Gram+ Cocci Flashcards
Name/list Gram+ Coccid pathogens.
Staphylococcus aureus
Staphylococcus epidermis
Streptococcus pyogenes
Streptococcus pneumoniae
Outline the features of S. aureus.
- Staphylococcus aureus is a Gram-positive spherically shaped bacterium, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin.
- S. aureus produces several enzymes including: Catalase & Coagulase.
- Commensal - all people carry s. aureus on normal skin flora and developed into opprtunistic pathogen (takes over in immunodeficient people)
- S. aureus causes several diseases(e.g. Impetigo, TSS, Septicemia)
Discuss the method of diagnosis/diagnostic methods of S. aureus.
Agar:
Haemolysis: a/b
-Gram + bacteria
-Occurs in clusters
-Faculties aerobes
-Produce yellow colonies
Biochemical tests:
-Catalase
-Coagulase
Other tests?
Molecular ?
Pros and cons for 2-3 of other methods
Discuss the treatment options and any prevention methods of Staph infections.
Decolonisation; is a medical intervention that attempts to rid a patient of an antimicrobial resistant pathogen, such as methicillin-resistant Staphylococcus aureus (MRSA).
- This can be done by: draining infected areas and cleaning (i.e. Topical treatments(e.g. antiseptic agents like chlorhexidine gluconate used for bathing.), Antibiotic prophylaxis, such as using antibiotics like neomycin or aminoglycosides.)
- Autoclaving contaminated bedding?
- Isolation of infected individuals.
Antibiotics
- Semi-synthetic penicillin: flucloxacillin, dicloxacillin.
- cephalosporins (cefazolin, cephalothin and cephalexin), clindamycin, lincomycin and erythromycin.
- MRSA infections clindamycin, minocycline, or doxycyclinevancomycin.
- Nosocomial strains of MRSA a combination of two oral antimicrobials, typically rifampicin and fusidic acid.
- New antibiotics such as linezolid and quinupristin/dalfopristin.
Outline the features of S. epidermis.
Discuss the method of diagnosis/diagnostic methods of S. epidermis.
Agar:
Haemolysis: Y
-Gram + bacteria
-Occurs in clusters
-Faculties anaerobes
-Produce white colonies
Biochemical tests:
-Catalase
-Coagulase
Other tests?
Molecular ?
Pros and cons for 2-3 of other methods
Explain the Pathogenesis and VF of S. aureus.
- Enzymes
- Coagulases; prevent plasma clots and coats bacterial cells (antigenic)- phagocyte evasion.
- Hyaluronidase; breaks hyaluronic acid down in animal tissues – spreads through host.
- Deoxyribonuclease (Dnase); nucleases which break down DNA.
- Lipase; enables lipid digestion (colonises oily skin).
- Beta lactamases (Penicillinase); resistance to beta-lactam antibiotics (inactivates penicillin).
- Staphylokinase; digests fibrin (Plasminogen to plasmin- digests fibrin clots).
- Toxins
- Endotoxins; food poisoning (Cytolysins)- 7 serotypes A B C D E G
- Alpha toxin (SEA) - Alpha haemolysin; pore-forming, chromosomal
- Beta toxin (SEB) – Sphingomyelinase damages membranes, carried on plasmid
- Delta toxin (SED) - peptide toxin- cytopathic
- Superantigens; Toxic Shock Syndrome Toxin-1; septic shock, expressed systemically- some non-menstrual cases
- Exofollatin; toxin (ETA; ETB)- scalded skin syndrome- separation within the epidermis.
- Exotoxins; Leukocidin acts on polymorphonuclear leukocytes
- Endotoxins; food poisoning (Cytolysins)- 7 serotypes A B C D E G
- Immuno-evasive Strategies(encompasses cell surface proteins, pigments)
- Polysaccharide Capsule (glycocalyx)
-
Surface Proteins
- Protein A binds IgG molecules- disrupts opsonisation and phagocytosis
- Some promote bacterial binding to host epithelial and endothelial surfaces
- Adhesins- Bind to Fibronectin & laminin
- Clumping factors which bind to fibrin/fibrinogen
Outline the features of Streptococcus (generally).
-Oxidase-negative and catalase-negative.
-Beta, gamma and alpha haemolysis on blood agar.
-Lactic acid bacteria- Fermenters of carbohydrates.
Discuss the method of diagnosis/diagnostic methods of S. pyogenes.
Agar:
Haemolysis: B
Gram+ Coccid
Occurs in chains
White colonies
aero-tolerant.
Fermentative
Biochemical tests:
-Catalase (-) Differentiates from Staphylococcus.
-Bacitracin test: distinguishes other beta-hemolytic streptococci.
MOLECUALR DIAGNOSIS
Nucleic Acid Amplification Tests
PCR: Usually target genes for amplification
Multiplexing: 16S
Rapid Identification Tests:
ELISA, Fluorescent Antibody
Newer Techniques
MALDI-TOF MS: many samples & cost effective
Pros and cons
Discuss the treatment options and any prevention methods of S. pyogenes.
Antibiotics
-Penicillin VK, Amoxicillin, Penicillin G
-Erythromycin, Clarithromycin, Azithromycin
-Cephalosporins: Cephalexin, Cefadroxil
-Vancomycins, Oritavancin
Vaccines
-Pneumococcal vaccines- conjugate or polysaccharide
Explain the Pathogenesis and VF of S. pyogenes.
-
Enzymes
- C5a Peptidase – destroys C5a compliment signals in recruitment and activation of phagocytes
- Hyaluronidase- breaks hyaluronic acid down in animal tissues – spread through host
- Nucleases– four antigenic types facilitate liquidation of pus
- Proteinases
- Streptokinases– lyse blood clots fibrinolysin
-
ToxinsStreptolysin O:
- oxygen labilecytolysinis responsible for beta hemolysis
- toxic to leukocytes & strongly immunogenic.
- oxygen stable and non-immunogeniccytolysin
- Superantigens in lysogenized strains
- Causes pyrogenic (fever producing) properties in hypothalamus and causes the rash characteristics of scarlet fever.
- Causes Streptococcal toxic shock syndrome, typified by multi-system involvement that includes renal and respiratory failure, rash and diarrhoea
-
Immunoevasive StrategiesHyaluronic Capsule
- Anti-phagocytic. Non-antigenic(chemical similarity to host connective tissue)
- Prevents opsonized phagocytosis.
- Adhesin production; Fibronectin (Fn) binding protein, lipoteichoic acids (LTA),
- M protein virulence factor
- Anti-phagocytic and anti-complement properties. It is cytotoxic for neutrophils.
- Acute glomerulonephritis can follow streptococcal infection of the pharynx
Discuss MRSA and how its developed resistance.
Methicillin-resistant Staphylococcus aureus is a group of gram-positive bacteria that are genetically distinct from other strains of Staphylococcus aureus. MRSA is responsible for several difficult-to-treat infections in humans. It caused more than 100,000 deaths attributable to antimicrobial resistance in 2019.
- Resistance is caused by mecA gene acquired from HGT.
- mecA encodes penicillin-binding protein 2a (PBP2a).
- Low affinity for β-lactam antibiotics.
- Some β-lactamase-producing strains.
- Many other mechanisms depend on stains.
(Watkins et al., 2019)
