How Are Antimicrobial Resistant Bacteria Examined?

Post date:2018.01.09

We often hear the word of antimicrobial resistant bacteria from the media, however, most people don’t seem to understand the correct meaning and definition of this term. The World Health Organization (WHO) defines antimicrobial resistance as “the ability of microbes to resist the effects of drugs”. Thus, it is a phenomenon in which antibiotics lose their effectiveness for treating infectious diseases. The WHO also explains that antimicrobial resistance occurs naturally, but is facilitated by the inappropriate use of medicines. As I have mentioned before, bacteria have a survival instinct like humans, and have the ability to survive against antibiotics like invaders.

Then, how can we determine the antibiotic concentration for treatment? It is performed by an examination called antibiotic susceptibility testing in vitro. Disc diffusion antibiotic sensitivity testing is usually done. In this test, wafers containing antibiotics are placed on an agar plate where bacteria have been placed, and the plate is left to incubate. We judge whether bacteria are resistant or sensitive for antibiotics by the size of area around the wafer where the bacteria have not grown enough to be visible. Bacteria showing resistance to the antibiotics will create small or no zone, while a sensitive bacteria creates a larger zone. This test can be performed inexpensively and easily, however, there are few discs of veterinary antibiotics, and the criterion for the bacteria being resistant or sensitive is not clear. Clinical veterinarians need much knowledge and experience in order to use this testing to select the best antibiotics suited to the condition. Owners can never judge for themselves.

The dilution method, using liquid or agar medium mixed with antibiotics diluted each concentration to inoculate and incubate, is a more precise method to determine the minimum inhibitory concentration (MIC) which is the lowest concentration of antibiotics to prevent bacterial growth. Even if we determine the MIC of bacteria, we can’t evaluate whether it is resistant or sensitive.  The result is evaluated according to the breakpoints published by public institutions in U.S. and others. If the MIC is greater than this value, the bacteria is regarded as resistant. Public breakpoints for animals are not established yet although they should be determined through medical practice.

It is remarkable that bacteria which have already been judged as resistant have their growth prevented by increasing the concentration of antibiotics. Thus, the resistance is viewed as a relative concept rather than an absolute concept. However, we need to control the increase of concentration of antibiotics, because it can unexpectedly cause side effects. That is why we are required to strictly observe dosage and administration backed by data, considering the problems of residual drugs for food animals.