Bacterial strain

A bacterial strain is a maintained lineage of bacteria descended from a particular isolation in pure culture. It is more specific than a species name. The naming code describes descent from an isolation and subsequent cultures; it does not require every cell forever to retain an identical genome.^[ref]

A concrete example is Pm70, the avian isolate used for the published Pasteurella multocida genome. Naming both the species and the strain identifies the particular material behind a sequence or assay. The 2001 genome paper reported 2,014 predicted coding sequences for Pm70, not a measurement of every member of the species.^[ref]

For an illustrative experiment, compare a parent strain with a derivative in which one candidate gene has been removed, keeping culture conditions comparable. A change in the measured phenotype tests a claim about that derivative under those conditions. It does not make the two cultures different species, nor establish that all other strains would respond identically.

A useful laboratory record therefore connects the strain designation, source culture and any deliberate genetic changes. Otherwise two reports can use the same species name while testing materially different biological preparations.

Bacterial host range describes the hosts in which a specified bacterial strain can establish a defined biological interaction, such as sustained colonization or disease. The endpoint must be stated: detecting the bacterium, observing growth and measuring host damage are different observations. In plant-pathogen classification, a pathovar summarizes disease-response characteristics in one or more hosts.^[ref]

Pseudomonas syringae pv. tomato DC3000 provides a concrete example of a label that does not enumerate the complete range. Its genome paper identifies both tomato and Arabidopsis thaliana as susceptible hosts.^[ref] The pathovar word tomato therefore cannot be interpreted as an exclusive host list.

For a hypothetical three-host panel, strain X gives the chosen response in hosts A and B, while strain Y does so in hosts A and C. Both have two positive hosts, yet their tested ranges differ; counting positives discards which hosts they were. An untested host D remains unclassified by this panel. Even a negative observation is conditional on the readout and setting.

A 2011 experiment illustrates that context: prior colonization by some Sphingomonas strains reduced DC3000 growth and disease symptoms in Arabidopsis thaliana.^[ref] A changed disease outcome in one host need not imply a change in host identity or a newly established species-wide host boundary.

A pathovar is an infrasubspecific grouping of bacterial strains distinguished by disease responses in one or more hosts. The criterion concerns pathogenic characteristics and bacterial host range, rather than simply the collection from which a strain came. The 2022 revision of the prokaryotic naming code lists pathovar in its advice on infrasubspecific subdivisions; Rule14a places designations below subspecies outside the code's principal Rules and Recommendations.^[ref]

The name Pseudomonas syringae pv. tomato DC3000 shows the levels of information. Pseudomonas syringae is the species designation, pv. tomato supplies the pathovar designation and DC3000 specifies the bacterial strain. The published DC3000 genome study describes disease on both tomato and Arabidopsis thaliana.^[ref] The word tomato in the name therefore does not mean that only tomato can be affected.

A bacterial host range observation is bounded by the hosts and conditions tested. If two strains share a species name but produce different responses across a host panel, the panel can reveal a pathogenic distinction. It does not by itself establish that the strains are genetically identical within each group, nor determine responses in every untested host. A pathovar designation summarizes a biological pattern while the individual strain record preserves which material was actually measured.