ICS - Microbiology Flashcards
1
Q
What is meant by gram positive bacteria?
A
They have a thick peptidoglycan layer and no outer lipid membrane. They retain the colour of the crystal violet stain in the gram stain
2
Q
What is meant by gram negative bacteria?
A
They have a thin peptidoglycan layer and have an outer lipid membrane
3
Q
What does a gram stain tell you?
A
If you have a bacterial infection - will either show gram positive or gram negative
4
Q
Approach to identifying bacteria - 3
A
- Gram +’ve / -‘ve
- Morphology/ shape - cocci, bacilli, etc.
- In clusters or chains
5
Q
Colour of gram stain positive ?
A
purple
6
Q
Colour of gram stain negative ?
A
pink
7
Q
How many membranes do gram stain negative bacteria have?
A
2
8
Q
Bacteria shapes
A
Cocci, bacilli, spirochaete
9
Q
Coccus bacteria shape
A
Oval/ spherical
10
Q
Bacilli bacteria shape
A
Rod shaped
11
Q
Staphylcocci bacteria
A
gram-positive cocci that grow in clusters
12
Q
Strepococci bacteria
A
gram-positive cocci that grow in pairs or chains
13
Q
Define pathogen
A
disease causing microorganism
14
Q
Define commensal
A
organism which colonises the host but does not cause disease
15
Q
Define opportunist pathogen
A
Microbe that only causes disease if the host defenses are compromised
16
Q
Define virulence/ pathogenicity
A
degree to which an organism is pathogenic
17
Q
Define asymptomatic carriage
A
When a pathogen is carried harmlessly at a tissue site where it causes no disease
18
Q
Purpose of the catalase test?
A
to differentiate between staphylococci and streptococci
19
Q
Explaining the results of the catalase test
A
Enzyme in bacteria which catalyses the decomposition of H2O2 –> H2O and O2. Therefore, when a bacterium possesses catalase, when H2O2 is added, oxygen bubbles are observed. particular types of bacteria (e.g. Staphylococci) are catalase-positive
20
Q
Catalase test
A
Add H2O2 to bacteria and see for bubbles (oxygen) = positive result
21
Q
Results of catalase test?
A
Bubbling = staph
No bubbling = non staph
22
Q
Coagulase enzyme
A
produced by Staphylococcus. aureus that converts soluble fibrinogen –> insoluble fibrin
23
Q
What does the coagulase test differentiate between?
A
Staphylococci. aureus and other Staphylococci
24
Q
Results of the coagulase test?
A
Staphylococcus. aureus - coagulase positive - clumping
Other Staphylococci - coagulase negative - no clumping
25
Gram stain test procedure
1. Apply primary stain such as crystal violet to bacteria
2. Add iodide which binds to crystal violet and fixes it to the cell wall
3. Decolourise with ethanol/ acetone
4. Counterstain with safranin (pink)
26
Haemolysis is the ability of bacteria to
Break down RBC in blood agar, it requires the expression of haemolysin
27
Substance in bacteria that causes haemolysis
Haemolysin
28
Beta haemolysis
Clear colourless zone appears around the bacteria colonies
| e.g. Staphylococci pyogenes, Staphylococci agalactiae, etc.
29
Alpha haemolysis
Indistinct zone of partial lysis of RBCs around the colonies of bacteria, often gives a green/brown discolourisation
30
Important bacteria that you need to distinguish that is alpha haemolytic
streptococcus pneumoniae - causes lobar pneumonia and meningitis
31
How to test for streptococcus pneumoniae?
Perform the optochin test
32
Result of optochin test?
streptococcus pneumoniae are optochin +'ve, whereas other alpha streptococci are resistant
33
alpha streptococci that cause infective endocarditis?
streptococci viridans
34
result of optochin test for streptococci viridans?
resistant, no clear ring
35
Positive result of optochin test?
clear demarcated ring around optochin disc
36
Optochin
Antibiotic that will inhibit the growth of sensitive bacteria e.g. pneumococci
37
Staphylococcus aureus is beta haemolytic, therefore what two tests can identify this strain?
- Positive coagulase test (clumping)
| - Positive beta haemolysis test - clear ring around colony
38
Gamma haemolysis
Implies no haemolysis
39
Bacteria sensitive to optochin
streptococci pneumoniae
40
Bacteria resistant to optochin
viridans streptococci and other alpha haemolytic streptococci
41
Oxidase test purpose?
