Bacteriology lab Flashcards
What are some common diagnostic techniques
Culture (sterile and non sterile sites)
Serology
Molecular techniques
Antimicrobial susceptibility testing
Give examples of sterile and non-sterile sites
Blood, CSF
Skin
Describe the process of culturing for bacteria
Watch cell growth
Mostly grown for 5 days at 37 degrees
Disc changes colour due to bacteria reproducing + producing CO2 → alter pH
Describe the use of serology for bacterial diagnosis
1st infection: IgG peaks later than IgM
2nd infection: IgG rises rapidly first + higher than IgM
Describe the use of molecular techniques
PCR chlamydia in genital specimens
Rapid PCR for MRSA
Describe specimen collection
In the acute phase of illness and before staring antimicrobials
Collection from proper site, avoiding contamination by normal flora
Prompt transport to lab since micro-organisms multiply in transit
Adequate quantity and appropriate number of specimens
Acute sera and Convalescent sera (paired), for rising antibody titres
Describe urine testing
Bedside - naked eye (clear, clouds, haemorrhagic)
Dipstick - ketones may indicate infection, nitrates suffuse bacteriuria
Microscopy - WBC (pyuria -> infection)
Culture on MacConkey agar
Antibiotic sensitivity testing of bacterial growth
Describe faeces testing
Naked eye - consistency, blood stain, colour, worm presence
Microscopy - ova, cysts, parasites
Culture on inhibitory media e.g. deoxycholatecitrate agar (DCA) selenite
Toxin detection (clostridium difficile)
Special stains e.g. cryptospridia
What may the cause of diarrhoea be
Bacterial, viral, or parasitic
Bacteria: salmonella, shigella, campylobacter, E Coli 0157, C. difficile, cholera
Describe the grams stain
Separates into gram positive and negative
Gram +ve - thick wall - purple
Gram -ve - thin wall + peptidoglycan wall - pink
Describe the test for staphylococci
Microscope with gram stain
Purple (gram positive)
Appears like clumps of grapes
Can be separated by using a coagulase test
Describe staphylococcus aureus
Includses MRSA
severe infections e.g. skin/soft tissue, endocarditis. osteomyelitis
Compare coagulase -ve and +ve staphylococci
-ve = skin commensals of low pathogenic potential. Probably contaminants from taking blood
+ve = appears in chains - S. aureus (might be MRSA)
Which investigations are available for stool samples
Bacteria
Culture on agar plates (salmonella, campylobacter routinely)
Clostridium difficile (toxin detection or PCR for toxin gene)
Parasite - concentrate and use special stains
What is the MIC
Minimum Inhibitory Concentration
Minimum concentration of antibiotic needed to inhibit bacterial growth
What its peptidoglycan made up of
NAM=N-acetylglucosamine
NAG=N-acetylmuramic acid
What tissue do gram positive and negative bacteria generally infect
Gram positive = skin and soft tissue
Gram negative = abdomen and urinary tract
What is chocolate agar and what is it used for
Cooked blood - certain bacteria can’t lyse RBCs
Cooking releases nutrients in blood agar to allow certain bacteria to grow e.g. Haemophilus influenzae
What is MacConkey and neomycin agar
MacConkey Agar = designed to grow Gram negative organisms
Neomycin agar = growth of anaerobic microorganisms
What is important when sending for agar plates
Usually incubate for 24 hours on plate
Where possible, try + send cultures BEFORE giving patients ABs
However, give antibiotics ASAP in meningitis or meningococcal septicaemia
What is coagulase
Coagulase converts fibrinogen to fibrin
Coagulase is virulence factor which helps S. aureus to cause infection
It doesn’t tend cause infection unless opportunistic circumstances e.g. central lines
What infections can coagulase cause
Can infect prosthetic material causing line, pacemaker infections
How are groups of streptococci divided
Divided depending on blood agar
How is alpha haemolysis detected and give example of bacteria that may cause it
Incomplete haemolysis that turns the agar green
streptococci e.g. S. pneumoniae = pneumonia + meningitis
How is beta haemolysis detected
Complete haemolysis - clears agar
Give an example of a group A beta-haemolytic streptococci and what it may cause
S. pyogenes
Causes skin and soft tissue infections e.g. necrotising fasciitis
Causes rheumatic and scarlet fever
Give an example of a group B beta-haemolytic streptococci and what it may cause
S. agalactiae
Causes sepsis in young children (neonatal septicaemia)
Infections in diabetics
Can streptococci infections be treated with antibiotics
There is no penicillin resistance in these bacteria
Some (e.g. group A) are aggressive but treatable with antibiotics
Describe the use of breakpoints with MIC
Set breakpoints correlate MIC when using the antibiotic
MIC below breakpoint - good correlation with clinical success if use that antibiotic
MIC ABOVE breakpoint - reported as resistant
