Before actual antimicrobial therapy can begin, consideration must be given to at least three factors:
1. the nature of the microorganism causing the infection,
2. the degree of the microorganism’s susceptibility (also called sensitivity) to various drugs, and
3. the overall medical condition of the patient.
Identifying the Agent
Identification of infectious agents from body specimens should be attempted as soon as possible. It is especially important that such specimens be taken before any antimicrobial drug is given to prevent loss of the infectious agent. Direct examination of body fluids, sputum, or stool can provide a rapid detection method for bacteria or fungi. Newer methods of molecular diagnosis have greatly improved the speed of identification in many cases. A doctor often begins the therapy on the basis of such immediate findings. The choice of drug will be based on experience with drugs that are known to be effective against the microbe. For instance, if a sore throat appears to be caused by Streptococcus pyogenes, the physician may prescribe penicillin, because this species seems to be almost universally sensitive to it so far. If the infectious agent is not or cannot be isolated, epidemiological statistics may be required to predict the most likely agent in a given infection. For example, Streptococcus pneumoniae accounts for the majority of cases of meningitis in children, followed by Neisseria meningitidis and Haemophilus influenzae.
Testing for the Drug Susceptibility of Microorganisms
Testing is essential in those groups of bacteria commonly showing resistance, primarily Staphylococcus species, Neisseria gonorrhoeae, Streptococcus pneumoniae, Enterococcus faecalis, and the aerobic gram-negative enteric bacilli. However, not all infectious agents require antimicrobial sensitivity testing. When certain groups, such as group A streptococci and most anaerobic bacteria, are known to be uniformly susceptible to penicillins, testing may not be necessary unless the patient is allergic to penicillin. Testing methods are available for fungi, protozoa, and viruses, although testing might not be done as often for these groups.
Selection of a proper antimicrobial agent begins by demonstrating the in vitro activity of several drugs against the infectious agent by means of standardized methods. In general, these tests in volve exposing a pure culture of the bacterium to several different drugs and observing the effects of the drugs on growth.
The Kirby-Bauer technique is an agar diffusion test that pro vides useful data on antimicrobial susceptibility. In this test, the surface of a plate of special medium is spread with the test bacterium, and small discs containing a premeasured amount of antimicrobial are dispensed onto the bacterial lawn. After incubation, the zone of inhibition surrounding the discs is measured and compared with a standard for each drug (figure 1). The profile of antimicrobial sensitivity, or antibiogram, provides data for drug selection. The Kirby-Bauer procedure is less effective for bacteria that are anaerobic, highly fastidious, or slow-growing (Mycobacterium).
Fig1. Techniques for preparation and interpretation of disc diffusion tests. (b): Lisa Burgess/McGraw Hill; (c): Lisa Burgess/McGraw Hill
An alternative system that provides a quantitative rating of drug effectiveness is the Etest® (figure 2). In addition to providing a precise numerical rating (discussed in the next paragraph), this method allows testing for a wide variety of drugs and microbial types, including anaerobes, mycobacteria, and fungi.
Fig2. Etest®: A quantitative method for determining sensitivity. This test uses a plastic strip that contains a premeasured gradient of a drug and is labeled with a scale that indicates varying concentrations of the drug. The concentration ranges from higher at the top of the strip to lower toward the bottom. When the strip is placed on a plate swabbed with the test microbe and incubated, an oval area called the ellipse of inhibition develops adjacent to the strip. The lowest point on the scale where the ellipse begins to form corresponds with the minimum inhibitory concentration (MIC). In this test, for benzylpenicillin, the MIC is 0.094 µg/ml. Sirirat/Shutterstock
More sensitive and quantitative results can also be obtained with tube dilution tests. First the antimicrobial is diluted serially in tubes of broth, and then each tube is inoculated with a small uniform sample of pure culture, incubated, and examined for growth (turbidity). The smallest concentration of drug in the series that visibly inhibits growth is called the minimum inhibitory concentration, or MIC. The MIC is useful in determining the smallest effective dosage of a drug and in providing a comparative index against other antimicrobials (figures 3 and 4, and table 1). In many clinical laboratories, these antimicrobial testing procedures are performed in automated machines that can test dozens of drugs and microbes simultaneously.
