Phage typing in virus

1. Phage typing
Phage typing is a phenotypic method that uses bacteriophages ("phages" for short) for
detecting and identifying single strains of bacteria.[1] Phages are viruses that infect bacteria
and may lead to bacterial cell lysis.[2] The bacterial strain is assigned a type based on its lysis
pattern.[3] Phage typing was used to trace the source of infectious outbreaks throughout the
1900s, but it has been replaced by genotypic methods such as whole genome sequencing for
epidemiological characterization.[1]
A culture of bacteria infected by bacteriophages, the "holes" are areas where the bacteria have been killed by the virus.
The culture is 10cm in diameter.
Principle

2. Phage typing is based on the specific binding of phages to antigens and receptors on the
surface of bacteria and the resulting bacterial lysis or lack thereof.[4] The binding process is
known as adsorption.[5] Once a phage adsorbs to the surface of a bacteria, it may undergo
either the lytic cycle or the lysogenic cycle.[6]
Virulent phages enter the lytic cycle where they replicate and lyse the bacterial cell.[7] Virulent
phages can differentiate between different species of bacteria based on their specific lytic
action.[8] Lysis will only occur if the virulent phage adsorbs to the bacterial surface,
configuring species specificity to phages.[5]
Temperate phages enter the lysogenic cycle and do not immediately lyse the cell.[7] The
phage is instead integrated into the bacterial genome as a prophage during lysogenization,
which protects the cell from being lysed by phages which are serologically identical or
related.[9] Since it is incorporated into the genome, the prophage is also passed down to the
bacteria's progenies.[7]
The bacterial strain carrying the prophage is known as a lysogenic
strain.[9] Lysogenization is strain-specific, so it allows for differentiation among different
strains of bacteria within the same species.[10] The prophage may be chemically or physically
induced to revert to the lytic pathway.[6]
The bacterial strain to be characterized is cultured on an agar Petri dish and dried.[11]
Once
dry, a grid or another recognizable pattern is drawn on the base to mark out different
regions.[11]
Each region is inoculated with a different phage at its routine test dilution and
then incubated for 5-48 hours.[11]
The susceptible phage regions will display a clearing where
the bacteria have been lysed, and this is used in differentiation.[12]
The size, morphology, and
pattern of the lysed region are important criteria for differentiating bacterial species and
strains.[13] They are compared against a standard scheme of lysis patterns to assign a type
to the strain.[14]
Routine Test Dilution (RTD)
Routine test dilution (RTD) is typically defined as the lowest phage dilution that still yields
lysis of its host.[11]
This technique prevents a phenomenon known as "lysis from without",
which is bacterial lysis induced by high multiplicity phage adsorption rather than phage
replication.[15]
Method
History

3. The first reported use of bacteriophages to identify bacteria was in 1925 when Sonnenschein
used typhoid and paratyphoid phages to diagnose typhoid.[16] In 1934, it was discovered that
some strains of Salmonella typhi displayed Vi antigens on the surface.[17] This led to the
isolation of Vi phages capable of lysing typhoid bacteria strains but only if they displayed the
Vi antigen[18] enabling the differentiation of typhoid species expressing the Vi antigen and
those which do not.
In 1938, Craigie and Yen adapted Vi phages by selective propagation and used them at their
critical test dilutions to differentiate 11 types of B. typhosus.[19]
In 1943, Felix and Callow
extended the method to Salmonella paratyphi B. in 1943 and differentiated 12 types with 11
phages.[20]
The International Committee for Enteric Phage Typing was established in 1947,
and these phage typing methods were soon standardized.[21]
Improvements to the specificity of phage typing schemes were made throughout the next
few decades. In 1959, Callow improved her initial scheme to differentiate 34 types of
Salmonella typhimurium with 29 phages.[22] In 1977, this was extended to 207 types by
Anderson at the Enteric Reference Laboratory in London.[22] Since then, phage typing
schemes have been developed for Salmonella typhi, Salmonella paratyphi B., Salmonella
typhimurium, Shigella sonnei, Staphylococcus aureus, and Escherichia coli to name a few.[23][24]
Phage typing requires the use of a comprehensive number of phages, so it is typically only
used in reference laboratories.[25]
It also relies on the interpretation of the individual lysis
pattern and comparison to a standard which has led to conflicting results from different
laboratories in the past.[26] Furthermore, bacteriophages mutate so reference phages must
be maintained.[25]
Limitations
Phage Isolation
References

4. Last edited 8 months ago by AAhap36
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