Alpha-toxin and gamma-toxin jointly promote Staphylococcus aureus virulence in murine septic arthritis

Septic arthritis is a common and feared complication of staphylococcal infections. Staphylococcus aureus produces a number of potential virulence factors including certain adhesins and enterotoxins. In this study we have assessed the roles of cytolytic toxins in the development of septic arthritis by inoculating mice with S. aureus wild-type strain 8325-4 or isogenic mutants differing in the expression of alpha-, beta-, and gamma-toxin production patterns. Mice inoculated with either an alpha- or beta-toxin mutant showed degrees of inflammation, joint damage, and weight decrease similar to wild-type-inoculated mice. In contrast, mice inoculated with either double (alpha- and gamma-toxin-deficient)- or triple (alpha-, beta-, and gamma-toxin-deficient)-mutant S. aureus strains showed lower frequency and severity of arthritis, measured both clinically and histologically, than mice inoculated with the wild-type strain. We conclude that simultaneous production of alpha- and gamma-toxin is a virulence factor in S. aureus arthritis.     

The accessory gene regulator (agr) controls Staphylococcus aureus virulence in a murine arthritis model.

We have studied the role of the accessory gene regulator (agr) of Staphylococcus aureus as a virulence determinant in the pathogenesis of septic arthritis. At least 15 genes coding for potential virulence factors in Staphylococcus aureus are regulated by a putative multicomponent signal transduction system encoded by the agr/hld locus. agr and hld mutants show a decreased synthesis of extracellular toxins and enzymes, such as alpha-, beta-, and delta-hemolysin, leucocidin, lipase, hyaluronate lyase, and proteases, and at the same time an increased synthesis of coagulase and protein A as compared with the wild-type counterpart. We have used a recently described murine model of S. aureus-induced arthritis to study the virulence of S. aureus 8325-4 and two agr/hld mutants derived from it. Sixty percent of the mice injected with the wild-type strain developed arthritis, whereas agrA and hld mutants displayed joint involvement in only 10 and 30%, respectively. In addition, 40% of the mice inoculated with the wild-type strain displayed an erosive arthropathy; such changes were not detectable at all in mice inoculated with the agrA mutant. Serum levels of interleukin-6, a potent B-cell differentiation factor, were significantly higher (P < 0.001) in the mice inoculated with the wild-type strain than in those inoculated with the agrA mutant counterpart. Overall, our results suggest that the agr system of S. aureus is an important virulence determinant in the induction and progression of septic arthritis in mice.     

Ribonucleotide reductase class III, an essential enzyme for the anaerobic growth of Staphylococcus aureus, is a virulence determinant in septic arthritis

Staphylococcus aureus is the most common cause of joint infections. It also contributes to several other diseases such as pneumonia, osteomyelitis, endocarditis, and sepsis. Bearing in mind that S. aureus becomes rapidly resistant to new antibiotics, many studies survey the virulence factors, with the aim to find alternative prophylaxis/treatment regimens. One potential virulence factor is the bacterial ability to survive at different oxygen tensions. S. aureus expresses ribonucleotide reductases (RNRs), which help it to grow under both aerobic and anaerobic conditions, by reducing ribonucleotides to deoxyribonucleotides. In this study, we investigated the role of RNR class III, which is required for anaerobic growth, as a virulence determinant in the pathogenesis of staphylococcal arthritis. The wild-type S. aureus strain and its isogenic mutant nrdDG mutant were inoculated intravenously into mice. Mice inoculated with the wild-type strain displayed significantly more severe arthritis, with significantly more synovitis and destruction of the bone and cartilage versus mutant strain inoculated mice. Further, the persistence of bacteria in the kidneys was significantly more pronounced in the group inoculated with the wild-type strain. Together these results indicate that RNR class III is an important virulence factor for the establishment of septic arthritis.     

Role of neutrophils in experimental septicemia and septic arthritis induced by Staphylococcus aureus.

