Principles and Practice of Clinical Bacteriology Second Edition - Free

14 β-HAEMOLYTIC STREPTOCOCCI
isolates of ST-17 are serotype III clones, and this ST apparently
defines a homogenous clone that had a strong association with
neonatal invasive infections. The variation in serotype with a single
ST and the presence of genetically diverse isolates with the same
serotype suggested that the capsular biosynthesis genes of GBS are
subjected to relatively frequent horizontal gene transfer, as observed
with S. pneumoniae (Coffey et al. 1998; Jones et al. 2003). A single
gene therefore confers serotype specificity in GBS of capsular types
III and Ia (Chaffin et al. 2000), and recombinational replacement of
this gene with that from an isolate of a different type results in a
change of capsular type. Therefore, confirmation of serotype identity
is possible when a DNA sequence-based serotyping method is
used, such as the one described recently by Kong et al. (2003). They
used three sets of markers – the capsular polysaccharide synthesis
(cps) gene cluster, surface protein antigens and mobile genetic
elements. The use of three sets of markers resulted in a highly
discriminatory typing scheme for GBS as it provides useful phenotypic
data, including antigenic composition, thus important for
epidemiological surveillance studies especially in relation to potential
GBS vaccine use. Further studies are needed on the distribution of
these mobile genetic elements and their associations with virulence
and pathogenesis of GBS disease.
International Quality Assurance Programmes for GAS
Characterisation
The various reported additions to the classical serotyping methods for
these organisms have increased the likelihood of differences both in
the determination of previously unknown types and in the confirmation
of previously unrecognised serotypes. Although international reference
laboratories have informally exchanged strains and confirmed the
identification of unique isolates for many years, the ‘new’ molecular
techniques have made precise confirmation and agreement even more
important to clinical and epidemiological laboratory research. The
first international quality assurance programme was established
amongst six major international streptococcal reference centres (two
in United States and one each in United Kingdom, Canada, New
Zealand and Czech Republic). Five distributions were made, and both
traditional and genotypic methods were used. The correlation of
results between centres was excellent, albeit with a few differences
noted. This continuing self-evaluation demonstrated the importance of
comparability, verification, standardisation and agreement of methods
amongst reference centres in identifying GAS M and emm types. The
results from this programme also demonstrate and emphasise the
importance of precisely defined criteria for validating recognised
and accepted types of GAS (Efstratiou et al. 2000). There have been
more recent distributions amongst European and other typing centres
(20 centres), within the remit of a pan-European programme ‘Severe
Streptococcus pyogenes infections in Europe’ (Schalén 2002).
ANTIMICROBIAL RESISTANCE
β-Haemolytic streptococci of Lancefield groups A, B, C and G remain
exquisitely sensitive to penicillin and other β-lactams, but resistance
to sulphonamides and tetracyclines has been recognised since the
Second World War. Penicillin has, therefore, always been the drug of
choice for most infections caused by β-haemolytic streptococci, but
macrolide compounds, including erythromycin, clarithromycin,
roxithromycin and azithromycin, have been good alternatives in
penicillin-allergic patients.
β-Lactam Resistance
In striking contrast to erythromycin no increase in penicillin resistance
has been observed in vitro among clinical isolates of β-haemolytic
streptococci, in particular GAS. Penicillin tolerance, however,
amongst GAS, defined as an increased mean bactericidal concentration
(MBC) : minimum inhibitory concentration (MIC) ratio (usually >16),
has been reported at a higher frequency in patients who were clinical
treatment failures than in those successfully treated for pharyngitis
(Holm 2000). The presence of β-lactamase-producing bacteria in the
throat has been assumed to be another significant factor in the high
failure rate of penicillin V therapy. Although many β-lactamaseproducing
bacteria are not pathogenic to this particular niche, they can
act as indirect pathogens through their capacity to inactivate penicillin
V administered to eliminate β-haemolytic streptococci.
Macrolide Resistance
Macrolide resistance amongst GAS has increased between the 1990s
and early 2000s in many countries (Seppala et al. 1992; Cornaglia et al.
1998; Garcia-Rey et al. 2002). The resistance is caused by two
different mechanisms: target-site modification (Weisblum 1985) and
active drug efflux (Sutcliffe, Tait-Kamradt and Wondrack 1996).
Target-site modification is mediated by a methylase enzyme that
reduces binding of macrolides, lincosamides and streptogramin B antibiotics
(MLS B resistance) to their target site in the bacterial ribosome,
giving rise to the constitutive (CR) and inducible (IR) MLS B resistance
phenotypes (Weisblum 1985). Another phenotype called noninducible
(NI) conferring low-level resistance to erythromycin (MIC