Genetic analysis of type 5 capsular polysaccharide expression by Staphylococcus aureus

Comprehensive evaluation and analysis of the salinity stress response mechanisms based on transcriptome and metabolome of Staphylococcus aureus

Staphylococcus aureus possesses an extraordinary ability to deal with a wide range of osmotic pressure. This study performed transcriptomic and metabolomic analyses on the potential mechanism of gradient salinity stress adaptation in S. aureus ZS01. The results revealed that CPS biosynthetic protein genes were candidate target genes for directly regulating the phenotypic changes of biofilm. Inositol phosphate metabolism was downregulated to reduce the conversion of functional molecules. The gluconeogenesis pathway and histidine synthesis were downregulated to reduce the production of endogenous glucose. The pyruvate metabolism pathway was upregulated to promote the accumulation of succinate. TCA cycle metabolism pathway was downregulated to reduce unnecessary energy loss. L-Proline was accumulated to regulate osmotic pressure. Therefore, these self-protection mechanisms can protect cells from hypertonic environments and help them focus on survival. In addition, we identified ten hub genes. The findings will aid in the prevention and treatment strategies of S. aureus infections.

Molecular characterization of the capsule locus from non-typeable Staphylococcus aureus

Most Staphylococcus aureus express a serotype 5 or 8 capsular polysaccharide (CP). However, 20-25% of human isolates and up to 86% of bovine strains of S. aureus are non-typeable (NT), i.e. non-reactive with antibodies to CP types 1, 2, 5 or 8. A vaccine that targets the S. aureus CP would not protect against NT strains. The aim of this study was to characterize NT S. aureus isolates at the molecular level to explain their lack of type 5 or 8 capsule production. The cap5(8) locus was present in all 22 NT clinical isolates from humans, eight of 21 bovine isolates, and in all eight sequenced strains. NT strains positive for the cap5(8) transcript had mutations within essential capsule genes and could be complemented in trans. S. aureus strains with reduced cap5(8) transcript had mutations within the cap5A promoter, decreased RNAIII levels, or a truncated arlR gene product. More than one mutation was identified in several isolates. The cap5(8) locus was replaced by IS257 in 13 of 21 NT bovine isolates of S. aureus. Lack of capsule expression in NT S. aureus can be explained by multiple mechanisms, and the data argue against the existence of capsule serotypes other than 1, 2, 5 and 8.

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.

Biosynthesis and assembly of Group 1 capsular polysaccharides in Escherichia coli and related extracellular polysaccharides in other bacteria

Extracellular and capsular polysaccharides (EPSs and CPSs) are produced by a wide range of bacteria, including important pathogens of humans, livestock, and plants. These polymers are major surface antigens and serve a variety of roles in virulence, depending on the biology of the producing organism. In addition to their importance in disease, some EPSs also have industrial applications as gelling and emulsifying agents. An understanding of the processes involved in the synthesis and regulation of CPSs and EPSs therefore potentially contributes to an understanding of the disease state, surface expression of protective antigens, and modulation of polymer structure to give defined physical properties. Escherichia coli has provided important model systems for EPS and CPS biosynthesis. Here we describe current knowledge concerning assembly of the Group 1 CPSs of E. coli and the conservation of similar mechanisms in other bacteria.

Regulation of Staphylococcus aureus type 5 capsular polysaccharides by agr and sarA in vitro and in an experimental endocarditis model

The expression of antiphagocytic polysaccharide capsules is an important pathogenetic step in establishing Staphylococcus aureus infections. Using a green fluorescent protein reporter gene (gfp) system, we examined the expression and genetic regulation of the cap5 promoter (capsular polysaccharide 5 genes) by two major global regulators of S. aureus (agr and sarA) in vitro and in a rabbit endocarditis model. In vitro, cap5 expression substantially increased during the post-exponential phase in parental, as well assarA mutant constructs. However, cap5 expression was greatly reduced in agr and agr/sarA double mutants. In the endocarditis model, the extent of cap5 expression in vegetations infected with the parental strain was substantially higher than that observed with the agr/sarA double mutants (P<0.05). Similar trends were noted in renal, but not splenic abscesses. Collectively, these data suggest that agr positively regulates cap5 expression both in vitro and in vivo, while the contribution of sarA to cap5 regulation, although modest, is readily discerned in vivo in agr minus background. In addition, the regulation ofcap5 expression by these global regulators may vary in distinct anatomic niches in vivo.