We have previously described a murine model of hematogenously induced Staphylococcus aureus sepsis and arthritis. In this model, large numbers of granulocytes can be observed both in the circulation and locally in the inflamed synovium within 24 h after bacterial inoculation. To assess the role of neutrophils in this severe infection, mice were given granulocyte-depleting monoclonal antibody RB6-8C5 before being inoculated with S. aureus. All the control mice survived their intravenous injection with 3 x 10(7) CFU of S. aureus, whereas all the mice given RB6-8C5 antibody died of sepsis within 2 to 3 days. Even when the inoculum size was reduced sixfold (i.e., 6 x 10(6) CFU/mouse), 50% of the RB6-8C5-treated animals died within 6 days. The RB6-8C5-treated mice had a significantly higher burden of bacteria in their blood and kidneys 24 and 48 h after bacterial inoculation. In addition, when a suboptimal dose of bacteria was administered, the neutrophil-depleted animals displayed a higher frequency of arthritis than did the controls. The granulocyte-depleted animals exhibited increased levels of the proinflammatory cytokines tumor necrosis factor alpha, interleukin-6, and gamma interferon, reflecting the severity of their disease. This is the first direct demonstration of neutrophils playing a crucial protective role in the early phase of S. aureus infection.     

Roles of alpha-toxin and beta-toxin in virulence of Staphylococcus aureus for the mouse mammary gland.

Mutants of Staphylococcus aureus which fail to express alpha-toxin (Hly), beta-toxin (Hlb), or both have been constructed by site-specific mutagenesis. The virulence of the mutants was compared with that of wild-type toxigenic strains by intramammary inoculation of lactating mice. A bovine strain, M60, and a laboratory strain, 8325-4, caused acute mastitis and death within 48 h for 60% of the mice inoculated. Animals inoculated with Hly mutants also developed acute mastitis, but no deaths occurred. Comparisons of Hly- or Hlb-positive strains with the double mutation Hly Hlb showed that both toxins led to a significantly higher recovery of S. aureus from the gland 48 h postinfection. Histopathological examination of mammary glands showed that phagocytosis of bacteria occurred irrespective of toxigenicity, but toxigenic strains, particularly those which were Hly+, continued to multiply, invaded the interalveolar tissues, and produced severe lesions. Stimulation of an inflammatory response by inoculation of the mammary gland with endotoxin prior to challenge with S. aureus reduced recovery of the bacteria 10- to 100-fold and, under these conditions, neither alpha-toxin nor beta-toxin contributed significantly to growth and survival.     

Staphylococcus aureus strains that express serotype 5 or serotype 8 capsular polysaccharides differ in virulence

Most isolates of Staphylococcus aureus produce a serotype 5 (CP5) or 8 (CP8) capsular polysaccharide. To investigate whether CP5 and CP8 differ in their biological properties, we created isogenic mutants of S. aureus Reynolds that expressed CP5, CP8, or no capsule. Biochemical analyses of CP5 and CP8 purified from the isogenic S. aureus strains were consistent with published structures. The degree of O acetylation of each polysaccharide was similar, but CP5 showed a greater degree of N acetylation. Mice challenged with the CP5(+) strain showed a significantly higher bacteremia level than mice challenged with the CP8(+) strain. Similarly, the CP5(+) strain survived preferentially in the bloodstream and kidneys of infected mice challenged with a mixed inoculum containing both strains. The enhanced virulence of the CP5(+) strain in vivo correlated with its greater resistance to in vitro killing in whole mouse blood. Likewise, in vitro opsonophagocytic killing assays with human neutrophils and sera revealed greater survival of the Reynolds (CP5) strain, even though the kinetics of opsonization by C3b and iC3b was similar for both the CP5(+) and CP8(+) strains. Electron micrographs demonstrated C3 molecules on the cell wall beneath the capsule layer for both serotype 5 and 8 strains. Purified CP5 and CP8 stimulated a modest oxidative burst in human neutrophils but failed to activate the alternative complement pathway. These results indicate that CP5 and CP8 differ in a number of biological properties, and these differences likely contribute to the relative virulence of serotype 5 and 8 S. aureus in vivo.     

