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Table 1 Comparison of minimum inhibitory concentration results for MSSA, MRSA and E. coli strains

From: Feasibility study demonstrating that enzymatic template generation and amplification can be employed as a novel method for molecular antimicrobial susceptibility testing

S. aureus ATCC 29213

    

Bacteria from purified cultures

Bacteria harvested from a positive blood culture bottle

Drug

Phenotype

CLSI MIC interpretation

Macrobroth MIC and interpretation

ETGA MIC and interpretation

gsPCR MIC and interpretation

TGA MIC and interpretation

gsPCR MIC and interpretation

    

4 hr

6 hr

22 hr

4 hr

6 hr

22 hr

4 hr

6 hr

4 hr

6 hr

Oxacillin

Susceptible

S ≤ 2 R ≥ 4

0.25 S

1 S

0.5 S

0.5 S

0.5 S

1 S

1 S

0.25 S

0.25 S

0.25 S

0.25 S

Vancomycin

Susceptible

S ≤ 2 I = 4-8 R ≥ 16

< 0.25 S

<0.25 S

<0.25 S

<0.25 S

<0.25 S

<0.25 S

<0.25 S

<0.125 S

<0.125 S

<0.125 S

<0.125 S

S. aureus NRS241

    

Bacteria from purified cultures

Bacteria harvested from a positive blood culture bottle

Drug

Phenotype

CLSI MIC interpretation

Macrobroth MIC and interpretation

ETGA MIC and interpretation

gsPCR MIC and interpretation

ETGA MIC and interpretation

gsPCR MIC and interpretation

    

4 hr

6 hr

22 hr

4 hr

6 hr

22 hr

4 hr

6 hr

4 hr

6 hr

Oxacillin

Resistant

S ≤ 2 R ≥ 4

16 R

8 R

16 R

> 16 R

N/Aa

16 R

>16 R

8 R

16 R

8 R

2 Sc (VME)

Vancomycin

Susceptible

S ≤ 2 I = 4-8 R ≥ 16

< 0.25 S

<0.25 S

<0.25 S

<0.25 S

<0.25 S

<0.25 S

<0.25 S

<0.25b S

<0.25 S

<0.25d S

<0.25d S

E. coli ATCC 25922

    

Bacteria from purified cultures

Bacteria harvested from a positive blood culture bottle

Drug

Phenotype

CLSI MIC interpretation

Macrobroth MIC and interpretation

ETGA MIC and interpretation

gsPCR MIC and interpretation

ETGA MIC and interpretation

gsPCR MIC and interpretation

    

4 hr

6 hr

22 hr

4 hr

6 hr

22 hr

4 hr

6 hr

4 hr

6 hr

Ciprofloxacin

Susceptible

S ≤ 1 I = 2 R ≥ 4

0.008 S

0.016 S

0.016 S

0.031 S

0.016 S

0.016 S

0.031 S

0.008 S

0.008 S

0.004 S

0.008 S

Tetracycline

Susceptible

S ≤ 4 I = 8 R ≥ 16

1 S

0.5 S

0.5 S

1 S

1 S

1 S

1 S

0.5 S

0.5 S

0.25 S

0.5 S

  1. All MIC values are in units of μg/mL. Interpretation of MIC values are S susceptible, I intermediate resistance, and R resistant. MICs are determined from the molecular assays as the culture with the lowest concentration of drug that produces a difference in Ct value that remains less than 3.33 cycles between its Ct value and the Ct value of the culture with the highest concentration of drug, where growth is fully inhibited. Four discrepancies are noted: aAt 4 hours, the MIC value of the gsPCR method of MRSA versus oxacillin could not be determined since the difference in Ct values moved above and below the cut-off value between several concentrations. bAt 4 hours, the MIC value of <0.25 μg/mL from the ETGA method of MRSA harvested from blood culture versus vancomycin is interpreted as susceptible and is in agreement with the macrobroth method. However, the 16 μg/mL culture from the AST series produced a Ct value that indicates resistance. cAt 6 hours, the MIC value of the gsPCR method of MRSA harvested from blood culture versus oxacilin is interpreted as susceptible, while the macrobroth method MIC is interpreted as resistant. This is defined as a very major error (VME). dThe gsPCR results from the MRSA harvested from blood culture versus vancomycin produced several reactions with negative results. The baseline was arbitrarily adjusted to account for the lack of signal for these reactions. All discrepancies are discussed in the text.