The susceptibility test detects the type and amount of antibiotic or chemotherapeutic agent required to inhibit bacterial growth. Often, culture and susceptibility tests are ordered together. Susceptibility studies also may be indicated when an established regimen or treatment is to be altered.

A common and useful test for evaluating antibiotic susceptibility is the disk diffusion method (Figure 7.4). A set of antibiotic-impregnated disks on agar is inoculated with a culture derived from the specific bacteria being tested. After a suitable period of incubation, the degree of bacterial growth within the different antibiotic zones on the disks is determined and measured. Growth zone diameters, measured in millimeters, are correlated to the minimum inhibitory concentration (MIC) to determine whether the organism is truly susceptible to the antibiotic. Another method is a broth dilution test (Figure 7.5). The organism is grown in the presence of doubling dilutions of the antibiotic. The lowest concentration of the antibiotic agent that inhibits the organism’s growth is the MIC. Many commercial systems are based on this method.

1. The terms sensitive and susceptible imply that an infection caused by the bacterial strain tested is likely to respond favorably in the presence of the indicated antimicrobial agent.

2. The terms intermediate, partially resistant, and moderately susceptible mean that the bacterial strain tested is not completely inhibited by therapeutic concentrations of a test drug.

3. Indeterminate means that the bacterium has an MIC that approaches achievable blood and tissue concentrations. It implies clinical efficacy in body sites where the antibiotic agent is physiologically concentrated. The intermediate category also includes a buffer zone, which should prevent major errors due to technical factors.

4. The term resistant implies that the organism is not inhibited by the antibiotic. This infection is not likely to respond to treatment.

5. Some antimicrobial agents act in a bactericidal manner, meaning that they kill the organism. Others act in a bacteriostatic manner, meaning that they inhibit growth of the organism but do not necessarily kill it.

   1. Bactericidal agents

      1. Aminoglycosides

      2. Cephalosporins

      3. Metronidazole

      4. Penicillins

      5. Quinolones

      6. Rifampin

      7. Vancomycin

   2. Bacteriostatic agents

      1. Chloramphenicol

      2. Erythromycin

      3. Sulfonamides

      4. Tetracycline

6. Emergence of strains of penicillin-resistant N. gonorrhoeae, carbapenem-resistant Enterobacteriaceae and extended-spectrum beta-lactamases (ESBLs), methicillin-resistant (or oxacillin-resistant) S. aureus, amikacin-resistant Pseudomonas spp. or other Gram-negative rods, and vancomycin-resistant Enterococcus spp. presents challenges to the healthcare provider in regard to treatment. Many hospitals screen for methicillin-resistant S. aureus and vancomycin-resistant Enterococcus spp. so as to isolate patients infected with these organisms.