The Staphylococcus aureus collagen adhesin is a virulence determinant in experimental septic arthritis.

The importance of a collagen-binding adhesin in the pathogenesis of septic arthritis has been examined by comparing the virulence of two sets of Staphylococcus aureus mutants in an animal model. Collagen adhesin-negative mutant PH100 was constructed by replacing the chromosomal collagen adhesin gene (cna) in a clinical strain, Phillips, with an inactivated copy of the gene. Collagen adhesin-positive mutant S. aureus CYL574 was generated by introducing the cna gene into CYL316, a strain that normally lacks the cna gene. Biochemical, immunological, and functional analyses of the generated mutants and their respective parent strains showed that binding of 125I-labeled collagen, expression of an immunoreactive collagen adhesin, and bacterial adherence to cartilage were directly correlated with the presence of a functional cna gene. Greater than 70% of the mice injected with the Cna+ strains developed clinical signs of arthritis, whereas less than 27% of the animals injected with Cna- strains showed symptoms of disease. Furthermore, mice injected with the Cna+ strain Phillips had remarkably elevated levels of immunoglobulin G1 and interleukin-6 compared with mice injected with the Cna- mutant PH100. Taken together, these results demonstrate that collagen adhesin plays an important role in the pathogenesis of septic arthritis induced by S. aureus.     

Capsule-negative Staphylococcus aureus induces chronic experimental mastitis in mice

Staphylococcus aureus capsular polysaccharides (CP) have been shown to enhance staphylococcal virulence in numerous animal models of infection. Although serotype 5 CP (CP5) and CP8 predominate among S. aureus isolates from humans, most staphylococcal isolates from bovines with mastitis in Argentina are capsule negative. This study was designed to evaluate the effects of CP5 and CP8 expression on the pathogenesis of experimental murine mastitis. Lactating mice were challenged by the intramammary route with one of three isogenic S. aureus strains producing CP5, CP8, or no capsule. Significantly greater numbers of acapsular mutant cells were recovered from the infected glands 12 days after bacterial challenge compared with the encapsulated strains. Histopathological analyses revealed greater polymorphonuclear and mononuclear leukocyte infiltration and congestion in the mammary glands of mice infected with the encapsulated strains compared with the acapsular mutant, and the serotype 5 strain elicited more inflammation than the serotype 8 strain. In vitro experiments revealed that the acapsular S. aureus strain was internalized by MAC-T bovine epithelial cells in significantly greater numbers than the CP5- or CP8-producing strain. Taken together, the results suggest that S. aureus lacking a capsule was able to persist in the murine mammary gland, whereas encapsulated strains elicited more inflammation and were eliminated faster. Loss of CP5 or CP8 expression may enhance the persistence of staphylococci in the mammary glands of chronically infected hosts.     

Virulence and immunity of Staphylococcus aureus BB and certain deficient mutants.

Coagulase-negative and deoxyribonuclease-negative mutants were isolated from Staphylococcus aureus BB by treatment with N-methyl-N'-nitro-N-nitrosoguanidine. Comparison of virulence (50% lethal dose) to mice of these six mutant strains and S. aureus BB was determined by both intravenous and intraperitoneal routes. The ratios of the 50% lethal dose of coagulase-negative mutants to that of the parental strain S. aureus BB ranged from 201 to 403 for intravenous infection and 30.7 to 52.7 for intraperitoneal infection. The virulence of deoxyribonuclease-negative mutants was essentially the same as that of S. aureus BB. When mice were immunized subcutaneously with live S. aureus BB or its deoxyribonuclease-negative mutants, the resulting protection against the intravenous challenge of S. aureus BB was remarkable. The ratios of the 50% lethal dose for the mice that were immunized by these strains to that for the untreated mice extended from 40.1 to 60.6 for intravenous infection and 6.61 to 11.5 for the intraperitoneal route. However, no effect against S. aureus BB challenge was shown in the mice that were immunized with coagulase-negative mutants.     

