Download Virulence factors

Streptococcus and Enterococcus :
They member of family streptococcacea. Catalase and
cytochrome enzyme (oxidase) differentiated them from other
micrococal and staphylococci. Elongated cocci (more than
spherical) arranged in chains when grown in broth.
Facultative anaerobic, fastidious (require special condition
and enriched media) and some spp are capnophilic (require
CO2 for growth).
Genus Streptococci
Classification schemes:
Four commonly used classification schemes are:
i. Hemolytic pattern on SBA.
There are 3 type of hemolytic pattern:
1. Alpha α-hemolysis greenish discoloration around the
colony (AH).
2. Beta hemolysis clear zone around the colony(BH).
3. Non haemolytic (NH).
ii.
iii.
iv.
Serological grouping of C CHO (Lancefield
classification) &capsular polysaccharide. Lancefield
classification depend on extraction of C CHO from the
cell by heating or acid. The Lancefield gp seen in
human infection (A,B,C,D,F,G,N).
The antigen detected in Lancefield grouping include
either cell wall polysaccharide (C CHO) (A,B,C,F & G
human groups) or on lipotiechoic acid in group D &
Enterococci. Viridians istreptococc do not posses any
of the recognized Lancefield grouping antigen.
Physiological characteristics.
Biochemical characteristics.
I.
Group A B hemolytic streptococci (S. pyogenes):
Virulence factors :
1. M- protein: is the best defined virulence factor. There are
more than 80 serotype of M protein . it play a role in
resistance to phagocytosis and adherence of mo to mucosal
cells.
2. Fibronectin binding proteins (FBPs): these proteins play a
role in adherence of mo to keratinocyte.
3. Capsule: which is composed of hyaluronic acid it is
indistinguishable from the ground substance of connective
tissue which explain it is lack of immunogensity.
4. Streptolysin O SLO (O2 labile and antigenic ) &
Streptolysin S SLS (O2 stable and non antigenic).
5. Streptococcal pyrogenic exotoxin (SPE A , B & C)
responsible for the rash of scarlet fever and pathogensity in
streptococcal toxic shock syndrome, SPE A & C act as
superantigens that cause hypotension and shock.
6. Other immunological active substances are: a.
hyaluronidase deploymerizes connective tissue b.
streptokinase hydrolyse fibrin clot c. deoxy ribonucleases
(DNAase used in debridement) d. SIC protein that inhibit
complements.
Clinical significant :
Human is natural reservoir of S. pyogenes transmitted from
person to person.
1. streptococcal pharyngitis is most common infection
caused by S. pyogenes children (5-15 years).
2. Pyodermal infection including
a. Impetigo: local infection of superficial layers of skin,
especially in children. It consists of superficial vesicles
that break down and eroded areas whose denuded surface
is covered with pus and later is encrusted. It spreads by
continuity and is highly communicable.
b. Erysiplus : results with massive brawny edema and a
rapidly advancing margin of infection.
c. Cellulitis: Streptococcal cellulitis is an acute, rapidly
spreading infection of the skin and subcutaneous tissues.
3.
Scarlet fever which is red spreading rash caused by SPE
A. it commonly affects children. Signs and symptoms include
sore throat, fever, and a characteristic red rash.
4.
Necrotizing fasciitis This is infection of the subcutaneous
tissues and fascia. There is extensive and very rapidly spreading
necrosis of the skin and subcutaneous tissues. it is called
“flesh-eating bacteria.”
5.
Streptococcal toxic shock syndrome
resemble
staphylococcal toxic shock syndrome.
6.
Post streptococcal diseases :
a. Rheumatic fever (after pharyngitis) This is the most
serious sequela of S pyogenes because it results in damage
to heart muscle and valves. Certain strains of group A
streptococci contain cell membrane antigens that crossreact with human heart tissue antigens. Sera from patients
with rheumatic fever contain antibodies to these antigens.
Typical symptoms and signs of rheumatic fever include
fever, malaise, a migratory nonsuppurative polyarthritis,
and evidence of inflammation of all parts of the heart
(endocardium, myocardium, pericardium).
a. Acute glamerulonephritis ( after skin infection and
pharyngitis) . Glomerulonephritis may be initiated by
antigen-antibody complexes on the glomerular basement
membrane. In acute nephritis, there is blood and protein in
the urine, edema, high blood pressure, and urea nitrogen
retention; serum complement levels are also low. A few
patients die; some develop chronic glomerulonephritis
with ultimate kidney failureand the majority recover
completely.
Treatment of streptococcal infection
pencillin is the drug of choice erythromycin is alternative in
allergic patients. Routine susceptibility testing for BH is not
required (sensitive to penicillin).
II. Group B streptococci (S. agalactiae):
Virulence factor :
the capsule is important virulence of group B it prevent
phagocytosis.
Clinical infection: S. agalactiae cause invasive disease in
newborn . two clinical syndrome:
1. Early onset appear(less than 7 days old) 80% of clinical
case of new born in the form of bactermia , pneumonia and
meningitis.
2. Late onset infection occur between 1 week- 3month
mainly in the form of meningitis.
The treatment of choice is ampicillin +aminoglycoside.
