Special phenotypic methods for detecting antibacterial resistance

1-Detection of oxacillin resistance (Methicillin) resistance in
Staphylococci:
Oxacillin and other penicillinase-resistant penicillins, constitute the
drug class of choice for treating staphylococcal infections. When an
isolate shows resistance to one of the penicillinase-resistant penicillins, it
must be considered resistant to the entire group. Oxacillin has been the
class representative most commonly used to detect resistance in
staphylococci and has produced more reliable results than testing one of
the other agents (Tenover, 2009).
Detection of oxacillin (methicillin) resistance in staphylococci is
difficult primarily because oxacillin-resistant strains tend to grow more
slowly and often show heteroresistance, i.e., only a fraction of the
bacterial population actually manifests the resistance phenotype. Various
strategies have been used to increase the likelihood of detecting the
resistant subpopulation, including growing the strains at 35°C instead of
37°C, adding 2% NaCl to the testing medium, and incubating the test for
a full 24 h (Swenson et al., 2001).
The extended incubation allows the more slowly growing resistant
subpopulation sufficient time to grow to detectable numbers. In addition
test plates should be examined very closely. Zones of inhibition must be
examined by using transmitted light (holding the plate up to the light
source) and any growth is considered significant (Andrews, 2008).
Cefoxitin-based disk diffusion test serves to induce greater
expression of PBP2a in mecA containing strains Staphylococci. The test
can be read in 16–18 h and replaces the use of the oxacillin disk for disk
diffusion testing (Swenson and Tenover, 2005). 5-Detection of high level aminoglycoside resistance in enterococci:
In vitro tests for detection of high level aminoglycoside resistance
in Enterococci include broth, agar, or disk diffusion methods. For
screening gentamycin is tested at concentration of 500 µg/ml and
streptomycin at 2000 µg/ml (agar) or 1000 µg/ml (broth). Growth at high
concentrations indicates that the isolate has high level resistance to the
agent tested. Special discs containing high concentrations of gentamycin
(120 µg) or streptomycin (300 µg) can also be used (CLSI, 2009).