Enhancement by Candida albicans of Staphylococcus aureus, Serratia marcescens, and Streptococcus faecalis in the establishment of infection in mice.

Candida albicans has been shown to stimulate infection in mice by a number of bacteria. Mice inoculated intraperitoneally with as little as 500 colony-forming units of Staphylococcus aureus along with a nonlethal dose of 10(8) colony-forming units of C. albicans (one-third the dose causing 50% mortality in 7 days) developed widespread staphylococcal infection. S. aureus injected alone at that or considerably higher doses did not establish infection. Phage typing methods demonstrated that the staphylococcal infection was due to the bacterial organisms originally injected. A minimum dose of C. albicans between 1.1 X 10(5) and 1.1 X 10(7) colony-forming units was necessary for the amplification of virulence of S. aureus to take place. Serratia marcescens and Streptococcus faecalis inoculated intraperitoneally at small nonlethal doses could also be recovered from blood and tissues 5 days after animals were dually injected with C. albicans but not from animals which were inoculated with the same amounts of these bacteria alone.     

Preferential induction of septic arthritis and mortality by superantigen-producing staphylococci.

Staphylococcal enterotoxins A through D (SEA through SED) and toxic shock syndrome toxin-1 display superantigen properties, i.e., they stimulate a great fraction of T cells expressing certain T-cell receptor V beta sequences. Using a newly established rat model of septic Staphylococcus aureus arthritis, we have recently shown that an S. aureus strain producing SEA showed marked arthritogenic properties. In the present study we decided to employ another five S. aureus strains, each one producing a distinct exotoxin. Almost all rats injected with superantigen-producing strains developed arthritis. In contrast, only 20% of rats injected with an S. aureus strain lacking superantigen production displayed mild and transient arthritis. Mortality was preferentially induced among the rats inoculated with the S. aureus strains producing SEB and SED. This study emphasizes that superantigen production is an important virulence factor in the development of septic S. aureus arthritis. Differences concerning mortality between staphylococcal strains producing different exotoxins may be dependent on the degree of activation of the immune system.     

Virulence of a hemB mutant displaying the phenotype of a Staphylococcus aureus small colony variant in a murine model of septic arthritis

Persistence of Staphylococcus aureus during invasive infections has been associated with a small-colony variant (SCV) phenotype. SCVs are frequently auxotrophic for menadione or hemin, two compounds involved in the biosynthesis of the electron transport chain. SCVs have been shown to be more resistant to antibiotics such as aminoglycosides, grow slowly and persist intracellularly. The aim of this study was to assess the virulence of an hemB mutant, which has been shown to display the typical characteristics of clinical SCVs, in a murine model of septic arthritis. NMRI mice were inoculated intravenously with either the wild type strain Newman or with its mutant displaying the SCV phenotype. The clinical, bacteriological, and histopathological progression of the disease was studied. Mice inoculated with the hemB mutant displayed a higher frequency and a significantly higher severity of arthritis than mice inoculated with the wild type Newman strain. Despite that, the mutant inoculated mice displayed significantly lower bacterial burden in their kidneys and joints compared with mice exposed to the wild parental strain. Notably, the hemB mutant produced almost 20 times more protease in vitro than the parental strain. We conclude that the small colony variants of S. aureus are more virulent on a per organism basis than its isogenic parental strain in the model of septic arthritis. This can at least in part be explained by the ability of SCV to produce high amounts of destructive proteases.     

