Introduction to Diagnostic Microbiology Flashcards
2
Q
How differentiate Staphyloccocus aureus from other Staphylococcus species?
A
- -* coagulase positive, other species are coagulase negative
- beta-hemolytic, other species are non-hemolytic
3
Q
What does catalase do?
A
- prevents bacteria oxidative damage by reactive oxygen species, by catalysing H2O2 to water and oxygen
- all Staphyloccus species are catalase-positive
4
Q
What does coagulase do?
A
- leads to fibrin formation around bacteria, protecting it from phagocytosis
- found within Staphylococcus aureus
5
Q
Where is Staphylococci found?
A
- ubiquitous (found everywhere)
- Coagulase-negative staphylocci are part of skin normal flora
: ~30% of normal healthy adults are persistant nasopharyngeal carriers of S. aureus
6
Q
How is Staphylococci spread?
A
- person-to-person spread through direct contact or exposure to contaminated fomites (e.g. bed linens, clothing, etc.)
- among the most common causes of community and/or hospital acquired infections
(some strains are resistant to different antibiotics)
7
Q
How do you differentiate Staphylococcus epidermidis from Staphylococcus aureus?
A
- S. epidermidis is coagulase-negative
- considered less virulent than S. aureus
8
Q
Where is S. epidermidis usually associated with and what is its pathogenesis associated with?
A
- most commonly found associated with prosthetic devices
- pathogenesis associated with slime layer and biofilm
9
Q
What causes hemolytic patterns?
A
- bacteria secrete proteins / metabolites that lyse red blood cells
10
Q
What are the different hemolytic patterns and their characteristics?
A
alpha: incomplete lysis; green
beta: complete lysis; clear
gamma: no lysis
11
Q
Mannitol-salt agar
A
- contains mannitol sugar and pH indicator (phenol red)
- if an organism can ferment mannitol, an acidic byproduct is formed that will cause the pheno red in the agar to turn yellow
12
Q
Which is (are) the catalse-negative species?
a) enterococci
b) staphylococci
c) streptococci
d) micrococci
A
- a) enterococci
- c) streptococci
13
Q
Description of Streptococcus
A
- gram-positive cocci in chains
- catalase-negative
14
Q
What classifications can be used for Streptococcus?
A
1) Lancefield grouping
: based on cell wall carbohydrate antigens
- Group A-W (Strep test RAPID)
2) Hemolytic pattern
15
Q
How can you differentiate the following in a lab:
- S. pyogenes*
- S. pneumoniae*
- S. agalactiae*
Viridans streptococcus
A
1) hemolytic pattern
- S. pneumoniae & Viridans strep are alpha-hemolytic
- S. pyogenes & S. agalactiae are beta-hemolytic
2) Optochin test & capsule presence
- S. pneumoniae is optochin sensitive & has a capsule
- Viridans strep is optochin resistant & does not have a capsule
3) Bacitracin test
- S. pyogenes is bacitracin sensitive
- S. agalactiae is bacitracin resistant
16
Q
S. agalactiae
: Group?
: virulence factors?
: epidemiology?
: transmission?
A
- Group B streptococcus
- virulence factors: polysaccharide capsule
- epidemiology: colonise lower GIT & UGT
: commonly associated with infections of newborns (Meningitis, pneumonia)
- transmission: increased risk if mother is vaginally colonised
17
Q
Viridins streptococci
A
- heterogenous collection of alpha-hemolytic and non-hemolytic streptococci
- colonises the oropharynx, GIT and UGT
- avirulent
: can cause dental carries (S. mutans)
: common cause of subacute endocarditis
18
Q
Differentiation of S. pneumoniae and Viridans streps
A
optochin test
S. pneumoniae: sensitive
Viridans streps**: resistant
bile solubility test
S. pneumoniae: sensitive
Viridans streps**: resistant
19
Q
Biology of Bacillus anthracis
A
- gram-positive rods; single or paired
- aerobic
- spore-forming
20
Q
Virulence factors of B. anthracis
A
- polypeptide capsule
: consists of poly-D-glutamic acid
- toxins
: Protecting Antigen (PA)
: Lethal factor (LF)
: Edema factor (EF)
21
Q
Epidemiology & Transmission of B. anthracis
- reservoir
- transmission
A
reservoir
- animals, skins, soil
- human infections via exposure to reservoirs
transmission
- innoculation (most common form of transmission)
- ingestion
- inhalation (i.e. Wool sorter’s disease)
potential for bioterrorism- anthrax
22
Q
Diseases caused by B. anthracis
: cutaneous anthrax
A
- painless swelling
- progresses to ulcer surrounded by vesicles
- necrotic eschar, central, with erythematous border often with painful regional lymphadenopathy
23
Q
Diseases caused by B. anthracis
: gastrointestinal anthrax
A
- rare
- symptoms are based on site of infection
- swelling
- vomiting, bloody diarrhea
24
Q
Diseases caused by B. anthracis
: pulmonary anthrax
A
- life-threatening
- leads to shock if without treatment
- macrophages ingest bacteria and transport to lymph nodes
- fever, malaise, swelling, dyspnea
25
Q
Diagnosis of B. anthracis
A
- gram-stain: gram-positive rods
- culture: non-hemolytic
- serology
- PCR
- public health reference lab (CDC select agent)
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Treatment for *B. anthracis*
- Ciprofloxacin
- doxycycline
- vaccine: toxoid, AVA, high risk only
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*Bacillus cereus* epidemiology & transmission
- ubiquitous (found in nearly all environments)
- transmission: foodbourne
: associated with fried rice
: food kept warm (not hot)
: buffets
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Pathogenesis & Disease of *B. cereus*
**eye infections**
**gastroenteritis** (most severe form)
- heat-stable form
- heat-labile form
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Heat-stable form of *B. cereus*
