Microbio Flashcards
Define pathogen
Organism that causes or is capable of causing disease
Commensal
Organism which colonises the host but causes no disease in normal circumstances
Opportunist Pathogen
Microbe that only causes disease if host defences are compromised
Virulence definition
The degree to which a given organism is pathogenic
Asymptomatic carriage
When a pathogen is carried harmlessly at a tissue site where it causes no disease
what color do gram-positive vs gram negative bacteria stain
+ve - purple
-ve - red
gram +ve vs gram -ve
+ve
thick peptidoglycan membrane
stains purple
single membrane
-ve
thin peptidoglycan membrane
have capsule
stains red
double membrane
endotoxin secretion from lipopolysaccharide layer
endotoxin vs exotoxins in bacteria
ENDOTOXIN
Component of the outer membrane of bacteria, eg lipopolysaccharide in Gram negative bacteria
released when the bacterial cells lyse
EXOTOXIN
Actively secreted proteins of Gram positive and Gram negative bacteria
what is bacterial conjugation
how bacteria swap genetic material
what factors give bacteria genetic variation
1) Mutation
Base substitution
Deletion
Insertion
2) Gene transfer
Transformation eg via plasmid
Transduction eg via phage
Conjugation eg via sex pilus
What is coagulase
enzyme produced by bacteria that clots blood plasma. Fibrin clot formation around bacteria may protect from phagocytosis.
how do coagulase +ve staphylococci protect themselves from phagocytosis
Fibrin clot formation around bacteria may protect from phagocytosis.
what is the normal habitat of staphylococci
Nose and skin
outline how staphylococcus aureus spreads, it’s virulence factors, what it is resistant to, and whether it is gram +ve/-ve
Spread by aerosol and touch
carriers & shedders (can infect wounds) - some people are specific carriers of staph aureus on skin
Virulence factors
Pore-forming toxins (some strains)
Proteases
Toxic Shock Syndrome toxin (stimulates cytokine release)
Protein A (surface protein which binds Ig’s in wrong orientation)
MRSA (Methicillin-resistant Staphylococcus aureus) – resistant to
beta-lactams
gentamicin, erythromycin,
tetracycline
name two coagulase-negative streptococci
S.epidermidis and S.saprophyticus
what kind of infections is S.epidermidis good at and what is its virulence factor
Infections in debilitated, prostheses (opportunistic)
Main virulence factor - ability to form persistent biofilms
what kind of infection is S. saprophytic good at and what is its virulence factor
common in UTI
Acute cystitis –> infection of the bladder
has hemagglutinin for adhesion and produces urease to neutralise acidic environment for bladder
how do the streptococci clum up compared to the staphylococci
streptococci form chains
staphylococci form clumps
what are the 3 haemolytic groups of streptococci and how do you tell them apart
alpha - haemolytic –> you get greening
beta-haemolytic –> You get transparent rim around colony
non-haemolytic –> no lysis
what is sero-grouping and what is the name of the test used
used in beta-haemolytic groups to tell apart bacteria based on their antigens
test used: Lancefield A-H and K-V
What are the group A and group B beta-haemolytic species?
S. pyogenes and S. algalactiae
what is the optochin test
used to differentiate between S pneumoniae and Viridas strep in alpha-haemolytic streptococci groups
name two gram-positive cocci
streptococcus and staphylococcus
what problems do the viridans group of bacteria cause
infective endocarditis
deep organ abscesses