Genetic variation among hospital isolates of methicillin-sensitive Staphylococcus aureus: evidence for horizontal transfer of virulence genes

Staphylococcus aureus strains often carry in their genomes virulence genes that are not found in all strains and that may be carried on discrete genetic elements. Strains also differ in that they carry one of four classes of an accessory gene regulator (agr) locus, an operon that regulates virulence factor expression and that has been proposed to be a therapeutic target. To look at their distribution among hospital strains, we investigated 38 methicillin-sensitive S. aureus isolates, classifying the isolates by agr class and screening them for the presence and restriction fragment length polymorphisms (RFLPs) of 12 core and 14 accessory virulence genes. Twenty-three (61%) were agr class I, 10 (26%) were agr class II, and 5 (13%) were agr class III. None were agr class IV. The S. aureus strains had distinguishable RFLP profiles, although clusters of isolates with clearly related core gene profiles were found among our strains, including all five agr class III strains, two sets of six strains within agr class I, and six strains within agr class II. Within these clusters there was evidence of horizontal acquisition and/or loss of multiple accessory virulence genes. Furthermore, two isolates from the same patient were identical except for the presence of the sea gene, indicating that movement of mobile elements may occur in vivo. Several strong correlations with the carriage of virulence genes between strains were seen, including a positive correlation between tst and agr class III and negative correlations between tst and lukE-splB and between lukE-splB and seg-sei. This suggests that the core genome or the presence of accessory genetic elements within a strain may influence acquisition and loss of other elements encoding virulence genes.

Staphylococcus aureus Cap5O has UDP-ManNAc dehydrogenase activity and is essential for capsule expression

The Staphylococcus aureus serotype 5 capsular polysaccharide (CP5) has a repeating unit composed of (-->4)-3-O-acetyl-beta-D-ManNAcA-(1-->4)-alpha-L-FucNAc (1-->3)-beta-D-FucNAc-(1-->)(n). Sixteen chromosomal genes (cap5A through cap5P) are involved in the synthesis of CP5. We recently demonstrated that Cap5P, a 2-epimerase, catalyzes the conversion of UDP-N-acetyl glucosamine (UDP-GlcNAc) to UDP-N-acetylmannosamine (UDP-ManNAc). In this study, we show that UDP-ManNAc is oxidized to UDP-N-acetylmannosaminuronic acid (UDP-ManNAcA) by a UDP-ManNAc dehydrogenase encoded by S. aureus cap5O. We expressed Cap5O in Escherichia coli and purified the recombinant protein. The UDP-ManNAc dehydrogenase activity of purified Cap5O was assessed by incubating Cap5P and UDP-GlcNAc (to produce UDP-ManNAc), together with Cap5O, NAD(+), and a reducing agent. Enzymatic activity was quantitated indirectly by measuring the increase in absorbance at 340 nm resulting from NADH formation. The product of the reaction was confirmed as UDP-ManNAcA by gas chromatography-mass spectroscopy. A cap5O mutation, created by deletion of 727 bp in the 5' end of the gene, was introduced by allelic replacement into S. aureus Reynolds, rendering it CP5 negative. Mice inoculated intravenously or subcutaneously with the wild-type strain Reynolds had greater numbers of S. aureus recovered from their kidneys (P = 0.019) or their subcutaneous abscesses (P = 0.0018), respectively, than did animals inoculated with the cap5O mutant. The results of this study indicate that S. aureus cap5O is essential for capsule production and that capsule promotes staphylococcal virulence in mouse models of abscess formation.

