Transcript Gram-positive: Staphylococci

Gram-positive: Staphylococci
and Streptococci
Medical Microbiology
SBM 2044
Staphylococcus aureus
• The most common pus-forming (pyogenic)
bacteria
• Can produce focal abscess, from the skin
(furuncles, boils) to the lungs, osteomyelitis,
kidneys and endocarditis
• Include S. aureus, S. epidermidis, S.
saprohyticus (UTI)
• S. aureus can persist in the body because they
have numerous cell surface virulence, exotoxins
and enzymes
S. aureus
• Staphylo = grape
clusters (Greek)
• A large 1µm Gram-positive coccus growing
in cluster-of-grapes shape.
• Can survive in long period of dry, on
inanimate object. Heat resistant.
• Identification: large, creamy colonies on
nutrient agar; catalase +
Staphylococci
S. aureus
Encounter
• Major reservoirs = humans
• Live on skin – grow at high salt and lipid concentrations
because they make lipases and glycerol ester
hydrolases, that degrade skin lipids
• Colonise skin and mucosal surfaces using MSCRAMMs:
– Fibronectin–binding proteins (FnbpA, FnbpB)
– Collagen-binding
– Clumping factors A and B
• Spread person-to-person by direct contact or airborne
S. aureus pathogenesis
• Entry
• Tissue penetration upon skin or mucosal
membrane damaged by cut
• Spread and Multiplication
• Survival in tissues dependon
–
–
–
–
no. of entering microorganisms
site involved
speed of body’s inflammatory responses
immunological history of the host
S. aureus pathogenesis
• Damage
• Local infections  pus collection, i.e. abscess
• Staphylococci can spread into subcutaneous
and submucosal tissues and caused cellulitis
• Activate acute inflammatory reaction, pouring in
chemotactic factors
• Damaged area are usually localised by the
formation of thick-walled fibrin capsule : center
of abscess is necrotic with debris of dead cells
• Why many virulence factors?
• Surface structures:
• Capsules – inhibit phagocytosis
• Peptidoglycan – interacts with TLR-2, activate
alternative pathway
• Teichoic acid – C’ activation and adherence to mucosal
cells
• Protein A – binds to Fc terminus of IgG
• Secreted factors:
• Catalase – H2O2  H2O
• Coagulases – fibrinogen  fibrin
• Pore-forming toxins – create channels to disturb cellular
homeostasis
• Haemolysins –
• Leukocidin
• Hyaluronidase – hydrolyse matrix of connective tissues
• Β-lactamase – hydrolyse penicillin
• Penicillin-binding protein (PBP2a)
S. epidermidis
• Normal flora, rarely caused disease
• Infections of S. epidermidis with other
catalase-negative staphylococci in patients
implanted with artificial devices e.g.
prosthetic joints or IV catheters
• Results in septicaemia and endocarditis
• Possibly peptidoglycan or slime layer
allows the organisms to stick to the
surface of plastics
S. saprophyticus
• Caused cystitis in young women
Staphylococcal toxin diseases
• Staphylococcal scalded skin syndrome (SSSS)
– Exfoliative toxins A and B – highly tissue specificserine
proteases that causes separation of the layers of the epidermis
at the desmosomes
• Staphylococcal toxic shock syndrome (TSS)
– characterised by fever, skin rash, hypotension, peeling of the
skin
– use tampons – oxygenated vagina and stimulate toxin
production
– TSST-1, staphylococcal enterotoxins AE
• Virulence gene regulation – two-component regulatory
systems
– Accessory gene regulator (Agr), staphylococcal respiratory
response (Srr)
•
•
•
•
Diagnosis
Gram stain and culture
Treatment
Methicillin-sensitive S. aureus – Rx: semisynthetic penicillins and cephalosporins
• Methicillin-resistant S. aureus – Rx: vancomycin
• vancomycin-resistant S. aureus –acquired the
genes of resistance from vancomycin-resistant
Enterococcus species
Streptococci
• Classification
– Haemolytic pattern – in blood agar media,
colonies formed may be surrounded by a
clear zone of haemolysis (α,β, γ)
– Group-specific antigens (Lancefield
classification) – by serological reactivity of
extracted cell wall antigens (A  U)
– Species – biochemical tests
Streptococcus




Gram-positive
Grow in chains
Non-motile
Facultative anaerobes
 Homofermentative [Glucose
lactic acid]
 Early studies distinguished 3 broad groups on blood agar
• non-haemolytic
• a-haemolytic
• b-haemolytic
• Streptococci
 GAS diseases – changing patterns
• Changes in virulence of prevailing GAS strains ?
