Bacteriophage infection at the poles
Bacteriophages are among the smallest but most abundant organisms on earth. For most phages, the tail mediates the anchoring of the phage to generally abundant bacterial outer membrane proteins that serve as specific receptors for their substrates. For example, the receptor for the temperate phage λ is the Escherichia coli maltoporin receptor LamB, which functions in amylomaltose uptake. The establishment of a stable phage–host interaction relays signals that allow injection of DNA from the phage capsid through the tail and into the host, leaving the empty capsid (head) attached to the cell surface. Following phage λ DNA injection, a decision between the lytic or lysogenic pathways of bacteriophage λ is made. The poles (ends) of bacterial calls have specialized functions related to the mobilization of DNA and certain proteins. To monitor the infection of Escherichia coli cells by light microscopy, scientists developed procedures for the tagging of mature bacteriophages with quantum dots:
Surprisingly, most of the infecting phages were found attached to the bacterial poles. This was true for a number of temperate and virulent phages of E. coli that use widely different receptors and for phages infecting Yersinia pseudotuberculosis and Vibrio cholerae. The infecting phages colocalized with the polar protein marker IcsA–GFP. ManY, an E. coli protein that is required for phage λ DNA injection, was found to localize to the bacterial poles as well. Furthermore, labelling of λ DNA during infection revealed that it is injected and replicated at the polar region of infection. The evolutionary benefits that lead to this remarkable preference for polar infections may be related to λ’s developmental decision as well as to the function of poles in the ability of bacterial cells to communicate with their environment and in gene regulation.
By labelling different phages with quantum dots and following adsorption using a fluorescence microscope the researchers were able to investigate the initial steps of binding (adsorption) and phage DNA injection. The surprising results showed that at low multiplicities of infection, phages preferentially adsorb, inject and replicate their DNA at the bacterial poles. This spatial preference was independent of host proteins, ManY and Pel, required for phage λ DNA injection. The significance of the pole, the binding of the phage to the pole and its implications in lytic-lysogenic decision are discussed in the paper.
Bacteriophage infection is targeted to cellular poles. Molecular Microbiology 21 Apr 2008
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Tags: Bacteria, Biology, Biotechnology, Microbiology, Science, Virology
Dr. Cann, I’m unable to get full access to the article so I’m not sure which conclusions the authors came to in regards to the evolutionary significance of this finding. However, based on my limited working knowledge of lambda phage gene expression the observation that low m.o.i results in the lytic pathway is not surprising to me. Please correct me if I am wrong in my assumptions: Low m.o.i would mean that the host cell has large amounts of constitutively expressed proteases to degrade cII protein. cII protein would remain below the threshold level and the lytic pathway would dominate. The reverse would be true for high m.o.i. with decreased amounts of free protease available to cleave cII. Evolutionarily this would be beneficial; with low m.o.i the virus would need to replicate more to increase the number of infective particles. If this is true, it still does not answer the question as to why lytic infection is pole specific. If bacterial gene regulation is closely tied to the poles, is it possible that interactions between viral and host cell polar proteins can lead increase bacterial gene expression of viral proteases??? I hope this makes sense . . .Thanks!!
The authors conclude:
Why then do phages attach to the polar regions of the cell? It is possible that the polar region is the preferred place for DNA injection to take place, as uptake of DNA by naturally competent bacteria also occurs at the pole. The polar localization of ManY and FtsH supports this notion, as does the reported polar localization of FhuD. The identification of host components essential for DNA injection of other phages and the localization of these host components remains to be investigated. The nucleoid-free cytoplasm at the cell poles could also provide a space for rapid phage DNA replication following infection. Regardless of the evolutionary forces, the data presented here indicate that phages have evolved mechanisms to utilize the asymmetry that is present in Gram-negative bacteria.