that 76 of peroxide-regulated genes were PerR dependent. The PerR-regulated genes, which encode purine and deoxyribonucleotide biosynthesis enzymes, peptide transport and heme uptake, were mostly down-regulated. The 75 up-regulated genes encoded for proteins involved in the detoxification of ROS, repair of damaged DNA, cofactor metabolism and pilus biosynthesis.

The strong activation of metabolic enzymes and DNA damage repair mechanisms may play a key role in GAS survival in oxidative environments in vivo , similar to S. PerR-dependent regulation was restored by complementation of the perR mutant with the wild-type PerR protein, but not with a modified PerR containing a mutation in one of the two metal-binding sites Grifantini et al. Metal content analysis revealed that PerR binds zinc and iron, and that iron oxidation plays a key role in the PerR response Grifantini et al.

Comparative transcriptomic analysis of wild-type M5 GAS and an isogenic perR mutant cultured to mid-log growth phase in the absence of oxidative stress revealed one highly down-regulated gene czcD , a newly identified GAS virulence factor Ong et al. Of the five up-regulated genes, only pmtA , encoding for PmtA involved in metal homeostasis and transport, contained a PerR-binding site in the promoter region and was directly repressed by PerR Brenot, Weston and Caparon Mutagenesis of all genes in the perR mutant background revealed that only pmtA contributes to H 2 O 2 stress resistance in vitro Brenot, Weston and Caparon The indirect effect on the remaining genes was attributed to metal ion starvation mediated by AdcR Brenot, Weston and Caparon The overexpression of pmtA also up-regulated the AdcR-regulated genes, suggesting a link between metal ion homeostasis and the PerR and AdcR oxidative stress responses.

However, PmtA overexpression increased metal efflux and may potentiate metal starvation, which could at least in part explain the reduced virulence of perR mutants in vivo. However, compared to wild-type bacteria, perR mutant survival was dramatically enhanced in lethal concentrations of H 2 O 2 King, Horenstein and Caparon Primer extension analysis and DNase 1 protection assays indicate that the transcriptional regulator PerR binds to a single promoter upstream of ahpC Brenot, King and Caparon MrgA is regulated by PerR and plays a key role in oxidative stress resistance in S.

Mitogen factor 3 MF3 and streptodornase 1 Sda1 are bacteriophage-encoded, whereas Spd and SpnA are chromosomally encoded Hasegawa et al. The expression of sda1 , encoding for the bacteriophage-encoding DNase Sda1, is up-regulated under oxidative stress conditions in wild-type M1 GAS bacteria, but not in a perR -deficient mutant.

Gel mobility shift assays revealed that PerR directly binds to the sda1 promoter region. Rgg, also known as RopB, is a DNA-binding global transcriptional regulatory protein that plays a key role in coordinating the expression of cell wall-associated and secreted virulence factors Chaussee, Ajdic and Ferretti , Chaussee et al. Chaussee, Callegari and Chaussee used a comparative proteomic approach to identify the Rgg-regulated cytoplasmic proteins from mid-log and stationary phase cultures of wild-type serotype M49 GAS and an isogenic rgg mutant strain.

Cytoplasmic proteins were identified by 2D gel electrophoresis and tandem mass spectrometry and revealed a growth phase-dependent Rgg regulation of proteins associated with arginine metabolism ArcABC , histidine HutI and serine SdhA in the exponential growth phase Chaussee, Callegari and Chaussee Thermal and oxidative stress response proteins, including ClpE and ClpL, were expressed in the rgg mutant, but not wild-type Chaussee, Callegari and Chaussee Compared to wild-type, the Rgg-deficient strain was more resistant to heat-shock and puromycin Chaussee, Callegari and Chaussee , an aminoacyl-tRNA analog that inhibits protein synthesis and induces a heat-shock-like response in Gram-negative and Gram-positive bacteria VanBogelen and Neidhardt ; Frees and Ingmer ; Steiner and Malke The rgg mutant was also hypersensitive to killing by superoxide free radicals induced by 50 mM paraquat exposure Chaussee, Callegari and Chaussee Transcriptomic analysis revealed that the ahpCF operon expression was up-regulated in rgg mutant GAS Pulliainen et al.

