MY Microbiology Flashcards
1
Q
Define a pathogen
A
An organism that causes or is capable of causing disease
2
Q
Define commensal
A
An organism that colonises the host but no disease is caused in normal circumstances
3
Q
Define an opportunist pathogen
A
A pathogen that only causes disease if the host’s defences are compromised
4
Q
Virulence =
A
The degree to which an organism is pathogenic
5
Q
What is asymptomatic carriage
A
Where a pathogen is carried harmlessly at a tissue site where it does NOT cause disease
6
Q
T/F
Viruses are larger than bacteria
A
False
7
Q
T/F
Viruses can be viewed on a microscope
A
False
8
Q
What areas of the body are sterile
What happens if bacteriaa enters these organs
A
- lungs
- blood
- kidneys
- gall bladder
- an immune response is raised if bacteria enters these organs
9
Q
What colour does gram negative bacteria stain
A
Pink
10
Q
What colour does gram positive bacteria stain
A
Purple
11
Q
Describe the 2 main basic bacterial morphology
A
Coccus - round
Bacillus - rod
12
Q
Diplococcus =
A
Pair of cocci
13
Q
Streptococcus =
A
Chain of cocci
14
Q
Staphylococcus =
A
Cluster of cocci =
15
Q
Vibrio =
A
Curved rod
16
Q
Spirochaete =
A
Spiral rod
- classified separately to rods
17
Q
6 basic components of a bacterial cell
A
- Cell wall
- Inner membrane
- Outer membrane
- Chromosome of Circular double stranded DNA
- Pili
- Capsule
5+6 not always present
18
Q
Components of gram +ve bacterial cell envelope
A
- capsule
- peptidoglycan
- cytoplasmic membrane
19
Q
Components of gram -ve bacterial cell envelope
A
- capsule
- lipopolysacharide ( endotoxin)
- outer membrane
- peptidoglycan
- inner membrane
20
Q
Which has a thicker peptidoglycan layer - gram +ve or -ve
A
Positive
P for peptidoglycan
21
Q
What is an endotoxin
A
A component of the outer membrane of bacteria
- e.g. lipopolysaccharide in gram -ve bacteria.
22
Q
What is exotoxin
A
- A target specific cellular component
- secreted by both gram +ve and -ve
- e.g. tetanus toxin targets the nervous system ➡️ muscle rigidity
23
Q
What is an exotoxin made of
A
Protein
24
Q
What is an endotoxin made of
A
Lipopolysaccharide
25
How do the action of exo and endotoxins differ
- Exotoxins have specific actions
- Endotoxins have non-specific actions
26
T/F
Both endotoxins and exotoxins are heat labile
False
Endotoxins are stable under heat
27
T/F
Both gram +ve and -ve bacteria produce exo and endotoxins
False
- Both produce exotoxins
- only gram -ve produce endotoxins only they have LPS
28
Which one has strong antigenicity - endotoxins or exotoxins
Endotoxins
[exotoxins have weak antigenicity]
29
What is a toxoid
A toxin treated with formaldehyde so that it loses its toxicity but maintains its antigenicity
30
Which can be converted into a toxoid - endotoxin or exotoxin
Endotoxin
31
How is genetic variation achieved in bacteria
- **Genetic mutation**
- Base substitution
- Deletion
- Insertion
- **Gene transfer**
- Transformation eg via plasmid
- Transduction eg via phage
- Conjugation eg via sex pilus
32
What is the first level of bacterial classification
- Obligate intracellular bacteria Vs
- Bacteria that may be cultured on artificial media
33
Examples of obligate intracellular bacteria
- rickettsia
- chlamydia
- Coxiella
34
Describe the stages of gram staining
**Come In And Stain**
1. Fixation of clinical materials to microscope slide using heat/methanol
2. Application of primary stain: **Crystal violet** ➡️ all cells turning purple
3. Application of mordant = **Iodine** ➡️ crystal violet-iodine complex formed
4. Decolourisation step: distinguishes gram +ve and gram -ve, use **Alcohol** [acetone or ethanol]
5. Application of counterstain: **Safranin** to stain gram -ve pink
