The concept of typing staphylococci with bacteriophages
was first developed in the 1940s. It was
noticed that some S. aureus contained temperate
phages which lysed other bacteria of the same
species. Those with a narrow host range could be
used in groups to produce a pattern of lysis characteristic
for individual strains.
Uniquely amongst staphylococcal typing systems
phage typing was standardized by the International
Subcommittee on Phage Typing of Staphylococci.11
An approved set of phages was recommended
which, with minor modifications, has been used
world-wide ever since. Currently 23 standard
phages are applied to an agar plate covered with the
test organism. There is also room for two locally
selected phages, which can be useful if indigenous
isolates are unreactive to the International Set.12
Areas of lysis (plaques) caused by each phage are
noted as either a strong or a weak reaction. The former,
defined as more than 50 plaques, is used to distinguish
between isolates. A lower number of
plaques is a weak reaction which is recorded but
should not influence the final phage type.11 If a
staphylococcus shows no lysis at the Routine Test
Dilution (RTD), then a concentration 100 times
greater (RTD X 100) is employed.11 Unfortunately
standardization of the phages used has not led to
uniformity of practice and interpretation. A multicentre
study showed that a variety of different media
was used and the method was often modified to fit in
with local conditions.12 Many laboratories routinely
tested at RTD X 100 in parallel with phages at RTD
and some did not use the latter concentration at all.
Furthermore, the relative influence of weak and
strong reactions on the results was inconsistent.
For many years, phage typing was the method of
choice for the investigation of MRSA epidemiology.
Several outbreaks have been defined with this technique
and its discriminatory power is demonstrably
greater than phenotypic tests such as capsular typing
and zymotyping.13–16 Despite its popularity, the
problems inherent in phage typing have never been
adequately resolved. It is a time consuming and
technically demanding procedure which is most efficiently
done on large batches. This and the necessity
for keeping stocks of phages and the propagating
strains, has confined it to larger laboratories and reference
facilities.
When used in an outbreak situation, the main disadvantage
of phage typing is the high proportion of
isolates which are non-typable. Typically this
reaches 20–30% for collections of MRSA7,16–18 but in
some cases has been as high as 75%.19 The value of
any information obtained is markedly reduced as
there is no way of knowing whether the non-typable
bacteria are related. Indeed, isolates non-typable by
phages but linked initially by epidemiological