Amino acid requirements for the production of enterotoxin B by Staphylococcus aureus S-6 in a chemically defined medium

Putrescine Detected in Strains of Staphylococcus aureus

Most forms of life, including the archaea, bacteria, and eukaryotes synthesize the polyamine putrescine. Although putrescine is widely distributed, several Gram-positive bacteria, including Staphylococcus aureus (S. aureus), appear to be the exceptions. We report here that strains of S. aureus can produce the polyamine putrescine, as well as the derivative N-acetyl-putrescine. Three strains of S. aureus from the American Type Culture Collection (ATCC), one strain listed in the National Center for Biotechnology Information (NCBI) database, whose genomic sequence is well defined, and well as eight strains from S. aureus-induced brain abscesses of individual patients from multiple geographic locations were evaluated. Each strain was grown in complete chemically defined medium (CDM) under stringent conditions, after which the partially purified conditioned medium (CM) was analyzed by mass spectroscopy (MS), and the data were reported as the ratio of experimental results to controls. We confirmed the synthesis of putrescine by S. aureus by using ¹³C/¹⁵N-labeled arginine as a tracer. We found that agmatine, N-acetyl-putrescine, ornithine, citrulline, proline, and NH₃ were all labeled with heavy isotope derived from ¹³C/¹⁵N-labeled arginine. None of the strains examined produced spermine or spermidine, but strains from either ATCC or human brain abscesses produced putrescine and/or its derivative N-acetyl-putrescine.

Strain-Specific Metabolic Requirements Revealed by a Defined Minimal Medium for Systems Analyses of Staphylococcus aureus

Staphylococcus aureus is a Gram-positive pathogenic bacterium that colonizes an estimated one-third of the human population and can cause a wide spectrum of disease, ranging from superficial skin infections to life-threatening sepsis. The adaptive mechanisms that contribute to the success of this pathogen remain obscure partially due to a lack of knowledge of its metabolic requirements. Systems biology approaches can be extremely useful in predicting and interpreting metabolic phenotypes; however, such approaches rely on a chemically defined minimal medium as a basis to investigate the requirements of the cell. In this study, a chemically defined minimal medium formulation, termed synthetic minimal medium (SMM), was investigated and validated to support growth of three S. aureus strains: LAC and TCH1516 (USA300 lineage), as well as D592 (USA100 lineage). The formulated SMM was used in an adaptive laboratory evolution experiment to probe the various mutational trajectories of all three strains leading to optimized growth capabilities. The evolved strains were phenotypically characterized for their growth rate and antimicrobial susceptibility. Strains were also resequenced to examine the genetic basis for observed changes in phenotype and to design follow-up metabolite supplementation assays. Our results reveal evolutionary trajectories that arose from strain-specific metabolic requirements. SMM and the evolved strains can also serve as important tools to study antibiotic resistance phenotypes of S. aureus IMPORTANCE As researchers try to understand and combat the development of antibiotic resistance in pathogens, there is a growing need to thoroughly understand the physiology and metabolism of the microbes. Staphylococcus aureus is a threatening pathogen with increased antibiotic resistance and well-studied virulence mechanisms. However, the adaptive mechanisms used by this pathogen to survive environmental stresses remain unclear, mostly due to the lack of information about its metabolic requirements. Defining the minimal metabolic requirements for S. aureus growth is a first step toward unraveling the mechanisms by which it adapts to metabolic stresses. Here, we present the development of a chemically defined minimal medium supporting growth of three S. aureus strains, and we reveal key genetic mutations contributing to improved growth in minimal medium.