III. Viridans streptococci:
1. They are constituents of the normal flora of the URT ,
female genital tract and GIT. (viridans mean green).
2. Many spp are α hemolytic and few spp are non hemolytic
and very rarely beta hemolytic.
3. They do not posses any recognized Lancefield grouping
antigen lack of C CHO.
4. Virulence factors include polysaccharide capsule and
adhesions implicated in adherence and colonization in
endocarditis.
5. Clinical infection :
a. It is the most common cause of subacute bacterial
endocarditis, bacterimia, gingivitis and dental caries,
abscess formation.
6. Viridians are general susceptible to penicillin, over the last 2
decade there is increase resistance to penicillin,
cephalosporin and aminoglycoside. Accurate susceptibility
to penicillin is required to viridians when isolated from
serious infection.
IV. Streptococcus pneumonia:
Virulence factor:
1. Capsular polysaccharide is main virulence factor by which
the mo resist opsonization and phagocytosis. There at least
90 capsular type ,23 of these type account for over 88% of
pneumoncoccal meningitis and bacterimia.
2. Several toxin produced include hemolysin, IgA1 proteases,
neuraminidase , hyaluronidases, pneumolysin, teichoic
acid and lipotiechoic acid.
Clinical infection:
1. It causes pneumonia (2ry pneumonia), otitis media
(recurrent infection in children under 3years of age)
,sinusitis , meningitis 2ry to otitis media affect all age
group and bactermia.
Antimicrobial resistance:
Over the last three decades S pneumoniae have become
increasingly resistant to penicillin and generally treat with
erythromycin.
Genus Enterococcus:
1. These mo are normal residents of GIT and in lower
number of the vagina and male urethra.
2. E. feacalis is the most common isolate being associated
with 80-90% of human enterococcal infection. E. faecium
ranks second and is isolated from 10-15% of infection.
3. It is the second most common cause of nosocomial UTI
and wound infection and the third most common cause of
bacterimia. Unlike streptococci ,enterococci have the
ability to grow under extreme conditions (the presence of
6.5% NaCl, the presence of bile or at 45C or alkaline PH).
4. The ability of enterococci to hydrolyse esculin
differentiated it from non hemolytic or alpha hemolytic
streptococci other than gp D.
5. The ability of enterococci to hydrolyse PYR and growth at
6.5 NaCl is useful in differentiate them from group D
streptococci (S. bovis bile esculin +).
6. Isolates that fit the criteria for diagnosis of enterococci (
bile esculin +, PYR +, growth at 6.5% NaCl growth at 1045 C).
7. E.faecalis differentiated from faecium) by hippurate
hydrolysis ( faecalis + faecium _-) .
8. They are generally
resistant to penicillin and
cephalosporin. Also resistant to aminoglycoside and now
emergence of vancomycin resistance.
.
Laboratory diagnosis of Streptococci:
I.
Direct detection:
1.
Gram stain of clinical specimen show gram positive cocci
arranged in chains or pairs.
2.
Direct detection of Ag of group A, B and pneumococcal
capsular Ag. by slide agglutination or latex or EIA.
3.
Quelling reaction for capsular type of pneumococcus
seldom used.
4.
DNA probe has been used for direct detection of gp A.
II.
Culture:
1. SBA 5% is suitable media for isolation where we can
evaluate hemolysis . group A, B, C, F and G Beta
hemolytic (hemolysis is enhanced under anaerobic
condition e.g inoculation by stabbing inside the media ),
while majority of group D and enterococci are either non
hemolytic or AH.
S. pneumoniae Surrounded with intense green AH.
Viridans streptococci are AH or NH.
2. Other media used Columbia base medium. The colonies is
then tested for catalase.
III. Biochemical activities:
Presumptive identification can be accomplished using few
biochemical test which are selected depending on the
hemolytic pattern.
1. Bacitracin susceptibility : This test used to identify Beta
Haemolytic gp A (Streptococcus pyogens) which sensitive
to Bacitracin while BH gp is resistant (S. agalactiae).
Bacitacin disk contain 0.04 unit of bacitracin any zone of
inhibition considered sensitive.
2. Susceptibility to trimethoprim and sulfamethoxazole (STX)
used to improve the accuracy of gp A identification both
groups A&B are resistant, while BH gp C& G are sensitive
to STX.
3. CAMP test used to differentiate gp B from other BH
streptococci. Arrowhead
shaped area of enhanced
4.
5.
6.
7.
8.
hemolysis are formed when 2 streak staph aureus and gp
B streptococci approach each other.
Hippurate hydrolysis : differentiate agalactia (+) from other
beta hemolytic strep. Some isolates of gp D give +ve
hippurate hydrolysis but are not BH.
PYR hydrolysis (pyrrolidonyl aminopeptidase) to identify
gp A.
Bile esculin hydrolysis to differentiate gp D & Enterococcus
from other Strep (viridans gp) .
Salt tolerance test : to differentiate gram positive cocci that
grow 6.5% (enterococci) from those inhibited by it ( gp D).
Optochin Susceptibility and bile solubility : used to
differentiate S. pneumoniae from viridians gp.