Posts Tagged ‘ transduction ’

Evolution of Bacterial Virulence

Point Mutations

Much like humans, bacteria are subject to evolution. Evolution occurs through beneficial mutations in the DNA. Because bacteria have much shorter generation times than humans, the process of evolution occurs much more rapidly. An example of how bacteria evolve through mutation is point mutation.

Point mutation is the random mutation of DNA nucleotides which occurs due to incorrect replication of DNA. The types of mutation that can occur are substitution of a nucleotide, deletion of a nucleotide or the addition of a nucleotide. For example, in a sequence of DNA a fragment of which codes for something (such as a protein) consists of multiple codons (sections of 3 nucleotides) if one of these nucleotides is altered in a point mutation, the codon (which codes for a specific amino acid) will be altered. Some alterations of codons will mean they encode for a different amino acid:

Initial DNA:                  … … TTA … …

After point mutation: … … TAA … …

In this case the selected codon in the initial DNA encodes for the amino acid leucine, however after the mutation, it no longer encodes for an amino acid at all as TAA is a stop codon (this closes the reading frame, meaning the rest of the gene is not read).

Point mutations may sometimes have no effect however, as multiple codons can encode for the same amino acid, for example:

Initial DNA:                  … … GCT … …

After point mutation: … … GCA … …

Both GCT and GCA codons encode for the amino acid alanine, so there are no alterations to the final protein which is encoded for by this gene.

Horizontal Gene Transfer

Bacteria are also able to ‘evolve’ by means other than mutation. Horizontal gene transfer is the sharing of genetic information between individuals in the population. Typical gene transfer occurs vertically i.e. parent to offspring, however with horizontal gene transfer genetic information can be spread to completely unrelated individuals.

There are three primary forms of horizontal gene transfer, these are:

  • Transformation – DNA is taken up from the environment
  • Conjugation – DNA is transferred directly between individual bacteria
  • Transduction – DNA transfer is mediated by bacteriophages (a virus which infects bacteria)

Transformation

Many bacteria are naturally transformable and have various means of DNA uptake from their environment. Bacteria able to do this have genes which encode for protein machinery which transports the DNA across the cell membrane from the environment. It is then possible for this DNA to be incorporated into the bacterial chromosome.

This ability has important implications in bacterial pathogenicity for example; Neisseria gonorrhea are able to uptake DNA from the environment which enables them to undergo possible conformational changes that make them more pathogenic. Another example would be Streptococcus pneumoniae and its possibility to acquire penicillin resistance.

Conjugation

Conjugation is the direct transfer of DNA between bacteria, this is of major advantage as advantageous genes (such as those which encode for antibiotic resistance) can be easily spread throughout the population.

DNA is transferred in the form of a plasmid (a circular DNA structure) in a unidirectional manner, that is only the donor requires the plasmid transfer-associated genes (i.e. those which encode for plasmid transfer mediating pili) for horizontal transfer via conjugation to occur.

Plasmids

Plasmids are typically circular in structure and consist of double stranded DNA. They replicate independently of the bacterial chromosome and are located in the cytoplasm, they are usually much smaller than the bacterial chromosome ranging from 1-100kb long. The genes found within the plasmid can be expressed by the bacterium and so proteins encoded for by the plasmid can be formed in the bacterium.

The most common plasmids are those which encode for antibiotic resistance. They are highly prevalent because of the great advantage they give to bacterial survivability. The are multiple genes found on plasmids which promote antibiotic resistance all with different mechanisms of producing resistance.

For example β-lactam antibiotics (such as penicillin derivatives) work by inhibiting cell wall synthesis, however β-lactamase is a bacterial enzyme which breaks the ring structure of these antibiotics are removes their antibacterial properties. These enzyme is encoded for by genes found within plasmids, which can easily be spread around the population thus increasing the resistance of the population to β-lactam antibiotics.

Other forms of plasmid-conferred attributes include:

  • Bacteriocins – Proteinaceous toxins which inhibit the growth of competitor species a specific example of this is colicin, a bacteriocin produced by Escherichia coli which exerts a cytotoxic effect within competitor species’ cytoplasm.
  • Toxin production – Toxins which affect other species such as the heat-labile enterotoxin produced by E. coli which causes diarrhoea or the Clostridium tetani tetanus toxin, the neurotoxin responsible for causing tetanus.
  • Protein delivery systems – Such as the Yersinia pestis secretion system, which allows them to inject protein materials into other organisms (such as immune cells), these injected proteins are known as YOPs (Yersinia outer proteins).
  • Adhesins – Typically pili (protein surface projections) which allow adherence to bind to cells, which is important in pathogenicity. Such adhesins can be found in E. coli and Shigella.
  • Metabolic activities – For example, the ability of Salmonella species to ferment lactose

Transduction

Transduction is the transfer of DNA which is mediated by bacteriophages (viruses which infect bacteria, combining their DNA with the bacterium host). Many bacteriophages have both lytic and lysogenic cycles of reproduction (typically viruses are one or the other). The lytic cycle is where the virus enters the cell, replicates using the cell’s own machinery and then lyses the cell releases large numbers of virus. The lysogenic cycle is where the virus incorporates its genetic material into the cell where it lies dormant until a later date, the viral genes are replicated when the cell replicates allowing them to reactivate and continue into a lytic cycle (formation of phages).

