Hi,
I am studying the resistance to antibiotics in bacteria and I often happen to read papers where the authors report minimal inhibitory concentration tests performed on these resistant bacteria. I didn't understand why the authors use a wide range of antibiotics for these MIC tests. I mean, I read a few papers in which carbapenem resistant bacteria were tested on carbapenems, as well as on cephalosporins, aminoglycosides..So, I was wondering what s the point to test also cephalosporins or other antibioitcs if the strains are resistant to carbapenems?
Thank you very much in advance,
Silvia
Hi:
MIC is defined as the lowest concentration of the agent that inhibits visible growth. The methods are not widely used for routine testing in clinical labs in most countries. Determination of MIC is recommended by CLSI to evaluate rapidly multidrug resistance in the bacterial isolates.
Dear Silvia,
On forehand you do not know for which type of antimicrobials the isolated bacteria is resistant and susceptible. Specially when a bacterium is resistant against carbapemases you hope to find that the bacterium is still susceptible for an aminoglycoside or quinolone. Furthermore, you want to have the the information as quick as possible to improve the patients outcome. Therefore, you test all your therapeutical options at once.
Once you've isolated bacteria, you don't know about their antimicrobial resistance. You must check. Usually you check all antimicrobials in the same time, not one after another.
You need to do that in order to understand the spectrum/range of resistance to the different antibiotics. And later, you or other researchers can identify the genes responsible for the resistance.
In minimum inhibitory concentration methods, the
susceptibility of organisms to serial twofold dilutions
of agents in agar or broth is determined. The
MIC is defined as the lowest concentration of the
agent that inhibits visible growth. The methods arenot widely used for routine testing in clinical laboratories
in most countries, although commercial
systems with restricted-range MICs in a variety of
formats are available from a number of companies.
In clinical laboratories MIC methods are used to
test the susceptibility of organisms that give equivocal
results in diffusion tests, for tests on organisms
where disc tests may be unreliable, as with slowgrowing
organisms, or when a more accurate result
is required for clinical management, e.g. infective
endocarditis. MIC methods are accepted as the
standard against which other methods are assessed
and for testing new agents. MICs may also be
derived from the size of the zone of inhibition in
disc diffusion tests using a regression curve produced
by plotting the diameter of the zone of inhibition
against the MIC of a range of strains of a
given species. In some cases it is necessary to know
the MBC of an organism. This is determined by
subculturing the broth of tubes showing no turbidity
in an MIC test on to antimicrobial agent-free
media and observing for growth after further incubation.
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