Genetic and serologic evaluation of capsule production by bovine mammary isolates of Staphylococcus aureus and other Staphylococcus spp. from Europe and the United States

Bovine mastitis caused by Staphylococcus aureus is responsible for major economic losses to the dairy industry, and more-effective therapeutic or preventive approaches are sorely needed. The predominance of staphylococcal capsular polysaccharide types 5 and 8 among human isolates from many sources is well documented, but there seems to be a greater variation in the distribution of capsular serotypes among isolates from cows. A total of 636 isolates of S. aureus from cases of bovine mastitis in Sweden, Denmark, Finland, Iceland, Ireland, and the United States were investigated for production of capsular polysaccharide types 5 and 8. Approximately half of all the European isolates tested were of serotype 8, although variation among countries and among isolates of clinical and subclinical origin was observed. Sweden had the highest frequency (87%) of serotypeable isolates, and Finland had the lowest (48%). Capsule types 5 and 8 accounted for only 42% of the U.S. isolates tested. A few isolates showed weak reactivity with CP5 antiserum in a colony blot assay, and an enzyme-linked immunosorbent assay inhibition method confirmed that the levels of capsule produced by these strains were <10% of those produced by control strains. Fifty isolates that failed to react with capsular antisera all possessed the genes for production of capsular polysaccharide type 5 or 8. These results underscore the variability in capsule production by bovine isolates of S. aureus from different geographic regions. This information is important for the rational design of a capsule-based vaccine to prevent S. aureus bovine mastitis.

Capsule expression by bovine isolates of Staphylococcus aureus from Argentina: genetic and epidemiologic analyses

Staphylococcus aureus is an important cause of bovine mastitis worldwide, and effective preventive or therapeutic modalities are lacking. Although most human S. aureus isolates produce capsular polysaccharides (CPs), few reports have described the prevalence of capsules on bovine isolates. This information is important for the rational design of a vaccine for the prevention of staphylococcal mastitis. We serotyped 195 S. aureus strains isolated between 1989 and 1997 from the milk of mastitic cows in Argentina. Only 14 (7.1%) of the strains were serotype 5, and all were recovered between 1989 and 1992. Thirteen serotype 8 strains were identified, and 12 of these were isolated between 1991 and 1994. The remaining 168 isolates were nonreactive (NR) with CP serotype 5 (CP5)- or CP8-specific antibodies. Hybridization studies performed with genomic DNA from eight NR strains revealed that only three of them carried the capsule genes. Pulsed-field gel electrophoresis (PFGE) performed with 127 of the 195 S. aureus isolates revealed that most (86%) strains belonged to one of four major PFGE groups. Although 8 of 14 CP5 isolates showed a common PFGE pattern (arbitrarily defined as A1), 31 other A1 isolates from the same time period (1989 to 1992) were not CP5 positive. In contrast, only nine PFGE type B3 isolates were recovered between 1990 and 1994, and eight of these were positive for CP8 (P < 0.0003). The results of this study underscore the variability in capsule expression by S. aureus strains isolated from different geographical regions and cast doubt on the roles of CP5 and CP8 in the pathogenesis and immunoprophylaxis of bovine mastitis in Argentina.

The cps genes of Streptococcus suis serotypes 1, 2, and 9: development of rapid serotype-specific PCR assays

We developed three type-specific PCR assays for the rapid and sensitive detection of Streptococcus suis serotype 1 (plus 14), serotype 2 (plus 1/2), and serotype 9 strains in tonsillar specimens from pigs. The PCR primers were based on the sequences of type-specific capsular genes of S. suis serotype 1, 2, and 9 strains. We recently characterized a major part of the capsular biosynthesis (cps) locus of S. suis serotype 2. Here we extended these studies and characterized major parts of the cps loci of S. suis serotypes 1 and 9. Type-specific genes were identified by cross-hybridization experiments between the individual cps genes and chromosomal DNAs from the 35 different serotypes. Four genes of S. suis serotype 1 specifically hybridized with serotype 1 and 14 strains only. Five genes of S. suis serotype 2 specifically hybridized with serotype 2 and 1/2 strains only, and two genes of S. suis serotype 9 specifically hybridized with serotype 9 strains. Until now rapid and sensitive diagnostic tests were available only for pathogenic strains of serotype 2 and highly pathogenic strains of serotype 1. The serotype-specific PCR assays can therefore be useful tools for the identification of serotype 1, 14, 2, 1/2, and 9 strains both for diagnostic purposes and in epidemiological and transmission studies. Therefore, these tests may facilitate control and eradication programs.