The ahpCF gene products are involved in reducing organic peroxides, decomposing H 2 O 2 and enhancing resistance against oxidative stress Ellis and Poole ; Poole et al. Voyich and coworkers Voyich et al. Eleven GAS virulence genes were up-regulated including sic streptococcal inhibitor of complement , speH streptococcal pyrogenic exotoxin H , ndoS endoglycosidase S , smeZ streptococcal mitogenic exotoxin Z and speB streptococcal pyrogenic exotoxin B , which are known to contribute to neutrophil resistance or modulation of the human innate immune system Lukomski et al.

Phagocytosis of GAS by neutrophils induced the up-regulation of nine genes encoding for proteins that participate in cell wall biogenesis, perhaps in response to the cell wall damage induced by phagocytosis and ROS Voyich et al. Metabolic pathway genes were also up-regulated, presumably because an increase in energy metabolism is essential for GAS survival during phagocytosis. Phagocytosis altered the expression of genes encoding for proteins involved in the global transcriptional regulation of GAS.

The mga regulon, encoding for a transcriptional regulator of virulence factors Kihlberg et al. The genes encoding the two-component gene regulatory system, ihk and irr , were up-regulated, suggesting a role for this regulatory system in host innate immunity resistance Voyich et al. A serotype M6 GAS strain JRS4 lacking the irr gene was hypersensitive to killing by human neutrophils following phagocytosis, indicating that Ihk-Irr plays a key role in the expression of genes necessary for GAS to survive phagocytosis.

Several genes involved in oxidative stress resistance were differentially regulated by ihk-irr , including nrdH glutaredoxin , trx thioredoxin , trxR thioredoxin reductase , nox NADH peroxidase , rnr ribonucleotide reductase and bsaA glutathione peroxidase Voyich et al. Multiple virulence-associated genes, including fbp putative fibronectin-binding protein-like protein A , mf and mf3 DNases , and sagA SLS , and multiple genes involved in cell wall biosynthesis were regulated by the Ihk-Irr regulon Voyich et al.

The up-regulation of cell wall biosynthesis genes may protect the bacteria from cationic neutrophil antimicrobial peptides such as LL and cathepsin G, which disrupt the bacterial cell membrane Ganz et al. During phagocytosis bacteria are exposed to ROS and antibacterial granule components stored within phagocytic vacuoles. The irr -deficient strain was more susceptible than wild-type to killing by H 2 O 2 across a range of different concentrations, and was hypersensitive to killing by the cell envelope active antimicrobial peptides LL and cathepsin G.

Inactivation of irr delayed the formation of skin abscesses and resulted in smaller abscesses, indicating that Irr is required for full GAS virulence in a mouse model of subcutaneous infection. In a bacteremia model of infection, irr mutant bacteria were more rapidly cleared from the blood 24 h post-infection compared to wild-type, indicating that Ihk-Irr plays an important role in GAS pathogenesis Voyich et al.

Iron is important for many bacterial metabolic functions including the electron transport chain and DNA synthesis repair. However, excess iron is potentially lethal to the bacterial cell through the generation of highly toxic oxygen radicals by the Fenton reaction.

In order to maintain iron homeostasis, bacteria express metal-dependent transcription regulators belonging to the Fur or the DtxR family Andrews, Robinson and Rodriguez-Quinones GAS acquires iron from heme, hemoglobin, haptoglobin—hemoglobin complexes, ferritin, myoglobin and catalase but not transferrin or lactoferrin Francis, Booth and Becker ; Eichenbaum et al.

