Microbiology Flashcards
What is a gram stain?
- A type of stain which differentiates gram positive and gram negative bacteria.
What colour will gram positive bacteria go on a gram stain?
- Purple (retain crystal violet)
- Mainly cocci
What colour will gram negative bacteria go on a gram stain?
- Pink (fuchsin or safranin)
- Mainly bacilli
What is ‘Come In And Stain’?
- Crystal Violet
- Iodine
- Acetate / Alcohol
- Safarin counterstain
What are some exceptions to the rule that most cocci are gram positive?
- N. meningitidis and N. gonorrhoea are both gram negative (diplo-)cocci
What are some exceptions to the rule that most bacilli are gram negative?
- C. diff and Listeria are both gram positive bacilli
What is blood agar?
- Either horse or sheep’s blood
What is chocolate agar?
- Blood agar + steroids, which has been cooked at 80 degrees for 5 minutes.
- Useful for more fastidious organisms such as H. influenzae
What is CLED agar?
- Cysteine Lactose Electrolyte Deficient agar
- Mainly for urinary bacteria analysis
- Can differentiate lactose fermenting positive bacteria.
What is MacConkey agar?
- Contains natural red dye and lactose
- Differentiates lactose fermenting gram negative bacilli.
How would you differentiate E. coli, Shigella and Salmonella using agar?
- Either CLED agar or MacConkey agar.
- CLED agar. E. coli goes yellow. Salmonella and Shigella goes blue
- MacConkey agar. E. coli goes Pink. Salmonella and Shigella goes yellow/colourless.
- Lactose fermenting and non-lactose fermenting respectively
What is Gonococcus agar?
- Used to grow Neisseria cultures
What is XLD agar?
- Used to differentiate Salmonella and Shigella.
- Salmonella appears red with black centres
- Shigella appears red only
What is Sabourard agar?
- A fungal culture.
- e.g. Candida albicans or aspergillus
What is Löwenstein–Jensen medium?
- A medium used for the culture of mycobacterium
- e.g. Tuberculosis
How would you differentiate S. aureus from other staph bacteria and why is it important?
- S. aureus is much more virulent as it has Coagulase and DNAase
- Two ways:
1. Coagulase test. S. aureus will coagulate rabbit plasma, other staph will not.
2. Culture on blood agar. S. aureus colonies are gold (aureus = gold), other staph will be colourless.
What are the sterile sites within the body and what are the infections with each associates site?
- Blood - Sepsis -> Septicaemia -> Septic shock
- CSF - Meningitis
- Pleural fluid - pericarditis, pleural effusion
- Peritoneum - SBP
- Joints - septic arthritis
- Urinary tract - UTI
- Lower respiratory tract - TB and pneumonia
What are the two ways that viral infection can be detected?
- Viral detection - electron microscopy or PCR
- Serology testing - ELISA, immunofluorescence, complement fixation test
What are the problems with electron microscopy?
- It takes too long
What are the advantages and disadvantages of PCR?
- It is very fast and very sensitive
- It has a risk of a false positive and you need to suspect a specific virus beforehand
What are the disadvantages of using serology testing for viruses?
- You are looking for the Igs vs the actual virus
What does VZV cause?
- Shingles - A red painful rash confined to a single dermatome
What HIV markers should you test for?
- HIV IgG
- HIV RNA
- p24 antigen
What pathogens are likely to cause meningitis in neonates (< 3 months)?
- Group B beta haemolytic streptococcus
- E. coli
- Listeria
What pathogens are likely to cause meningitis in infants (3 months - 6 years)?
- S. pneumonia
- N. meningitidis
- H. influenza (now rare due to vaccine)
What pathogens are likely to cause meningitis in adults (6-60 years)?
- S. pneumonia
- N. meningitidis
What pathogens are likely to cause meningitis in elderly (60+ years)?
- S. pneumonia
- N. meningitidis
- Listeria
What is the most common cause of meningitis?