• Changes in social conditions – less crowding?
• Changes in herd immunity to prevailing virulent strains?
 Reemergence of severe invasive infections
• Sporadic cases since mid-1980s – new virulent strains?
• Streptococcal toxic shock
o Some cases associated with obviously severe tissue infections
o Many others – shock following mild or unapparent infections
• Sporadic – implies predisposing factors
Group A Streptococci
• Epidemiology
– Ubiquitous worldwide
– Common diseases are acute pharyngitis or pyoderma infections
(skin and soft tissues)
• Encounter
– Carriers appear asymptomatic
– Person-to-person spread is mediated by respiratory droplets or
by direct contact to skin
• Entry
– For pyodermal infections, streptococci need to gain entry into
deeper layers of skin
– In pharyngeal infections, to prevent from being swept away, GAS
must lipoteichoic acid (LTA),protein F and M protein
• Spread and multiplication
– Most GAS remain localised to the site of initial
infection
– In pharynx and tonsils, may result in erythema and
exudate associated with strep throat
– Peritonsillar abscess (quinsy) or spread to adjacent
structures (mastoid and middle ear)
– Impetigo in skin
– Erysipelas and cellulitis in deeper layers of skin
– GAS may spread laterally in deep tissues, by
secreting enzymes
• necrotizing fasciitis and myositis
Secreted protein
Function
Proteases
Hyaluronidase
spreading factor
Streptolysins S and O form pores in the host
membranes
Deoxyribonucleases
(DNase)
Streptokinase
– bind to human protein
plasminogen, and convert
this to plasmin
plasmin degrades fibrin
GAS virulence factors – Excreted products
 Cytolytic toxins
• Streptolyxin O (SLO)
Produced by all strains
• Streptolysin S (SLS)
• Both lyse wide range of cells, including PMNs
suppuration and/or necrosis
• Various subtle effects at sub-lytic concentrations
• SLO – ‘sensitive’ to O2
• SLS – stable in O2
b-haemolysis
“Thiol-activated” toxin
(NB: Salyers & Whitt misleading - neither are “enzymes” )
SLO- and SLS-defective mutants
 murine s.c. model - weight loss at 24h post infection
Sterile
~3 x 109 cfu
+ 0.5
~3 x 108 cfu
~3 x 107 cfu
~3 x 106 cfu
~3 x 105 cfu
Weight gain (grams)
0.0
- 0.5
- 1.0
- 1.5
- 2.0
- 2.5
PBS
WT
sloΔ1
sagBΔ1
sloΔ1sagBΔ1
M protein
• Important for cell adherence to keratinocytes
• Prevent opsonization by complement
– bind to fibrinogen and interferes with the alternative
pathway
– bind with host complement control proteins and
inhibit opsonins formation
• Hypervariable regions of M protein are antigenic,
but there are > 100 M protein serotypes
Hyaluronic acid capsule
• Antiphagocytic structure on bacterial
surface
• Hyaluronic acid is abundant in human
connective tissue - hence GAS can
camouflaged themselves
• But capsule may interfere with the
adherence of GAS to epithelial cells
– so GAS shed the capsule during the early
stages of infection using hyaluronidase
Damage
• GAS can evoke an intense inflammatory
responses in tissues
• Streptococcal pyrogenic exotoxins (SPE)
– SPE A, B and C cause rash, a characteristic of scarlet
fever
– SPE A and C are bacterial superantigens that activate
a large subset of T cells
• Immunologically mediated disease
(nonsuppurative sequelae)
– acute rheumatic fever (ARF)
– acute post-streptococcal glomerulonephritis
Diagnosis
• Impetigo – a cluster of small vesicles on a
pink base that breaks down to honeycoloured crusts
• Erysipelas – a raised, bright red patch of
skin
• More difficult to diagnose streptococcal
pharyngitis
– rapid strep tests
– throat cultures (throat swab)
Treatment and Prevention
• Penicillin – 10 day oral therapy
• Erythromycin or other macrolide antibiotics
for individuals allergic to penicillin
Streptococcus pyogenes
 Natural habitat: Humans
 Strains distinguished by M serotyping
• Devised by Lancefield in 1920s, using panels of absorbed sera
to hot-HCl extracted antigen, she called “M antigen”
• M-type specific antigen was sensitive to proteases
M proteins
• > 100 distinct M types of GAS distinguished since then
- called M1, M2, M3, M4,……..etc.