Promoter analysis of the cap8 operon, involved in type 8 capsular polysaccharide production in Staphylococcus aureus

The production of type 8 capsular polysaccharide (CP8) in Staphylococcus aureus is regulated in response to a variety of environmental factors. The cap8 genes required for the CP8 production in strain Becker are transcribed as a single large transcript by a primary promoter located within a 0.45-kb region upstream of the first gene of the cap8 gene cluster. In this study, we analyzed the primary cap8 promoter region in detail. We determined the transcription initiation site of the primary transcript by primer extension and identified the potential promoter sequences. We found several inverted and direct repeats upstream of the promoter. Deletion analysis and site-directed mutagenesis showed that a 10-bp inverted repeat of one of the repeats was required for promoter activity. We showed that the distance but not the specific sequences between the inverted repeat and the promoter was critical to the promoter activity. However, insertion of a DNA sequence with two or four helix turns in this intervening region had a slight effect on promoter activity. To demonstrate the biological significance of the 10-bp inverted repeat, we constructed a strain with a mutation in the repeat in the S. aureus Becker chromosome and showed that the repeat affected CP8 production mostly at the transcriptional level. By gel mobility shift assay, we demonstrated that strain Becker produced at least one protein capable of specific binding to the 10-bp inverted repeat, indicating that the repeat serves as a positive regulatory protein binding site. In addition, reporter gene fusion analysis showed that the cap8 promoter activity was influenced by various growth media and affected most by yeast extract. Our results suggest that yeast extract may exert its profound inhibitory effect on cap8 gene expression through the 10-bp inverted repeat element.

Genetic analysis of the cap5 locus of Staphylococcus aureus

Staphylococcus aureus expresses at least eight distinct serotypes of capsular polysaccharide (CP). Gene clusters involved in the expression of serotypes 1, 5 and 8 have been cloned and sequenced. In this report we describe the isolation and analysis of serotype 5 capsular polysaccharide-defective mutants. A naturally occurring cap mutation in the laboratory strains 8325-4 and RN4220 was mapped to the cap5E gene by genetic complementation. The cap5H-K genes were shown to be responsible for CP5 serotype specificity by transduction and complementation.

Molecular analysis of the capsule gene region of group A Streptococcus: the hasAB genes are sufficient for capsule expression

Enzymes directing the biosynthesis of the group A streptococcal hyaluronic acid capsule are encoded in the hasABC gene cluster. Inactivation of hasC, encoding UDP-glucose pyrophosphorylase in the heavily encapsulated group A streptococcal strain 87-282, had no effect on capsule production, indicating that hasC is not required for hyaluronic acid synthesis and that an alternative source of UDP-glucose is available for capsule production. Nucleotide sequence and deletion mutation analysis of the 5.5 kb of DNA upstream of hasA revealed that this region is not required for capsule expression. Many (10 of 23) group A streptococcal strains were found to contain insertion element IS1239' approximately 50 nucleotides upstream of the -35 site of the hasA promoter. The presence of IS1239' upstream of hasA did not prevent capsule expression. These results elucidate the molecular architecture of the group A streptococcal chromosomal region upstream of the has operon, indicate that hasABC are the sole components of the capsule gene cluster, and demonstrate that hasAB are sufficient to direct capsule synthesis in group A streptococci.