- Enteroviruses (coxsackie, echovirus)
- HSV-2
- CMV
- HIV
What would you expect to see in the CSF for: appearance, protein content, WCC and glucose levels for a bacterial meningitis?
- Appearance - Yellow
- Protein - increased
- WCC - neutrophilia
- Glucose - <50% serum (decreased)
What would you expect to see in the CSF for: appearance, protein content, WCC and glucose levels for a viral meningitis?
- Appearance - Clear
- Protein - increased
- WCC - Lymphocytosis
- Glucose - >60% serum (the same)
What would you expect to see in the CSF for: appearance, protein content, WCC and glucose levels for a TB or fungal meningitis?
- Appearance - Yellow, Fibrinous
- Protein - increased
- WCC - Lymphocytosis
- Glucose - <50% serum (decreased)
What type of bacteria is N. meningitidis?
- A gram negative diplococcus.
- Causes a blanching purulent rash
What would the diagnoses be if there is a purulent rash and meningism? what is the treatment?
- Gonococcal meningitis
- DO NOT NEED CSF SAMPLE
- IV Ceftriaxone or IV Cefotaxamine and IV dexamethasone
What is the prophylactic treatment for meningitis symptoms within the community?
- IM Benzylpenicillin
- Send them to hospital for admission
What would be the antibiotics for Listeria induced meningitis?
- IV amoxicillin
What are the close contact rules for meningitis?
- Notify Public Health England Immediately
- Close contacts should be given single dose ciprofloxacin prophylaxis
What is the treatment for encephalitis caused by HSV-1?
- IV acyclovir
What would the appropriate treatment be for pneumonia caused by s. pneumoniae?
- Amoxicillin
What would the appropriate treatment be for pneumonia caused by legionella pneumonia?
- Clarithromycin
- Can also use: Erythromycin
Is Legionella a notifiable disease?
- Yes. Public Health England must be notified
What would the treatment be H. influenzae in COPD?
- A gram negative bacillus
- B lactamase +ive (resistant) - Co-amoxiclav
- B lactamase -ive (non resistant) - Amoxicillin
What would the treatment be S. pneumoniae in COPD?
- Amoxicillin
What is a Ziehl-Neelsen stain?
- An acid fast stain used for mycobacterium
Why can’t mycobacterium use gram staining?
- They have mycolic acid in their cell wall which means very waxy and it will not take up the stains.
What is the problems with culturing TB? what are the alternatives?
- It grows very slowly, 15-20 hours doubling time, therefore may take up to 6 months.
- MGIT (Mycobacteria growth indicator tube) or PCR can be used
What is meant by an AIDS defining illness?
- A CD4+ count of less than 200
What is the treatment for pneumocystis pneumonia?
- Cysts of p. Jirovecci fungi
- Co-trimoxazole
- and prednisolone if type 1 respiratory failure
What do 1/3 cases of bronchiectasis have a history of?
- TB
- Whooping cough
- Measles
- CF
What organisms are implicated in bronchiectasis?
- Pseudomonas aeruginosa (an oxidase negative, gram negative bacillus)
- S. pneumoniae
- H. influenzae
- NOTE. CF organisms are the same
What is the treatment for bronchiectasis?
- S. pneumoniae and H. influenzae -> Amoxicillin
- Pseudomonas aeruginosa -> Piperacillin/Tazobactam
What are the side effects of vancomycin and gentamicin?
- Nephrotoxicity
What are the side effects of gentamicin?
- Nephrotoxicity
- Ototoxicity
What are the UTI organisms?
K: Klebsiella spp.
E: Enterococcus faecalis / Enterobacter cloacae - 80%
E: Escherichia coli
P: Pseudomonas aeruginosa / Proteus mirabilis
S: Staphylococcus saprophyticus / Serratia marcescens
What is the management for uncomplicated UTI?
