The potential benefits of rapid antimicrobial susceptibility screening procedures are obvious, particularly in aggressive infections or rapidly progressing nosocomial infections of immunocompromised patients where appropriate antimicrobial selection is critical.
RAPID EVALUATION PROCEDURES
In most of the tests mentioned in the other articles results are not
available, until visible microbial growth occurs, at least in the controls.
This usually takes 24 hours or more. The potential benefits of rapid antimicrobial
susceptibility screening procedures are obvious, particularly in aggressive
infections or rapidly progressing nosocomial infections of immunocompromised
patients where appropriate antimicrobial selection is critical. To date only a
few ‘rapid’ methods for detecting microbial viability or growth are presently employed
in assessing the efficacy of antimicrobials. These include epifluorescent and
bioluminescence techniques. The former relies on the fact that when exposed to
the vital stain acridine orange and viewed under UV light, viable cells
fluoresce green or greenish yellow, while dead cells appear orange. Live/dead
staining of sessile bacterial populations has the potential to yield important
data with respect to antimicrobial susceptibility, but requires skilled
personnel and specialized microscopy equipment.
With tests involving liquid systems the
early growth of viable cells can be assessed by some light scattering processes,
blood culture techniques have classically used the production of CO2 as an indicator of bacterial metabolism and
growth. In addition, the availability of molecular techniques, such as
quantitative PCR, may be useful in demonstrating the presence or growth of
microorganisms that are slow or difficult to culture under usual laboratory
conditions, e.g. viruses. This may obviate the need to neutralize residual
disinfectant with some assays.
Recently, rapid colorimetric assays for
antimicrobial susceptibility have been developed including the commercially
available Vitek and Vitek2 systems (Biomerieux) and colourimetric tests based
on the extracellular reduction of tetrazolium salts
2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2, 4-disulfophenyl)-2
H-tetrazolium monosodium (WST-8) and 2,3-bis[2-methyloxy-4-nitro-5-sulfophenyl]-2
H-tetrazolium-5-carboxanilide (XTT). These latter studies have demonstrated the
potential for the tetrazolium salts WST-8 and XTT to be used in the rapid,
accurate and facile screening of antimicrobial susceptibility and MIC
determination in a range of bacteria, including staphylococci, extended
β-lactamase producing clinical isolates (E. coli, Ent. faecalis)
and Ps. aeruginosa (see Tunney et al., 2004). Using this method, MIC values in
agreement with those obtained using standard methods were obtained after 5
hours.
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