Irregular Bacterial Cells

Chlamydiae

Chlamydiae are a bacterial phylum and class whose members are obligate intracellular pathogens.

Learning Objectives

Discuss the evidence that supports Chlamydiae as a unique bacterial evolutionary group

Key Takeaways

Key Points

  • Chlamydiae replicate inside the host cells and are termed intracellular.
  • Most intracellular chlamydiae are located in an inclusion body or vacuole.
  • Chlamydiae is a unique bacterial evolutionary group that separated from other bacteria approximately a billion years ago. It falls into the clade Planctobacteria in the larger clade Gracilicutes.
  • Chlamydia infection is a common sexually transmitted infection (STI) in humans caused by the bacterium Chlamydia trachomatis.

Key Terms

  • chlamydiae: Chlamydiae is a bacterial phylum and class whose members are obligate intracellular pathogens.
  • inclusion body: Inclusion bodies are nuclear or cytoplasmic aggregates of stainable substances, usually proteins.

Chlamydiae are a bacterial phylum and class whose members are obligate intracellular pathogens. Many chlamydiae coexist in an asymptomatic state within specific hosts. It is widely believed that these hosts provide a natural reservoir for these species. All known chlamydiae only grow by infecting eukaryotic host cells. They are as small or smaller than many viruses.

Chlamydiae replicate inside the host cells and are termed intracellular. Most intracellular chlamydiae are located in an inclusion body or vacuole. Outside of cells they survive only as an extracellular infectious form. Chlamydiae can only grow where their host cells grow. Therefore, chlamydiae cannot be propagated in bacterial culture media in the clinical laboratory. Chlamydiae are most successfully isolated while still inside their host cell.

Chlamydiae is a unique bacterial evolutionary group that separated from other bacteria approximately a billion years ago. Cavalier-Smith has postulated that the Chlamydiae fall into the clade Planctobacteria in the larger clade Gracilicutes. The species from this group can be distinguished from all other bacteria by the presence of conserved indels in a number of proteins such as RNA polymerase alpha subunit, Gyrase B, Elongation factor-Tu and Elongation factor-P, and by large numbers of signature proteins that are uniquely present in different chlamydiae species. Reports have varied as to whether Chlamydiae is related to Planctomycetales or Spirochaetes. However, genome sequencing indicates that 11% of the genes in Candidatus Protochlamydia amoebophila UWE25 and 4% in Chlamydiaceae are most similar to chloroplast, plant, and cyanobacterial genes. Phylogeny and shared presence of conserved indels in proteins such as RNA polymerase Beta subunit and lysyl-tRNA synthetase indicate that Verrucomicrobia are the closest free-living relatives of these parasitic organisms.

There are three described species of chlamydiae that commonly infect humans:

1. Chlamydia trachomatis, which causes the eye-disease trachoma and the sexually transmitted infection chlamydia.

2. Chlamydia pneumoniae, which causes a form of pneumonia.

3. Chlamydia psittaci, which causes psittacosis.

Chlamydia infection is a common sexually transmitted infection (STI) in humans caused by the bacterium Chlamydia trachomatis. The term Chlamydia infection can also refer to infection caused by any species belonging to the bacterial family Chlamydiaceae. C. trachomatis is found only in humans. Chlamydia is a major cause of blindness today, especially in developing countries.

Risk factors include a history of chlamydial or other sexually transmitted infection, new or multiple sexual partners, and inconsistent condom use. C. trachomatis infection can be effectively cured with antibiotics once it is detected. Current guidelines recommend: azithromycin, doxycycline, erythromycin, or ofloxacin. Agents recommended for pregnant women include erythromycin or amoxicillin.

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Chlamydias bacteria group: Light microscope view of cells infected with chlamydiae as shown by the brown inclusion bodies.

Planctomycetes

Planctomycetes are a phylum of aquatic bacteria and are found in samples of brackish, marine, and fresh water.

Learning Objectives

Describe the characteristics associated with Planctomycetes

Key Takeaways

Key Points

  • In structure, the organisms of this group are ovoid and have a holdfast, called the stalk, at the non-reproductive end that helps them to attach to each other during budding.
  • The organisms belonging to this group have a glycoprotein rich in glutamate instead of murein in their cell wall.
  • The nuclear material in planctomycetes can sometimes be enclosed in a double membrane.

Key Terms

  • nucleoid: The irregularly-shaped region within a prokaryote cell where the genetic material is localized.
  • operon: A unit of genetic material that functions in a coordinated manner by means of an operator, a promoter, and structural genes that are transcribed together.
  • planctomycetes: A phylum of aquatic bacteria that are found in samples of brackish, and marine and fresh water.
  • budding: a form of asexual reproduction in which a new organism develops from an outgrowth or bud on another one

Planctomycetes are a phylum of aquatic bacteria. They are found in samples of brackish, marine, and fresh water. They reproduce by budding. In structure, the organisms of this group are ovoid and have a holdfast, called the stalk, at the non-reproductive end that helps them to attach to each other during budding.

