MicroBiology Flashcards
What is a Human pathogen?
organism capable of causing disease through infection (or other means)
Describe the 5 Classes of human pathogens
Bacteria, Virus, Fungi, Proin, Protazoa
Bacteria - single celled prokaryote
Virus - obligate parasite (ie non-living)
Fungi - plant like eukaryote organism, eg thrush
Prion - protein based infective agent (CJD)
Protazoa - single celled animals/eukaryotes
Cell wall + staining colours
Gram positive - stains Purple
Gram negative - stains Pink
Why does Gram- positive stan purple?
-thick cell wall with many peptidoglycan layers
-absorbs more of the stain
-hence you get a darker
Key differences between GRAM + AND GRAM -?
- Gram + wall composed of many ropes of peptidoglycan layered on top of each other.
- Gram - has wall composed of single layer of peptidoglycan, however contains an outer membrane
- Gram - wall contains large periplasmic space
- Gram - wall contains LPS chains which project from outer membrane, known as endotoxins
Cocci/ Coccus
Spheres
Largely exist in 3 different ways → Diplococcus, Streptococcus or Staphlycoccus:
Diplococci
examples
division occurs in 1 plane, isolated pairs
Key examples → Strep pneumoniae (gram +), Neisseria species (gram -)
Staphylococcus
examples
division occurs in 3 planes to produce a ‘clump’ (staff meeting)
Staph Aureus (gram +), staph epidermidis
Streptococci
examples
division occurs in 1 plane, forms continues long chains (strep form strips)
Strep pneumoniae, Strep Pyogenes (GAS)
Baccilus
examples
Rod shaped bacteria
Contain flagella to help with motility/ movement
e.g. Escherichia (ie E.coli), Klebsiella , Shigella, Salmonella
most bacteria causing diarrhoea tend to be?
gram NEGATIVE
Coccobacillus
somewhere between a cocci (sphere) and bacillus (rod), rod w blunted edges
e.g. H. influenza
Curved Rod/ Vibrios
examples
curved rod shape
e.g. Vibrio Cholera (causes cholera lol)
Spirillum and spirochaete
Syphilis and Lyme disease are both caused by spirochaetes
Virulence
the ability of a microbe to cause damage to the host
Commensals
an organism that is part of the normal flora, often mutualistic relationship, found in gut and on skin
Opportunistic pathogen
an organism that is not normally pathogenic, however when circumstances arise (such as when you are ill, recovering from infection etc) jump at the chance to proliferate and cause infection (C.diff).
Properties of Bacteria
Aerobic - grow in oxygen/air.
Obligate aerobes - require oxygen.
Obligate anaerobes - killed by oxygen.
Capnophilic - prefer high CO2 levels.
How to distinguish between organisms?
Gram stain → Gram positive or Negative
Shape form? → Cocci in strips? Cocci in clusters? Rod shaped/ bacillus?
Catalase enzyme present? → Staph are catalase +ve, strep are catalase negative
Staph Test
Coagulase test, Oxidase test, Catalase test (+)
Strep test
Blood agar/ haemolysis
What is Haemolysis?
essentially the amount of blood cells the bacteria can chew through
3 types of Haemolysis
Beta haemolytic → complete haemolysis, appears clear on blood agar plate (Strep pyogenes, other GAS)
Alpha haemolytic → partial haemolysis or green haemolysis (Strep viridans, strep pneumo)
Gamma haemolytic→ no haemolysis occurs (Enterococcus sp)
Golden haemolysis
Very specific, buzzword for Staph Aureus
eve tho haemolysis is for strep not staph
How do we distinguish between different Staphylococcus types?
ALL staph are catalase POSITIVE
Next test for coagulase:
Coagulase +ve = Staph. Aureus
Coagulase -ve = staph epidermidis, other staph species
Browns Classification
Examples of Gram -ve cocci
N. meningitidis
N. gonorrhoeae
Examples of Gram +ve cocci
Group A Strep (GAS) - Strep pyogenes.
Strep pneumoniae - alpha haemolytic cocci
Non-haemolytic strep eg enterococcus faecalis and enterococcus faecium
All staphylococcus species
Staph Aureus Treatment
Flucloxacillin
What is Staph Aureus
Staph aureus causes, skin commensal (eg ally) however causes wound infections, prosthetic infections + cardiac implant infections. PWID (people who inject drugs) is a buzzword for a staph aureus infection.
Examples of Gram -ve rods
- Salmonella
- Gut commensals and Pathogens - E. coli, klebsiella, enterobacter, proteus.
- Coliforms - term used to describe species of gram negative bacilli that look like E. coli on gram film and when cultured on agar. Many part of normal bowel flora.
Example of Gram +ve rods
C.diff (spore forming anaerobe)
3 Mechanisms of antibiotics
- Target cell wall
- Target Prokaryotic nucleic acid (target bacterial DNA)
- Target Bacterial Protein synthesis
3 examples of antibiotics that target cell wall?
Penicillins
Cephalosporins
Glycopeptides
3 examples of antibiotics that target prokaryotic nucleic acid (target bacterial DNA)?
Fluoroquinolones (class of Abx)
Metronidazole (singular abx type)
Trimethoprim (singular abx)
3 examples of antibiotics that target Bacterial Protein synthesis?
Macrolides
Tetracyclines
Aminoglycosides
How do Penicillins + cephalosporins Target cell wall?
- Contain structure called Beta-lactam ring
- which binds to the ‘penicillin-binding-protein’ (PBP) found on cell wall of bacteria
- Prevents addition of peptidoglycan layers formation, killing bacteria
How do Glycopeptides Target cell wall?
bind to the LPS side chains on cell wall, preventing formation. Only work on Gram -ve bacteria since only gram -ve contain Lipopolysaccharides (LPS)
Vancomycin
How do quinolones Target Prokaryotic Nucleic acid?
Quinolones → inhibit DNA synthesis by preventing DNA ‘supercoiling’
Eg. ciprofloxacin
How do Trimethoprim Target Prokaryotic Nucleic acid?
Trimethoprim → inhibits folic acid synthesis
Causes tendonitis (achilles heel)
How do Metronidazole Target Prokaryotic Nucleic Acid?
Metronidazole → causes DNA structure to unwind
NO ALCOHOL
Used against anaerobes + protazoa
How do Tetracyclines Target Bacterial Protein synthesis?
Tetracyclines → bind to 30s and stops protein chain elongation
Eg doxycycline
How do Macrolides Target Bacterial Protein synthesis?
Macrolides → binds to 50s subunit of ribosome, interferes with protein synthesis
Eg erythromycin (Note, this macrolide is safe in pregnancy)
How do Aminoglycosides Target Bacterial Protein synthesis?
Aminoglycosides → Binds to aminoacyl site of 16s within 30s subunit to cause misread of genetic code
Gentamicin
What are the 4 Naughty Cs?
Cephalosporins
Ciprofloxacin
Clindamycin
Co-amoxiclav
Don’t give to over 65s, higher risk of C.diff infection
How to treat C.Diff?
oral vancomycin
