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
