ANTIBIOTIC RESISTANCE

Antibiotic: A drug that kills or inhibits the growth of microorganisms Resistant: An antimicrobial that will not inhibit bacterial growth at clinically achievable concentrations Susceptible: An antimicrobial will inhibit bacterial growth at clinically achievable concentrations

Minimum Inhibitory Concentration MIC (Minimal inhibitory concentration), the lowest concentration of antimicrobial that inhibits growth, is an important concept. This test is fundamental to susceptibility testing and determining resistance in bacteria. This slide shows such testing done in test tubes, where the far left tube with a concentration of 100mcg/mL is given through serial dilutions to 0.4 mcg/mL. A standard amount of bacteria is added to each tube and incubated overnight. The lowest concentration of antibiotic showing no bacterial growth (seen as cloudiness) is the minimal inhibitory concentration: in this case 6.25 mcg/mL. MIC = 6.25 mcg/mL

Other Methods for Determining Susceptibility In addition to broth-dilution, other methods for determining antimicrobial susceptibility include Kirby-Bauer disk diffusion, agar dilution and E-test. E-test and agar dilution can determine MICs directly while Kirby-Bauer disk diffusion is done by measuring the diameter of growth inhibition and comparing to a chart. This test can determine only susceptible vs resistant and not an MIC. These methods are used less-often in most hospital clinical laboratories because of increased labor, costs, and increased time to results. The methods are still used for special MIC testing. E-test® Disk Diffusion Agar dilution

misuse (misdiagnosis) Causes of Resistance Problem Antibiotic overuse, abuse or misuse (misdiagnosis)

Antibiotics promote resistance If a patient taking a course of antibiotic treatment does not complete it Or forgets to take the doses regularly, Then resistant strains get a chance to build up

Antibiotic use and abuse Viral infections are not stopped by antibiotics Yet doctors still prescribe (or are coerced into prescribing) antibiotics to treat them

Resistance gets around When antibiotics are used on a person, the numbers of antibiotic resistant bacteria increase in other members of the family

Multiple resistance It seems that some resistance was already naturally present in bacterial populations Resistant bacteria that survived, rapidly multiplied They passed their resistant genes on to other bacteria (both disease causing pathogens and non-pathogens)

Mechanisms Of Antibiotic Resistance Bacteria are capable of becoming resistant through several mechanisms One or many mechanisms may exist in an organism Multidrug-resistant bacteria often have multiple mechanisms Genes encoding resistance may exist on plasmid or chromosome Alteration in Target Molecule Decreased Permeability