Microbiology Flashcards
Define commensal
Organism which colonise the host but cause no disease in normal circumstances
Define oppurtunist pathogen
Microbe that only causes disease if host defences are compromised
Define asymptomatic carriage
Pathogen is carried harmlessly at tissue site without causing disease
What is the relationship between gram stain and membrane of bacteria?
Gram +ve = single membrane
Gram -ve = double membrane
Describe the process of gram staining
Come In And Stain
1. Crystal violet
2. Iodine
3. Alcohol/acetone
4. Safranin
How do gram +ve and gram -ve bacteria appear?
Gram +ve - purple
Gram -ve - pink
When would Ziehl-Neelsen stain be used?
Mycobacteria
-acid-fast bacilli do not take up gram stain
How do Ziehl-Neelsen stain?
- Acid-fast bacilli = Red (mycobacterium)
- Non acid-fast bacilli = Blue
What is the catalase test?
- Add peroxide to bacteria and see for bubbling reaction
- Used to differentiate between staphylococci (+ve) and streptococci (-ve)
What is a haemolysis test?
- Haemolysis = ability of bacteria to break down RBCs in blood agar
- Expression of haemolysin dependent
- Used to classify streptococci
What is alpha haemolysis?
- Indistinct zone of partial destruction of RBC
- Medium discolouration to greenish brown
Give some examples of alpha haemolytic bacteria
- Strep. Pneumoniae
- Strep. Viridans
What is beta haemolysis?
- Clear colourless zone appears around colonies; RBCs have undergone complete lysis
Give some examples of beta haemolytic bacteria
- Strep. pyogenes
- Staph. aureus
How can you differentiate between strep. and staph. aureus in beta haemolysis?
Staph aureus beta haemolysis will appear creamy yellow on blood agar
What is gamma haemolysis?
No haemolysis occurs
What is the Optochin test?
- Tests to differentiate between Strep. pneumoniae and viridans strep. and other alpha haemolytic strep.
- Optochin soaked disc placed in agar, growth observed
How does the Optochin test?
-Sensitive = strep. pneuomniae = clear zone of no growth around disc
-Resistant = Viridans strep. and other alpha haemolytic strep. grow around disc
How does the oxidase test?
- oxidase +ve = blue (aerobic) = V. cholerae
- oxidase -ve = no colour change = bacteria may be aerobic or anaerobic
What is the use of Macconkey agar?
- Only grows gram -ve bacilli
- Useful to differentiate between lactose fermenting and non-lactose fermenting gram -ve bacilli
- pink/red = lactose fermenting = E.Coli
- White/transparent = non lactose fermenting = Salmonella
What is the use of XLD agar?
- Differentiate salmonella vs shigella
- Red/pink colonies with some black spots = Salmonella
- Red/pink colonies = Shigella
How to find, diagnose and treat Staph. aureus
Clinical presentation
- Pain in shoulder
- Elevated temperature
- Osteomyelitis (bone infection) C6 & C7
Diagnose
- Blood culture shows staph. aureus
- Coagulase +ve
Treat
- FLUCLOXACILLIN 3 MONTHS
What is the cause,test and treatment of impetigo, boils, cellulitis, endocarditis, toxic shock syndrome?
CAUSE - Strep. Aureus
TEST - Gram +ve, Coagulase +ve
TREAT - Flucloxacillin, co-amoxiclav, macrolides
What is the cause,test and treatment of MRSA?
CAUSE - Staph. Aureus
TEST - Gram +ve, Coagulase +ve
TREAT - Gentamicin, vancomycin
What is the cause,test and treatment of surgical wound infections, septicaemia, endocarditis?
CAUSE - Strep. epidermidis
TEST - Gram +ve, Coagulase -ve
TREAT - Co-Amoxiclav
What is the cause,test and treatment of acute cystitis?
CAUSE - Strep. saphrophiticus
TEST - Gram +ve, Coagulase -ve
TREAT - Nitrofurantoin, Trimethoprim
What are the key differences between gram +ve and -ve bacteria?
- Gram +ve = single cytoplasmic membrane, large amount of peptidoglycan, no endotoxin, STAIN PURPLE
- Gram -ve = Inner & outer membrane, small amount of peptidoglycan, endotoxin present, STAIN PINK
State the different types of antibiotics
- Beta lactams glycopeptides
- Metronidazole and Rifampicin
- Fluroquinolones
- Aminoglycosides,Tetracyclines, Macrolides, Chloramphenicol
- Sulphonamides, Trimethoprim
How do beta lactam gylcopeptide antibiotics work?
- Bind to bacterial cell wall
- Inhibition of cell wall synthesis
How do Metronidazole and Rifampicin antibiotics work?
Interfere with nucleic acid synthesis or function
How do Fluroquinolones antibiotics work?
Inhibit DNA gyrase
How do Aminoglycosides, Tetracyclines, Macrolides, Chloramphenicol antibiotics works?
Inhibit ribosomal activity and protein synthesis
How do Sulphonamides and Trimethoprim antibiotics work?
Inhibit folate synthesis
What are the 4 species of protazoa responsible for malaria?
- P. falciparum
- P. Ovale
- P. vivax
- P. malariae
Which mosquito is responsible for malaria?
Female ANOPHELES mosquito
What is the pathology of malaria?
- Anaemia
- Cytokine release
- Widespread organ damage due to impaired microcirculation
What are the clinical features of malaria?
- Incubation period = 10-21 days
- Chills & sweats
- Myalgia (muscle pain)
- Fatigue
- N,D&V
What is the life cycle of malaria?
Mosquito > sporozoites > human blood > liver > merozoites > RBC > gametocytes > mosquito
