Bacteriology lab Flashcards
What is meant by best-guess diagnosis
The initial diagnosis of infection is based on the principles of making an informed best-guess clinical diagnosis.
Present illness: date of onset, symptoms, signs (esp. rashes)
Past history: previous infections particularly with resistant organism e.g. MRSA, hospitalizations, travel, antimicrobial use.
Use these to inform what you want the lab to look for- important when taking samples from non-sterile sites
State some microbiological possibilities for Billy’s case
Infectious Diarrhoea Endocarditis (SBE) -sub-acute bacteria Syphilis - STI causing skin rash Toxoplasma- lymphadenopathy and reactivation in immunocompromised Tuberculosis Brucellosis - unpasteurised milk Melliodosis
State the common diagnostic techniques used in microbiology
Culture i) Sterile sites (blood/CSF) ii) Non sterile sites
Serology
Molecular Techniques
Antimicrobial Susceptibility Testing
Why is culture still used over molecular methods
Culture still used – to analyse sensitivity
Molecular methods- need to know mechanism of resistance – but don’t need to know for cultures
Whole genome sequencing- to correlate with phenotype
Describe the difference between sterile and non-sterile sites
Sterile- should be no bacteria- bone- csf, blood
Non sterile- commensal bacteria
Non-sterile- only report what may be relevant from clinical details and travel history- look for your differential
Describe serology
Serology- look for anitbodies in immunitry
Evidence of infection- when organisms are difficult to grow or molecular tests not sensitive enough- i.e syphilis
Describe how skin is not a sterile site
Rosebury estimates that 50 million individual bacteria live on the average square centimeter (5x107/cm2) of human skin
[5x107/cm2 x 20,000 cm2/person = 1011 bacteria]
He describes the skin surface of our bodies as akin to a “teeming population of people going Christmas shopping.”
What bacteria is commensal in the nasopharynx
Streptococci
What’s important to remember about microbial susceptibility testing
- Antimicrobial susceptibility testing – used to test AB resistance but takes a long time
Describe the conditions that blood cultures are kept in.
Place into plastic bottles with growth medium
Machine agitates bottle (to ensure homogeneous spread of nutrients), warm it- 37 degrees- to increase growth
Rich broth and favourable conditions promote the growth of bacteria
Describe how a positive result is detected by the machine
Aerobic and anaerobic
Paediatric blood culture- small amounts of blood- strep
Indicator- bacteria produce toxic products- which will change colour- machine detects colour change
Beads to absorb antibiotics- to maxmise growth
What should be done once a positive blood result is flagged
Flag positive after 16-20 hours- then take from machine – sesntivity test
Agar plates- susceptibility testing
Antibotics can give false negatives- no growth- so place pus there – patient had no antibiotics- no growth
Send samples before you start patient on antibiotics
As you spred out- diluted antibiotic– so you get growth
You should then go gram-testing
Summarise the key features of gram negative and gram positive bacteria
Gram +ve = thick wall, purple stain, retains dye.
Gram –ve = thin wall, pink stain, loses dye and outer LPS layer
Many AB target the cell wall but may not be able to get past the outer membrane in gram- bacteria – e.g. vancomycin works only on +ve.
o Gram POSITIVE = skin and soft tissue.
o Gram NEGATIVE = abdomen and urinary tract
What are two components of the bacterial cell wall
NAM=N-acetylglucosamine NAG=N-acetylmuramic acid
Summarise staphylococci
Gram+ cocci are the most common bacterium.
Staphylococci often form clumps and look like bunches of grapes as they bud divide.
You can do a coagulase test to test between coagulase± staphylococci:
o Coagulase + = Staphylococci aureus.
o Coagulase - = Common skin microbes.
Prosthetic materials are prone to biofilm formation by staph. Infections which can be very bad – e.g. in hip replacements.
Describe the morphology of gram + cocci
Gram positive- retain crystal violent niadine gets trapped- deep purple
Shape- identification
Clumps- staphylococci- grapes
Name two antibiotics that can be used to treat staphylococcus
Methicillin or vancomycin
What is the purpose of the coagulase test
Two groups- coagulase test (but other ways)
Shows ability to form a clot in horse plasma
Lots of coagulase negative in skin- so blood cultures- will see gram positive cocci which are coagulase negative- so test differentiates between virulent can commensal/contaminant staph cooci
Compare the virulence of coagulase positive and negative Staphylococci
Staphylococcus aureus (including Methicillin Resistant Staphylococcus aureus, MRSA)
Severe infections eg: skin/soft tissue, endocarditis, osteomyelitis
Coagulase Negative Staphylococci
Skin commensals of low pathogenic potential. Can infect prosthetic material causing line, pacemaker infections and endocarditis
How can we kill coagulase negative staphylococci when takin blood sample
Let alcohol dry to kill commensal stapy cocci
Summarise streptococci
Streptococci generally form chains in the gram stain.
On blood agar, streptococci separate into two groups:
o Alpha haemolysis – incomplete haemolysis, turns green.
E.G. Streptococcus pneumoniae.
o Beta haemolysis – complete haemolysis, clears agar.
E.G. Group A – Streptococcus pyogenes – skin/soft tissue infection
E.G. Group B – Streptococcus agalactiae – sepsis in the young
Strep – sensisitve to penicllins and cephalosporins
What are two likely causes of gram positive cocci in chains
Divide into chains- will be enterococci or streptococic
