Automated susceptibility testing methods

A series of commercially available automated and semi automated
methods are available to assist laboratories in testing and reporting the
results of antimicrobial susceptibility tests. Most of the methods combine
bacterial identification and susceptibility testing reagents in a single panel
or card to enhance the speed at which antimicrobial susceptibility testing
results can be reported. Results may be available for some bacterial
species in as little as 6 h, versus the 16–18 h often required for disk
diffusion testing or standard MIC tests (Evangelista and Truant, 2002).
The overall, automated systems work well, although they have
traditionally shown problems with certain resistance phenotypes
including oxacillin-resistant S. aureus strains (Swenson et al., 2001) and
Pseudomonas aeruginosa strains that are resistant to newer β-lactam
agents, such as piperacillin (Juretschko et al., 2007).
■ Epsolimeter test (Etest):
Principle:
Etest incorporates MIC testing into a format similar to the setup of
a disk diffusion test. The antibiotic is microencapsulated on the back of a
plastic strip and, when placed on the surface of an agar plate, the
antibiotic diffuses off the strip into the agar medium in a rapid and
predictable fashion forming a gradient. The Etest strip evaluates the
inhibitory potential of a single antimicrobial agent over a large range of
concentrations (Tenover, 2009).
Advantages:
Several Etest strips containing different antimicrobial agents can be
arranged on a single agar plate. The Etest method is particularly useful
for testing fastidious microorganisms such as campylobacters,
neumococci, and anaerobic bacteria, for which only a limited number of
antimicrobial agents need to be tested (Marsik, 2011).