Capsule production and growth phase influence binding of complement to Staphylococcus aureus

Complement-mediated opsonization of bacteria by C3 binding is an important component of the host innate immune system. Little information is available concerning the interaction between complement proteins and capsule type 5 and 8 Staphylococcus aureus strains, even though these isolates are responsible for approximately 70% of human staphylococcal infections. To investigate the importance of an intact complement pathway in an experimental staphylococcal infection, control and C3-depleted mice were challenged intravenously with 10(7) CFU of a serotype 5 S. aureus isolate. Whereas only 8% of the control mice succumbed to the infection, 64% of the complemented-depleted animals died. In vitro parameters of C3 binding to two heavily encapsulated (CP++) strains, three encapsulated (CP+) strains, and an isogenic capsule-negative (CP-) mutant were examined. The alternative pathway contributed 90% of C3 binding in 20% serum at 30 min, whereas it accounted for only 13% of C3 binding in 2% serum. Stationary-phase organisms bound only 10% as much C3 as mid-log-phase organisms; this was only in part due to capsule. When the S. aureus strains were cultivated on solid medium, the CP++ isolates bound 50% less C3 than CP+ strains; a CP+ strain bound 42% less C3 than the CP- mutant. Both C3b and iC3b fragments of C3 bound to S. aureus cells, and about one-third of the bound C3 was shed from the staphylococcal surface as iC3b, regardless of the CP phenotype of the strain. Thus, the phase of growth and presence of capsule are critical to opsonization.     

Effect of strain of Staphylococcus aureus on synergism with Candida albicans resulting in mouse mortality and morbidity.

Nine Staphylococcus aureus strains isolated from patients with toxic shock syndrome (TSS), two strains from non-disease-associated sources, and four strains from disease (not TSS)-associated sources were characterized for the intraperitoneal dose necessary to kill 50% of exposed animals (LD50) and toxic shock toxin production and studied for synergistic effects on mouse mortality and morbidity when combined with a sublethal dose of Candida albicans and inoculated intraperitoneally. Representative toxic shock toxin-producing strains (free of other enterotoxins) exhibited the following unique set of characteristics when inoculated intraperitoneally into mice and compared with all other strains tested: (i) lowest virulence when inoculated alone into mice as determined by the LD50; (ii) greatest synergistic decrease in LD50 (up to 70,000-fold as compared to up to 200-fold for other strains) when combined with C. albicans and injected intraperitoneally; and (iii) induced a characteristic, dose-independent, temporal death pattern in dually injected animals. When sublethal dual doses were used, animals receiving disease (TSS and not TSS)-associated S. aureus in combination with C. albicans developed symptoms, but some differences in symptomatologies, depending on the strain, were observed. The symptoms included conjunctivitis; gastrointestinal, neurological, and circulatory abnormalities; rash followed by desquamation; and patchy baldness. Although overlap in symptoms between animal treatment groups was observed, certain symptoms (neurological sequeae and petechial hemorrhages) were observed only in animals inoculated with a specific S. aureus strain combined with C. albicans. Animals receiving sublethal dual doses, which included non-disease-associated S. aureus, did not develop symptoms. When Staphylococcus epidermidis was combined with C. albicans and inoculated into mice, no synergistic effects on morbidity or mortality were observed.     

Respiratory activity is essential for post-exponential-phase production of type 5 capsular polysaccharide by Staphylococcus aureus.

Capsule formation is believed to have a significant role in bacterial virulence. To examine the possible involvement of capsular polysaccharide (CP) from Staphylococcus aureus in the pathological mechanisms associated with staphylococcal infections, we investigated the influence of respiratory activity on type 5 CP production by S. aureus grown in the presence of various concentrations of dissolved oxygen or nitrate. The effects of several metabolic inhibitors (arsenite, cyanide, azide, trimethylamine N-oxide, 2-heptyl-4-hydroxyquinoline N-oxide, and 2,4-dinitrophenol) were also tested. The metabolism of the bacteria was estimated by measuring their reductive capacity and by monitoring the pH and concentrations of fermentation products. Type 5 CP was always produced by S. aureus during the exponential phase of growth under all culture conditions tested. In contrast, post-exponential-phase CP production appeared to be strictly dependent on the respiratory activity. Since post-exponential-phase CP production contributes at least two-thirds of the total CP obtained, the influence of S. aureus respiration on CP production might be of some importance in the process of infection.     