35
Purpose of gram staining
To determine whether bacteria is gram positive or negative
36
What is a catalase test used for
Used in gram positive cocci to determine whether chains or clusters are present
37
Catalase +ve test means what is present
Staphylococcus
Clusters
38
Catalase -ve test means what is present
Streptococcus
Chains
39
T/F
Most gram +ve bacteria that commonly cause disease are anaerobic
False - most are aerobic
40
What is the purpose of the coagulase test
To differentiate between staph. Aureus Vs staph epidermidis/ staph. saphrophiticus
41
Coagulase +ve test indicates the presence of…
Staph aureus
42
Coagulase -ve test indicates the presence of…
staph epidermidis or staph. saphrophiticus
43
What is blood agar haemolytic used for
- to determine the type of gram positive streptococcus
44
What are the possible outcomes of haemolysis on blood agar
- alpha haemolytic
- partial lysis ➡️ green streaks on agar
- beta haemolytic
- complete lysis ➡️ colourless streaks on agar
- gamma haemolytic
- no lysis ➡️ no colour change
45
How do the blood agar haemolysis test yield results
- alpha haemolysis ➡️ green streaks because these bacteria produce hydrogen peroxide which partly breaks down haemoglobin
- beta haemolysis ➡️ colourless streaks because these bacteria produce streptolysin [O or S] and that fully breaks down haemoglobin
- gamma haemolysis ➡️ no change as there is no agent that affects haemoglobin
46
When is antigenic servo grouping used
Only in beta haemolytic strep groups
47
What is antigenic sero grouping
- the test for Carbohydrate cell surface antigens
- Lancefield groups A-H and K-V
48
Which sero-groups are most important
Groups A and B
49
Example of a group A strep
Strep. Pyogenes
50
How bacteria classified into antigenic groups
**Lancefield microbead agglutination test**
- Antiserum (antibodies) made that recognise each group are tagged to tiny plastic beads
- they added to a suspension of bacteria
- Antibodies bind bacteria and beads clump together
- Visible to naked eye
51
What is the optochin test used for
To differentiate between alpha haemolytic strep bacteria
- strep. Pneumonia OR
- Strep. Viridens
52
Optochin sensitive bacteria indicates presence of …
Streptococcus pneumoniae
53
Optochin resistant bacteria indicates presence of …
Streptococcus viridens
54
What is MacConkey agar
and what is it used for
- A selective and differential culture medium for bacteria
- used to selectively isolate gram negative enteric bacteria AND differentiate them based on ability to ferment lactose
55
Colourless/ yellow MacConkey agar =
Non-lactose fermenting bacteria present
56
Pink MacConkey agar =
Lactose fermenting bacteria present
57
What colour does a non lactose fermenting bacteria show up as on MacConkey agar
Colourless / yellow
58
What colour does a lactose fermenting bacteria show up as on MacConkey agar
Pink
59
T/F
MacConkey agar can be used for gram -ve bacilli and cocci
FALSE
Only used for gram -ve bacilli
60
Give 3 examples of lactose fermenting bacteria
E.coli
Enterobacter
Klebsiella
61
What is the oxidase test used for
Oxidase test is used to determine if a bacterium produces certain cytochrome c oxidases - enzyme used in electron transport chain.
**this means the bacteria is aerobic**
- therefore oxidase test tests for aerobic or anaerobic bacterium
62
Outcome of a +ve oxidase test
Blue end product produced = aerobic bacterium
63
Outcome of a -ve oxidase test
- no colour change = anaerobic
- e.g:
- shigella
- salmonella
- proteus
64
2 examples of oxidase -ve bacteria
- shigella
- salmonella
65
An example of oxidase +ve bacteria
Pseudomonas [gram negative bacilli]
66
When would an oxidase test be used?
