Antimicrobial resistance

A bacterium is considered resistant to a certain agent when bacterial growth cannot be inhibited by achievable serum concentrations of the agent. The minimal inhibitory concentration (MIC) is the antibiotic concentration required to suppress visible growth. In diagnostic microbiology laboratories, the disc diffusion method is used to determine resistance, whereby a certain diameter for the inhibition zone corresponds to the MIC (Fig. 3.40.2). Some bacteria show inherent or innate resistance to certain antibiotics (e.g. Pseudomonas aeruginosa is always resistant to benzylpenicillin). Other bacteria have acquired resistance as a result of genetic change. Resistance may result from chromosomal mutation or transformation (Fig. 3.40.3). Spontaneous mutation of the chromosome may change protein synthesis to create bacteria that have a selective advantage and will, therefore, outgrow the susceptible population. Certain species such as Streptococcus viridans, Neisseria and Haemophilus spp. can pick up genes from dead bacteria and transform their corresponding gene with the newly acquired DNA sequence, resulting, for example, in penicillin-resistant pneumococci.

Fig. 3.40.2 Disc diffusion susceptibility testing. A. Organisms susceptible to the chemical will not grow, leaving a ‘zone of inhibition’. B. The discs are impregnated with chemicals (e.g. antibiotics), which diffuse out into the surrounding area.