Exam 1: Gram-Positive Cocci Flashcards
Pyogenic Cocci
Pus producing organisms:
- Gram ⊕
- Staphylocci
- Streptococci
- Gram ⊖
- Neisseria spp
Pyogenic pathogens cause ___ through ___.
acute suppurative infections
neutrophil mobilization
Staphylococcus
Overview
- Pyogenic bacteria
- Survive for a long time on dry inanimate objects
- Common source of hospital acquired infections
- Associated diseases includes:
- Abscess
- Food poisoning
- Osteomyelitis
- Endocarditis
- Toxin related diseases
Staphylococcus
Characteristics
- Large gram ⊕ cocci
- Usually grouped in irregular “grape-like” clusters
- Divides in 3 planes
- Facultative anaerobes
The most pathogenic species of Staphylococcus genus is…
Staph. aureus
Staph. aureus is part of the normal flora of the…
skin, nasal pharynx, urethra, and eye
S. aureus is resistant to __ and can therefore be acquired from ___.
drying
fomites
S. aureus colonizes ~ 20% of healthy individuals in the…
nasal nares
Type of disease caused by S. aureus depends on…
- Location in the body
- Environment within the host
- Virulence factors expressed by that particular strain
S. aureus
Surface Components
- Capsule
- Protein A
S. aureus
Capsule
Found in some strains:
- Prevents phagocytosis
- Promotes adherence to cells and prosthetic devices
S. aureus
Protein A
- Binds Fc terminus of IgG
- Prevents opsonization and Fc mediated phagocytosis
- Prevents IgG complement activation
S. aureus
Enzymes
- Catalase
- Coagulase
- Hyaluronidase
- Lipases, Proteases, DNAases
- Beta lactamase
S. aureus
Catalase
Converts H2O2 ⇒ H2O + O2
Limits respiratory burst of phagocytes
S. aureus
Coagulase
Converts fibrinogen ⇒ fibrin
Causes clotting & promotes abscess formation
S. aureus
Hyaluronidase
Hydrolyzes CT matrix
Enhances bacterial spread
S. aureus
Lipases, Proteases, DNAses
Allows tissue and cell digestion
Facilitates substrate use & enhances virulence
S. aureus
Beta Lactamase
Inactivates pencillin and ampicillin
Plasmid mediated
S. aureus
Toxins
A single isolate can produce several exotoxins:
- Enterotoxins
- Exfoliatin
- Toxic Shock Syndrome Toxin (TSST-1)
- Leukocidins
- Hemolysins
S. aureus
Enterotoxins
7 serotypes (A-G) produced by 50% of S. aureus strains
Responsible for Staph. food poisoning
S. aureus
Exfoliatin
Causes skin sloughing in scaled skin syndrome
S. aureus
Toxic Shock Syndrome Toxin
(TSST-1)
Superantigen
Induces excessive cytokine release from T-cells
Results in fever, shock, and multi-organ failure
S. aureus
Leukocidins
Pore forming proteins that lyse WBCs
S. aureus
Hemolysins
Pore forming proteins that lyse RBCs
S. aureus
Disease Causing Mechanism
- Most commonly causes disease by inciting an acute inflammatory reaction
- Many neutrophils mobilized to site
- Bacterial and host derived chemotactic factors
-
Abscess formation common
- Walled off structure makes it harder for immune components and abx to penetrate
- May remain localized within abscess or disseminate to other sites
S. aureus
Lab Characteristics
-
Catalase ⊕
- Differentiates all Staph (⊕) from Strep (⊖)
- Coagulase ⊕
-
Beta-hemolytic on blood agar
- Golden colonies
-
Grow on 7.5% NaCl Mannitol
- All Staph. species grow under high salt conditions
- Only S. aureus ferments mannitol
-
Nucleic acid tests
- Used to ID S. aureus from clincal specimens
S. aureus
Treatment
-
All Staph resistant to penicillin
- Due to Beta Lactamase
- Hydrolyzes beta lamtam ring of penicillin
- Transmitted by plasmid
- Due to Beta Lactamase
-
Semi-synthetic penicillins are effective
- Nafcillin
- Methicillin
- Cephalosporin agents effective
- Strains that are methicillin resistent are resistant to all beta lactam abx
- Encoded by a penicillin binding protein (PBP2’)
-
Vancomycin may be used to treat MRSA
- Resistance also starting to develop against this drug
S. aureus
Immunity & Prevention
-
Immunity
- Clearance mediated by opsonizing IgG
-
Incomplete and short-lived
- Repeated infections possible
-
Best prevention
- Good handwashing
- Good disinfectant practices
Staph. epidermidis is normal flora of the…
skin, nose, and throat
Staph. epidermidis causes infections of…
prosthetic devices, shunts, grafts, and catheters
Staph. epidermidis is the most common cause of…
prosthetic valve endocarditis
Staph. epidermidis major virulence factor is…
high molecular weight polysaccharide slime
Enchances adherence.