On a gram negative bacilli that has tested negative of MacConkeys agar [colourless] so is non-lactose fermenting
67
What is XLD agar
and how does it work
- A selective growth medium used in the isolation of Salmonella and Shigella
- the agar contains sugar which can be fermented be salmonella but not shigella ➡️ differentiation
68
What is XLD agar used for
To differentiate between shigella and salmonella
- shigella ➡️ red/pink colonies on red agar
- salmonella ➡️ black colonies on red agar
69
What is the Ziehl-Neelson stain
A stain used to identify acid fast organisms, mainly mycobacteria
70
What colour are positive and negative ziehl-neelson stains
- positive = pink
- negative = blue / colourless
71
What does a positive Ziehl Neelson test indicate
Presence of acid fast bacteria = mycobacteria
72
When would you use a ziehl-neelson stain
To test a sample of rods
73
T/F
Ziehl-neelson stain can be used on rods, cocci and spirochaetes
FALSE
- only used on rods
74
what is chocolate agar
heated to 80C for breakdown of haem & release of factors that grow organisms w/ fastidious growth requirements [H.influenzae]
75
what is CLED agar
- cysteine lactose electrolyte deficient agar - contains cytosine + lactose
- a non-inhibitory growth medium that differentiates between organisms in urine & allows classification of lactose fermenting [yellow] & non lactose fermenting [blue] gram-negative bacilli
76
what is blood agar
agar with horse/sheeps blood
used for growing lots of bacteria
least selective
77
What is MacConkey Agar
grows gram negative bacilli (it has bile salts that inhibit the growth of gram positive). Has lactose & red dye
lactose fermenting = pink colonies
non lactose fermenting = white colonies
78
What is Gonococcus Agar?
Contains growth factors to promote the growth of Neisseria species as well as antibiotics & antifungal agents to inhibit the growth of other organisms
79
What is Sabouraud’s Agar?
used to culture fungi. Has antibiotics to inhibit bacterial growth.
80
What is XLD Agar?
- a selective growth medium used to isolate Salmonella and Shigella [non lactose fermenting, gram negative].
- most other gut bacteria turn the agar yellow.
- Salmonella - Red with black centre (produces H2S)
- Shigella - Red only
81
What colour will Staphylococcus aureus appear on blood agar and what colour would other staphylococcus species appear?
S.aureus - gold
S.other - white
82
How can you tell the difference between various Gram Positive bacteria?
+tve Catalase Test = Staphylococcus:
(if positive then do Coagulase Test:)
+tve = S.aureus
-tve = S.epidermidis/S.saprophyticus
-tve Catalase Test = Streptococcus
(if negative then do Haemolysis on blood agar)
alpha haemolysis (partial/green) = requirement for Optochin Test
Optochin Sensitive = S.pneumoniae
Optochin Resistant = S. viridans group
Beta haemolysis (full) = requirement for Lancefield grouping via serology)
A = S.pyogenes
B = S.agalactiae
A,C, G = Tonsilitis, pharyngitis, skin infection
B = Neonatal sepsis + meningitis
Gamma Haeomolgysis (none) = Enterococcus
83
How can you tell the difference between various Gram Negative bacteria?
MacConkey Agar for lactose fermentation Test:
+tve lactose fermentation = E.coli/Klebsiella
-tve Lactose fermentation = Requirement to do Oxidase Test
Oxidase Test:
+tve Oxidase = Pseudomonas, Aueruginosa
-tve Oxidase = Requirement to do XLD test:
Colonies on XLD which are Red with black centers = Salmonella
Colonies on XLD which are Red only = Shigella
84
What are the 2 main approaches to diagnosing viral infection?
Electron Microscopy / PCR
Serology Testing - ELISA, IF, Complement Fixation Test.
85
What is an advantage and disadvantage to PCR?