The GAS multimetal transport system mts is involved in the acquisition of manganese, zinc and iron Janulczyk, Pallon and Bjorck , ; Ge and Sun , and is essential for GAS growth in metal-restricted media and full virulence in a mouse model of GAS infection Janulczyk, Ricci and Bjorck Transcriptional regulator MtsR is a member of the DtxR family of metal-dependent regulatory proteins involved in the coordination of iron homeostasis, oxidative stress resistance and virulence Jakubovics, Smith and Jenkinson Analysis of an mtsR mutant in serotype M49 GAS strain NZ by western blot and RNA analysis revealed that constitutive transcription of the gene sia streptococcal iron acquisition operon Bates et al.

Electrophoretic mobility gel shift assays indicated that MtsR directly binds to the sia promoter in an iron- and manganese-dependent manner to repress the expression of the sia operon Bates et al. Excess iron accumulation causes oxidative stress via the Fenton reaction Ratledge and Dover ; Touati ; Bates et al. The mtsR mutant accumulated more intracellular iron compared to wild-type in 55 Fe uptake assays in complete medium, and was hypersensitive to H 2 O 2 , indicating that MtsR plays a role in GAS resistance to oxidative stress Bates et al.

In zebrafish models of intramuscular and intraperitoneal GAS infection, the mtsR mutant was attenuated for virulence Bates et al. CiaRH is a two-component regulator of GAS gene expression that is up-regulated under conditions of oxidative stress Riani et al.

Similar to the function of the sensor kinase CiaH in S. Pneumococcal CiaH directly up-regulates HtrA, which is involved in resistance to oxidative stress, as described above Seol et al. However, in serotype M1 GAS, the expression of HtrA in the ciaH -null mutant was not down-regulated compared to wild-type, suggesting that the contribution of the CiaH sensor kinase to oxidative stress resistance may not be mediated via HtrA Tatsuno et al. Bacterial pathogens have evolved a plethora of sophisticated defense mechanisms to counter oxidative damage and highly toxic ROS generated from atmospheric oxygen and the oxidative burst from phagocytes.

ROS, including H 2 O 2 , hydroxyl radicals and superoxide anions, are capable of damaging proteins, DNA, membrane lipids, and may induce cell death Nunoshiba et al.

Neutrophils are the first line of defense for the host innate immune system and promote bacterial clearance at the infection site through phagocytosis, a process whereby bacteria are killed by ROS and microbicidal granule components stored within phagocytic vacuoles Mayadas et al. Bacterial ROS resistance mechanisms include direct detoxification of harmful reactive oxygen molecules by enzymes catalases, peroxidases and Sods , repair mechanisms and alteration of intracellular metal ion concentrations Faulkner and Helmann GAS resistance to ROS generated by the human innate immune response enables this preeminent human pathogen to survive in the human host under the harsh conditions of oxidative stress.

Compared to other bacterial pathogens, GAS is equipped with surface-associated and secreted factors and unique molecular mechanisms to promote aerotolerance and combat ROS-induced stress in vitro and in vivo. In Gram-positive bacteria, peroxide-sensing transcriptional regulators are responsible for regulating the oxidative stress response Imlay ; Dubbs and Mongkolsuk Future work is needed to increase our understanding of the mechanisms of ROS resistance, the complex regulatory networks that coordinate GAS—host interactions and the response to ROS, and how these contribute to GAS pathogenesis and human infection.

In addition, many of the proteins involved in ROS resistance are virulence factors. Therefore, these proteins are potential targets for the development of novel anti-GAS therapeutics and immune-boosting agents for the prevention and treatment of streptococcal diseases.

National Center for Biotechnology Information , U. Published online Feb Jason N. Author information Article notes Copyright and License information Disclaimer. Received Jul 25; Accepted Dec This article has been cited by other articles in PMC. Keywords: Group A Streptococcus , innate immunity, oxidative stress resistance, reactive oxygen species, Streptococcus pyogenes , virulence. Open in a separate window. Graphical Abstract Figure. Table 1. Figure 1. M protein GAS strains are distinguished serologically on the basis of immunovariable surface-anchored M proteins Facklam et al.

