Microbiology Flashcards
Define pathogen
An organism that causes/can cause a disease
Define commensal
Organism colonising the host but usually causes no disease
Define opportunist pathogen
Microbe only causing disease if host diseases compromised
Define virulence/pathogenicity
Degree to which an organism is pathogenic
Define asymptomatic carriage
Pathogen carried harmlessly at a tissue site, causing no disease
How is a gram stain done (4 steps)?
- Add crystal violet
- Add iodine
- Add acetate/alcohol to decolourise
- Add sapranin counter stain
What colour will gram positive bacteria be after staining?
Retain crystal violet- PURPLE
What colour will gram negative be after staining?
Safranin stains PINK
What are 3 gram positive cocci?
Staphylococci
Streptococci
Enterococci
What are 3 gram positive bacilli?
Bacillus
Clostridia
Corynebacteria
Why are gram positive and gram negative different colours?
Gram-negative cells have a thinner peptidoglycan layer that allows the crystal violet to wash out on addition of ethanol. They are stained pink or red by the counterstain
What are 2 gram negative cocci?
Neisseria
Moraxella
What are 5 gram negative bacilli?
E. Coli
Campylobacter
Salmonella
Shingles
Proteus
What are 7 types of agar cultures?
Blood agar
Chocolate agar
Cled agar
MacConkey agar
XLD agar
Sabourard agar
How is chocolate agar made and why I it used?
Blood agar cooked at 80 degrees for 5 mins
-Nutrients are released
What type of microorganism are grown on MacConkey agar?
Only gram negative bacilli
Why is MacConkey agar used?
Differentiates lactose fermenting from non-lactose fermenting
What temperature can bacteria survive at?
-800 to 80 degrees
1200c for spores
What pH can bacteria survive at?
4-9
How long can bacteria survive in water?
2 hours-3 months
spores can live >50 years
Describe the structure of a bacterial cell?
Capsule- sugar polymer that can inhibit immune system
Cell wall- phospholipid membrane
Usually no nuclear membrane
1 circular chromosome
Describe the cytoplasmic membranes of gram positive and gram negative bacteria
Positive: single membrane
Negative: 2 membranes
Describe the peptidoglycan membranes of gram positive and negative bacteria
Gram positive: thick membrane
Gram negative: thin layer between the 2 cytoplasmic membranes
Where are lipoteichoic and teichoic acids present?
Gram positive
When is endotoxin present?
Gram negative
What is endotoxin?
Outer membrane of gram negative bacteria
What locations in the body are sterile?
Blood
CSF
Joints
Bladder
Lower resp tract
Pleural fluid
Peritoneal cavity
What locations in the body are colonised with flora?
GI tract
Skin
Oral cavity
Vagina
Urethra
How is bacterial growth rate measured?
Shining light and measuring absorption
What is exotoxin?
Proteins secreted from gram negative and gram positive bacteria
-mainly gram positive
Describe the antigenicity (binding to immune system) of endotoxin and exotoxin
Endotoxin: weak
Exotoxin: strong
Describe the specificity of endotoxin and exotoxin
Endotoxin: unspecific
Exotoxin: specific
Describe the heat specificity of endotoxin and exotoxin
Endotoxin: heat stable
Exotoxin: unstable
What can be converted to toxoid (non-active toxin)?
Endotoxin
What effect does botulism have?
Inhibits nervous system
What effect does tetanus have?
Stimulates nervous system
Describe the process of bacterial infection
- Th cell activation
- Th cells secrete INF + TNF and IL-2
- Macrophage recruitment
How does genetic variation occur on a chromosome?
Base substitution
Deletion
Insertion
What is a plasmid?
Small circular DNA
Antibiotic resistant genes
How are genes transferred by transformation?
Uptake of exogenous substance (ie plasmid) leads to genetic alteration of cell
How does gene transfer occur by transduction?
Foreign DNA introduced via vector/ virus
How does gene transfer occur via conjugation?
Direct cell to cell contact that transfers genetic material
Is staphylococcus aerobic or anaerobic?