The ClpCP Complex Modulates Respiratory Metabolism in Staphylococcus aureus and Is Regulated in a SrrAB-Dependent Manner

The staphylococcal respiratory regulator (SrrAB) modulates energy metabolism in Staphylococcus aureus Studies have suggested that regulated protein catabolism facilitates energy homeostasis. Regulated proteolysis in S. aureus is achieved through protein complexes composed of a peptidase (ClpQ or ClpP) in association with an AAA⁺ family ATPase (typically, ClpC or ClpX). In the present report, we tested the hypothesis that SrrAB regulates a Clp complex to facilitate energy homeostasis in S. aureus Strains deficient in one or more Clp complexes were attenuated for growth in the presence of puromycin, which causes enrichment of misfolded proteins. A ΔsrrAB strain had increased sensitivity to puromycin. Epistasis experiments suggested that the puromycin sensitivity phenotype of the ΔsrrAB strain was a result of decreased ClpC activity. Consistent with this, transcriptional activity of clpC was decreased in the ΔsrrAB mutant, and overexpression of clpC suppressed the puromycin sensitivity of the ΔsrrAB strain. We also found that ClpC positively influenced respiration and that it did so upon association with ClpP. In contrast, ClpC limited fermentative growth, while ClpP was required for optimal fermentative growth. Metabolomics studies demonstrated that intracellular metabolic profiles of the ΔclpC and ΔsrrAB mutants were distinct from those of the wild-type strain, supporting the notion that both ClpC and SrrAB affect central metabolism. We propose a model wherein SrrAB regulates energy homeostasis, in part, via modulation of regulated proteolysis.IMPORTANCE Oxygen is used as a substrate to derive energy by the bacterial pathogen Staphylococcus aureus during infection; however, S. aureus can also grow fermentatively in the absence of oxygen. To successfully cause infection, S. aureus must tailor its metabolism to take advantage of respiratory activity. Different proteins are required for growth in the presence or absence of oxygen; therefore, when cells transition between these conditions, several proteins would be expected to become unnecessary. In this report, we show that regulated proteolysis is used to modulate energy metabolism in S. aureus We report that the ClpCP protein complex is involved in specifically modulating aerobic respiratory growth but is dispensable for fermentative growth.

The DUF59 Containing Protein SufT Is Involved in the Maturation of Iron-Sulfur (FeS) Proteins during Conditions of High FeS Cofactor Demand in Staphylococcus aureus

Proteins containing DUF59 domains have roles in iron-sulfur (FeS) cluster assembly and are widespread throughout Eukarya, Bacteria, and Archaea. However, the function(s) of this domain is unknown. Staphylococcus aureus SufT is composed solely of a DUF59 domain. We noted that sufT is often co-localized with sufBC, which encode for the Suf FeS cluster biosynthetic machinery. Phylogenetic analyses indicated that sufT was recruited to the suf operon, suggesting a role for SufT in FeS cluster assembly. A S. aureus ΔsufT mutant was defective in the assembly of FeS proteins. The DUF59 protein Rv1466 from Mycobacterium tuberculosis partially corrected the phenotypes of a ΔsufT mutant, consistent with a widespread role for DUF59 in FeS protein maturation. SufT was dispensable for FeS protein maturation during conditions that imposed a low cellular demand for FeS cluster assembly. In contrast, the role of SufT was maximal during conditions imposing a high demand for FeS cluster assembly. SufT was not involved in the repair of FeS clusters damaged by reactive oxygen species or in the physical protection of FeS clusters from oxidants. Nfu is a FeS cluster carrier and nfu displayed synergy with sufT. Furthermore, introduction of nfu upon a multicopy plasmid partially corrected the phenotypes of the ΔsufT mutant. Biofilm formation and exoprotein production are critical for S. aureus pathogenesis and vancomycin is a drug of last-resort to treat staphylococcal infections. Defective FeS protein maturation resulted in increased biofilm formation, decreased production of exoproteins, increased resistance to vancomycin, and the appearance of phenotypes consistent with vancomycin-intermediate resistant S. aureus. We propose that SufT, and by extension the DUF59 domain, is an accessory factor that functions in the maturation of FeS proteins. In S. aureus, the involvement of SufT is maximal during conditions of high demand for FeS proteins.

