How do capsules help the bacterium

Future studies will be required to uncover how this different scale of host range may determine not only the ecology but also the genetic cargo of MGE.

Funding: The author s received no specific funding for this work. Download: PPT. Fig 1. Capsule trading boosts the acquisition of new mobile DNA. References 1. In: Jann K, Jann B, editors.

Bacterial Capsules. Berlin, Heidelberg: Springer Berlin Heidelberg; Mobile genetic elements associated with antimicrobial resistance. Clin Microbiol Rev. American Society for Microbiology. Appl Environ Microbiol. Modular prophage interactions driven by capsule serotype select for capsule loss under phage predation.

ISME J. Interplay between the cell envelope and mobile genetic elements shapes gene flow in populations of the nosocomial pathogen Klebsiella pneumoniae.

PLoS Biol. View Article Google Scholar 6. Both the chronic and subclinical forms generally remain undiagnosed until activated by a traumatic event or a decrease in immunocompetence [ 25 ]. At the time our studies were initiated some cell-associated antigens had been identified and characterized in B. The EPS produced by B. The role of EPS in virulence was not known, but sera from patients with melioidosis had been shown to contain antibodies against EPS [ 30 ].

Two other EPS structures were also identified; a branched 1,4-linked glucan polymer CP-1a and a triple-branched heptasaccharide repeating unit composed of rhamnose, mannose, galactose, glucose, and glucuronic acid CP-2 [ 33 ]. The genes involved in the synthesis of these capsules, and the role of these capsules in virulence had not been identified. The LPS of B. Based on biochemical, immunological, and genetic data, B.

However, these two organisms differ in a number of ways and have been classified into two different species [ 38 ]. The rRNA sequence of B.

The biochemical profiles of these two species differ in that B. The most distinct difference between these two species, however, is their relative virulence. It has also been shown that the two species can be differentiated based on their propensity to cause disease in humans.

Environmental strains isolated in Thailand that are able to assimilate L-arabinose are not associated with human infection, whereas clinical isolates are not able to utilize L-arabinose [ 41 ]. To identify the genetic determinants that confer enhanced virulence in B. Subtractive hybridization was carried out between the virulent B. The genomic DNA sample from B. Tester and driver DNAs were digested and subjected to two rounds of hybridization. The remaining unhybridized sequences were considered tester-specific sequences.

To enrich for tester-specific sequences, excess driver DNA was added in the hybridizations. Screening of the subtraction library revealed a number of DNA sequences unique to B. Fifteen distinct plasmid inserts from the library were sequenced. One of the plasmid inserts, pDD, was found to share limited homology with WbpX, a glycosyltransferase, from Pseudomonas aeruginosa [ 43 ].

The resulting plasmid, pSR, was mobilized into wildtype B. Since the insert from pDD was found to demonstrate homology to a glycosyltransferase from P. Since three carbohydrate structures had been previously purified and characterized, antibodies to each of these polysaccharides were available. Immunogold electron microscopy studies using rabbit polyclonal sera specific for a type I O-PS—flagellin conjugate was performed on the parent strain, b, and SR Figure 1.

Unlike B. Western blot analysis of proteinase K-digested whole cells from B. These results indicated that we had identified and insertionally inactivated a gene involved in the synthesis of the type I O-PS of B.

SR was tested for virulence in the Syrian hamster model of acute septicemic melioidosis. The LD 50 for SR after 48 h was 3. This demonstrated that SR is severely attenuated for virulence in this animal model of melioidosis and that type I O-PS is a major virulence determinant of B. Immunogold electron microscopy of B. Bacteria were reacted with polyclonal rabbit antiserum directed against an O-PS—flagellin protein conjugate absorbed with B. Original magnification x, Two methods were used to clone the genes involved in the production and export of type I O-PS.

We also used transposon mutagenesis to clone the genes involved in production of the polysaccharide; this was done to obtain any unlinked genes that may be involved in polysaccharide production.