Virulence of Staphylococcus aureus mutants altered in type 5 capsule production.

Most clinical isolates of Staphylococcus aureus produce microcapsules (uronic acid-containing extracellular polysaccharides) that are detectable by serologic methods but are not visible by negative staining. Among the 11 reported serotypes, capsule types 5 and 8 comprise approximately 75% of all isolates. Transposon mutagenesis was performed on S. aureus to create mutants altered in capsule expression. Tn918 was introduced into the capsule type 5 strain Reynolds by filter mating, and a capsule-deficient transconjugate, JL236, was isolated. The wild-type strain was transformed with JL236 chromosomal DNA to confirm that transfer of the appropriate-size chromosomal fragment containing Tn918 generated a capsule-deficient transformant. Strain Reynolds was mutagenized with ethyl methanesulfonate to obtain a capsule-negative mutant (strain JL240). Capsular phenotypes were determined by colony immunoblots, antibody adsorption experiments, and transmission electron microscopy. The virulences of the parental and mutant strains in mice were compared. The 50% lethal doses for strains Reynolds, JL236, and JL240 were similar (10(8.59), 10(8.98), and 10(8.93) CFU, respectively). Animals injected intraperitoneally with either wild-type or mutant strains had comparable levels of bacteremia at 3 and 24 h after challenge. Quantitative cultures of blood and kidneys from animals challenged intravenously with sublethal doses of the S. aureus strains also showed no differences in bacterial clearance or renal abscess formation. These studies indicate that the type 5 S. aureus microcapsule does not promote bacterial virulence in the animal models tested.     

Expression of an antisense hla fragment in Staphylococcus aureus reduces alpha-toxin production in vitro and attenuates lethal activity in a murine model.

Isogeneic bacterial strains that differ only in the production of a single microbial factor have been invaluable in studying the pathogenesis of bacterial infections. The targeted, intentional inactivation of a gene encoding a potential virulence determinant generally requires homologous recombination to replace the gene with an inactivated allele. To determine whether the insertion and expression of a fragment of a bacterial gene in an antisense orientation could be used as a rapid alternative to allelic inactivation for producing paired isogeneic isolates, we inverted a 600-bp fragment of the Staphylococcus aureus gene encoding alpha-toxin, hla, behind its native promoter on an Escherichia coli-S. aureus shuttle vector. A transformant of an S. aureus strain carrying the antisense hla fragment produced antisense hla RNA and made 16-fold less alpha-toxin than either its parent or an isogeneic transformant containing vector DNA without hla. Also, intraperitoneal injection of 1.5 x 10(9) CFU of the antisense hla-containing transformant was significantly less lethal in a murine model than that of the parent (1 of 10 versus 7 of 10 mice expired [P < 0.02]) or the transformant without hla (1 of 10 versus 7 of 7 mice expired [P < 0.001]). We conclude that the expression of a fragment of hla in an antisense orientation in S. aureus on a plasmid vector reduces alpha-toxin production and the lethal activity of the strain in a murine model. The antisense strategy for creating isogeneic strains of bacteria may facilitate molecular investigations into the pathogenesis of infection. It also may be useful in creating novel live-attenuated strains of bacteria for use as vaccine candidates.     