Hyaluronan capsule The surface capsule of GAS is composed solely of hyaluronan HA ; a high-molecular-mass polysaccharide comprised of glucuronic acid and N -acetylglucosamine Kendall, Heidelberger and Dawson Mac-2 A second Maclike protein has been identified, designated Mac-2 Lei et al. Figure 2. Glutathione peroxidase GpoA Glutathione peroxidase Gpo is a selenoprotein oxidoreductase important for maintaining cellular redox homeostasis and for protecting cells from the deleterious effects of ROS Arthur High-temperature requirement A HtrA High-temperature requirement A HtrA, also designated DegP is a dual-functional serine protease and chaperone protein that either refolds or degrades damaged proteins destined for secretion into the extracellular environment Spiess, Beil and Ehrmann PerR-regulated metal transporter A PmtA In a genome-wide transcriptome analysis between serotype M14 GAS strain HSC5 and an in-frame perR mutant in the absence of a stress stimulus, six genes were identified with at least 3-fold differential expression Brenot, Weston and Caparon Streptococcal hemoprotein receptor Shr Iron availability is essential for bacterial growth, survival and the establishment of infection within the human host Payne PerR regulon Regulation of the inducible peroxide resistance response in GAS is primarily coordinated by the peroxide-sensing PerR transcriptional regulator, a amino acid zinc-containing metalloprotein, which is a member of the ferric uptake regulator Fur family of metal-binding transcriptional regulators Herbig and Helmann ; Mongkolsuk and Helmann ; Brenot, King and Caparon ; Moore and Helmann Figure 3.

Table 2. M type ORF no. Two-component regulator Ihk-Irr Voyich and coworkers Voyich et al. Transcriptional regulator MtsR Iron is important for many bacterial metabolic functions including the electron transport chain and DNA synthesis repair. Conflict of interest statement. None declared. Insight of host immune evasion mediated by two variants of group A Streptococcus Mac protein. J Biol Chem. Reactive oxygen species induced by Streptococcus pyogenes invasion trigger apoptotic cell death in infected epithelial cells.

Cell Microbiol. Infect Immun. Conversion of amino-acid residues in proteins and amino-acid homopolymers to carbonyl derivatives by metal-catalyzed oxidation reactions. Growth phase-dependent modulation of Rgg binding specificity in Streptococcus pyogenes. J Bacteriol. The fibrinogen-binding M1 protein reduces pharyngeal cell adherence and colonization phenotypes of M1T1 group A Streptococcus. Bacterial iron homeostasis.

Manganese import is a key element of the OxyR response to hydrogen peroxide in Escherichia coli. Mol Microbiol. Peroxisomes are oxidative organelles. Antioxid Redox Sign. Defense factors of vaginal lactobacilli. Am J Obstet Gynecol. The glutathione peroxidases.

Cell Mol Life Sci. Post-proteomic identification of a novel phage-encoded streptodornase, Sda1, in invasive M1T1 Streptococcus pyogenes. The respiratory burst of phagocytes.

J Clin Invest. Biological defense mechanisms. The production by leukocytes of superoxide, a potential bactericidal agent. Identification and characterization of a Streptococcus pyogenes operon involved in binding of hemoproteins and acquisition of iron. Characterization of MtsR, a new metal regulator in group A Streptococcus , involved in iron acquisition and virulence.

Of inflammasomes and pathogens — sensing of microbes by the inflammasome. Genome sequence of a serotype M3 strain of group A Streptococcus : phage-encoded toxins, the high-virulence phenotype, and clone emergence.

Sequence heterogeneity of PsaA, a kilodalton putative adhesin essential for virulence of Streptococcus pneumoniae. Hydrogen peroxide-mediated killing of Caenorhabditis elegans : a common feature of different streptococcal species.

Granules of the human neutrophilic polymorphonuclear leukocyte. DNA strand scission by enzymically generated oxygen radicals. Arch Biochem Biophys. The PerR regulon in peroxide resistance and virulence of Streptococcus pyogenes. Contribution of glutathione peroxidase to the virulence of Streptococcus pyogenes. A PerR-regulated metal transporter PmtA is an interface between oxidative stress and metal homeostasis in Streptococcus pyogenes.