Six mutants were identified, and the DNA flanking the transposon insertion was cloned and sequenced. Sequence analysis of the cloned fragments revealed the presence of 26 potential open reading frames involved in the synthesis and export of type I O-PS [ 42 ]. The open reading frames that predicted proteins involved in polysaccharide biosynthesis were found to demonstrate homology to proteins involved in the synthesis of a polysaccharide structure composed primarily of mannose.

The other reading frames in the locus predicted proteins involved in the transport of capsular polysaccharides in a variety of bacteria, particularly those that produce group 2 and group 3 capsular polysaccharides [ 8 ].

The genes responsible for the production of type I O-PS were found to be similar to other loci encoding for capsular polysaccharides in that they are divergently transcribed [ 4 ]. The gene cluster involved in the production of this polysaccharide is also similar to group 3 capsule gene clusters in that there are no genes encoding KpsF and KpsU, which are present in group 2 capsule gene clusters [ 8 ].

However, the organization of the B. The biosynthetic genes identified are not organized into one continuous transcriptional unit; instead, wcbB , manC , and wcbP are separated from the rest of the biosynthetic genes. The genes involved in the production of this polysaccharide were named according to the bacterial polysaccharide gene nomenclature scheme [ 47 ].

The gene products associated with this cluster are shown in Figure 4. Mutations constructed in a number of these genes confirmed their role in the production of this polysaccharide [ 42 ]. However, our results suggested that this polysaccharide was a capsule rather than an O-PS moiety. The genes involved in the production of this capsule demonstrated strong homology to the genes involved in the production of capsular polysaccharides in many organisms, including N.

In addition, the export genes associated with this cluster are not associated with the previously characterized O-PS gene cluster [ 36 ]. Western blot analysis of proteinase K cell extracts and silver staining demonstrated that this polysaccharide has a high molecular mass kDa and lacks the banding pattern seen with O-PS moieties. Studies by our laboratory indicated that mutants in the production of the core oligosaccharide of the LPS are still capable of producing this polysaccharide [ 48 ].

Based on the above criteria and the genetic similarity to group 3 capsules, we proposed that this polysaccharide is a group 3 capsule and designated this capsule CPS I. This conclusion was further supported by Isshiki et al who separated this polysaccharide from a smooth lipopolysaccharide preparation of B.

These experiments were facilitated by constructing a deletion strain harbouring a mutation in one of the CPS I genes and by complementation of this strain.

An in-frame deletion was constructed in wcbB , a gene which encodes a glycosyltransferase, resulting in the capsule-minus strain SZ To confirm the role of wcbB in the biosynthesis of capsule, SZ was complemented by the introduction of a wild-type copy of the wcbB gene cloned into the mobilizable broad-host-range plasmid pBHR1 MoBiTec. Western blot analysis of proteinase K-digested whole cells was performed using mouse monoclonal antibody directed to B.

Similar to the capsule minus strain SR and B. Complementation of SZ by providing the wild type wcbB gene in trans restored capsule production. Whole-cell extracts from the complemented strain SZ pSZ reacted to the capsule antibody producing the kDa band corresponding to the B.

Out of the 55 clinical strains tested for capsule production, 52 were found to produce this capsule. Three strains, a, c, and a were found to be negative for capsule production, similar to B.

However, one of the capsule genes, wzt2 , was successfully amplified from these three strains and following inoculation in the animal model, all three of these strains were found to produce capsule by western blot analysis. This indicated that CPS I production may be regulated in some strains and its expression may be induced in vivo. Therefore all of the 55 clinical strains of B.

Syrian golden hamsters were inoculated intraperitoneally with 10 1 to 10 5 cells of either wild type B. After 48 h, the LD 50 values were calculated, and the blood of the infected animals was diluted and plated for bacterial quantitation. The addition of purified capsule significantly increased the virulence of the capsule mutant strain SR In contrast, the LD 50 value for SR without the addition of purified capsule was calculated to be 3.

In addition, purified capsule enhanced the survival of SR in the blood. Bacteria could not be detected in the blood of hamsters inoculated with SR alone.