Test if the bacteria contain cytochrome oxidase enzymes (in oxidative phosphorylation) - i.e. respires aerobically
42
All bacteria that are oxidase positive are -->
aerobic
43
bacteria that are oxidase negative are -->
either aerobic or anaerobic
44
Oxidase positive
blue result - bacteria is aerobic
45
Oxidase negative
no colour change
46
Macconkey agar only grows
gram-negative bacilli
47
Staphylococcus are gram
positive
48
Lancefield grouping
Method of grouping catalyse negative, coagulase negative bacteria based on bacterial carbohydrate surface antigens
49
Staphylococcus
- > 40 species
- Coagulase +'ve or -'ve
- Catalase positive
- S. aureus - coagulase +'ve
- Normal habitat - nose or skin
50
Staphylococcus aureus - common clinical presentation
pain in shoulder, elevated temperature, causes osteomyelitis (bone infectio)
treat with flucloacillin for 3 months
51
Staphlococcus aureus
- Coagulase positive
| - Spread by aerosol and touch
52
MRSA =
methicillin resistant Staphylococcus aureus
53
MRSA is resistant to
Beta-lactams antibiotics, gentamicin, erythromycin, tetracycline
54
Virulence factors of staphylococcus aureus
- Pore-forming toxins
- Proteases
- Toxic shock syndrome
55
Associated conditions with staphylococcus aureus
Wound infections, osteomyelitis, toxic shock syndrome, food poisoning
56
Two examples of coagulase negative staphylococci?
staphylococcus epidermis, staphylococcus saprophyticus
57
Three ways of classifying streptococci?
1. haemolysis
2. Lancefield typing
3. Biochemical properties
58
Lancefield typing of both catalase and coagulase negative bacteria based on carbohydrate cell surface antigens - important groupings
Group. A - S. pyogenes
| Group B - S agalactiae
59
Infections caused by Streptococci pyogenes
Cellulitis, tonsilitis, scarlet fever
60
Streptococci pneumoniae - common presentation
heavy smoker with nasal congestion and fever, gets cough and chest pain, rust coloured sputum, chest x-ray shows fluid in lung
61
Viridans streptococci is the collective name for
oral streptococci
62
Where is the normal commensal of Streptococci pneumoniae?
oropharynx
63
Infections caused by Streptococci pneumoniae?
Pneumonia, meningitis, otitis media (middle ear infection), sinusitis
64
Virulence/pathogenic factors of Streptococci pneumoniae?
- Polysaccharide capsule
| - Cytotoxin
65
Types of bacterial toxins?
Endo and exotoxins
66
Endotoxin
Component of the outer membrane of bacteria - e.g. lipopolysaccharide in gram -'ve bacteria
67
Exotoxin
secreted proteins of gram +'ve and gram -'ve bacteria
68
Major difference betwen gram +'ve and gram -'ve bacteria?
Presence of lipopolysaccharide on gram-negative bacteria
69
Lipopolysaccharides (LPOs)
Constituent of the outer membrane of gram-negative bacteria and are also referred to as endotoxins
70
Function of LPOs
Help protect bacterium from host defences and can contribute to illness in the host
71
Toxic portion of the LPO
Lipid A
72
Why do gram negative bacteria have more difficulty secreting toxins?
Needs to cross 2 membranes instead of one
73
4 major groups of gram-negative pathogens?
Proteobacteria, bacteroids, chlamydia, spirochaetes
74
Important gram +'ve bacilli
Listeria monocytogenes, propionibacterium acne (acne)
75
What does Macconkey agar indicate?
whether the bacteria can ferment lactose
76
Two key pathogenicity factors/ virulence factors of gram negative bacteria?
colonisation factors, toxins
77
colonisation factors of gram negative bacteria
adhesins and invasins
78
toxins released from gram negative bacteria?
secreted proteins
79
Enterobacteria (coliforms)
- Bacilli
- Most are motile due to flagella
- Macconkey agar can differentiate between lactose and non-lactose fermenting
80
Infections caused by E.COli
- wound infections
- UTIs
- Gastroenteritis
- Traveller's diarrhoea
81
Shigella
Family of gram negative bacteria which cause damage to the intestinal mucosa, causing acute infection of the large intestine and painful diarrhoea
82
Infections caused by salmonella?
Gastroenteritis, enteric fever (typhoid), bacteraemia
83
Example of a slow growing bacteria?
TB
84
Example of a fast growing bacteria?