Identification of a gene essential for O-acetylation of the Staphylococcus aureus type 5 capsular polysaccharide

The Staphylococcus aureus serotype 5 capsular polysaccharide (CP5) has a trisaccharide repeating unit of (-->4)-3-O-Ac-beta-D-ManNAcAp-(1-->4)-alpha-L-FucNAcp-(1-->3 )-beta-D-FucNAcp-(1-->). Tn918 mutagenesis of strain Reynolds yielded a mutant that produced wild-type levels of O-deacetylated CP5. The site and orientation of the single transposon insertion in mutant JL232 were determined by analysis of Southern blots and amplification of DNA flanking the transposon. DNA sequencing revealed that Tn918 was inserted within an open reading frame of 627 bp. The predicted amino acid sequence encodes a protein of approximately 26 kDa with homology to members of the NodL-LacA-CysE family of bacterial acetyltransferases. Southern blot analysis showed that genes similar to cap5H were present only in strains of S. aureus belonging to capsular serotypes 2, 4 and 5. In an in vitro assay, the parental strain was more resistant to opsonophagocytic killing than the mutant strain. In a mouse model of staphylococcal infection, the parental strain was able to seed the bloodstream from the peritoneal cavity and colonize the kidneys more efficiently than the O-deacetylated mutant. When cap5H was provided to the mutant in trans, it fully restored CP5 O-acetylation. The virulence of the complemented mutant strain closely approximated that of the parental strain.

Staphylococcus aureus cap5O and cap5P genes functionally complement mutations affecting enterobacterial common-antigen biosynthesis in Escherichia coli

The Staphylococcus aureus cap5P and cap5O genes of the type 5 capsule biosynthetic locus restore enterobacterial common-antigen expression to Escherichia coli mutants defective in rffE and rffD gene expression, respectively. Cap5P and Cap5O likely function as UDP-GlcNAc 2-epimerase and UDP-ManNAc dehydrogenase enzymes, respectively, in the synthesis of the capsule precursor UDP-ManNAcA.

The Staphylococcus aureus allelic genetic loci for serotype 5 and 8 capsule expression contain the type-specific genes flanked by common genes

The nucleotide sequences of two gene clusters, cap5 and cap8, involved in the synthesis of Staphylococcus aureus type 5 and type 8 capsular polysaccharides (CPs), respectively were determined. Each gene cluster contained 16 ORFs, which were named cap5A through cap5P for type 5 CP and cap8A through cap8P for type 8 CP. The cap5 and cap8 loci were allelic and were mapped to the SmaI-G fragment in the standard SmaI map of Staph. aureus strain NCTC 8325. The predicted gene products of cap5A through cap5G and cap5L through cap5P are essentially identical to those of cap8A through cap8G and cap8L through cap8P, respectively, with very few amino acid substitutions. Four ORFs located in the central region of each locus are type-specific. A comparison of the predicted amino acid sequences of cap5 and cap8 with sequences found in the databases allowed tentative assignment of functions to 15 of the 16 ORFs. The majority of the capsule genes are likely to be involved in amino sugar synthesis; the remainder are likely to be involved in sugar transfer, capsule chain-length regulation, polymerization and transport.

Molecular characterization and transcriptional analysis of type 8 capsule genes in Staphylococcus aureus

A 20.5-kb contiguous DNA fragment from Staphylococcus aureus Becker affecting type 8 capsule (CP8) biosynthesis was previously cloned. Sequencing analysis indicated that 16 open reading frames (ORFs) encoded within this fragment might be involved in CP8 synthesis. Using various plasmids containing DNA inserts derived from the 20.5-kb region, we showed by complementation of chemical mutants that 8 of the 16 ORFs were required for CP8 synthesis. To determine the involvement of the remaining eight ORFs, nonpolar gene-specific chromosomal mutations located in each of these ORFs were constructed. We found that three additional ORFs were also involved in the CP8 synthesis. Thus, 11 of the 16 ORFs were shown to affect CP8 synthesis. Complementation analyses of these 11 type 8 capsule (cap8) genes affecting CP8 production showed several promoters within the cap8 gene cluster. However, by Northern hybridization using either the entire cap8 gene cluster or the internal fragments of individual ORFs as probes, one 17-kb cap8-specific transcript was detected. Using xylE as the reporter gene, we found that the promoter at the beginning of the cap8 operon was much stronger than any of the internal promoters. These results suggest that the cap8 genes are transcribed mainly as a single large transcript. In addition, Southern hybridization analyses showed that cap8H, cap8I, cap8J, and cap8K, located in the central region of the cap8 gene cluster, were CP8 specific.

