Antibiotics used in routine Antimicrobial susceptibility testing (AST)

Last updated on June 21st, 2021

World Health Organization has published the basic set of drugs for routine susceptibility testing in various situations. The drugs are divided into two sets.

Set 1: It includes the drugs that are available in most hospitals and for which routine testing should be carried out for every strain.

Tests for drugs in set 2 are to be performed only at the special request of the physician, or when the causative organism is resistant to the first-choice drugs, or when other reasons (allergy to a drug, or its unavailability) make further testing justified. According to WHO, In very rare cases, one or more additional drugs should be included when there is a special reason known to the physician, or when new and better drugs become available.

Periodic revision of this table is therefore desirable, and this should be done after appropriate discussions with clinical staff. Many problems arise in practice, because clinicians are not always aware that only one representative of each group of antimicrobials is included in routine tests. The result obtained for this particular drug may then be extrapolated to all, or most, of the other members of the group.

Staphylococcus

Set 1: First choice

1. Benzylpenicillin
2. Oxacillin
3. Erythromycin
4. Tetracycline
5. Chloramphenicol

Set 2: Additional drugs

1. Gentamicin
2. Amikacin
3. Co-trimoxazole
4. Clindamycin

Intestinal Enterobacteriaceae

Set 1: First choice

1. Ampicillin
2. Chloramphenicol
3. Co-trimoxazole
4. Nalidixic acid
5. Tetracycline

Set 2: Additional drugs

1. norfloxacin

Urinary Enterobacteriaceae

Set 1: First choice

1. Sulfonamide
2. Trimethoprim
3. Co-trimoxazole
4. Ampicillin
5. Nitrofurantoin
6. Nalidixic acid
7. Tetracycline

Set 2: Additional drugs

1. Norfloxacin
2. Chloramphenicol
3. Gentamicin

Enterobacteriaceae isolated from blood and tissues

Set 1: First choice

1. Ampicillin
2. Chloramphenicol
3. Co-trimoxazole
4. Tetracycline
5. Cefalotin
6. Gentamicin

Set 2: Additional drugs

1. Cefuroxime
2. Ceftriaxone
3. Ciprofloxacin
4. Piperacillin
5. Amikacin

Pseudomonas aeruginosa

Set 1: First choice

1. Piperacillin
2. Gentamicin
3. Tobramycin

Set 2: Additional drugs

• Amikacin

1.  The benzylpenicillin disc is used to test susceptibility to all β-lactamase-sensitive penicillins (such as oral phenoxymethylpenicillin and pheneticillin).
2. Ampicillin is the prototype of a group of broad-spectrum penicillins with activity against many Gram-negative bacteria. As it is susceptible to β-lactamase, it should not be used for testing staphylococci. Generally, the susceptibility to ampicillin is also valid for other members of this group: amoxycillin, pivampicillin, talampicillin, etc.
3.  Oxacillin disc is representative of the whole group of β-lactamase-resistant penicillins (including meticillin, nafcillin, cloxacillin, dicloxacillin, and flucloxacillin).  Moreover, there is good clinical evidence that cross-resistance exists between the meticillin and the cephalosporin groups. Therefore, it is useless and misleading to include cefalotin in the antibiogram for staphylococci. The oxacillin disc is much more resistant to deterioration and is therefore preferred for the standardized diffusion test.
4. The results for the tetracycline disc may be applied to chlortetracycline, oxytetracycline, and other members of this group. However, most tetracycline-resistant staphylococci remain normally sensitive to minocycline. A disc of minocycline may thus be useful to test multiresistant strains of staphylococci.
5. The result with the chloramphenicol disc may be extrapolated to thiamphenicol, a related drug with a comparable antibacterial spectrum, but without known risk of aplastic anemia.
6. Erythromycin is used to test the susceptibility to some other members of the macrolide group (oleandomycin, spiramycin)
7. Only one representative sulfonamide (sulfafurazole) is required in the test.
8. Cefalotin. The only cefalotin needs to be tested routinely, as its spectrum is representative of all other first-generation cephalosporins (cefalexin, cefradine, cefaloridine, cefazolin, cefapirin). Where second-and third-generation cephalosporins and related compounds (cefamycins) with an expanded spectrum are available, a separate disc for some of these new drugs may be justified in selected cases (cefoxitin, cefamandole, cefuroxime, cefotaxime, ceftriaxone). Although some cephalosporins can be used to treat severe staphylococcal infections, the susceptibility of the infecting strain can be derived from the result of oxacillin
9. The co-trimoxazole disc contains a combination of trimethoprim and sulfonamide (sulfamethoxazole).  In co-trimoxazole the two components of this synergistic combination have comparable pharmacokinetic properties and generally act “as a single drug”.
10. Aminoglycosides group includes streptomycin, gentamicin, kanamycin, netilmicin, and tobramycin. Their antimicrobial spectra are not always close enough related to permitting assumption of cross-resistance, but against susceptible pathogens, these agents have been shown to be equally effective.WHO strongly recommended that each laboratory select a single agent for primary susceptibility testing. The other agents should be held in reserve for the treatment of patients with infections caused by resistant organisms.
11. Nitrofurantoin is limited to use only in the treatment of urinary tract infections, and should not be tested against microorganisms recovered from material other than urine.