Very fast but high risk of false positives as it is very sensitive and so can easily be contaminated.
86
What is an advantage and disadvantage to ELECTRON MICROSCOPY testing?
Very specific and sensitive but too expensive and too long
87
What is a differential Diagnosis to Glandular fever and how is this ruled out?
S.pyogenes throat infection - Both EBV and S.pyogenes present with purulent (pus) infection over the tonsils
Take black charcoal swab to rule out S.pyogenes
88
What is BALF?
Bronchoalveolar lavage fluid
Fluid collected from the lungs
Bronchi are washed and this fluid is sent for analysis using PCR
89
What condition can cytomegalovirus cause?
CMV colitis
Characteristic finding is OWL EYE INCLUSION BODIES
90
What is the treatment for CMV infection?
IV ganciclovir
91
What HIV markers can be found in the blood
HIV Igs
HIV RNA
p24 antigen
92
What is chocolate agar
blood agar that's been cooked to release the contents of RBCs
93
what pathogen grows exclusively on chocolate agar
haemophilus influenzae
[loves haem]
94
what samples would you collect for suspected community acquired pneumonia
sputum and blood culture
95
what does A show
beta haemolysis
96
what does B show
alpha haemolysis
97
what does C show
no haemolysis or gamma haemolysis
98
what is the most common causative pathogen of CAP
streptococcus pneumoniae
99
what is invasive pneumococcal disease
who is most susceptible to IPD
- strep. pneumoniae that goes beyond the respiratory tract, e.g. into the blood
- immunocompromised pts, pts who smoke.
100
what is CURB-65 and what is it used for
- a severity score that indicates risk of death secondary to CAP.
- used to guide antibiotic treatment.
C = confusion/delerium
U = urea >7mmol/L
R = Respiratory rate ≥30
B = Blood pressure: diastolic ≤60 OR systolic<90
Age = >65
101
in what scenario is an infection caused by legionella most likely
pts that have been abroad and around AC - e.g. Spain.
102
what samples are collected for suspected legionella infection
blood for culture
urine for urinary legionella antigen test [like a lateral flow]
103
what makes an atypical pathogen atypical
- less common
- not easily cultured
- can't be seen on gram film
- don't respond to beta lactams abx
104
classical causes of CAP
- strep pneumoniae
- H. influenzae
- mortadella catarralis
105
what samples would you take for an acute exacerbation of COPD
- sputum sample - for bacterial culture
- throat swab for virus PCR
106
what is never the first line treatment for atypical CAP and why
amoxicillin
atypical pathogens are not sensitive to beta lactams
107
what is ziehl Nelson staining and what pathogen does it find
- a staining method used for acid and alcohol fast bacteria.
- most commonly find mycobacteria - TB
108
what is an alternative better way of detecting mycobacteria
+ what are the benefits
- auramine phenol fluorescent stain using a flurescent microscope.
- more sensitive and faster than Ziehl Neelson
109
what medium is used to grow mycobacteria
Lowenstein Jensen Slope.
- Contains nutrients that promotes mycobacterial growth.
- contains antimicrobial agents to prevent growth of other organisms. e.g. penicillins
110
histological finding from a lymph node biopsy that indicates TB
caseating granulomata
111
what is a common causative pathogen of pneumonia in HIV pts or immunocompromised pts
pneumocystis jiroveci - a fungus
112
how is a microbial sample of the lower lungs obtained for sample in lower respiratory tract infections
- brocho alveolar lavage = looking down into the lungs and a saline solution is put through the bronchoscope to wash the airways and collected as a fluid sample
- induced sputum sample = breathing in different concentrations of saline through a nebuliser and then carrying out cough-like movements to produce a sample of sputum.
113
what type of pathogens can be treated with co-trimoxazole
- bacteria
- fungi
- Protozoa
combo of trimethoprim and sulfamethoxazole
[tri = 3 different pathogen types]
114
what CD4 count is indicative of AIDs
<200