Staph. epidermidis
Lab Characteristics
- Gram ⊕ cocci
- Catalase ⊕
- Coagulase ⊖
-
Non-hemolytic ⇒ gamma-hemolytic
- White colonies on blood agar
Staph. saprophyticus is a common cause of…
UTI in young healthy women
Staph. saprophyticus
Lab Characteristics
- Gram ⊕ cocci
- Catalase ⊕
- Coagulase ⊖
- Non-hemolytic ⇒ gamma-hemolytic
- White colonies on blood agar
Streptococcus
Diseases
Includes:
- Pharyngitis
- Otitis media
- Meningitis
- Pneumonia
- Sepsis
Streptococcus
Classification
Classified in three ways:
-
Serologically
- Lancefield grouping
- Patterns of hemolysis
- Species
Streptococcus
Characteristics
- Gram ⊕ cocci arranged in chains
- Catalase ⊖
-
Facultative anaerobe
- Metabolism is fermentative
- Tolerates and can grow in oxygen
- Growth enhanced by 5-10% CO2
- Small translucent colonies
Lancefield’s Groupings
Serological grouping of Strep based on major cell wall carbohydrate Ag.
A⇒U
Some species cannot be assigned to any of these groups.
Group A Strep includes ___ which causes ___.
Strep. pyogenes
Pharyngitis & Scarlet fever
Group B strep includes ___ which causes ___.
Strep. agalactiae
Neonatal sepsis & Meningitis
Group D strep includes ___ which causes ___.
Strep. bovis
Endocarditis & Bacteremia
Non-Typable Strep
Some species cannot be grouped according to Lancefield system.
Some important pathogens include:
-
Viridans group ⇒ endocarditis
- S. mutans
- S. sanguis
- S. salivarius
-
S. pneumonia
- Pneumonia, meningitis, otitis media
-
Peptostreptococcus
- Strict anerobes
Enterococcus
Characteristics
-
Gram ⊕ cocci
- Formerly part of Group D Strep
- Now seperate genera
- Opportunistic pathogens
Enterococcus are normal flora of…
skin, URT, GI and urogenital tract
Enterococcus that causes infective endocarditis, UTI, bacteremia/septicemia, and meningitis are…
Enterococcus faecalis
&
Enterococcus faecium
Strepococci are commonly classified based on type of ___ when grown on blood agar plates.
hemolysis
Alpha (α) Hemolysis
Partial hemolysis of RBCs
Colony surrounded by green halo
Includes Strep. viridans & Strep. pneumonia
Beta (β) Hemolysis
Complete hemolysis of RBCs
Colonies surrounded by a clear zone
Includes S. pyogenes and S. agalactiae
Gamma (γ) Hemolysis
Non-hemolytic
No change in blood agar
Includes Group D Strep and Enterococci
Bacitracin Sensitivity
Used to distinguish between two β-hemolytic Streps:
Group A ⇒ bacitracin sensitive ⇒ S. pyogenes
Group B ⇒ bacitracin resistant ⇒ S. agalactiae
Optochin Sensitivity
Used to distinguish between α-hemolytic Strep:
- S. pneumoniae* ⇒ sensitive
- Viridans group* ⇒ resistant
Gram Positive Chart