- emetic (vomitting) form associated with enterotoxins
: ingestion of enterotoxins, not bacteria
: incubation period short (1-6 hours)
: illness duration short (~24 hours)
: vomitting, nausea, abdominal cramps
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Heat-labile form of *B. cereus*
- diarrheal form
: stimulates adenylate cylate-cAMP system in intestinal epithelial cells
: profuse watery diarrhea
: true infection with bacteria
: contaminated meat and sauces
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Diagnosis of *B. cereus*
**culture and gram-stain**
- clinical specimens
- implicated foods
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Treatment of *B. cereus*
- vancomycin (glycopeptide)
- ciprofloxacin (quinolone)
- gentamicin (amino glycoside)
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Biology of *Corynebacterium*
- Gram-positive; irregular, "club-shaped" rods
- aerobic
- catalase-positive
- non-spore forming
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Virulence factors of *Corynebacterium diphtheriae*
**diphtheria toxin**
- lysogenic bacteriophage
- beta-phage
- introduces exotoxin encoded on tox gene
- toxin inhibits protein synthesis by inactivating elongating factor 2 (EF-2)
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Epidemiology & Transmission of *C. diphtheriae*
- found worldwide
- maintained in population by **asymptomatic carriage** in **oropharynx** & **skin**
- transmission via **respiratory droplets**
- immunisation program in US controls disease
36
Pathogenesis & Disease of *C. diphtheriae*
- respiratory
: sudden onset, malaise, sore throat
: exudative pharyngitis progresses into **psuedomembrane** (whitish plaque)- bacteria, dead cells, fibrin: firmly adheres to underlying tissue
: systemic complications (myocarditis, recurrent laryngeal nerve palsy)
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Diagnosis of *C. diphtheriae*
**- clinical diagnosis**
**- culture:** selective medium- cysteine-tellurite blood agar (CTBA)
: tellurite inhibits growth of most URT bacteria and gram-negative rods
: reduced by *C. diphtheriae-* produces **gray/black colour** on agar
**- PCR:** detect tox gene
**- toxigenicity testing:** producing of exotoxin
**:** Elk test
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Diagnosis of *C. diphtheriae* with Elk test
- sterile paper with diphtheria antitoxin imbedded into agar medium
- *C. diphtheriae* isolates are streaked across plate ar 90 degrees to the antitoxin strip
- toxigenic *C. diphtheriae* is detected because secreted toxin diffuses from area of growth and reacts with antitoxin to form lines of precipitin
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Prevention & treatment of *C. diphtheriae*
- penicillin or vancomycin
- antitoxin
- education on vaccination
- DPT vaccine (diphtheria, pertussis, tetanus antigens)
40
*Myobacterium* biology
- aerobic rods
- cell wall is rich in lipids
: mycolic acids- increases resistance to desiccation and many chemicals
- acid-fast bacilli
41
*Myobacterium tuberculosis* biology
- facultative intracellular pathogen (can live/reproduce inside/outisde of cell)
- aerobic rod
42
virulence factors *M. tuberculosis*
- dormant in macrophages
- cord factor
: 2 mycolic acids together with a disaccharide (2 molecules of glucose)
: inhibits neutrophil migration and damages mitochondria
- sulfatides
: prevent phagosome fusion with lysosome containing bacterocidal enzymes
43
Epidemiology & transmission of *M. tuberculosis*
- reservoir: human lungs
- transmission via respiratory droplets
: coughing, sneezing
: ventilation & isoloationi important to prevent transmission
44
pathogenesis & disease of *M. tuberculosis*
1. enters airways and penetrates alveoli
2. phagocytosed by alveolar macrophages
3. bacteria multiply (days-weeks) in macrophages
4. infected macrophages travel to regional lymph nodes (hilar, mediastinal) and tissues
5. alveolar macrophages release cytokines (IL-12 and TNF-alpha) that increases local inflammation and recruits T-cells
6. T-cells are activated and secrete IFN-gamma, which activates macrophages to increase phagosome-lysosome fusion and intracellular killing of bacteria (phagosome has bacteria)
45
granuloma of *M. tuberculosis*
- prevents further bacteria spread
- necrotic mass surrounded by dense wall of CD4, CD8 and NK cells and macrophages
- bacteria can remain dormant in this stage or can be reactivated years later when / if the patient's immunity decreases
46
clinical manifestations of *M. tuberculosis*
- initially, non-specific complaints of malaise, weight loss, cough and night sweats
: primary infection with resolution or with progression
- reactivation
- disseminated (military) tuberculosis
- sputum may be scant or bloody and purulent (pus)
: cavitary disease with tissue destruction is associated with sputum production with hemoptysis
47
How is clinical diagnosis of *M. tuberculosis* supported
1. **radiographic evidence** of pulmonary disease
2. positive **skin test reactivity**
3. **lab dectection**
48
Lab Diagnostics of *M. tuberculosis*
- Auramine-rhondamine staining bacilli (fluroscent apple green)
- Acid fast stain (Ziehl Neelson method)
- growth is slow on Lowenstein Jensen medium
49
Acid fast stain for *Mycobacterium*
**Ziehl Neelson Method**
- carbolfuschin (primary stain- red colour)
- acid fast organisms resist decolourization with acid alcohol
- after decolourisation, methyelene blue (secondary stain) is added to organisms to counterstain any material that is not acid fast
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