E.Coli, S. aureus
85
What types of bacteria release endotoxin?
Gram -'ve
86
What types of bacteria release exotoxin?
Gram +'ve and -'ve
87
What are endotoxins?
Lipopolysaccharides, outer membrane component released when bacteria are damaged
88
What are exotoxins?
Proteins secreted from gram positive and gram negative bacteria
89
test to distinguish between staphylococcus and streptococcus?
catalase test
| add H2O2, bubbles of oxygen identifies staphylococcus
90
Flagella is the whole structure, whereas the slender threadlike portion of the flagella is called the?
H antigen
91
Does shigella have an H antigen?
No, non-motile therefore no flagella and no H antigen
92
Does Salmonella have an H antigen?
Yes, since it has a flagellum
93
Does E.coli have an H antigen?
Yes, since it has a flagellum
94
MacConkey agar result
Positive - pink - lactose fermenting
| Negative - yellow/ colourless - non-lactose fermenting
95
MacConkey agar result for shigella and salmonella?
Negative - yellow/ colourless
96
MacConkey agar result for E.Coli?
Positive - pink
97
Why are there different pathogenic strains of E.Coli?
Due to the acquisition of genes from other bacteria
98
Strain of E.Coli associated with traveller's diarrhoea?
Enterotoxigenic E.Coli (ETEC)
99
How does ETEC cause traveller's diarrhoea?
Pilli enable it to adhere to the tissue of the small intestine, binding leads to release of toxin that alters the function of a protein on the GI cell surfaces. Leads to the release of Cl- ions into the GI tract and thus water follows down an osmotic gradient - results in diarrhoea
100
Symptoms of a shigella infection?
severe bloody diarrhoea and frequent passage
101
Shigella is acid intolerant - how is this advantageous?
Can pass through the acidic stomach without being destroyed and move to the intestine
102
Action of shigella in the small intestine?
Macrophage apoptosis, causing release of cytokines and neutrophils --> inflammation
103
What is gastroenteritis?
Frequent cause of food poisoning
104
Symptoms of v.cholerae? Why is it so dangerous?
huge volumes of watery stools, losing huge amounts of water which can result in hypovolemic shock and severe dehydration
105
Pathogenesis of gastroenteritis
1. Endocytosis
2. Chemokine release
3. Neutrophil recruitment
4. Neutrophil induced tissue injury
5. Fluid and electrolyte loss --> diarrhoea
106
In which individuals do you normally get fungal infections?
Immunocompromised
107
Fungal skin infections are usually caused by which type of fungi?
Dermatophytes
108
Dermatophytes reside in the epidermis and require what for growth?
Keratin
109
What is meant by dimorphic fungi?
Can exist as both yeast and mould - switch between the two forms
110
Aim of antimicrobial drug therapy?
Achieve inhibitory levels of agent at the site of infection without host cell toxicity
111
Why are fungal infections rare?
Unable to grow at 37C and are often killed by body's innate and adaptive immune responses
112
3 common fungal infections
nappy rash, tinea pedis (athlete's foot), onychomycosis (fungal nail infection)
113
What might antifungal treatments target?
cell wall and the ergosterol containing plasma membrane
114
What is ergosterol?
Steroid alcohol that serves the same purpose as cholesterol does in membranes for fungi. Crucial for fungi survival, therefore acts as a good target for drugs
115
Antifungal treatments - how does amphtericin work?
targets ergosterol in the plasma membrane and causes pore formation, leading to cell death
116
Antifungal treatments - how do azoles work?
they affect the ergosterol synthetic pathway
117
What is candida?
a yeast, grows in warm moist areas e.g. vagina, penis
118
Test to identify fungal antigens?
Beta-D-glucan test
119
Pneumocystis pneumonia
fungal infection causing fluid build up and inflammation in the lungs, often in the immunocompromised people, presents as a symptom due to HIV infection
120
What are moulds composed of?
branched filamentous filaments called hyphae
121
Difference between bacteria and mycobacteria?
Mycobacteria have an outer membrane, no capsule. Their distinguishing feature is that their cell wall is thicker than in many other bacteria
122
Characteristics of mycobacteria
Aerobic, non-motile, non-spore forming, bacilli, slow reproduction, slow response to treatment, slow growing
123
Two examples of mycobacteria?
M. tuberculosis (TB)
| M. leprae (leprosy)
124
Why is it difficult to perform a gram stain on mycobacteria?
