Diagnosis of Bacterial Infection

Question 1

what are the results of a gram stain?

Answer 1

pink= negative 
purple= positive

Question 2

what substances can you culture things in?

and what human substances would you use to produce them?

Answer 2

solid media, liquid media, or blood

culture with:
pus
swab
fluid- CSF, joint aspirate, urine
faeces

Question 3

what is the problem with diagnosis with antibody detection?

Answer 3

test rapid itself but it may take around 2 weeks after initial infection for antibodies to be produced

Question 4

other methods of bacterial diagnosis include?

Answer 4

toxin detection
antigen detection
PCR

Question 5

name some gram positive cocci and what colour will they stain?

Answer 5

staphlococci, streptococci (enterococci, like part of strep)
will stain purple

Question 6

name some gram negative bacilli and what colour will they stain?

Answer 6

e.coli, shigella, pseudomonas, salmonella, campylobacter

Question 7

gram positive cocci in:
chains=?
clusters=?

Answer 7

chains= strep
clusters= staphlo

Question 8

You have found staphlococcus under the microscope- what test would you do now?
and what bacteria is it based on the results

Answer 8

do coagulase/DNA test (remember- coagulate in clusters)

if coagulase +ve : s. aureus (remember +ve auras)
coagulase -ve : s. epidermidis or

Question 9

you have found streptococci under the microscope- what test do you do now?
+ what do results look like?

Answer 9

hemolysis- alpha, beta or none
alpha- greening
beta- clearing (remember ABC)
none- no clearance or greening

Question 10

Under the hemolysis test on your streptococci, there is clearance around the bacteria- what type of hemolysis is this and what test would you do now?

Answer 10

(A)BC
beta- clearing
lancefield grouping (A-G)

Question 11

Under the hemolysis test on your streptococci, there is greening around the bacteria- what type of hemolysis is this and what test would you do now?

Answer 11

greening- alpha hemolysis
do the optochin resistance test

(AGRO alpha,greening,resistance to optochin)

Question 12

there is an alpha hemolytic strep, and you wants to find out which strain it is. based on the optochin test- describe the results and suggest which bacteria it is.

Answer 12

resistant to optochin- still grows around the disk: strep viridans

not resistant- there is clearing around the disk: strep pneumonia…

Question 13

you have found a gram negative bacillus under the microscope- what test would you do now and why?

Answer 13

appearance on MacConkey plate

as it inhibits growth of gram positive bacteria and will tell you if its a lactose fermenter or not

Question 14

the bacteria is shown to be a lactose fermenter- how will it appear on the maconkey plate and what bacteria might it be?

Answer 14

if it goes pink, it is a lactose fermenter (sugar=pink)
if it stays pale, it is a non lactose fermenter

therefore the plate is pink and it is likely an enterobacterea (coliform) such as e.coli or klebsiella

Question 15

the bacteria is shown to be a non lactose fermenter- how will it look on the macconkey plate and what test would you do? what do the results mean?

Answer 15

it would be clear and you would do oxidase test
oxidase positive: pseudomonas
negative: enterbacterae

Question 16

gram negative bacilli non lactose fermenters include:

Answer 16

shigella, salmonella, pseudomonas

Question 17

what plate is used to differentiate between shigella and salmonella?

Answer 17

XLD
(stays pink= shigella,
if goes yellow/black dots= salmonella)

Question 18

example of a gram negative cocci is?

Answer 18

neisseria

Question 19

example of a gram positive bacillus is?

Answer 19

listeria

Question 20

why might you not see anything on the gram stain?

Answer 20

needs to be cultured first/too few bacteria

Question 21

why might you not bother doing a gram stain?

Answer 21

if the sample is taken from an unsterile site, where it would have other bacteria which are not causing disease, so would be difficult to interpret results

Question 22

name all the sterile site in the body?

Answer 22

blood, CSF, urinary tract, peritoneal cavity, pleural fluid, joints, lower respiratory tract

Question 23

where would you expect to culture normal flora?

Answer 23

urethra, GI tract(mouth, large intestine) vagina, skin

Question 24

what can we do to prevent just growing normal flora + actually grow the disease causing pathogen?

Answer 24

selective media (contains chemicals/pH/antibiotics to inhibit grow of others)
media containing different nutrients (blood etc)
grown at different temps
grown at in different conditions (co2, anaerobic)

Question 25

what is the difference between blood agar and chocolate agar?

Answer 25

choco agar is blood agar heated to 80+ degrees, this is to release extra nutrients to grow more/different bacteria

Question 26

mackonkey agar is used for?

Answer 26

growing gram -ve bacilli only (bile salts inhibit gram +ive)

contains lactose and dye- those which ferment the lactose will take up the dye so lactose fermenters are pink

Question 27

XLD plate is used to grow which two organisms?

Answer 27

shigella and salmonella

Question 28

sabourourd’s medium is used to grow?

Answer 28

funghi

Question 29

what is the difference between gram positive and gram negative bacteria and staining?

Answer 29

gram positive has much more peptidoglycan in the cell wall, therefore retains the crystal violet dye of the gram stain when you wash with alcohol (stays purple= pos)

gram negative has much less peptidoglycan, therefore decolorises with alcohol, and is then counter stained pink with carbol fushin/safranin
(pink= neg)