The surface protein Pls of methicillin-resistant Staphylococcus aureus is a virulence factor in septic arthritis

Pls, a surface protein of certain methicillin-resistant Staphylococcus aureus strains, is associated with poor bacterial adherence to solid-phase fibronectin and immunoglobulin G, as well as with reduced invasion of cultured epithelial cells. Here the importance of Pls for the development of septic arthritis and sepsis was investigated by using a mouse model. Mice inoculated with a pls knockout mutant developed a much milder arthritis and showed less grave weight reduction than mice infected with the wild-type Pls(+) clinical isolate. Also, the pls mutant induced a significantly lower frequency of mortality than the wild-type strain. The bacterial load of the kidneys was larger in mice infected with the Pls(+) strain than in animals challenged with the pls mutant. However, there was no evident inflammatory effect due to the Pls molecule alone, as indicated by knee injection of purified Pls. In conclusion, the results show that Pls is a virulence factor for septic arthritis and sepsis.     

The Salmonella typhimurium virulence plasmid increases the growth rate of salmonellae in mice.

The virulence plasmids of Salmonella typhimurium and other invasive Salmonella serovars have long been associated with the ability of these bacteria to cause systemic infection beyond the intestines in orally inoculated animals. Genetic analysis of virulence genes on the high-molecular-weight plasmids has revealed that no more than five genes spanning a 6.2-kb region are sufficient to replace the entire plasmid for conferring virulence. However, the exact virulence function(s) encoded by these genes has not been elucidated. In this report, we measured the possible effect of the virulence plasmid on the growth rate of S. typhimurium in mice by two complementary procedures. The first procedure used segregation of a temperature-sensitive plasmid in vivo to provide a measure of bacterial divisions and the number of recovered marker plasmid-containing salmonellae as a measure of killing. In the second procedure, aroA deletions were transduced into virulence plasmid-containing and plasmid-cured S. typhimurium. Since AroA- salmonellae are inhibited for growth in vivo, if the virulence plasmid affected only growth rate, no difference in the recoveries of the paired AroA- strains would be seen. Virulence plasmid-containing S. typhimurium segregated the marker plasmid more rapidly than did the virulence plasmid-cured strain, and AroA- derivatives of both strains were recovered equally from mice. Therefore, the S. typhimurium virulence plasmid increased growth rate but had no detectable effect on killing or bacterial movement into deep tissues. To examine whether the plasmid accomplished this function by affecting the intracellular/extracellular location of bacteria, orally infected mice were injected with gentamicin to kill the extracellular bacteria. Wild-type and plasmid-cured S. typhimurium strains were equally resistant to gentamicin in vivo and hence most likely located intracellularly to equal degrees. When wild-type and plasmid-cured S. typhimurium strains were sequestered within peritoneal chambers in mice, the resulting extracellular growth was equal. Therefore, the virulence plasmid increases the growth rate of S. typhimurium in mice, probably within mouse cells.     

Ribonucleotide reductase class III,an essential enzyme for the anaerobic growth of Staphylococcus aureus,is a virulence determinant in septic arthritis

Staphylococcus aureus is the most common cause of joint infections. It also contributes to several other diseases such as pneumonia, osteomyelitis, endocarditis, and sepsis. Bearing in mind that S. aureus becomes rapidly resistant to new antibiotics, many studies survey the virulence factors, with the aim to find alternative prophylaxis/treatment regimens. One potential virulence factor is the bacterial ability to survive at different oxygen tensions. S. aureus expresses ribonucleotide reductases (RNRs), which help it to grow under both aerobic and anaerobic conditions, by reducing ribonucleotides to deoxyribonucleotides. In this study, we investigated the role of RNR class III, which is required for anaerobic growth, as a virulence determinant in the pathogenesis of staphylococcal arthritis. The wild-type S. aureus strain and its isogenic mutant nrdDG mutant were inoculated intravenously into mice. Mice inoculated with the wild-type strain displayed significantly more severe arthritis, with significantly more synovitis and destruction of the bone and cartilage versus mutant strain inoculated mice. Further, the persistence of bacteria in the kidneys was significantly more pronounced in the group inoculated with the wild-type strain. Together these results indicate that RNR class III is an important virulence factor for the establishment of septic arthritis.