Principles and Practice of Clinical Bacteriology Second Edition - Free

4 β-HAEMOLYTIC STREPTOCOCCI
Table 1.2
Cell-surface molecules
Putative virulence factors of group A streptococcus (GAS)
M-protein family (Emm, Mrp, Enn)
Fibronectin-binding proteins
(multiple)
C5a peptidase
Hyaluronic acid capsule
Excreted molecules
Streptolysin O/streptolysin S
Streptokinase
Superantigens (multiple)
Protein Sic
SpeB (cysteine protease)
literature in this area. For a more encompassing view, readers are
referred to the many excellent reviews covering the role of putative
virulence factors identified in GAS (Cunningham 2000; Nizet 2002;
Bisno, Brito and Collins 2003; Collin and Olsén 2003). Throughout
the ensuing section the reader should bear in mind that the repertoire
of putative virulence factors is now known to vary between strains and
that even when the same factors are present there may be extensive
genotypic and phenotypic diversity. It is thus increasingly apparent
that GAS pathogenesis involves a complex and interacting array of
molecules and that the fine details may vary between different strains.
The best-studied virulence factors are listed in Table 1.2, though this table
is far from exhaustive as there are numerous less-well-characterised
molecules that could be involved in pathogenesis.
Cell-Associated Molecules
Major function(s)/role
Resistance to phagocytosis
Multiple ligand-binding capacities
Adhesion, invasion
Inhibits leukocyte recruitment
Resistance to phagocytosis
Adherence
Probable function/role
Haemolytic toxins
Lysis of blood clots and tissue spread
Nonspecific immune stimulation
Inhibitor of complement
Generates biologically active
molecules
Inflammation, shock and tissue
damage
The M protein has long been considered the major cell-surface protein
of GAS. M protein was originally defined as an antiphagocytic protein
that determines the serotype of a strain and evokes serotype-specific
antibody that promotes phagocytosis by neutrophils in fresh human
blood. Despite extensive antigenic variation all M proteins have an
α-helical coiled–coil conformation and possess an array of properties
that may be important in their antiphagocytic role. The best-characterised
activities are binding of factor H, a regulatory component of complement
control system, and binding of fibrinogen that diminishes alternate
complement pathway-mediated binding of C3b to bacterial cells,
thus impeding recognition by polymorpholeukocytes (PMLs). The
M-protein-encoding gene (emm) is now known to belong to a family
of so-called M-like genes linked in a cluster that appear to have
evolved by gene duplication and intergenic recombination (Whatmore
and Kehoe 1994). A variety of genomic arrangements of this family
has been identified, the simplest of which consists of only an emm
gene flanked by a regulatory gene mga (multiple gene activator) and
scpA (see Virulence Gene Regulators). However, many isolates contain
an upstream gene (usually designated mrp) and/or a downstream gene
(usually designated enn), and several members of the M-like gene
family have been found to bind a whole array of moieties including
various immunoglobulin A (IgA) and IgG subclasses, albumin,
plasminogen and the factor H-like protein C4bBP. In recent years
it has become clear that resistance to phagocytosis is not solely
dependent on the M protein itself. For example, inactivation of emm49
has little impact on resistance to phagocytosis, with mrp49, emm49 and
enn49 all appearing to be required (Podbielski et al. 1996; Ji et al. 1998).
The capacity of some strains (notably M18) to resist phagocytosis
is virtually entirely dependent on the extracellular nonantigenic
hyaluronic acid capsule (Wessels and Bronze 1994). Many clinical
isolates, particularly those epidemiologically associated with severe
disease or RF, produce large mucoid capsules. These may act to mask
streptococcal antigens or may act as a physical barrier preventing
access of phagocytes to opsonic complement proteins at the bacterial
cell surface, as well as being involved in adherence.
In addition to the antiphagocytic molecules described above many
other cellular components have postulated roles in pathogenesis. All
GAS harbour scpA mentioned above that encodes a C5a peptidase.
This protein destroys C5a complement protein that is a chemotactic
signal that initially attracts neutrophils to sites of infection. GAS have
been shown to possess an array of fibronectin-binding proteins,
indicating the importance of interactions with this molecule.
Fibronectin is a Principal glycoprotein in plasma, body fluids and the
extracellular matrix. Perhaps the best-characterised GAS fibronectinbinding
protein is protein F (SfbI) that has roles in adherence and
invasion (see Adherence, Colonisation and Internalisation). However,
an ever-expanding array of other molecules has been shown to
possess this activity, including proteins such as SfbII, SOF, proteinF2,
M3, SDH, FBP-51, PFBP, Fba and glyceraldehyde-3-phosphate
dehydrogenase.
With the recent completion of genome sequencing projects many
novel cell-surface proteins have been identified. Most remain to be
fully characterised, but their surface location facilitates potential roles
in host interaction. Examples include Slr, a leucine-rich repeat protein,
isogenic mutants of which are less virulent in an intraperitoneal mouse
model and more susceptible to phagocytosis by human PMLs (Reid
et al. 2003). Two novel collagen-like proteins, SclA and SclB, have
also been identified. Their function is unclear, but they may be
important in adherence and are of potential interest in the pathogenesis
of autoimmune sequelae (Lukomski et al. 2000; Rasmussen, Eden and
Bjorck 2000; Whatmore 2001). A further potential virulence factor
identified from the genome sequence is protein GRAB, present in most
GAS and possessing high-affinity binding capacity for the dominant
proteinase inhibitor of human plasma α2-macroglobulin. Again, mutants
are attenuated in mice and it has been suggested that α2-macroglobulin
bound to the bacterial surface via protein GRAB entraps and inhibits
the activity of GAS and host proteinases, thereby protecting other
virulence determinants from proteolytic degradation (Rasmussen,
Muller and Bjorck 1999).
Excreted Products
Most GAS produce two distinct haemolysins, streptolysin O (SLO)
and streptolysin S (SLS). SLO is an oxygen-labile member of the
cholesterol-binding thiol-activated toxin family that elicits antibodies
useful for documenting recent exposure and is toxic to a variety of
cells. SLS belongs to the bacteriocin family of microbial toxins and is
not immunogenic in natural infection but shares similar toxic activities to
SLO. Streptokinase facilitates the spread of organisms by promoting
the lysis of blood clots. Streptokinase binds to mammalian plasminogen,
and this complex then converts other plasminogen molecules to the
serum protease plasmin that subsequently acts on a variety of proteins
including fibrin. GAS have cell-surface receptors capable of binding
plasminogen, which following conversion to plasmin in the presence
of streptokinase may generate cell-associated proteolytic enzyme capable
of causing tissue destruction.
GAS produce an array of superantigens. These protein exotoxins
have the ability to trigger excessive and aberrant activation of T cells
by bypassing conventional major histocompatibility complex (MHC)-
restricted antigen processing (Llewelyn and Cohen 2002). Subsequent
excessive and uncoordinated release of inflammatory cytokines such as
tumour necrosis factor-α (TNF-α), interleukin-2 (IL-2) and interferon-γ
(IFN-γ) results in many of the symptoms consistent with toxic shock