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To perform a D-test, an agar plate is inoculated with the bacteria in question and two drug-impregnated disks (one with erythromycin, one with clindamycin) are placed 15–20 mm apart on the plate. If the area of inhibition around the clindamycin disk is D-shaped, the test result is positive.
Detailedly, broad-spectrum antibiotics can kill or inhibit a wide range of microorganisms; extended-spectrum antibiotic can kill or inhibit Gram positive bacteria and some Gram negative bacteria; narrow-spectrum antibiotic can only kill or inhibit limited species of bacteria. [1] [2] [3]
This is called a D-zone test, or D test. If a 'D' shape is formed around the clindamycin disk (distinguished from a circular zone of inhibition) then the isolate is reported as resistant to clindamycin. [5] This occurs due to erythromycin inducing the bacteria's erm gene, and thus making it resistant to clindamycin (MLS-B phenotype). [6] [7]
Once a bacterium has been identified following microbiological culture, antibiotics are selected for susceptibility testing. [5] Susceptibility testing methods are based on exposing bacteria to antibiotics and observing the effect on the growth of the bacteria (phenotypic testing), or identifying specific genetic markers (genetic testing). [6]
Gram stain (Gram staining or Gram's method), is a method of staining used to classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. It may also be used to diagnose a fungal infection . [ 1 ]
A colored electron microscopy image of methicillin-resistant staphylococcus aureus (), a bacterium commonly targeted by broad-spectrum antibioticsA broad-spectrum antibiotic is an antibiotic that acts on the two major bacterial groups, Gram-positive and Gram-negative, [1] or any antibiotic that acts against a wide range of disease-causing bacteria. [2]
β-Lactam antibiotics are indicated for the prevention and treatment of bacterial infections caused by susceptible organisms. At first, β-lactam antibiotics were mainly active only against gram-positive bacteria, yet the recent development of broad-spectrum β-lactam antibiotics active against various gram-negative organisms has increased their usefulness.
However, these drugs did not address the entire spectrum of resistance of Gram-negative bacilli. [159] [160] According to the WHO fifty one new therapeutic entities - antibiotics (including combinations), are in phase 1–3 clinical trials as of May 2017. [157] Antibiotics targeting multidrug-resistant Gram-positive pathogens remains a high ...