3: Bacteriology Flashcards
Common diagnostic techniques for bacterial infections?
- Culture - Sterile sites (blood, CSF) or Non-sterile sites
- Serology
- Molecular techniques
- Antimicrobial susceptibility testing
What are bacterial cultures used for?
Used to test antimicrobial resistance so you can determine which antibiotic to use
Problem: takes 24hrs to grow and another 24hrs to do susceptibility test
In sterile sites (blood, CSF) culture is easy because there should NOT be any bacteria present
In non-sterile sites (wounds, urine, skin) there are lots of bacteria that may not be causing any problems
What is serology used for?
Looking at body’s response to infection
Example: chicken pox - see if blood sample goes from IgM negative to positive after 10 days. If it goes positive it means you’ve had immune response
If you cant culture the organism this method isnt used (e.g. syphilis)
What are molecular techniques used for?
PCR - you need to know specifically what sequence you’re looking for
Whole genome sequencing - dont need to know what ur looking for
Useful for MRSA because resistance mechanism is coded by the MecA gene - so if you do PCR for this gene you will know its resistant
What is antimicrobial susceptibility testing?
Put microorganism onto agar, then put antibiotic discs on it
What are positive blood cultures?
Take different bottles for aerobic and anaerobic organisms
Bottle has broth inside that provides nutrients for bacteria
Incubated at 37c
Indicator at bottom of bottle changes colour due to waste products of bacteria. So if bacteria present, indicator changes color.
Colour change detected by machines
What do you do after blood cultures are confirmed positive?
Gram stain
Helps select antibiotics, since gram +/- bacteria are susceptible to different antibiotics
Use non-selective agar plates (designed to grow anything)
Chocolate agar = COOKED blood (some bacteria cant lyse RBCs themselves. Most common: Haemophilus influenzae
Macconkey agar = Designed for gram NEGATIVE
Then do sensitivity tests
When do you give antibiotics?
AFTER sending bacterial cultures
Exception: MENINGITIS / MENINGOCOCCAL SEPTICAEMIA
give antibiotics immediately
Difference between gram positive and negative
Gram Positive - Thicker Ptg wall, holds the gram stain and stains PURPLE
Gram Negative - Has an OUTER membrane embedded with LPS + proteins. Stains PINK
Antibiotics that target the cell wall may not get past the outer membrane of gram negative bacteria hence ineffective.
VANCOMYCIN - inhibits Ptg synthesis. doesn’t work for gram negative
How do you distinguish between gram positive cocci?
Staphylococci form CLUMPS
Streptococci form CHAINS
How do you confirm if it’s Staph. aureus?
Staphylococci Coagulase test
Coagulase POSITIVE = Staph aureus (including MRSA)
Coagulase negative = commensal skin microbes, low pathogenic potential. In most cases they’re just contaminants
Severe staph aureus infection can cause:
Endocarditis, osteomyelitis
Skin commensals can also infect:
Prosthetic material -> causes pacemaker infections and endocarditis
What are the general areas of infection in gram positive and negative bacteria?
Gram Positive = skin/soft tissue
Gram Negative = abdomen, urinary tract
How do you distinguish types of streptococci?
Depends on how they look when put on blood agar:
ALPHA haemolysis = INCOMPLETE haemolysis, agar turns green(ish)
BETA haemolysis = COMPLETE haemolysis, agar turns CLEAR
Which strain of strep. is alpha haemolytic?
Strep. pneumoniae - common cause of pneumonia/meningitis
Which strains of strep are beta haemolytic?
Group A strep (Strep. PYOGENES)
Causes skin/soft tissue infection, pharyngitis, rheumatic fever, scarlet fever
Group B strep (Strep. AGALACTIAE)
Causes SEPSIS in children (neonatal septicaemia)
Diabetics are susceptible to group B strep infection.
Gram negative bacilli
Takes up counter stain, appears pink
E.coli look SHORT + FAT on microscopy
List bacterial causes of diarrhoea
Salmonella Shigella Campylobacter E coli O 157 Clostridium difficile Cholera
List parasitic causes of diarrhoea
Amoeba
Giardia
Cryptosporidium
More about E coli O157
Affects young children + elderly
Can lead to haemolytic anaemia syndrome, leads renal failure
More about C difficile
Much more common in children
Antibiotic use -> C difficile colonises the gut
Very common in hospital wards
Viruses are associated with..
ACUTE diarrhoea + vomiting
What can you do with stool samples?
- Grow culture on agar plates
- Only salmonella, shigella, campylobacter looked for routinely
- Different types of agar plates used for different organisms
Parasite test: only for valid reasons, using special stains
C diff is hard to culture: use toxin (antigen) detection (EIA) and PCR for toxin gene
Test for Salmonella?
XLD agar (xylose lysine deoxycholate)
Should see BLACK SPOTS for 2 reasons:
- Salmonella cant ferment XYLOSE
- It forms hydrogen sulfide
Test for campylobacter?
Takes 48hrs to grow
Campylobacter can survive at 42c, so incubate at 42c to KILL other bacteria in stool sample
Put remaining campylobacter on selective agar dish
Test for Vibrio cholerae?
TCBS agar
Cholera makes it go GREEN
What is sensitivity testing?
Testing for Minimum Inhibitory Concentration (MIC)
= lowest amount of antibiotic needed to inhibit growth of bacteria in vitro
Start with HIGH conc of antibiotic, and lower the dose to find the minimum dose needed.
If bacteria has an MIC ABOVE the BREAKPOINT it should be reported as resistant
Can you get systemic disease with bacterial infection?
No
You can get it with parasitic infection and typhoid but ONLY if you’re very immunocompromised
Subacute Bacterial Endocarditis (SBE) can cause:
Rash, fever, possible weight loss
Syphilis/toxoplasma can cause:
Rash, fever, enlarged lymph nodes
Tuberculosis can cause:
Fever, weight loss
Brucellosis can cause:
Possible rash, fever, enlarged lymph nodes