Bacteriophage-encoded virulence factors include:

  • Streptococcus pyogenes toxin (which causes scarlet fever)
  • Corynebacterium diphtheriae toxin
  • Clostridium botulinum toxin

E. coli O157:H7 contains around 1.4Mb of extra DNA when compared to E. coli K12 (a much less pathogenic strain which is used in laboratories). It is suggested that around 50% of this extra genetic material has been derived from bacteriophages.

Vibrio cholera Toxin

Another bacteriophage encoded virulence factor is the Vibrio cholera toxin, however for this toxin to be produced by the bacterium, genetic material from two bacteriophages must be acquired.

The initial bacteriophage (VPI phage) infects a non-toxic Vibrio cholera bacterium, its transduced genetic material encodes for pili proteins which act as an attachment site for the secondary bacteriophage (CTX phage). When CTX phage binds, the genetic material it incorporates encodes for the production of the cholera toxin and thus the bacterium becomes toxogenic.

Recombination

Transformed and transduced (i.e. non-plasmid DNA) is inserted into the recipient bacterial chromosome by recombination. Recombination is also responsible for the rearrangement of DNA fragments around the genome (genomic plasticity) which enables the bacteria to survive in rapidly changing environments due to the alterations in the genes which are expressed. Recombination can be either homologous (RecA dependent – a DNA repair and maintenance protein) or non-homologous.

Homologous Recombination

DNA inserted into the bacterium can be incorporated into the bacterial chromosome if there are regions of homology expressed on both the foreign transmitted DNA and the bacterial chromosome DNA. The differences between the foreign and bacterial DNA will cause RecA to attempt to ‘repair’ the chromosome. In doing so, the foreign DNA may be incorporated into the bacterial chromosome, (there is also the chance that it will be disregarded).

Non-homologous Recombination

Non-homologous recombination allows genetic material to enter the bacterial chromosome without RecA. Small fragments of DNA known as insertion sequences (ISs) are what enter the chromosome. ISs typically consist of DNA fragments from bacteriophages or elsewhere, however, all that an IS codes for is its motility and insertion. ISs integrate into the bacterial chromosome through self-encoded transposition enzymes (as opposed to RecA) and when within the chromosome they facilitate replication of themselves. The short ISs then become numerous within the bacterial chromosome.

The presence of multiple ISs within the bacterial chromosome is what allows genetic plasticity (The movement or deletion of short fragments of DNA within the chromosome). Genetic plasticity leads to alterations in genes or the removal of genes altogether, this has the potential to increase pathogenicity. For example, Bordetella bronchiseptica is a gram negative bacterium responsible for causing infectious bronchitis, which rarely infects humans. B. pertussis on the hand is the major causative agent of pertussis (whooping cough) and is an obligate human pathogen. It is believed that genetic plasticity caused a reshuffling of the B. bronchiseptica chromosome which lead to the formation of the obligate human pathogen B. pertussis.

Transposons

Transposons are transmitted by similar means to ISs, however they are more structurally complex and much larger, due to the greater amount of genetic information they carry. This extra genetic material encodes for attributes such as antibiotic resistance, toxins and enzymes.

Some transposons (known as conjugative transposons) excise themselves from the bacterial chromosome to form circular intermediates of DNA, much like plasmids. These can also be transmitted around a population, again like plasmids, to spread resistance to antibiotics etc.

Pathogenicity Islands

Pathogenetic islands are also laterally-transmitted. They are large integrated DNA elements which carry multiple virulence-associated genes such as those which encode for adherence factors, secretion systems, toxins and invasion factors. A bacterium may have multiple pathogenicity islands and due to the incorporation of multiple virulence factors on one island, the transmission of one island often causes a non-toxogenic bacterium to become pathogenic.

They may be incorporated into the main bacterial chromosome or any plasmids which may be present in the cytoplasm, the DNA found within these islands may contain remnants from bacteriophages and IS elements. The GC-content (guanine-cytosine content) is often different from the rest of the genome.