The organisms belonging to this group lack murein in their cell wall. Murein is an important heteropolymer present in most bacterial cell walls that serves as a protective component in the cell wall skeleton. Instead, their walls are made up of glycoprotein rich in glutamate. Planctomycetes have internal structures that are more complex than typically expected in prokaryotes. While they do not have a nucleus in the eukaryotic sense, the nuclear material can sometimes be enclosed in a double membrane. In addition to this nucleoid, there are two other membrane-separated compartments; the pirellulosome or riboplasm, which contains the ribosome and related proteins, and the ribosome-free paryphoplasm.

Cavalier-Smith has postulated that the Planctomycetes are within the clade Planctobacteria in the larger clade Gracilicutes. RNA sequencing shows that the planctomycetes are related to the Verrucomicrobia and possibly the Chlamydiae. A number of essential pathways are not organized as operons, which is unusual for bacteria. A number of genes have been found (through sequence comparisons) that are similar to genes found in eukaryotes. One such example is a gene sequence (in Gemmata obscuriglobus) that was found to have significant homology to the integrin alpha-V, a protein that is important in transmembrane signal transduction in eukaryotes. The life cycle of many planctomycetes involves alternation between sessile cells and flagellated swarmer cells. The sessile cells bud to form the flagellated swarmer cells which swim for a while before settling down to attach and begin reproduction.

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Operon: In genetics, an operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter.

Verrucomicrobia

Verrucomicrobia is a recently described phylum of bacteria which is part of the PVC superphylum.

Learning Objectives

Describe the structure of Verrucomicrobia and its placement in the PVC superphylum

Key Takeaways

Key Points

  • The PVC group includes Chlamydiae, Lentisphaerae, Planctomycetes, Verrucomicrobia, Poribacteria and OP3.
  • Verrucomicrobia possess a compartmentalised cell plan with a condensed nucleoid and the ribosomes pirellulosome (enclosed by the intracytoplasmic membrane ) and paryphoplasm compartment between the intracytoplasmic membrane and cytoplasmic membrane.
  • Evidence suggests that verrucomicrobia are abundant within the environment, and important (especially to soil cultures ).

Key Terms

  • verrucomicrobia: Verrucomicrobia is a recently described phylum of bacteria which is part of the PVC superphylum and they possess a compartmentalised cell plan with a condensed nucleoid.

Verrucomicrobia is a recently described phylum of bacteria which is part of the PVC superphylum. The PVC group includes Chlamydiae, Lentisphaerae, Planctomycetes, Verrucomicrobia, Poribacteria and OP3. Support for this superphylum has been found by examining the RNA polymerase protein RpoB.

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RNA polymerase II: RNA polymerase II (also called RNAP II and Pol II) is an enzyme found in eukaryotic cells. It catalyzes the transcription of DNA to synthesize precursors of mRNA and most snRNA and microRNA.

RpoB is the gene that encodes the β subunit of bacterial RNA polymerase. This protein has a unique 3 amino acid insert in all sequenced Chlamydiae, Lentisphaerae and Verrucomicrobia species. In addition, a conserved protein of unknown function is present in all sequenced species from the phyla Chlamydiae, Lentisphaerae, Planctomycetes and Verrucomicrobia. This protein is absent in the Poribacteria. Study of additional proteins from this proposed superphylum suggests that the Poribacteria may be separate from this clade. The Planctomycetes may be basal to the Chlamydiae-Verrucomicrobia-Lentisphaerae clade.

Like the Planctomycetes species, Verrucomicrobia possess a compartmentalised cell plan with a condensed nucleoid and the ribosomes pirellulosome (enclosed by the intracytoplasmic membrane) and paryphoplasm compartment between the intracytoplasmic membrane and cytoplasmic membrane. Cavalier-Smith has postulated that the Verrucomicrobia belong in the clade Planctobacteria in the larger clade Gracilicutes. 16S rRNA data corroborate that view. In 2008, the whole genome of Methylacidiphilum infernorum (2.3 Mbp) was published. On the single circular chromosome, 2473 predicted proteins were found, 731 of which had no detectable homologs. These analyses also revealed many possible homologs with Proteobacteria.

Evidence suggests that verrucomicrobia are abundant within the environment, and are important especially to soil cultures. Verrucomicrobia have been isolated from fresh water, soil environments and human feces. A number of as-yet uncultivated species have been identified in association with eukaryotic hosts including extrusive explosive ectosymbionts of protists and endosymbionts of nematodes residing in their gametes. While verrucae is another name for the warts often found on the hands and feet, this phylum is so called not because it is a causative agent thereof, but because of its wart-like morphology.