understanding bacterial differentiation

Question 1

how do you distinguish between different bacteria

Answer 1

• morphology
• motility
• cell wall composition (gram type)
• repiration
• colony morpholgy on specific plates
• growth on specific media
• haemolysis
• substrate utilisation (what the bacteria can metabolise)
• presence and absence of specific enzymes
• tolerance to compounds

Question 2

list the tests used to determine what bacterial species a patient has

Answer 2

• haemolysis
• catalase test
• oxidase test
• coagulase test
• urease test
• fermentation assays

Question 3

discuss how haemolysis test works for bacteria diagnosis

Answer 3

• used to assess the ability o bacteria to lyse erythrocytes. when grown on blood agar some lyse or damage blood cells to access iron stored in hemoglobin (if bacteria can lyse erythrocytes it can lyse any cell type)
• tested on blood agar plate (enriched medium)

there are 3 different common descriptions for haemolysis:

• alpha: incomplete. produces green zone of lysis
• beta: complete destruction clear zone of lysis
• gamma: absence of any haemolysis

Question 4

discuss how catalse & oxidase testing works to diagnose bacteria

Answer 4

catalase: catalase catalyses the decomposition of hydrogen peroxide to water and oxygen. it helps protect cells from oxidative damage by reactive oxygen species.. catalase trigger decomposition of hydrogen peroxide so it fizzes as it produces oxygen

oxidase: identifies bacteria that produce cytochrome c oxidase, cytochrome c is an enzyme of the bacteria electron transport chain used by bacteria growing aerobically. the colour will change to blue when the test compound is reduced

Question 5

describe how the coagulase test works to diagnose bacteria

Answer 5

identifies the presence of bound coagulase or clumping factor, which is attached to the cell walls of bacteria. bound coagulase reacts with the fibrinogen in plasma, causing the fibrinogen to precipitate

main clinical use is to separate coagulase positive/negative staphylococcus aureus and also separate other groups of non-coagulase positive staphylococci S. epidermidis

Question 6

describe how the urease test works to diagnose bacteria

Answer 6

• pH based indicator test
• urease breaks down urea into CO2 and ammonia
• the indicator is more red the more alkaline it is. a more yellow is more acidic
• produces diagnosis within minutes

Question 7

describe how fermentation testing works to diagnose bacteria

Answer 7

• assays indicate the ability to ferment different sugars
• assays can be changed for different sugars (lactose, fructose, galactose, etc.)
• all assays are based on a pH change more acid if sugar can be fermented
• there is a pH indicator phenol red and is red at neutral pH

macconkey agar plate does fermentation test for lactose in a solid format
- bacteria which do not ferment lactose appear colourless on the medium and the agar surrounding the bacteria remains relatively transparent

Question 8

what are some common methods of bacteria typing will you come across in microbiology reptort

Answer 8

recognition of variable surface structures:

• serotyping
• phage typing

DNA varitation:

• ribosome sequencing
• multi locus sequence typing
• probe based tests
• PCR

other:

• mass spectrometry profiles

Question 9

describe how phage typing works

Answer 9

• uses lytic bacteriophage - virus that infects bacteria
• the pattern of susceptibility of a bacterial isolate establishes its phage type
• the phage attaches to surface structures
• these structures can subtly vary between different isolates of the same species or simply be present or absent
• multiple phage are assessed for their ability to infect the bacteria
• the pattern of susceptibility leads to a phage type

Question 10

how does serotyping work

Answer 10

uses antibodies that recognize variable structures. antibody-antigen interaction are very specific
-antibody targets are specific to groups of bacteria