2011
01/09
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Infectious Diseases
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The Somatosensory System

Introduction

The somatosensory system comprises of ‘senses’ known as sensory modalities, these include; tactition (touch), temperature, proprioception (body position awareness) and nociception (pain). It is possible there are others, and these categories may be broken down further, for example kinaesthesia is the awareness of muscle strain/tension which is a form of nociception/proprioception.

Sensory receptors and sensory (afferent) neurones of the somatosensory system can be found from the periphery (such as the skin, muscles and organs) through to the deeper neurones of the central nervous system. Specific receptors are able to detect different stimuli; the stimulation of a receptor causes information to be sent along neurones to the corresponding area of the brain.

General Organisation of the Somatosensory System

A typical somatosensory pathway will begin with a sensory receptor (for example a mechanoreceptor which is able to detect stress/stretch in the skin – helping to form the tactile sensory modality). The stimulation of the receptor will cause information to be sent to the brain, where it will be perceived (in this example as touch). The information is sent to the brain through the spinal cord, typically three long neurones will facilitate this.

The cell body of the first neurone is located in the dorsal root ganglion of the corresponding spinal nerve. The second neuronal cell body is located in the midbrain for motor/touch sensory modalities and the spinal cord for pain sensory modalities. Neurones involved in pain sensory modalities travel to the thalamus, up the spinal cord via the spinothalamic tract.

It is at this point that the ascending neurones cross-over (decussate) to the opposite side of either the spinal cord or midbrain (depending on the sensory modality – above), typically upon entry of the structure of decussation. The axons of these neurones mainly terminate in the thalamus, but may also terminate in the reticular system or cerebellum of the brain.

In the case of touch and pain, the third neurone has its cell body located in the ventral posterior nucleus of the thalamus. The axon of this final neurone terminates in the postcentral gyrus (sometimes referred to as the somatosensory cortex) of the parietal lobe – where sensory information from different modalities is integrated.

Ascending Somatosensory Pathways

Information from sensory modalities is transmitted to the brain, via the spinal cord. These ascending neurones are able to take multiple pathways to reach their destination. These pathways can be split into three main routes.

Dorsal Column Pathway

The dorsal column pathway:

  • This pathway carries tactile and proprioception sensory modality information. Touch discrimination is owed to this pathway.
  • Sensory information arrives through the dorsal horn and is carried to the dorsal columns (which consist of the Gracile & Cuneate fasciculi)
  • The neurones synapse in the Gracile & Cuneate fasciculi of the medulla, where they decussate
  • The neurones terminate at the thalamus; they travel there along the medial lemniscus. The role of the medial lemniscus is simply to carry the neurones from the Gracile & Cuneate fasciculi of the medulla to the thalamus.

Ventrolateral Pathway

The ventrolateral pathway carries all sensory modalities (except proprioception) but is specifically involved in the propagation of pain. This pathway can be divided into two, as there are two possible tracts which the sensory modalities can take. These are the spinothalamic tract and the spinoreticular tract.

The Spinothalamic Tract

  • Nociceptors (pain receptors) detect a stimulus and neurones carry this to the spinal cord
  • These neurones head directly to the thalamus from the spinal cord, without synapsing elsewhere (via the medial lemniscus)
  • This pathway is associated with nociception such as that from thermal stimuli or from a pinprick

The Spinoreticular Tract

  • Follows the same pathway as the spinothalamic tract except the neurones synapse in the reticular formation of the medulla (primarily associated with the sleep/awake cycle)
  • From the reticular formation the neurones continue to the thalamus
  • This pathway is associated with ‘true pain’

Spinocerebellar Pathway

This pathway is associated with muscle and joint proprioceptors primarily, involving it in postural reflexes. Many neurones which travel via this pathway do not decussate, as is common in the other pathways.

After entering the spinal cord from an appropriate proprioceptor (or kinaesthesia receptor etc.), the neurones synapse in the dorsal horn and then head straight to the cerebellum.

Segmental Organisation

The spinal cord can be divided into sections by which part of the body it serves; cervical (head/ immediate upper body & arms), thoracic (trunk), lumbar (lower back/legs) and sacral (hind). Each of these sections is then made up of 5-12 nerve pairs each serving a smaller sub section of the body/skin; they send sensory information to the brain from their corresponding section.

  • Cervical Nerve Pairs – 8
  • Thoracic Nerve Pairs – 12
  • Lumbar Nerve Pairs – 5
  • Sacral Nerve Pairs – 5

This is significant diagnostically, because the deratomes (small section of skin served by a spinal nerve pair) are served by a specific spinal nerve pair. This means pain deriving from a deratome (or area of skin) if located, can be tracked back to its spinal nerve source.

For example, a human with pain in the skin of the abdomen (Thoracic nerve 12 [Th12]) could point out this pain to a doctor. The pain would be a symptom of possible damage to Th12 and further action could be taken.

