A superinfection is generally defined as a second infection superimposed on an earlier one, especially by a different microbial agent of exogenous or endogenous origin, that is resistant to the treatment being used against the first infection. Examples of this in bacteriology are the overgrowth of endogenous Clostridium difficile which occurs following treatment with a broad-spectrum antibiotic, and pneumonia or septicemia from Pseudomonas aeruginosa in some immuno-compromised patients.
In virology, the definition is slightly different. Superinfection is the process by which a cell that has previously been infected by one virus gets co-infected with a different strain of the virus, or another virus, at a later point in time. Viral superinfections may be resistant to the antiviral drug or drugs that were being used to treat the original infection. Viral superinfections may also be less susceptible to the host's immune response.
Superinfection immunity in Lambda phages
When a cell is undergoing the lysogenic cycle and infected with a lambda phage (making it a lambda lysogen), other lambda phages that infect it are not able to undergo lytic development and produce progeny. The incoming phage can inject its DNA into the cell, but the DNA is immediately repressed and no transcription of genes or translation of phage proteins initiates. Therefore, lambda lysogens are immune to infection by other lambda phage particles. This occurs because the lambda lysogen is continuously producing cI repressor proteins to the point where the amount of cI proteins in the cell exceeds the amount needed to inhibit the replication of more than one phage. These repressor proteins bind to the superinfecting phage DNA operators to block transcription of the phage's genes by the cell and viral polymerase enzymes.
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