Nutrient limitation governs Staphylococcus aureus metabolism and niche adaptation in the human nose

Colonization of the human nose by Staphylococcus aureus in one-third of the population represents a major risk factor for invasive infections. The basis for adaptation of S. aureus to this specific habitat and reasons for the human predisposition to become colonized have remained largely unknown. Human nasal secretions were analyzed by metabolomics and found to contain potential nutrients in rather low amounts. No significant differences were found between S. aureus carriers and non-carriers, indicating that carriage is not associated with individual differences in nutrient supply. A synthetic nasal medium (SNM3) was composed based on the metabolomics data that permits consistent growth of S. aureus isolates. Key genes were expressed in SNM3 in a similar way as in the human nose, indicating that SNM3 represents a suitable surrogate environment for in vitro simulation studies. While the majority of S. aureus strains grew well in SNM3, most of the tested coagulase-negative staphylococci (CoNS) had major problems to multiply in SNM3 supporting the notion that CoNS are less well adapted to the nose and colonize preferentially the human skin. Global gene expression analysis revealed that, during growth in SNM3, S. aureus depends heavily on de novo synthesis of methionine. Accordingly, the methionine-biosynthesis enzyme cysteine-γ-synthase (MetI) was indispensable for growth in SNM3, and the MetI inhibitor DL-propargylglycine inhibited S. aureus growth in SNM3 but not in the presence of methionine. Of note, metI was strongly up-regulated by S. aureus in human noses, and metI mutants were strongly abrogated in their capacity to colonize the noses of cotton rats. These findings indicate that the methionine biosynthetic pathway may include promising antimicrobial targets that have previously remained unrecognized. Hence, exploring the environmental conditions facultative pathogens are exposed to during colonization can be useful for understanding niche adaptation and identifying targets for new antimicrobial strategies.

Staphylococcus δ-toxin induces allergic skin disease by activating mast cells

Atopic dermatitis (AD) is a chronic inflammatory skin disease that affects 15 to 30% of children and ~5% of adults in industrialized countries¹. Although the pathogenesis of AD is not fully understood, the disease is mediated by an abnormal immunoglobulin E (IgE) immune response in the setting of skin barrier dysfunction². Mast cells (MCs) contribute to IgE-mediated allergic disorders including AD³. Upon activation, MCs release their membrane-bound cytosolic granules leading to the release of multiple molecules that are important in the pathogenesis of AD and host defense⁴. More than 90% of AD patients are colonized with Staphylococcus aureus in the lesional skin whereas most healthy individuals do not harbor the pathogen⁵. Several Staphylococcal exotoxins (SEs) can act as superantigens and/or antigens in models of AD⁶. However, the role of these SEs in disease pathogenesis remains unclear. Here, we report that culture supernatants of S. aureus contain potent MC degranulation activity. Biochemical analysis identified δ-toxin as the MC degranulation-inducing factor produced by S. aureus. MC degranulation induced by δ-toxin depended on phosphoinositide 3-kinase (PI3K) and calcium (Ca²⁺) influx, but unlike that mediated by IgE crosslinking, it did not require the spleen tyrosine kinase (Syk). In addition, IgE enhanced δ-toxin-induced MC degranulation in the absence of antigen. Furthermore, S. aureus isolates recovered from AD patients produced high levels of δ-toxin. Importantly, skin colonization with S. aureus, but not a mutant deficient in δ-toxin, promoted IgE and IL-4 production, as well as inflammatory skin disease. Furthermore, enhancement of IgE production and dermatitis by δ-toxin was abrogated in Kit^W-sh/W-sh MC-deficient mice and restored by MC reconstitution. These studies identify δ-toxin as a potent inducer of MC degranulation and suggest a mechanistic link between S. aureus colonization and allergic skin disease.

Effect of low water activity on staphylococcal enterotoxin A and B biosynthesis

Staphylococcus aureus strains FDA 743 (staphylococcal enterotoxin A [SEA]-producing), FDA 778 (staphylococcal enterotoxin B [SEB]-producing), and S6 (SEA- and SEB-producing) were used to examine the effect of low water activity (a(w)) on SEA and SEB biosynthesis. In this report, we show that SEB production is more sensitive to low a(w) than SEA production. We also show that when proline is available as a compatible solute for S. aureus, SEB production is significantly stimulated at low a(w). This stimulatory effect was not observed when other compatible solutes (i.e., glycine betaine or carnitine) were added to low a(w) growth media. Finally, Northern blot analysis revealed that the stimulation of SEB production at low a(w) by added proline occurs at the level of transcription.

