Mycobacteriophage

A mycobacteriophage is a member of a group of bacteriophages known to have mycobacteria as host bacterial species. While originally isolated from the bacterial species Mycobacterium smegmatis and Mycobacterium tuberculosis, the causative agent of tuberculosis, more than 4,200 mycobacteriophage have since been isolated from various environmental and clinical sources. Almost 600 have been completely sequenced. Mycobacteriophages have served as examples of viral lysogeny and of the divergent morphology and genetic arrangement characteristic of many phage types.

All mycobacteriophages found thus far have had double-stranded DNA genomes and have been classified by their structure and appearance into siphoviridae or myoviridae.

A bacteriophage found to infect Mycobacterium smegmatis in 1947 was the first documented example of a mycobacteriophage. It was found in cultures of the bacteria originally growing in moist compost. The first bacteriophage that infects M. tuberculosis was discovered in 1954.

Thousands of mycobacteriophage have been isolated using a single host strain, Mycobacterium smegmatis mc2155, over 600 of which have been completely sequenced. These are mostly from environmental samples, but mycobacteriophages have also been isolated from stool samples of tuberculosis patients, although these have yet to be sequenced. About 30 distinct types (called clusters, or singletons if they have no relatives) that share little nucleotide sequence similarity have been identified. Many of the clusters span sufficient diversity that the genomes warrant division into subclusters (Figure 1).

There is also considerable range in overall guanine plus cytosine content (GC%), from 50.3% to 70%, with an average of 64% (M. smegmatis is 67.3%). Thus, phage GC% does not necessarily match that of its host, and the consequent mismatch of codon usage profiles does not appear to be detrimental. Because new mycobacteriophages lacking extensive DNA similarity with the extant collection are still being discovered, and as there are at least seven singletons for which no relatives have been isolated, we clearly have yet to saturate the diversity of this particular population.

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