On gram stain, what colour will appear if gram positive
Purple
Why do gram positive bacteria stain purple
Contains peptidoglycan that is stained by crystal violet
On gram stain, what colour will appear if gram negative
Pink
Why do gram negative bacteria stain pink
Has a lot less peptidoglycan that is not stained, by instead stained by safranin
Appearance of Staphylococcus
Gram-positive cocci in clusters
Appearance of Streptococcus
Gram-positive cocci in chains
What is another way of distinguishing between staph and strep (not clusters or chains)
Catalase test
Describe catalase test
Hydrogen peroxide + Staphylococci -> Gas bubbles
+ve for staph
-ve for not staph (or strep)
You know that bacteria is Staphylococcus as positive catalase test. How would you test for Staph.aureus
Coagulase test
+ve = staph aureus as it produces coagulase that converts fibrinogen to fibrin
-ve = coagulase -ve staph
Example of a coagulase negative staphylococcus
Staph epidermis
You know a bacteria is an Alpha Haemolytic Streptococcus. How would you further differentiate between species
Optochin test:
Sensitive means Strep. pneumoniae
Resistant means Viridans strep
How can you tell if a Strep is beta, alpha or gamma haemolytic
Blood agar
Beta haemolysis = complete haemolysis
Alpha = partial
Gamme - none
Appearance of Streptococcus pneumoniae on microscopy
Gram Positive Diplococci
How would you further differentiate between beta haemolytic streptococci
Lancefield test - groups bacteria based on carbohydrate composition of bacterial antigen on cell wall, normally for beta-haemolytic strep
Examples of beta haemolytic streptococci and their group
A: Streptococcus pyogenes
B: Streptococcus agalactiae
D: Enterococcus, Strep bovis (group D is not beta-haemolytic)
Examples of gram positive bacilli
Listeria monocytogenes
Propionibacterium acne – acne
Clostridium difficile – diarrhoea from antibiotic overuse (those starting with C)
What does MacConkey agar contain
Bile salts, lactose and neutral red (pH indicator)
What is the purpose of MacConkey agar
Identifies bacteria that can ferment lactose (normally gram -ve bacilli or enteric bugs)
How does MacConkey agar work
Ferment lactose -> produce acid -> pH drop <6.8= pink, or if pH>6.8, remains normal colour
Therefore:
Lactose fermenting gram -ve bacilli = pink
Non-lactose fermenting gram -ve bacilli = yellow
Examples of bacteria (bacilli) that appear pink on MacConkey agar
(Enterobacteriaceae (coliforms)):
Escherichia coli
Klebisella pneumoniae
Examples of gram -ve non-lactose fermenting bacilli and what colour they appear on MacConkey agar
Appear YELLOW
salmonella, shigella, pseudomonas aeruoginosa, proteus mirabilis
Further differentiation between Gram negative non-lactose fermenting bacteria
Oxidase test:
Black/purple = +ve = pseudomonas (additional anti-pseudomonal snesitivity tests required to identify this) or Neisseria(note it is cocci not bacilli)
Colourless = -ve = Shigella, salmonella, proteus
How could you specifically identify Proteus mirabilis (Gram-ve bacilli that is oxidase negative and non-fermenting)
Urease test
+ve
Examples of gram -ve cocci
Neisseria meningitidis, gonorrhoea
Moraxella catarrhalis
Appearance of Vibrio cholera
Gram -ve comma/curved shaped
Rice water stool
Examples of Gram-ve bacteria with a HELICAL SHAPE
Campylobacter jejuni – bloody diarrhoea
Helicobacter pylori – gastric and duodenal ulcer
Appearance of Haemophilus influenza