Fig3. Tube dilution test for determining the MIC. The antibiotic is diluted through tubes of liquid nutrient so that the concentration in successive tubes ranges from 6.4 (far right) to 0.1 μg/ml. All tubes are inoculated with an identical amount of a test bacterium and then incubated. The first tube on the left is a control that lacks the drug and shows cloudiness indicative of normal growth. Reading from left to right, the MIC is the dilution of the first tube in the series that has no growth (is not cloudy). AB BIODISK 2008. Re-printed with permission of AB BIODISK
Fig4. Microbroth dilution in a multiwell plate adapted for a pathogenic yeast. Drugs being tested are amphotericin B (AmB), flucytosine (5FC), fluconazole (Fluc), azole drugs (itraconazole [itra], ketoconazole [Keto], Voriconazole [vori], and caspofungin [caspo]). Pink indicates growth; blue indicates no growth. Reading across, numbers 2–12 are increasing concentrations of the drugs in μg/ml. Row 1 has no drug. The first dilution with no growth is crossed out, indicating the MIC for that drug. Results are given in the box on the left. McGraw Hill
Table1. Comparative MICs (μg/ml) for Common Drugs and Pathogens
The MIC and the Therapeutic Index
The results of antimicrobial sensitivity tests guide the physician’s choice of a suitable drug. If therapy has already commenced, it is imperative to determine if the tests bear out the use of that particular drug. Once therapy has begun, it is important to observe the patient’s clinical response, because the in vitro activity of the drug is not always correlated with its in vivo effect. When antimicrobial treatment fails, the failure is usually due to
1. the inability of the drug to diffuse into that body compartment (the brain, joints, skin);
2. a few resistant cells in the culture that did not appear in the sensitivity test; or
3. an infection caused by more than one pathogen (mixed), some of which are resistant to the drug.
If therapy does fail, a different drug, combined therapy, or a different method of administration must be considered.
Many factors influence the choice of an antimicrobial drug be sides microbial sensitivity to it. The nature and spectrum of the drug, its potential adverse effects, and the condition of the patient can be critically important. When several antimicrobial drugs are available for treating an infection, final drug selection advances to a new series of considerations. In general, it is better to choose the narrowest-spectrum drug of those that are effective if the causative agent is known. This decreases the potential for superinfections and other adverse reactions.
Because drug toxicity is of concern, it is best to choose the drug with high selective toxicity for the infectious agent and low human toxicity. The therapeutic index (TI) is defined as the ratio of the dose of the drug that is toxic to humans as compared to its minimum effective (therapeutic) dose. The closer these two figures are (the smaller the ratio), the greater the potential for toxic drug reactions. For example, a drug that has a therapeutic index of
is a riskier choice than one with a therapeutic index of
Drug companies recommend dosages that will inhibit the microbes but not adversely affect patient cells. When a series of drugs being considered for therapy have similar MICs, the drug with the highest therapeutic index usually has the widest margin of safety.
Patient Factors in Choosing an Antimicrobial Drug
The physician must also take a careful history of the patient to discover any preexisting medical conditions that will influence the activity of the drug or the response of the patient. A history of allergy to a certain class of drugs should preclude the administration of that drug and any drugs related to it. Underlying liver or kidney disease will ordinarily necessitate the modification of drug therapy, because these organs play such an important part in metabolizing or excreting the drug. Infants, the elderly, and pregnant women require special precautions. For example, age can diminish gastrointestinal absorption and organ function, and most antimicrobial drugs cross the placenta and could affect fetal development.
The intake of other drugs must be carefully scrutinized, because incompatibilities can result in increased toxicity or failure of one or more of the drugs. For example, the combination of aminoglycosides and cephalosporins increases nephrotoxic effects, antacids reduce the absorption of isoniazid, and the interaction of tetracycline or rifampin with oral contraceptives can abolish the contraceptive’s effect. Some drugs (penicillin with certain aminoglycosides, or amphotericin B with flucytosine) act synergistically, so that reduced doses of each can be used. Other concerns in choosing drugs include any genetic or metabolic abnormalities in the patient, the site of infection, the route of administration, and the cost of the drug.
Even when all the information is in, the final choice of a drug is not always straightforward. Consider the case of an elderly alcoholic patient with pneumonia caused by Klebsiella and complicated by diminished liver and kidney function. All drugs must be given par enterally because of prior damage to the gastrointestinal lining and poor absorption. Drug tests show that the infectious agent is sensitive to fourth-generation cephalosporins, gentamicin, imipenem, and ticarcillin. The patient’s history shows previous allergy to the penicillins, so these would be ruled out. Cephalosporins are associated with serious bleeding in elderly patients, so this may not be a good choice. Aminoglycosides such as gentamicin are nephrotoxic and poorly cleared by damaged kidneys. Imipenem is probably the best choice because of its broad spectrum and low toxicity.