This number was comparable to the number of wild-type B. The LD 50 value for the capsule mutant strain SZ containing an in frame deletion of the wcbB gene was calculated to be 9.

Complementation of this strain restored virulence in the animal model, resulting in an LD 50 value of 12 CFU, comparable to that of wild type B. So, why would some researchers bother differentiating between fimbriae and pili?

Pili are typically longer than fimbriae, with only present on each cell, but that hardly seems enough to set the two structures apart. It really boils down to the fact that a few specific pili participate in functions beyond attachment. The conjugative pili participate in the process known as conjugation , which allows for the transfer of a small piece of DNA from a donor cell to a recipient cell.

The type IV pili play a role in an unusual type of motility known as twitching motility , where a pilus attaches to a solid surface and then contracts, pulling the bacterium forward in a jerky motion.

Bacterial motility is typically provided by structures known as flagella. The bacterial flagellum differs in composition, structure, and mechanics from the eukaryotic flagellum, which operates as a flexible whip-like tail utilizing microtubules that are powered by ATP. The bacterial flagellum is rigid in nature, operates more like the propeller on a boat, and is powered by energy from the proton motive force. Bacterial movement typically involves the use of flagella, although there are a few other possibilities as well such as the use of type IV pili for twitching motility.

But certainly the most common type of bacterial movement is swimming , which is accomplished with the use of a flagellum or flagella. Rotation of the flagellar basal body occurs due to the proton motive force, where protons that accumulate on the outside of the cell membrane are driven through pores in the Mot proteins, interacting with charges in the ring proteins as they pass across the membrane.

The interaction causes the basal body to rotate and turns the filament extending from the cell. Rotation can occur in a clockwise CW or a counterclockwise CCW direction, with different results to the cell. Some spiral-shaped bacteria, known as the Spirochetes , utilize a corkscrew-motility due to their unusual morphology and flagellar conformation. These gram negative bacteria have specialized flagella that attach to one end of the cell, extend back through the periplasm and then attach to the other end of the cell.

When these endoflagella rotate they put torsion on the entire cell, resulting in a flexing motion that is particularly effective for burrowing through viscous liquids. Gliding motility is just like it sounds, a slower and more graceful movement than the other forms covered so far.

Gliding motility is exhibited by certain filamentous or bacillus bacteria and does not require the use of flagella. It does require that the cells be in contact with a solid surface, although more than one mechanism has been identified.

Some cells rely on slime propulsion, where secreted slime propels the cell forward, where other cells rely on surface layer proteins to pull the cell forward. In light microscopy, capsules appear to be amorphous gelatinous areas surrounding the cell. Capsule is located immediately exterior to the murein peptidoglycan layer of gram-positive bacteria and the outer membrane Lipopolysaccharide layer of gram-negative bacteria.

In electron microscopy, capsule appears like a mesh or network of fine strands. Most bacterial capsules are composed of polysaccharides i. These polymers are composed of repeating oligosaccharide units of two to four monosaccharides. Capsules composed of single kinds of sugars are termed homopolysaccharides.

For example, the capsule of Streptococcus mutans is made up of glucose polymers. If several kinds of sugars are present in a capsule, then it is called heteropolysaccharides , eg. The capsule of Bacillus anthracis is an exception. This polypeptide capsule is composed of polymerized D-glutamic acid.

The sugar components of polysaccharides vary within the species of bacteria, which determines their serologic types. Example: Streptococcus pneumoniae has 84 different serotypes discovered so far. Slime is a loose network of polymers extending outward from a cell whereas capsule is a dense and well-defined polymer layer surrounding the cell.

Both capsules and slime layers are important for the adherence of microorganisms and subsequent colonization but they differ in some of the properties. Capsules are anti-phagocytic. They limit the ability of phagocytes to engulf the bacteria. The smooth nature and negative charge of the capsule prevent the phagocyte from adhering to and engulfing the bacterial cell.

Polysaccharide capsule is the major virulence factor for Streptococcus pneumonia.