Gram negative coccobacilli
What is used to identify fastidious bacteria
Chocolate agar (Neisseria meningtidis also requires chocolate agar)
What is chocolate agar
blood agar but with lysed RBCs due to heating to 80 deg C (haem from factor X)
What medium is used to culture mycobacteria
Lowenstein-Jensen medium
What stain is used to identify mycobacteria
Ziehl-Neelsen stain
Antibiotic for MRSA
Vancomycin
Antibiotic for Entamoeba histolytica or Giardia
Metronidazole
Antibiotic for Pneumocystis pneumonia
Co-trimoxazole
Antibiotics that could be used for C. difficile
Vancomycin/metronidazole
When would you give Metronidazole antibiotic
Entamoeba histolytica
Giardia
How would you decide on treatment for pneumoniae
CURB65 score
Modes of action by antibiotics
Antibiotics that inhibit cell wall synthesis
Antibiotics that inhibit protein synthesis
Antibiotics which inhibit nucleic acid synthesis
Examples of antibiotics that inhibit cell wall synthesis
Glycopeptides
Beta lactams
Examples of glycopeptides
Vancomycin
Teicoplanin
Examples of beta lactams
Penicillins
Cephalosporins
Carbapenems
Examples of penicillins
Benzylpenicillin
Flucloxacillin
Amoxicillin
Examples of cephalosporins
1st gen - Cephalexin 2nd - Cefuroxime 3rd - Ceftazidime (penetrate CNS) 1st gen more gram positives 3rd gen more gram negatives
Example of carbapenems
Imipenem
Ertapenem
Antibiotics that inhibit protein synthesis
Chloramphenicol
Macrolides
Tetracyclines
Aminoglycosides
Examples of macrolides
Carithromycin
Erythromycin
Examples of tetracyclines
Tetracycline
Doxycylcine
Examples of Aminoglycosides
Gentamycin
Streptomycin
Antibiotics that inhibit nucleic acid synthesis
Inhibit folate synthesis
Inhibit DNA gyrase
Binds to RNA polymerase
DNA strand breaks
Antibiotics that break DNA strands
Metronidazole (nitroidimazoles)
Antibiotics that bind to RNA polymerase
Rifampicin
Antibiotics that inhibit gyrase
Fluoroquinolones:
Ciprofloxacin, Ofloxacin, Levofloxacin
Antibiotics that inhibit folate synthesis
Trimethoprim
Sulfonamides: Sulfamethoxazole
Co-trimoxazole (trimethoprim and sulfamethoxazole)
Examples of fungi
Candida albicans
Pneumocystis pneumonia
Aspergilliosis
Treatment of candida albicans
Treat with antifungal e.g. topical nystatin or oral fluconazole
Treatment of pneumocystis pneumonia
Co-trimoxazole (fungi treatment)
Which patients get infected by aspergilliosis
Cystic fibrosis
Bronchiectasis
examples of helminths
Wucheria bancroftii
Enterobius vermicularis
Schistosomiasis
Hookworm
What is pneumocystis pneumonia
Fungi
What is Wucheria bancroftii
Helminth
Filariasis causing lymphoedema
What is Enterobius vermicularis
Helminth
Common in UK, Itchy bum, spreads in household
What can result from schistosmiasis
Squamous cell bladder cancer
What is the worldwide cause of iron-deficiency anaemia
Hookworm
*Treatment of hookworm
Mebendazole
Examples of protozoa
Entamoeba histolytica
Malaria
Giardia
What is Enterobius vermicularis
Helminth
What is Entamoeba histolytica
Protozoa
What is candida albicans
Fungi
What is schistosmiasis
Helminth
What is Wucheria bancroftii
Helminth
What is Giardia
Protozoa
What is aspergilliosis
Fungi
What is malaria
Protozoa
What is Pneumocystis pneumonia
Fungi
Describe symptoms of Entamoeba histolytica
Bloody diarrhoea
Liver abscess
*Treatment of Entamoeba histolytica
Metronidazole
What causes diarrhoea in Giardia
Alteration of intestinal villi, reducing absorption
*Treatment of Giardia
Metronidazole