HtrA stress protein is involved in intramacrophagic replication of adherent and invasive Escherichia coli strain LF82 isolated from a patient with Crohn's disease. Neutrophil extracellular traps kill bacteria.

Cells from a broth culture grown at room temperature displayed the tumbling motility characteristic of Listeria Figure 6. Figure 6. An inoculated thioglycolate medium culture tube shows dense growth at the surface and turbidity throughout the rest of the tube. What is your conclusion? An inoculated thioglycolate medium culture tube is clear throughout the tube except for dense growth at the bottom of the tube.

Pseudomonas aeruginosa is a common pathogen that infects the airways of patients with cystic fibrosis. It does not grow in the absence of oxygen. The bacterium is probably which of the following? Streptococcus mutans is a major cause of cavities. It resides in the gum pockets, does not have catalase activity, and can be grown outside of an anaerobic chamber. Why do the instructions for the growth of Neisseria gonorrheae recommend a CO 2 -enriched atmosphere?

Four tubes are illustrated with cultures grown in a medium that slows oxygen diffusion. Match the culture tube with the correct type of bacteria from the following list: facultative anaerobe, obligate anaerobe, microaerophile, aerotolerant anaerobe, obligate aerobe. Skip to main content. Microbial Growth. Search for:. Oxygen Requirements for Microbial Growth Learning Objectives Interpret visual data demonstrating minimum, optimum, and maximum oxygen or carbon dioxide requirements for growth Identify and describe different categories of microbes with requirements for growth with or without oxygen: obligate aerobe, obligate anaerobe, facultative anaerobe, aerotolerant anaerobe, microaerophile, and capnophile Give examples of microorganisms for each category of growth requirements.

Figure 2. Diagram of bacterial cell distribution in thioglycolate tubes. Think about It Would you expect the oldest bacterial lineages to be aerobic or anaerobic? Which bacteria grow at the top of a thioglycolate tube, and which grow at the bottom of the tube? An Unwelcome Anaerobe Figure 4. Think about It What substance is added to a sample to detect catalase? What is the function of the candle in a candle jar? Key Concepts and Summary Aerobic and anaerobic environments can be found in diverse niches throughout nature, including different sites within and on the human body.

Microorganisms vary in their requirements for molecular oxygen. Obligate aerobes depend on aerobic respiration and use oxygen as a terminal electron acceptor. They cannot grow without oxygen. Obligate anaerobes cannot grow in the presence of oxygen.

They depend on fermentation and anaerobic respiration using a final electron acceptor other than oxygen. Facultative anaerobes show better growth in the presence of oxygen but will also grow without it.

Although aerotolerant anaerobes do not perform aerobic respiration, they can grow in the presence of oxygen. Most aerotolerant anaerobes test negative for the enzyme catalase. Optimum oxygen concentration for an organism is the oxygen level that promotes the fastest growth rate. The minimum permissive oxygen concentration and the maximum permissive oxygen concentration are, respectively, the lowest and the highest oxygen levels that the organism will tolerate.

Peroxidase , superoxide dismutase , and catalase are the main enzymes involved in the detoxification of the reactive oxygen species. Superoxide dismutase is usually present in a cell that can tolerate oxygen.

All three enzymes are usually detectable in cells that perform aerobic respiration and produce more ROS. A capnophile is an organism that requires a higher than atmospheric concentration of CO 2 to grow. Multiple Choice An inoculated thioglycolate medium culture tube shows dense growth at the surface and turbidity throughout the rest of the tube. The organisms die in the presence of oxygen The organisms are facultative anaerobes. The organisms should be grown in an anaerobic chamber.

The organisms are obligate aerobes. Gram-positive cocci are often isolated from clinical samples. Catalase is an enzyme that converts hydrogen peroxide to water and oxygen gas. The test is easy to perform; bacteria are simply mixed with H 2O 2. If bubbles appear due to the production of oxygen gas the bacteria are catalase positive.

If no bubbles appear, the bacteria are catalase negative.