Measurement of antimicrobial activity 2

Question 1

What are the characteristics of standardised tests?

Answer 1

• Technical details of the tests are normalised in the report of international collaborative studies
• The inhibition zone size breakpoints are valid only if all aspects of the defined methods are followed precisely

Question 2

Examples of organisations establishing standards for antibiotic susceptibility testing?

Answer 2

• British Society for Antimicrobial Chemotherapy (BSAC)

- Clinical and Lab Standards Institute (CLSI)

Question 3

READ: Some details about BSAC standards

Answer 3

Culture media
- Iso-Sensitest agar (Oxoid) is used for antibiotic testing of Enterobacteriaceae, Pseudomonas, enterococci and staphylococci (except for the antibiotics methicillin and oxacillin)
- Columbia agar with 2% sodium chloride is used for the detection of methicillin and oxacillin resistance in staphylococci
- And several other culture media are subject to
standardization by BSAC

Control organisms

• Control organisms should be used to monitor the performance of the test (by ensuring that the zone sizes obtained fall within the recommended limits)
• Also used to confirm that the method will detect resistance (but they are not used for the interpretation of susceptibility)

Question 4

Susceptible control strains are used to monitor

Answer 4

test performance of antimicrobial susceptibility testing

Question 5

Resistant control strains are used to confirm

Answer 5

that the method will detect resistance

Question 6

Where can we obtain control strains?

Answer 6

• CBS
• JCM
• ATCC
• NCTC
• DSMZ

Question 7

BSAC standards:

Columbia agar with 2% sodium chloride is used to detect

Answer 7

methicillin and oxacillin resistance in staphylococci

Question 8

Iso-Sensitest agar (Oxoid) is used for

Answer 8

antibiotic testing of Enterobacteriaceae, Pseudomonas, enterococci and staphylococci

Question 9

BSAC standards: Regarding the inoculum, the aim is to produce ___-_____ growth

Answer 9

semi-confluent

Question 10

BSAC standards for inoculum include:

Answer 10

• suspension of organisms in a suitable diluent (peptone water or broth; density equivalent to a 0.5 McFarland standard; using four or five colonies of a pure culture to avoid selecting atypical variants)

Question 11

a 0.5 McFarland standard gives a density equivalent to ≈

Answer 11

108 cfu/ml

Question 12

BSAC standards for incubation time and temp.

Answer 12

With most organisms plates should be incubated within 15 min of disc application at 35 - 37 oC in air for 18–20 h

Question 13

Potential disadvantages of doing MICs in test tubes

Answer 13

• time consuming
• takes up a lot of space and materials
• can be hard to determine whether the test organism has grown or not
• hard to distinguish between growth of contaminant and growth or intended organism.

Question 14

READ:

MIC determination in agar vs. disk diffusion method

Answer 14

• When MICs are determined in Petri dishes the antibiotic
  is dissolved in the agar (and is uniformly distributed through the gel when the test organism is inoculated onto the surface)
• This is a fundamental difference from the disk diffusion method for antibiotic susceptibility testing
  (where the antibiotic diffuses from the disk through the agar to create a growth inhibition zone)
  -When MIC determination is conducted in agar there is no diffusion and no zones of growth inhibition (the result depends on the presence or absence of growth of the test organism)
• Regardless of the method used, the principle is always the same: the MIC is the lowest concentration at which microbial growth is inhibited
• Values can be compared with standardized tables and the susceptibility/resistance may be assigned to the isolates