Lecture 15 - Staph/Strep/Enterococci Flashcards
Gram-? Shape?
Positive. Cocci (spherical).
Which leukotoxins are used by Strep pyogenes?
SLO and SLS
Motility?
non-motile
Why are these superbugs call nocosomial?
Hospital acquired
How are superbugs formed?
- Arise from antibiotic misuse (i.e.
overuse, insufficient use, inappropriate
use, use in livestock) - Incomplete antibiotic treatment that fails
to kill all bacteria – survival of strains
that evolved to resist the antibiotic
Besides Lancefield, another way to classify Streptococci/Enterococci?
Based on lysis of red blood cells
RBCs) by membrane-disrupting “hemolysins” (alpha, beta or gamma-hemolytic
What do alpha, beta and gamma classifications stand for?
Alpha- partial lysis (S pneumo, Enterococcus)
Beta - complete lysis (GAS, GBS)
Gamma - no lysis (Enterococcus)
Which organisms do GAS infect?
Humans, most pathogenic
Where is GAS usually located in host?
Respiratory tract
What kinds of diseases does GAS cause?
- Localized diseases: strep throat (pharyngitis), tonsillitis, Scarlet fever, impetigo (skin inf’n), wound infections, postpartum infections, endocarditis
- Invasive disease: necrotizing fasciitis (“flesh-eating disease”), streptococcal toxic shock syndrome (sepsis), pneumonia
3.Post-streptococcal illnesses: rheumatic fever, rheumatic heart disease, glomerulonephritis – autoimmune disease caused by GAS antigens
mimicking host antigens (eg. heart muscle)
What is Necrotizing fasciitis?
infection deep in subcutaneous tissues that spreads along fascial planes, destroying muscle and fat; initially cellulitis followed by bullae (fluid filled blisters), gangrene, systemic toxicity, multi-organ failure and mortality in more than 50% of patients
What is Streptococcal toxic shock syndrome? Which toxins/superantigens are involved?
Multi-system toxicity (sepsis) following soft tissue infection progressing to shock and organ failure – caused by toxins like streptococcal pyrogenic exotoxin (SPE), streptococcal mitogenic exotoxin (SME), streptococcal superantigen (SSA)
What are the exotoxins secreted by GAS?
Leukotoxins, NADase, Hyaluronidase, superantigens, SIC
What are leukotoxins?
Membrane disrupting toxins - form pores in
host cell membranes, including leukocytes (neutrophils, macrophages, lymphocytes), epithelial cells, RBCs (“hemolysins” ® b-hemolysis); lead to host cell apoptosis, cause tissue damage – allow invasion of the bacteria into tissues – may be cholesteroldependent pore-forming proteins (like L. monocytogenes LLO) - streptolysin O (SLO), SLS
What are NADases?
Enters host cells via
pores formed by streptolysins -
cleaves host NAD+
What are Hyaluronidases?
Can degrade
host connective tissue ->
invasion. Also present in GAS acid capsule.
What are superantigens expressed by Strep?
SPE (streptococcal pyogenic exotoxins; SpeA and SpeC), SME (streptococcal mitogenic exotoxin), and SSA (streptococcal superantigen)
What to SIC’s do?
Interferes with coagulation and fibrinolysis, interferes with wound healing.
What are GAS envelope virulence factors?
M and M-like protein, Pili, HA capsule, fibronectin binding proteins, LTA, SpyCEP, C5s peptidase, and many more.
What is M protein?
Binds extracellular matrix - immune evasion. Attached to cell wall. Immunogenic, but highly variable from strain to strain. MSCRAMMS are an example.
What is the HA in the HA capsule? What else is located in the HA capsule?
HA is a polysaccharide, resembles polysaccharides on
human connective tissue so is non-antigenic – masks bacterial antigens (-> immune evasion). Hyaluronidase found in GAS envelope.
What is the function of MSCRAMM’s?
Surface adhesins that bind to extracellular matrix that promote adherence, persistence and self-mimicry. Surface proteases and hyaluronidase can
degrade these matrices, providing nutrients and
allowing the bacteria to invade deeper into
tissues, enter bloodstream
How are MSCRAMMs and pili attached to cell wall when they are being made?
Sortases via C-terminal sortase motif LPXTG
What are sortases?
Sortases are enzymes anchored in the plasma membrane that catalyze attachment of proteins to the cell wall of Gram-positive bacteria
What is the acyl enzyme intermediate?
Substrate cell envelope protein (virulence factors) attached to sortase via Thr-Cys bond (proteolysis) after translocation across membrane and before final transpeptidation.
What is the final anchor for cell envelope virulence factors?
Proteins on the peptidoglycan wall or Lipid II (transpeptidation)
What are the functions of pili in Strep/Staph/Entero?
Function in attachment to epithelial cells (adhesion),
microcolony formation, pathogenesis
How are pilin subunits bound?
(1) non-covalent interactions plus (2) an intermolecular
(intersubunit) isopeptide (amide) bond between the C-term Thr (T) of one pilin and a Lys (K) side chain in the adjacent pilin
What are the three main types of pilin?
A - Main structural pilin (major)
B - Terminal pilin
C - Tip-associated pilin (adhesin)
Where are Staph located?
Nasal passages
What is MRSA?
Methicillin-resistant S. aureus (MRSA) – a superbug - is becoming more prevalent – MRSA is not just resistant to methicillin and other penicillium-based drugs but is in fact resistant to most antibiotics (is multi-drug resistant)
How can MRSA and CA-MRSA be acquired?
Casual social contact
What are Staph MSCRAMMS?
Protein A and clumping factor
What is protein A and why is it important?
Binds to the conserved Fc portion of IgG – prevents opsonin-mediated phagocytosis but also camouflages the bacteria so they are not recognised by the immune system – immune evasion (purified protein A is used in laboratories to purify antibodies and to capture antibody/antigen)
What does clumping factor do?
Inactivates C3b - prevents opsonin-mediated phagocytosis; binds to fibrinogen
What do superantigens do?
- Superantigens (SAg) bind directly to THC
receptors and to MHC class II molecules
and cross-link them, which activates the T
cells – superantigens are capable of
binding to and activating as much as 20%
of THC - Results in massive detrimental cytokine
release (a cytokine storm) by the T cells
(TNF-a, IL-1, IL-2) à PMN infiltration - Causes fever, nausea, vomiting, endothelial
damage, capillary leakage, hypotension
and, in extreme cases, “toxic shock
syndrome” – can be fatal
Why are superantigens so dangerous for hosts?
Superantigens bind to conserved regions on
MHC class II and T cell receptor, bringing
antigen presenting cells and TH cells together
and activating the T cells to release cytokines.
Many more T cells become activated than
would those specific for the MHC II:peptide
complex.
What are PMNs?
polymorphonuclear cells - monocytes, neutrophils, etc.
What are superantigens of Staph?
Staphylococcal enterotoxins (SEs: SEB and SEH), staphylococcal toxic shock syndrome toxin (TSST)
How does toxic shock syndrome develop in the host?
There is an initial T cell derived initial peak of IL-2, INFγ and TNF-α followed by a second peak from macrophage-derived cytokines such as IL-1, IL-6, IL-8 and TNF-α, along with chemokines, leukotrienes,
prostaglandins and complement, that in severe cases may result in vasodilatation associated with vascular leakage of fluid, leading to tissue hypoperfusion, shock and multiorgan failure.