- Trimethoprim 200mg twice daily for 3 days (contraindicated in pregnancy: tetragenic)
- Nitrofurantoin 50mg four times daily for 3 days if pregnant
What is the treatment for pyelonephritis?
- Co-amoxiclav for 7 days
- OR Cefalexin and Trimethoprim for 14 days
What is the main cause of infective endocarditis?
- S. aureus
What is the main salmonella serotype which causes diarrhoea?
- Salmonella enteritidis
What are the main salmonella serotypes which cause thyphoid fever?
- S. typhi
- S. paratyphi
What is the treatment for thpoid fever?
- Fluoroquinolones (ciprofloxacin)
- 4th generation cephalosporins (ceflatrixone)
- Azithromycin
What are the viral causes of diarrhoea and gastroenteritis?
- Adults -> Norovirus
- Children -> Rotavirus
What are the differences between gram positive and gram negative bacteria?
- Gram positive - contain a single thick peptidoglycan cell wall and secrete exotoxins with a specific action.
- Gram negative - contain two thinner cell walls made from peptidoglycan and secrete endotoxins (LPS) which have a non-specific action.
What are the differences between gram positive and gram negative bacteria?
- Gram positive - contain a single thick peptidoglycan cell wall and secrete exotoxins with a specific action.
- Gram negative - contain two thinner cell walls made from peptidoglycan and secrete endotoxins (LPS) which have a non-specific action.
What antibiotics affect the cell wall?
- B lactams.
- penicillin’s and cephalosporins
- Good coverage against gram positive bacteria - strep and staph
When would vancomycin be used?
- To treat MRSA or patients with a penicillin allergy
What antibiotics affect the folate pathway?
- Trimethoprim and Nitrofurantoin
- Cotrimoxazole for PCP
- They inhibit dihydrofolate reductase and stop mTHF -> THF
What are DNA gyrase antibiotics?
- Fluoroquinolones (e.g. Cirpfloxan)
- They are very good against gram negative bacteria
What are RNA polymerase antibiotics?
- Rifampicin which is used in TB.
What is the treatment for TB and what are the complications of each?
RIPES
- R - Rifampicin - Orange urine, hepatotoxicity
- I - Isoniazid - Peripheral neuropathy, neurotoxicity, hepatoxicity
- P - Pyrazinamide - hyperuricemia, arthralgia, hepatotoxicity
- E - Ethambutol - Optic neuritis
- S - Streptomycin - Peripheral neuropathy, neurotoxicity, hepatotoxicity
What are the antibiotics that effect protein synthesis?
- Macrolides (Clarithromycin, Erythromycin, Azithromycin)
- Aminoglycosides (Gentamicin)
- Tetracyclines (Doxycycline)
What is intrinsic antibiotic resistance?
- Natural ability to be resistant to a bacteria.
What is extrinsic antibiotic resistance?
- Spontaneous mutations
- OR horizontal gene transfer
What are the three types of horizontal gene transfer for bacteria?
- Conjugation - bacterial sex pilus transfers plasmid DNA
- Transformation - uptakes DNA plasmid from the environment
- Transduction - bacteriophage mediates DNA exchange between bacteria
What are the three types of protozoa?
- Amoeboids
- Sporozoa
- Flagellates
Give an example of an Amoeboid and the disease it causes?
- Entamoeba histolytica
- Causes parasitic diarrhoea
Give an example of a Sporozoa and the disease it causes?
- Plasmodium genus which causes malaria
- Toxoplasma gondii which causes encephalitis (rarely)
Give an example of a Flagellate and the disease it causes?
- Giardia lamblia which causes severe gastroenteritis
- Trichomonas vaginalis causes vaginal infection
Name the treatment of fungal infection?
- Azole antifungal
- Amphotericin B antifungal
What fungus causes athletes foot?
- Tinea Pedis (foot ringworm)
What are the three helminths/worms and what is an example of each?
- Nematode - Roundworm
- Trematodes - Flatworm
- Cestode - Tapeworm