The biochemistry and genetics of capsular polysaccharide production in bacteria

Bacterial polysaccharides are usually associated with the outer surface of the bacterium. They can form an amorphous layer of extracellular polysaccharide (EPS) surrounding the cell that may be further organized into a distinct structure termed a capsule. Additional polysaccharide molecules such as lipopolysaccharide (LPS) or lipooligosaccharide (LOS) may also decorate the cell surface. Polysaccharide capsules may mediate a number of biological processes, including invasive infections of human beings. Discussed here are the genetics and biochemistry of selected bacterial capsular polysaccharides and the basis of capsule diversity but not the genetics and biochemistry of LPS biosynthesis (for reviews see 100, 140).

Cloning of type 8 capsule genes and analysis of gene clusters for the production of different capsular polysaccharides in Staphylococcus aureus

Eleven serotypes of capsular polysaccharide from Staphylococcus aureus have been reported. We have previously cloned a cluster of type 1 capsule (cap1) genes responsible for type 1 capsular polysaccharide biosynthesis in S. aureus M. To clone the type 8 capsule (cap8) genes, a plasmid library of type 8 strain Becker was screened with a labelled DNA fragment containing the cap1 genes under low-stringency conditions. One recombinant plasmid containing a 14-kb insert was chosen for further study and found to complement 14 of the 18 type 8 capsule-negative (Cap8-) mutants used in the study. Additional library screening, subcloning, and complementation experiments showed that all of the 18 Cap8- mutants were complemented by DNA fragments derived from a 20.5-kb contiguous region of the Becker chromosome. The mutants were mapped into six complementation groups, indicating that the cap8 genes are clustered. By Southern hybridization analyses under high-stringency conditions, we found that DNA fragments containing the cap8 gene cluster show extensive homology with all 17 strains tested, including type 1 strains. By further Southern analyses and cloning of the cap8-related homolog from strain M, we show that strain M carries an additional capsule gene cluster different from the cap1 gene cluster. In addition, by using DNA fragments containing different regions of the cap8 gene cluster as probes to hybridize DNA from different strains, we found that the central region of the cap8 gene cluster hybridizes only to DNAs from certain strains tested whereas the flanking regions hybridize to DNAs of all strains tested. Thus, the cap8 gene clusters and its closely related homologs are likely to have organizations similar to those of the encapsulation genes of other bacterial systems.

Association of staphylococcal type-1 capsule-encoding genes with a discrete genetic element

Previous studies have shown that 13 genes located in a 14.6-kb region of the chromosome of Staphylococcus aureus (Sa) M are required for type-1 capsular polysaccharide (CP1) biosynthesis. In this report, a total of 17 Sa strains producing different CP serotypes were analyzed by Southern hybridization using DNA probes from the cap1 coding region and the flanking sequences. The results showed that the sequence encoding cap1 genes was specific to CP1-producing strains. In addition, DNA regions of at least 18 kb flanking the cap1 genes were absent in most of the non-type-1 strains. These data suggest that the cap1 genes are associated with a chromosomally located discrete genetic element. One end of the element, referred to as the cap1 element, is located in a 1.7-kb fragment about 11.1 kb upstream from the first gene of the cap1 locus and the other end is located in a 0.8-kb region about 7.6 kb downstream from the last gene of the cap1 locus. Thus, the size of the cap1 element is between 33.3 and 35.8 kb.