Medical Microbiology Flashcards
What are psychrophiles
Microorganisms that can grow in extremely cold temperatures (-20)
How many chromosomes do prokaryotes have?
1
What type of ribosomes do prokaryotes have?
70s
What type of ribosomes do eukaryotes have?
80s
How big are prokaryote cells?
1 - 10 um
How big are eukaryote cells?
10 - 100 um
What are the 6 shapes of bacteria?
Coccus Bacillus Spirochete Spirilium Coccobacillus Vibrio
Give an example of a coccus bacteria?
Streptococcus
Give an example of a vibrio bacteria
Vibrio cholerae
Give an example of a bacillus bacteria?
P aeruginosa
Give an example of a coccobacillus?
Chlamydia
What are the three growth types for cocci?
Chain
Packet
Cluster
What are the 4 components used in the gram stain?
Crystal violet primary stain
Iodine mordant
Alcohol decolouriser
Safarin counterstain
What is the primary stain in gram staining and what does it do?
Crystal violet dye - stains all cells purple
What is the mordant used in gram staining and what does it do?
Iodine. It forms insoluble complexes with the crystal violet dye
What is the decolouriser used in gram staining and what does it do?
Removes crystal violet iodine complexes from gram negative but not gram positive
What is the counterstain used in gram staining and what does it do?
Safarin. It stains both cell types pink
What does the peptidoglycan layer contain in gram positive cells?
Teichoic acids and lipteichoic acids
What is the role of the s layer in bacterial cells?
To hide from the immune system
What does the outer membrane of gram negative bacteria contain?
Porins and endotoxin. Endotoxin activates the innate immune system.
Where is the s layer found on gram postive and gram negative bacterial cells?
Peptidoglycan layer of gram positives. Outer membrane of gram negatives.
What is peptidoglycan made up of?
Repeating structure of 2 alternating sugars (nam and nag) crosslinked by peptides
Which enzymes are involved in peptidoglycan synthesis?
Transglycosylases which connect sugars and transpeptidases which form peptide crosslinks
How do beta lactams inhibit cell wall synthesis?
Inhibit transpeptidase reaction
How does vancomycin inhibit cell wall synthesis?
Binds to d-ala-d-ala to stop cross linking
What is the glycocalyx? Give examples
Generic name for extracellular polymers. Capsule and slime layer
What is the capsule used for?
Biofilm production
What is the slime layer used for?
Movement
What are 3 types of pili?
Fimbriae
Type 4 pili
Sex pili
What are fimbriae?
Short pili which are used to attach bacteria to surfaces. They are made up of helically arranged proteins tipped with adhesive proteins.
What are type 4 pili used for?
Movement
How do cells move?
Runs and tumbles
When a cell is near an attractant, how do its runs and tumbles change?
Tumbles are less frequent. Runs are longer.
Which type of bacteria form endospores and in what situation?
Gram positive bacteria form endospores when nutrients run out
What are the 4 arrangements of flagella?
Monotichous
Lophotrichous
Amphitrichous
Petritrichous
What are the 6 shapes of bacterial cells, give examples of each
Coccus (streptococci) Bacillus (p aeruginosa) Coccobacillus (chlamydia) Vibrio (vibrio cholerae) Spirilium Spirochete
What are the 3 growth types for cocci?
Chain
Packet
Cluster
Halophile
Requires salt to grow
Halotolerant
Can grow in mild salt concentrations but grow best without salt
Osmophiles
Can survive in high sugar environments
Xerophiles
Can survive in dry environments (usually fungi)
Neutrophile
Grows best between pH 5.4 and 8.5
Psychotrophs
Optimum is around room temperature but can grow in the fridge
Mesophiles
Optimum between 20 and 40 degrees (all animal pathogens)
Thermophiles
Optimum 50 to 80 degrees
Obligate aerobe
Has SOD and catalase
Requires oxygen
Facultative anaerobe
Has SOD and catalase
Does not require oxygen but grows best when oxygen is available
S aureus
Fermentation when oxygen is unavailable
Aerotolerant anaerobe
Has SOD but not catalase
Grows equally well with or without oxygen
Streptococcus pyogenes
Strict anaerobe
Has neither SOD nor catalase
Some tolerate oxygen, some killed by it
Microaerophile
Has SOD and some have catalase
Grows best in low oxygen conditions
Aerobe
An organism which uses oxygen as the terminal electron acceptor in a respiratory chain
Can aerobes carry out fermentation?
No
What charge does a biofilm matrix have?
Negative
What are persister cells?
They are found in biofilms. They neither grow nor die in the presence of antimicrobials. They are multi drug tolerant (MDT)
Why is natural flora important?
Prevents growth of pathogenic flora by competition and amensalism (secretion of inhibitory substances)
Factors determining microbiome
Host physiology Host pathobiology Immune system Lifestyle Genotype Environment
How is flora on the skin limited?