They have a very thick cell wall with a high lipid content
125
Clinical relevance of slow growth of mycobacteria?
Difficult for antibiotics to target the division phase
126
Clinical relevance of thick cell wall of mycobacteria?
Difficult for immune system to target/ damage
127
Acid-fastness is a physical property of some bacteria, what does it mean?
Their resistance to decolourisation by acids during laboratory staining procedures
128
How would you stain mycobacteria?
Using Ziehl-Neelsen stain for acid fast-bacili
129
Positive result for Ziehl-Neelsen stain
Red/pink
130
Nuclei acid detection
Amplification using PCR, produces a rapid result for mycobacterium tuberculosis
131
Mycobacteria immunology
- Acid fast bacilli which are phagocytosed by macrophages to form a phagolysosome
- They have adapted to withstand this phagolysosomal killing and escape into the cytosol
132
Why is antimicrobial treatment prolonged?
Bacteria are slowly replicating
133
Characteristics of viruses?
- Acellular - no membranes or cell organelles
- Outer protein coat and strand of nucleic acid (DNA or RNA)
- Do not carry out metabolic reactions of their own, require host organelles and enzymes
134
Stages of viral replication
1. Attachment - viral and cell receptors (e.g. HIV)
2. Entry - only the viral contents enters host cell (viral nucleic acid and proteins)
3. Interaction with host cell - uses cell materials for replication
4. Replication
5. Assembly - may occur in nucleus, cytoplasm or cell membrane
6. Release - bursting open of a cell or leaking from the cell
135
How do viruses cause disease? (5) COMED
1. Cell proliferation and immortalisation
2. Over-reactivity of the host in response to infection
3. Modification of host cell structure
4. Evasion of extracellular and intracellular host defences
5. Destruction of host cell
136
How might a virus evade host defences?
- Antigenic variability (ability to change antigens in order to evade the host's immune system
- Prevention of host cell apoptosis
- Down regulation of interferon and other intracellular host defence proteins
137
Function of interferons
Modulate the response of the immune system to viruses
138
3 classes of worms?
Nematodes (roundworms)
Trematodes (flatworms)
Cestodes (tapeworms)
139
Nematodes (roundworms)
- Intestinal e.g ascarius lumbricoides
- Skin e.g. larva migrans
- Tissue e.g. filaria
140
Trematodes (flatworms)
- Blood
- Liver
- Lungs
- Intestinal
141
Cestodes (tapeworms)
- Non-invasive
| - Invasive
142
What is the pre-patent period?
Period between infection and appearance of eggs in stools
143
How are intestinal nematodes (roundworms) normally diagnosed?
Stool microscopy looking for eggs
144
How to roundworms get into the host's body?
When the host eats food or drinks water containing the worms, the worm penetrates the hosts skin, an insect such as a mosquito
145
When might humans be infected with cestodes (tapeworms)?
By eating raw contaminated pork, beef or fish, or by consuming food contaminated by faeces from infected animals
146
How are intestinal nematodes spread?
Human to human transmission via eggs or larvae
147
Signs and symptoms of an ascarius lumbricoides infection
Loeffler's syndrome - larval migration to lungs results in a cough, fever and wheeze (infection however is often asymptomatic)
148
Wucheria bancroftii
Filariasis causing lymphoedema
149
Enterobius vermicularis
common in UK, itchy bum, spreads in households
150
Schistosomiasis
Causes squamous cell bladder cancer
151
Hookworm
worldwide cause of iron-deficiency anaemia
152
What are protozoa?
single-celled eukaryotic organisms
153
Main biological role of protozoa?
Consumers of bacteria, algae and microfungi
154
5 major groups of protozoa?
Flagellates, amoebae, sporozoans, ciliates, microsporida
155
How is malaria transmitted?
via the bite of a female mosquito from dusk till dawn
156
Why are there different clinical manifestations of malaria?
Due to the variation in the plasmodia life cycle - has stages in the human and the mosquito
157
Stages of the plasmodia lifestyle in humans?
Exo- and endo-erythrocytic stages
158
Malaria is a protozoan infection caused by ?
Plasmodia sporozoan
159
4 species of malaria causing protozoae?
P. falciparum
P. ovale
P. vivax
P. malariae
160
Why do you get anaemia as a result of malaria?