Reflexes

Certain sensory modalities such as nociception provide information which needs to be responded to rapidly, using the example of nociception the information received may be that a hot object is causing tissue damage and requires the removal/release of the object quickly. This type of action is usually processed without involvement of the conscious brain and is known as a reflex.

Comparing a conscious response to reflex:

Conscious Response


Reflex Response


Reflexes offer the chance to act quickly by using local processing in the spinal cord – without the need for information to travel to the brain, thus saving time. However there is another type of reflex (sometimes called a long loop, compared to a simple reflex – short loop) called an inter-segmental reflex. This type of reflex looks more like a conscious response, yet the conscious brain is still not involved, so it is deemed a reflex. The processing is done in the brainstem or a separate spinal cord segment, the complete pathway is as follows:

An example of this type of reflex is the ‘Tonic-Neck’ reflex; the reorientation of the head (and thus neck) causes a reflex repositioning of the body and limbs to accommodate the new posture.

Receptors

So we have discussed the transmission of somatosensory signals, but what about their detection? As said earlier, receptors found all over the periphery of the body (e.g. skin, muscle, and organs) detect specific stimuli and transmit the information to the brain, but there are multiple types of receptors available to detect the different stimuli.

Mechanoreceptors

Two key attributes of a mechanoreceptor are the size of its receptive field and the speed at which its fibres adapt. The receptive field is important for discriminating from where a stimulus arises. A small receptive field has better discrimination than a larger one. Typically smooth skin has a small receptive field.

The speed at which fibres adapts concerns how quickly the receptors become desensitised to a stimulus. Rapidly adapting fibres will quickly become desensitised and stop generating action potentials to a stimulus (they may fire action potentials when the stimulus is stopped), whereas slow adapting fibres generally continue to fire action potentials during the length of exposure to the stimulus.

Meissner’s Corpuscle

Found in smooth skin, these mechanoreceptors have a small receptive field and rapidly adapting fibres. They are said to perceive fluttering stimuli.

Pacinian Corpuscle

Found deep within all types of skin, these mechanoreceptors have a large receptive field and rapidly adapting fibres. They are able to perceive vibrations.

Merkel Discs

Found in all types of skin, fairly shallow. These mechanoreceptors have small receptive fields and slow adapting fibres. They are able to perceive pressure.

Ruffini Corpuscle

Found deep within all types of skin, these mechanoreceptors have large receptive fields and slow adapting fibres. They are able to perceive stretching.

Free Nerve Ending

Common receptors for temperature and nociception, they are able to express different types of receptors; mechanical, thermal nociception and polymodal nociception (slow burning pain from chemicals, temperature etc.)

Stimulation of Receptors

The majority of somatosensory receptors are modified ion channels, which when stimulated allow the influx of ions and depolarisation which results in the generation of an action potential and its transduction.

Mechanoreceptors like those listed above; require some sort of mechanical stress e.g. stretching, to stimulate them. This causes the shape of the receptor to distort and opens the ion channel.

Chemoreceptors and thermoreceptors are stimulated by their corresponding stimulus, either directly or by the binding of the (chemical) stimulus to the receptor or a protein linked to the receptor. Again, stimulation leads to depolarisation and action potential generation.

Speed of Signal Transmission

The different receptors propagate their signals along nerve fibres which differ in the speed at which they transmit action potentials. Nerve fibres associated with pain are often much slower than those associated with touch. The different nerve fibres are classified as:

  • Aα – The fastest nerve fibre (72-120ms-1)
  • Aβ – Fast (36-72ms-1)
  • Aδ – Small, slow, myelinated fibres associated with nociception and temperature (4-36ms-1)
  • C – Small, very slow, unmyelinated fibres associated with nociception (0.4-2.0ms-1)

Pain

Pain Perception

Pain is the perception of nociception, until ‘pain’ reaches the cortex it is not pain, but nociception. It is believed that there is a ‘pain gate’ in the dorsal horn, the theory is that by preventing a ‘pain’ stimulus from passing through this gate you can prevent its perception as pain – making it a target for drugs.

Triple Response

The triple response is a phenomenon which occurs after inflammation, it results in pain – caused by irritant chemicals released after physical injury/damage. The chemicals are released onto the skin and free nerve endings. This causes nociception information to be sent to the spinal cord for processing, but the chemicals are also able to spread to other local nerves causing the release of more chemicals and more nociception transduction to the spinal cord. The overall effect is the spreading of inflammation and pain to a larger area than was originally damaged.

The Route of the Somatosensory System through the Brain

Upon reaching the brain, the majority of somatosensory information travels through the thalamus and continues further into the brain. From the thalamus, information head to the sensory cortex. Processing here allows the sensory modalities to be perceived.

2010
05/14
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Neuroscience
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