WHO study on subtyping Listeria monocytogenes: results of phage-typing.

A multi-centered study on phage typing of Listeria monocytogenes was carried out using 80 cultures sent under code and tested in six different laboratories. Phage typing was performed using an international phage set in five laboratories and phage sets unique to two laboratories. Testing of cultures sent in duplicate showed similar levels of reproducibility to that previously reported. Analysis of results from groups of epidemiologically related cultures showed a high level of agreement in all laboratories. Patterns of phage susceptibility were relatively stable on retesting strains in the same laboratory after long periods of time. However, there was limited comparability between results obtained from testing the same cultures using the same phages in different laboratories. It is recommended that the phages in the international set be reviewed, and that better inter-laboratory reproducibility may be achieved by standardisation of phage suspensions, propagation strains and methodology, together with the use of centrally propagated phages.