Lab work

Question 1

MacConkey’s medium

Answer 1

=selective for gram -ve bacteria

–>due to inclusion of crystal violet & bile salts that inhibit growth of gram +ve bacteria

Question 2

Selective Medium

Answer 2

=contains compounds allowing growth of organisms of interest while inhibiting the growth of other organisms

Question 3

Differential Medium

Answer 3

=allows discrimination between two or more organisms by evoking different colours etc.

Question 4

T.b. rhodesiense
(acute) E & C Africa
Treatment:

Answer 4

Stage of disease
1 = suramin
2 = melarsoprol

Question 5

T.b gambiense
(95% of sleeping sickness infections due to this sub-species)
W &C Africa
Treatment:

Answer 5

Stage of disease
1= pentamidine
2= eflornithine or nifurtimox and eflornithine combination therapy (NECT)

Question 6

Why is isolating single colonies on a streak plate culture important

Answer 6

• single colonies allow you to isolate organisms in pure culture and check for contaminants based on colony characteristics
• the inoculum is streaked several times across the plate, each time spreading out the cells
• flaming loop inbetween each streak reduces cells

Question 7

What procedure would help you discriminate between E. coli and P. mirabilis?

Answer 7

-plate bacteria onto MacConkeys medium.
E.coli ferments the lactose in the medium, producing acid, turning the phenol red indicator pink, while P.mirabilis is a lactose non-fermenter and no colour change is observed

Question 8

Tests to discriminate between two forms of sleeping sickness

Answer 8

• Blood sample and perform a LATEX/T.b gambiense aggulation assay - if aggulation is observed, patient is infected with T.b gambiense
• No serological test for T.b rhodesiense, so PCR test for presence of the SRA gene (present in rhodesiense but not in gambiense) could be performed

Question 9

What are bacteriophage?

Answer 9

viruses that infect bacteria

-most are DNA viruses

Question 10

Replication of filamentous bacteriophage

Answer 10

-eg M13 do not lyse their host cells but are released via budding process

Question 11

Virulent bacteriophage

Answer 11

eg T4 , lyses and kills bacterial host as it replicates

Question 12

Temperate bacteriophage

Answer 12

-eg lambda, can undergo a lytic lifecycle or a lysogenic lifecycle where it is replicated in concert with the host genome and may integrate into genome

Question 13

How does a virus plaque form?

Answer 13

• plaques are formed by lytic viruses
• when the virus infects the host cell, it replicates & lyses the host cell, and then infects the neighbouring cells, in turn lysing them & spreading
• viruses are titred as plaque forming units/ml

Question 14

What temp do water and soil microbes grow at?

Answer 14

20-25 degrees C

Question 15

What temp do bacteria of faecal origin grow at?

Answer 15

37 degrees C

Question 16

Commensal organisms

Answer 16

• live in body without causing harm to host

- eg E.coli

Question 17

Opportunistic pathogens

Answer 17

-can cause disease if immune system of host becomes compromised eg Staphylococcus aureus

Question 18

Primary pathogens

Answer 18

• cause disease in healthy hosts

- eg. Trypanosoma brucei

Question 19

Cell counts

Answer 19

• viable count= cells viable under conditions

- total count = viable cells and dead cells

Question 20

What might plating a microbial isolate on Hugh and Leifson medium tell you about its biochemistry?

Answer 20

• hugh and leifson medium contains glucose as a C source & pH indicator (bromo blue) blue–> green –> yellow
• distinguish between exaditive & fermentative metabolism of glucose

Question 21

What media could you use to isolate Neisseria gonorrhoeae

Answer 21

• chocolate agar is a non-selective medium that enriches for Neisseria
• Thayer - Martin agar incorporates abs to suppress growth of normal flora

Question 22

Eosin-methylene blue agar:

Answer 22

• inhibits growth of gram +ve bacteria (methylene blue)
• contains lactose & lactose fermenters produce acid
• acid channges eosin from colourless –> red (some acid) or colourless –>black (lots of acid)
• hence differential for lactose fermenters (strong and weak) and non fermenters

Question 23

How would you discriminate between S.aureus and S.epidermidis in the lab?

Answer 23

• plate on nutrient agar: S.aureus colonies are yellow; S.epidermis colonies = white.
• Coagulase test: s.aureus is coagulase +ve; S.epidermidis is coagulase -ve.
• salt agar: S.aureus ferments mannitol turning medium yellow, S.epidermidis does not

Question 24

E. coli vs E. aerogenes vs E. faecalis in lab:

Answer 24

E. coli & E. aerogenes = gram -ve rods

Enterococcus faecalis = gram +ve coccus

Question 25

E.faecalis vs S. aureus in lab:

Answer 25

• s. aureus tends to form clumps of cells
• e. faecalis tends to form chains

-both are gram +ve cocci

Question 26

Epidemic

Answer 26

=one region or country

eg UK foot and mouth disease 2001

Question 27

Pandemic

Answer 27

=worldwide

eg Influenza A H1N1

Question 28

Gram +ve bacterial cell

Answer 28

• higher amount of peptidoglycan

- presence of teichoic acid

Question 29

Gram -ve bacterial cell

Answer 29

• two membranes (therefore periplasmic space)

- presence of lipopolysaccharide

Question 30

Why don’t gram -ve cells retain the violet stain>?

Answer 30

• due to thin layer of peptidoglycan
• this allows the insoluble complex of crystal violet &iodine that forms inside the cell in the first stages of gram staining to be readily extracted by the alcohol step

Question 31

Why is the heat step important during malachite green staining of fixed films of bacteria?

Answer 31

• malachite green staining is for visualising endospores
• tough coat on endospores requires vigorous treatment for stains to penetrate
• without heat step, endospores = colourless against the safranin - counter stained cells