Top 5 Misconceptions About Antimicrobials

J. Scott Weese, DVM, DVSc, DACVIM, FCAHS, Ontario Veterinary College, Guelph, Ontario, Canada

ArticleLast Updated February 20244 min readPeer Reviewed
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Antimicrobials are among the most common and important therapeutics in veterinary medicine. Ready access to antimicrobials prior to typical modern clinical research (eg, randomized clinical trials) resulted in data gaps, misconceptions, and illogical dogmas, and certain practices became entrenched through expert opinion and anecdotes. Many questionable approaches have persisted despite a lack of supporting evidence (or even abundant evidence to the contrary).

Following are the top antimicrobial misconceptions, according to the author.

1. Bactericidal antimicrobials are more effective than bacteriostatic antimicrobials.

Bactericidal antimicrobials kill bacteria, whereas bacteriostatic antimicrobials inhibit bacterial growth, making it easier for the immune system to eliminate infection. It may therefore appear that bactericidal antimicrobials would be preferred in patients with severe infections, in immunocompromised patients, or for infections at sites that may have a diminished immune response. Bactericidal and bacteriostatic activity, however, are in vitro properties that occur under specific growth conditions and at specific doses. Some antimicrobials may function as bactericidal or bacteriostatic depending on conditions and dose or may be bactericidal against some bacteria but not others. Bacteriostatic antimicrobials may also be highly effective at killing bacteria but fail to meet the 99.9% elimination cutoff used for in vitro classification of bacteriostatic versus bactericidal status.1

A recent review identified 56 studies that compared bacteriostatic and bactericidal drugs in humans and found no difference in 49 studies and superiority of bacteriostatic drugs in 6 studies.1 The remaining study showed superiority of bactericidal drugs, but the bacteriostatic drug dose was likely inadequate. Determining whether a drug is bacteriostatic or bactericidal is therefore unnecessary when considering antibacterial options.

2. There is a minimum antibiotic course.

Although a minimum antimicrobial treatment duration is commonly believed necessary to reduce emergence of resistance and inadequate duration of treatment is believed to result in recurrence with a resistant bacterium, no data support these beliefs. There is no plausible mechanism for short treatment durations to increase the risk for antimicrobial resistance.2 Antimicrobials inhibit susceptible bacteria and allow persistence of resistant strains. As doses are increasingly administered, more susceptible bacteria are eliminated, and more resistant bacteria remain. Inadequate durations increase the risk for treatment failure, but there is no evidence that recurrence is associated with resistance. Short treatment durations are desirable because of reduced risk for adverse effects, cost effectiveness, need for owner administration of medication, and antimicrobial resistance selection pressure and are accordingly emphasized in human and veterinary medicine.3-5

3. IV antimicrobials are required in patients with severe disease.

Drug concentration at the site of infection is more important than method of entry into the bloodstream. IV antimicrobials are indicated in cases in which an oral drug cannot be effectively administered, absorption of an oral drug may be impacted by concurrent disease (eg, GI stasis), or the optimal drug does not have good oral bioavailability. There are no additional indications for IV antimicrobials compared with oral antimicrobials.6 Maintaining IV access to administer an antimicrobial that could be administered orally increases costs and patient risks.

4. The drug with the lowest minimum inhibitory concentration is preferable.

The minimum inhibitory concentration (MIC) is the lowest concentration of an antimicrobial that inhibits bacterial growth in vitro. Low MIC does not necessarily indicate a drug is preferable (or even viable), as MIC is just one component of efficacy. Another key component is the amount of drug present at the infection site, which is influenced by bioavailability, distribution, and dose. A drug with a higher MIC may be as effective as or more effective than a drug with a lower MIC if it reaches the infection site at higher concentrations. Clinical break points are used by laboratories to determine whether the MIC for an infection/drug combination is within susceptible, intermediate, or resistant categories. These classifications should guide drug selection in addition to MIC.

5. Higher-tier antimicrobials are more effective.

Antimicrobials can be ranked or prioritized, typically based on their spectrum, activity against multidrug-resistant bacteria, and/or importance in human and veterinary medicine.7 Some higher-tier drugs (eg, third-generation cephalosporins, fluoroquinolones) are commonly used in veterinary medicine, but others (eg, meropenem, vancomycin, linezolid) are used sparingly in some regions for select resistant infections. Higher-tier drugs are important for infections resistant to other options but are not better (and are sometimes less effective) than lower-tier antimicrobials if the bacterium is susceptible to the lower-tier drug. For example, vancomycin, a higher tier drug, is less effective than lower tier beta-lactams for treatment of methicillin-susceptible Staphylococcus aureus in humans.8 Need for higher-tier drugs should be based more on bacterial (ie, known or reasonably suspected resistance) than patient (eg, level of illness) factors.