- Haemolysis of infected RBC
- Haemolysis of non-infected RBC
- Splenomegaly
- Folate depletion
161
Clinical features of malaria
- Fever
- Chills and sweats
- Headache
- Myalgia
- Fatigue
162
Exo-erythrocytic stage of plasmodia life cycle in humans
Hepatocytes become infected by sporozoites, the cells mature and develop, released as tropozoites
163
Endo-erythrocytic stage of plasmodia life cycle in humans
Tropozoites invade teh RBCs, parasite numbers expand rapidly with a sustained cycling of the parasite population
164
Which two specides of the plasmodia genus lie dormant and cause late relapse of malaria?
P. ovale and P. vivax
165
Genetic condition that may give you immunity to malaria?
Sickle cell anaemia
166
Clinical features of haemolysis?
Anaemia, jaundice (dark urine due to increased Hb)
167
Pathogenesis of P.falciparum?
Unique cerebral malaria, fatal infection, parasites mature in RBCs and these collect in the small vessels and cause blockage of cerebro-microvasculature - hypoxia
168
Diagnosis of malaria?
Thick and thin films
169
Diagnosis of malaria - thick films
Tells you if malaria is present, sensitive but low resolution
170
Diagnosis of malaria - thin films
Tells you species and parasite count
171
Can immunity to malaria be acquired?
recurrent infections lead to some immunity, but the immunity is lost if not reinfected after a couple of years
172
Humoral response to viral infections?
Antibodies (IgA - blocks binding of host cell and virus), opsonisation, complement cascade
173
Cell-mediated response to viral infections?
Kill infected cells, macrophages
174
How does virus' evade the host defences?
Interfere with defence, influenza changes coat antigen
175
Which type of antibody blocks the binding of the host cell and the virus?
IgA
176
Bacterial infections - entry to host via?
Respiratory tract, GI tract, GU tract, skin breakage
177
Evasion of host defences - mycobacteria?
Escape from the phagolysosome, live in cytoplasm
178
What does the immune response to a protozoan infection depend on?
Location of parasite within the host (blood, tissue, etc.)
179
immune response to a protozoan infection in the blood?
humoral immunity
180
immune response to a protozoan infection in the tissue?
cell mediated immunity
181
Immune response to worm infections
They do not multiply in humans and they are not intracellular. The immune response is poor and not sufficient enough to kill the worms
182
Infection control - the health act 2006
Infection control is every health worker's responsibility, no longer just the responsibility of the infection control team, possible to be prosecuted
183
Key components of infection prevention and control
Infection prevention and control team, ward teams, microbiology/ virology laboratories, domestic services, pharmacy
184
Diseases in hospital with important infection control policies?
MRSA, TB
185
Hand hygiene - infection control
Single most effective method of preventing cross infection
186
When to wash hands?
Before/ after handling patients, after using toilet, handling any soiled item, after an aseptic procedure
187
When to use alcohol gel?
Following hand washing prior to an invasive procedure or caring for a patient with barrier precautions
188
Endogenous infections
Infection of a patient by their own flora, important in hospitalised patients (especially those with invasive devices or surgical patients
189
How to prevent healthcare endogenous infections?
Good nutrition and hydration, antisepsis, control the underlying disease (drain pus, remove lines and cathethers as soon as appropriate)
190
Disposal of sharps - key points
- Disposal bin is the responsibility of the person using the equipment
- Sharps bin must be correctly assembled
- Never re-sheath or bend needles
- Never overfill a sharps bin
191
Antibiotics
Agents produces by micro-organisms that kill or inhibit the growth of other micro-organisms in high dilution
192
Where on bacteria would penicillin bind?
Bacterial cell wall
193
Define bacteriostatic?
Antibiotics that prevent bacterial growth by inhibiting DNA synthesis
194
Define bactericidal?
Antibiotics that kill bacteria by inhibiting cell wall synthesis
195
What is MIC?
Minimum inhibitory concentration - lowest concentration of a chemical that prevents growth of a bacterium
196
What is concentration dependent killing?
Antibiotics that eradicated pathogenic bacteria by achieving high concentrations at the site of binding
197
What is time dependent killing?
The time that serum concentrations remain above the minimum inhibitory concentration
198
4 diseases that haemophilus influenzae can cause?
Meningitis, otitis media, pharyngitis, exacerbations of COPD
199
What does the HIV envelope contain?
RNA, capsid, reverse transcriptase
200
What is ringworm?
fungal infection
201
Can mycobacteria withstand phagolysosomal killing?
yes, they have adapted to escape into the cytosol