 Highly versatile pathogen
• Suppurative infections
• Toxinogenic diseases
• Immunologically-mediated diseases
Group A Streptococci
Principle sites
of infection:
Pharynx
Skin
Deep-seated
tissues
Local spread (e.g.)
Invasive infections
Pharyngitis, tonsillitis,
otitis media, sinusitis.
Other tissues
pyroderma, erysipelas
Occasionally
Extensive necrosis
(necrotizing fasciitis)
Bacteraemia
or septicaemia
Streptococcal
Toxic Shock
• Puerperal fever (childbirth fever) in women – major killer in past
Streptococcus pyogenes
Tonsillitis
Follicular tonsillitis
Streptococcus pyogenes
Impetigo
Cellulitis
Erysipleas
Streptococcus pyogenes
Necrotizing fasciitis
(< 24 hours post surgery)
Streptococcus pyogenes
Streptococcus pyogenes
Scarlet fever
Toxic Shock
 In past – probably linked to scarlet fever
 Since mid-1980s – associated with ‘new’ highly virulent
strains - rapidly fulminating
• some cases in previously healthy young adults, no obvious
predisposing factors
 Associated with production of superantigenic toxins, but
other factors also involved
Acute rheumatic fever (ARF)
 autoimmune disease - triggered only by GAS pharyngitis
 associated with strong immune response to GAS.
• antibodies and/or T cells X-react with host antigens? (later)
 Symptoms arise > 10days + after GAS infection
• responsible GAS strain already ‘cleared’
 Symptoms may include:
• inflammation multiple sites, starting with major joints (arthritis)
• neurological disorders (Sydenhams chorea)
• rheumatic heart disease (RHD) (ca 50% cases)
- damage to heart valves, permanent scaring in survivors
Acute rheumatic fever (ARF)
 Initial attack rates low (3% in untreated pharyngitis)
 High recurrence (up to 50%) - increasing in severity
Widespread prescription of penicillin for ‘sore throats’
Remarkably, GAS have not (yet ?) developed resistance to penicillins
GAS infections - complications
Post-streptococcal acute glomerulonephritis (PSGN):
 Common, but rarely life-treating - some GAS infections
of either pharynx or skin.
 Symptoms arise some 10 days after infection
• reflect kidney dysfunction, probably involving inflammation
of glomeruli
 Pathogenesis:
• Most probably entrapment of GAS antigen-host antibody
complexes at basement membranes of glomeruli
• might also involve an ‘autoimmune’ response
Normal glomerulus
Glomerulonephritis
Mesangial cell
intrusion
Mesangial cell
Endothelial
cell, has
100 nm
pores
PMN
Basement membrane
Small complexes diffuse thro’
basement membrane into urine,
but the occasional larger complex
can’t & is normally removed by
mesangial cell
Inflammation
Too much large immune
complex entrapment ?
Example: Sequel of some
S. pyogenes infections
Group B streptococci
• Streptococcus agalactiae are aerobic G+
diplococci that are β-haemolytic on blood
agar plates
– found in lower GIT and female genital tracts
– GBS is a leading cause of neonatal sepsis
and meningitis
– prevent opsonization and phagocytosis with a
polysaccharide capsule
Enterococci
• Enterococcus faecalis cause UTI, wound
infections, endocarditis, intraabdominal
abscesses and bacteremia.
–
–
–
–
–
Normal flora of GIT and GUT
resistant to bile and high salt concentrations
nosocomial infections
resistance to many antibiotics, often bacteriostatic
bacterial killing must use a combination treatment of
a β-lactam and an aminoglycoside