Dessication
Lack of nutrients
Disinfectant secretions (lysozyme which cleaves peptidoglycan and cathelicidin which forms pores)
What is the microbiome of the oropharynx?
HACEK organisms
Like elevated CO2 and are slow growing
How are microbes removed from the respiratory tract?
Lysozyme
Macrophages
Mucus
Ciliated cells
What does the the colon microbiome secrete?
Vitamin k and B
What are antimicrobial secretions?
Lysozymes
Lactoferrin
Cathelicidin
Defensins
What is found in granules of NK cells?
Perforin and granzyme
ID50
Infectious dose 50. Number of cells that results in disease for 50% of population
How do pathogens attach to host cells?
Fimbriae
Capsule
How do microbes cross the mucosa?
The use M cells
Name the 4 classes of s aureus virulence factors
Adhesins
Invasins
Siderophores
Haemolysins
Name the 2 invasins in s aureus
Staphylokinase
Hyaluronidase
What does staphylokinase do?
Cleaves plasminogen to plasmin which degrades fibrin clots. Cleaves IgG. Cleaves C3b which inhibits phagocytosis
What does hyaluronidase do?
Lyses hyaluronic acid found in connective tissue which causes tissue breakdown
What 3 substances does s aureus use to evade the immune system?
Coagulase
Staphyloxanthin
S aureus protein A
What does coagulase do?
Reacts with thrombin forming staphylothrombin which cleaves fibrinogen to fibrin. S aureus coated with fibrin can evade the immune system
What does staphyloxanthin do?
Antioxidant that protects against ROS
What does s aureus protein do?
Binds to Fc portion of IgG and prevents phagocytosis by preventing opsonisation
What is PVL?
S aureus toxin which is cytotoxic to WBC
Name 3 s aureus toxins
Pyrogenic
Exfoliating
Membrane damaging
What does pyrogenic toxin cause?
Toxic shock syndrome
How does superantigen cause toxic shock syndrome?
More T cells activated so more cytokines activated
What does exfoliating toxin do?
Cleaves cadherin which forms junctions between skin cells. Leads to skin peeling
Infectivity
Propensity for transmission. Measured by secondary attack rate in a household
Secondary attack rate
Rates of infection among individuals exposed to a first case
Pathogenicity
Measure of disease causing propensity (ID50)
Virulence
Propensity to cause severe disease. Measured by fatality ratio or LD50
Incubation period
Time between exposure and onset of symptoms
What are the 5 Is?
Inoculation Isolation Incubation Inspection Identification
Chocolate agar
Blood heated to release nutrients
Name the 5 selective media
Phenylethyl alcohol Blood Macconkeys EMB Baird parker
Phenylethyl alcohol agar
Selects out gram negatives by reversibly inhibiting their dna synthesis
Baird parker agar
Selects staphylococci
Contains egg yolk
EMB media
Lactose fermenting - metal sheen
Non lactose fermenting - colourless
MacConkeys
Selects gram negatives (bile salts inhibit growth of gram positives)
Lactose fermenting - produces acid so indicator turns red
Non lactose fermenting - colourless
Capnophile
Likes elevated CO2 conc
Alpha haemolysis
Lots of haemolysis
Beta haemolysis
Some haemolysis
Green halo
What are 6 biochemical tests?
Catalase Oxidase Indole Urease Coagulase Carbohydrate fermenting
Oxidase test
Cytochrome c oxidase (purple when positive)
Indole test
Tests for tryptophanase which turns tryptophan into pyruvate, indole and urea. DMAB turns red when indole present
Urease test
Urease hydrolyses urea to ammonia which increases pH. Indicator turns pink
Coagulase test
Mix with plasma, look for clot
Carbohydrate fermentation
Positive - acid or gas production
Acid turns indicator yellow
Gas in collected in upturned durham tube
What are the cons of traditional methods?
Organism must be able to be cultured in vitro (obligate intracellular cannot)
Slow growing
Poor discrimination between similar microbes
What immunological methods are used?
Agglutination
Immunofluorescence
ELISA
Immunodiffusion assays
Direct immunofluorescence
Antigen from patient
Fluorescent antigen from lab
Indirect immunofluorescence
Antigen from lab mixed with patient serum. Antibody against patient antibody fluoresces
What are the molecular methods used?
PCR and real time PCR
Nucleic acid probes
RFLP
Plasmid fingerprinting
What are the 4 different types of CD4 T cells?
Th1
Th2
Th17
TFH
What is the role of Th1 cells?
They recognise mycobacteria derived antigens on macrophages and secrete cytokines which help overcome lysosome binding
What is the role of Th2 response?
Deals with infections at mucosal surfaces, parasite infections and allergies
Th17 response
Deals with extracellular bacteria and fungi
TFH response
Helps B cells produce antibody
