The proportion of S. aureus infections that are methicillinresistant
in hospitals in the UK has risen alarmingly by
nearly 20-fold in the second half of the 1990s to just under
40% [9]. The spread of MRSA is known to be clonal in the
UK; there are two main types, EMRSA-15 and EMRSA-
16, first identified by phage typing [10]. These account
for more than 95% of all MRSA isolated. The integrity of
the types has been verified by genotyping. Six strains of
S. aureus have been sequenced and there is conservation
in 78% of the genome with considerable diversity in the
remaining 22%; however the main epidemic MRSA
strains do show some common characteristics in that they
all carry the seg and sei enterotoxin genes but, unlike less
common MRSA, do not carry the lukE leucocidin and splB
serine protease genes [10]. The use of methicillin resistance
as a marker is misleading; S. aureus acquired resistance
to this penicillin in the 1960s. It was the acquisition
of aminoglycoside resistance with methicillin resistance
that generated treatment problems; however the bacteria
could still be treated with glycopeptides. In the mid
1990s, Hiramatsu identified strains of MRSA isolated
in Japan that had intermediate susceptibility to vancomycin,
with minimum inhibitory concentrations (MICs)
of 8mg/L [11]. Hiramatsu also demonstrated that strains
from the same area that were vancomycin susceptible
could become intermediate at relative high frequency
[12]. Further investigation revealed that the bacteria had
increased their cell wall production and now had a much
thicker cell wall than the parent strains, which provided
some barrier to vancomycin [13]. Infections caused by
these strains, now known as VISA (vancomycin-intermediate
Staphylococcus aureus) could still be treated. An
in vitro experiment 10 years ago demonstrated that the
much more efficient vancomycin resistance mechanism of
the vanA operon from VRE could transfer and survive in
MRSA [14]; fortunately this was not found in clinical
strains. The situation changed in 2002 when the first
MRSA strain harbouring a vanA operon was found in a
patient in Michigan [15]. This was followed by reports
of a strain in Pennsylvania [16,17] and later in New York.
Courvalin recently described at the 14th European Congress
of Clinical Microbiology and Infectious Diseases
(ECCMID) in Prague that the vanA operon is very similar
to that in VRE but that the mobile element on which it is
located is not stable in S. aureus. To survive, Tn1546 had
to have sufficient opportunity to ‘jump’ into a more stable
replicon in S. aureus. The rarity of reports of vancomycinresistant
S. aureus strains around the world, despite the
pressure that there must be to select them, suggests that
this operon has yet to find a sufficiently stable genetic
environment in MRSA, but surely it will.
De