Aerobic
What is the normal habitat of staphylococcus?
Skin and nose
What is coagulate?
Enzyme that forms fibrin clot around bacteria and protects from phagocytosis
How is staphylococcus aureus spread?
Aerosol and touch
Is staphylococcus aureus coagulase positive or negative?
Positive
How does staphylococcus aureus show on blood agar?
Creamy/yellow
What are the 4 virulence factors of staphylococcus aureus?
Pore forming toxins
Proteases
Toxic shock syndrome
Protein A
What does MRSA stand for?
Methicillin resistant staphylococcus aureus
What are some conditions associated with staphylococcus aureus?
TSS
Wound infections
Food poisoning
Scalded skin syndrome
What are 2 coagulate negative staphylococci?
Staphylococcus epidermis
Staphylococcus saprophyticus
What shape are staphylococcus aureus cells?
Spherical clusters
Is streptococcus catalase positive or negative?
Negative
Are staphylococcus and streptococcus gram positive or negative?
Positive
What are the 3 groups of streptococcus?
Alpha haemolytic
Beta haemolytic
Non haemolytic/ gamma haemolytic
What type of agar is streptococcus haemolysis done on?
Blood agar
How alpha haemolytic streptococci appear?
Partial lysis- green/browning
What is an example of an alpha haemolytic strep?
S. Oralis
S. Sanguis
S. Pneomonie
How does beta haemolytic strep present?
Complete lysis- colourless
What is an example of a beta haemolytic strep?
S. Pyogenes
S. Agalactiae
How does gamma haemolytic strep appear?
No lysis
What is an example of gamma/nom haemolytic strep?
S. Bovis
S. Mutans
Are enterococcus catalase positive or negative?
Negative
What is an example of enterococcus?
E. Faecalis
What is optochin sensitivity tested on?
Alpha haemolytic strep
What is an example of a optchin resistant strep?
Viridans strep
What is an example of an optochin sensitive strep?
S. Pneumoniae
-all other alpha strep are resistant
What is antigenic zero-grouping carried out on?
Beta haemolytic strep
What is zeihl-neelsen stain test used for?
Mycobacteria do not take up gram stain so this is used to identify acid-fast bacilli and non acid-fast bacilli
What colour will acid fast bacilli be?
Red
What colour will non-acid fast bacilli be?
Blue
What is catalase test used for?
Differentiating streptococci and staphylococci
What is catalase positive?
Staphylococci
What is catalase negative?
Streptococci
Why is a coagulase test used?
Coagulase produced by staph. Aureus
-distinguishes it from other staph
What indicates positive coagulase result?
Clumping
What generates a coagulase positive response?
Staphylococcus aureus
How are beta haemolytic bacteria differentiated?
Lancefield grouping
What is lancefield grouping A, C and G?
Tonsillitis and skin infection
What is lancefield grouping B?
Neonatal sepsis and meningitis
What is lancefield grouping D?
UTI- enterococci
What indicates optochin sensitivity?
Clear zone of no growth around disc
Are oxidase positive bacteria anaerobic or aerobic?
Aerobic
Are oxidase negative bacteria anaerobic or aerobic?
Either
What are some oxidative positive bacteria?
Campylobacter
Helicobacter
V. Cholerae
What indicates oxidase positivity?
Blue colour change
What does a pink/red colour on MacConkey agar indicate?
Lactose fermenting bacteria
What does a whit/transparent result show on MacConkey agar?
Non-lactose fermenting bacteria
Why is XLD agar used?
Differentiates salmonella and shingella
How does salmonella show on XLD agar?
Red/pink with black spots
How does shingella show on XLD agar?
Red/pink colonies
How are streptococci differentiated using lancefield typing?
Groups coagulase negative and positive bacteria
- group A and B
What is in strep group A?
Strep pyogenes
What is in strep group B?
Strep agalactiae
What are the common clinical presentations of staph aureus?
Pain in shoulder
High temperature
Osteomyelitis in C6 and 7
How much osteomyelitis is staph aureus responsible for?
90%
How is staph aureus treated?
Flucoxacillin for 3 months