Siderophore production by Staphylococcus aureus and identification of iron-regulated proteins

Siderophore activity of Staphylococcus aureus was detected in an iron-restricted chemically defined medium. The molecular mass of this siderophore, called aureochelin, was 577 Da. Surface-associated proteins of 120, 88, 57, 35, and 33 kDa were mainly expressed under iron restriction conditions. Results showed a relationship between siderophore production and the existence of the 120- and 88-kDa proteins. Western blotting of surface-associated proteins revealed that these proteins were recognized both by patients sera and polyclonal rabbit serum.

Staphylococcus aureus growth and type 5 capsular polysaccharide production in synthetic media

The production of type 5 capsular polysaccharide by Staphylococcus aureus in synthetic media was investigated. The influence of medium components on capsular polysaccharide synthesis appeared to relate to the presence or absence of the component rather than to concentration gradient. The production of type 5 capsular polysaccharide was linked to energy availability and energy source, but not to carbohydrate concentration or carbon/nitrogen ratio. Regulation of capsular polysaccharide production by S. aureus in response to medium changes would appear to differ from that typically displayed in other organisms that produce polysaccharides.

Media for study of growth kinetics and envelope properties of iron-deprived bacteria

Ion-exchange chromatography was used to remove iron from complex and chemically defined laboratory media. The kinetics of metal cation removal from the media was investigated by using atomic absorption spectrophotometry, and the results indicated that over 90% of the iron could be eliminated from certain complex media by this treatment. The treated medium was used for growth studies in a gram-positive and a number of gram-negative organisms that were isolated from infections in humans. High-molecular-weight outer membrane proteins that are known to be induced under iron-depleted growth conditions (iron-regulated membrane proteins) were observed when a number of gram-negative pathogens were cultivated in the treated media. Iron uptake by Staphylococcus aureus varied, depending on the iron content of the medium.

Modified method for production and purification of Staphylococcus aureus enterotoxin B

A medium containing 4% bio-trypcase and 1% yeast extract was used for the production of Staphylococcus aureus enterotoxin B. The yield obtained was estimated at 200 micrograms of enterotoxin per ml of S. aureus S-6 culture supernatant. The purification method involves chromatography on Biorex 70 resin, isoelectric focusing, and gel filtration on Sephadex G-100. The purified enterotoxin (isoionic point, pH 8.55) was shown to be homogenous protein with a molecular weight of 29,000 when tested by gel electrophoresis.

Effect of specific growth limitations on cell wall composition of Staphylococcus aureus H

Conditions are described for the continuous culture of a derivative of Staphylococcus aureus H in a fully defined minimal medium in which cysteine is the sole amino acid. The effects of growth under various nutrient limitations on the composition and properties of the cell wall have been studied. The proportion of ribitol teichoic acid present in the wall, and the extent to which it is substituted with N-acetylglucosamine, varies in bacteria grown under different conditions as does the composition and extent of cross-linking of the peptidoglycan. Neither the derivative nor the original strain H produced teichuronic acid when grown under phosphate limitation.

Heat-stable enterotoxin from Escherichia coli: factors involved in growth and toxin production

Six enterotoxigenic strains of Escherichia coli produced variable levels of heat-stable enterotoxin (ST) when grown under pH control at 8.5 in a simple synthetic medium containing neither amino acids nor vitamins. Bacterial growth and ST production were at levels as high as or higher than those observed in complex media. ST elaboration was detectable in the early logarithmic phase of growth and appeared to be related to disappearance of glucose in the growth medium. The results of this study did not suggest pH-dependent release of ST. Imposition of pH control in complex media resulted in increased growth rates, earlier detectable ST synthesis, and elevated levels of ST. In synthetic medium, attainment of the stationary growth phase was followed by a significant decrease in culture density and a concomitant increase in ST. Cellular autolysis experiments revealed that as much as 20% of the total ST activity was present in a cell-associated form.

Molar growth yields and enterotoxin B production of Staphylococcus aureus S-6 with amino acids as energy sources

Staphylococcus aureus S-6 was able to utilize glutamate, proline, histidine, aspartate, alanine, threonine, serine, or glycine as a major energy source, when these were added singly at a concentration of 2.5 g/liter to a base medium containing salts, vitamins, and 30 mg of each of 18 amino acids per liter. Molar growth yields (Y) for each of the eight amino acids were determined in aerobic batch cultures. Except with aspartate and serine, all molar growth yields decreased with increasing concentration of the energy source, whether or not it was present in excess. The average molar growth yields for both aspartate and serine were Y = 35 g/mol. The highest Y values obtained with the other amino acids were 79, 78, 56, 46, 58, and 21 g/mol for glutamate, proline, histidine, alanine, threonine, and glycine, respectively. Histidine was not utilized as an energy source at concentrations of less than 2 mM. Enterotoxin yields (mg of toxin per g [dry wt] of cells) higher than in the basal medium were obtained when proline, histidine, alanine, serine, and glycine were the major energy sources. When proline and alanine were the major energy sources, enterotoxin yields were not affected by the concentration, whereas toxin yields were reduced at high concentrations of the other six amino acids. The highest cell yields and Y values were obtained with glutamate, but its utilization as an energy source markedly reduced enterotoxin yields.

Effect of minerals on staphylococcal enterotoxin B production

Raising the magnesium level from 0.4 to 1.5 mM in a medium containing only amino acids (2.08%), salts, and vitamins increased enterotoxin B production by Staphylococcus aureus S-6 by about 80%. The level of phosphates in the medium was lowered to one-tenth (to 2.87 mM) the original amount without adversely affecting growth and enterotoxin production. The optimum level of potassium was between 15 and 45 mM. Ammonium salts were not essential in the amino acid medium. Sufficient quantities of trace metals were already present. Revising the salt mixture according to the information obtained resulted in a doubling of the enterotoxin B production.

Nutrition and enterotoxin synthesis by enterotoxigenic strains of Escherichia coli: defined medium for production of heat-stable enterotoxin

A defined medium has been developed which supports synthesis of heat-stable enterotoxin (ST) by porcine and bovine strains of enterotoxigenic (ENT+) Escherichia coli in levels equivalent or better than a complex Casamino Acids-salts medium. The medium components did not support production of heat-labile enterotoxin (LT) but were similar for ST synthesis by ENT+ strains producting only ST and those which produced ST in addition to LT. The amino acids in Casamino Acids found to be necessary for growth and enterotoxin synthesis were proline, serine, aspartic acid, and alanine. Maximal growth and toxin levels were obtained after 8 h of incubation. Improved growth, but not an increase in synthesis of ST, was observed in the presence of Mg2+, Mn2+ and Fe3+ compared with Mg2+ alone. A chelator, tricine, was necessary for maximal cell densities,, probably to solubilize trace ions and make them more available to the bacteria. Increased growth was observed upon addition of glucose to both complex and defined media; however, glucose as well as gluconate and pyruvate appeared to cause repression of toxin synthesis. Addition of vitamins, oleic acid, or DL-lactic acid to the defined medium slightly increased levels of ST.

Factors affecting competence for transformation in Staphylococcus aureus

A chemically defined medium has been developed for isolation of amino acid-requiring mutants of Staphylococcus aureus strain 8325, and for use as a selective medium in transformation assays. Variables affecting transformation of both plasmid and chromosomal markers have been studied. The optimal pH and temperature for transformation are 6.75 to 7.0 and 30 C, respectively. Ca ions are required for transformation, and only cells lysogenic for the phage phi11 can be transformed. Superinfection of competent cells with phi11 does not increase the transformation frequency. Maximal number of transformants is obtained after 20 min of contact between cells and deoxyribonucleic acid. The transformation frequencies for the plasmid marker erythromycin resistance (ero) and the chromosomal markers trp, thy, and cyt are of the same order of magnitude, whereas the frequency for the chromosomal marker tyr is approximately one order of magnitude lower.

Polyvalent antiserum agar system for the detection of staphylococcal enterotoxins A, B, C, and E

A polyvalent antiserum agar system in capillary tubes was developed and evaluated for the detection of enterotoxins A, B, C, and/or E present in culture supernatant fluids.