Intrinsic Antibiotic Resistance: Meaning, Mechanism, and Examples

A urine culture grows Citrobacter, and the laboratory does not even test it against ampicillin. This is not an oversight. Citrobacter is naturally, predictably resistant to ampicillin, so testing it would waste a disc and risk a misleading result.

When most people hear "antibiotic resistance," they picture a once-susceptible bacterium that has newly acquired a defense through a mutation or a borrowed gene. But a large amount of resistance was there all along, written into the species itself. That is intrinsic resistance, and knowing it turns a long list of drug-bug facts into a set of rules you can reason from.

What is intrinsic antibiotic resistance?

Intrinsic resistance (also called natural or inherent resistance) is resistance that comes from the normal genetic, structural, or physiological make-up of a bacterial species. It is carried on the chromosome and inherited by every member of the species, and it exists whether or not the bacterium has ever met the antibiotic. In other words, the drug never worked against that organism in the first place.

This is the key contrast with acquired resistance, in which a previously susceptible strain gains resistance through mutation or horizontal gene transfer. Only acquired resistance spreads between bacteria; intrinsic resistance is simply a fixed property of the species. Both sit within the wider picture of antibiotic resistance, where intrinsic resistance is the "natural" category and acquired resistance is the "gained" one.

Why intrinsic resistance matters

Intrinsic resistance is clinically useful precisely because it is predictable. It shapes practice in three ways:

1. It guides empiric therapy. Knowing that a species is naturally resistant to a drug means that drug is never an option, so it is not chosen while awaiting culture. There is no point giving vancomycin for a gram-negative infection.
2. It streamlines laboratory testing. As CLSI puts it, "intrinsic resistance is so common that susceptibility testing is unnecessary." Routine testing of vancomycin against gram-negative bacilli, or ampicillin against Citrobacter, is not needed, because the answer is already known.
3. It helps identify bacteria. A species' intrinsic-resistance pattern is a fingerprint. An unexpected result (an organism appearing susceptible to a drug it should be intrinsically resistant to) is a red flag for a misidentification or a testing error.

Mechanisms of intrinsic resistance

Intrinsic resistance comes from features a species simply has, rather than anything it acquires. There are four core mechanisms:

1. No target for the drug. The bacterium lacks the structure the antibiotic attacks. Mycoplasma has no cell wall, so cell-wall drugs like penicillin have nothing to act on, and gram-positive bacteria lack the penicillin-binding protein that aztreonam needs.
2. The drug cannot get in (impermeability). The drug cannot reach its target. Vancomycin is too large to cross the gram-negative outer membrane, and Pseudomonas keeps many drugs out through a low-permeability envelope.
3. The drug is pumped out (efflux). Chromosomally encoded efflux pumps expel the drug before it can act, a major contributor in Pseudomonas aeruginosa.
4. The drug is inactivated or cannot be activated. Some species make chromosomal enzymes that destroy the drug (for example, the AmpC beta-lactamase of many Enterobacterales), while others cannot activate a prodrug (aerobic bacteria cannot reduce metronidazole to its active form, and anaerobes cannot drive the oxygen-dependent uptake of aminoglycosides).

The quick rules worth memorizing

Most exam questions on this topic come down to a few whole-group rules:

• Gram-negative bacteria are intrinsically resistant to the "gram-positive only" drugs, because their outer membrane keeps these large or poorly penetrating molecules out: glycopeptides (vancomycin, teicoplanin), daptomycin, macrolides, clindamycin, linezolid, and fusidic acid.
• Gram-positive bacteria are intrinsically resistant to aztreonam, polymyxin B and colistin, and nalidixic acid.
• Anaerobes are intrinsically resistant to aminoglycosides (no oxygen-driven uptake) and to aztreonam.
• Aerobes are intrinsically resistant to metronidazole (they cannot activate it).

Intrinsic resistance by organism

The table below collects the common intrinsic-resistance patterns of clinically important bacteria. These are naturally resistant drugs that should not be used or, in most cases, tested.

A note on the enterococcal entries above: enterococci may appear active against cephalosporins, aminoglycosides, clindamycin, and trimethoprim-sulfamethoxazole in vitro, but they are ineffective clinically and should not be reported as susceptible.

How to remember

Intrinsic means "born with it," acquired means "picked it up." Intrinsic resistance is a fixed species trait carried on the chromosome; acquired resistance is gained later by mutation or gene transfer and can spread.

The two big envelope rules: gram-negatives keep the big gram-positive drugs out (no vancomycin, daptomycin, macrolides, clindamycin, linezolid), and gram-positives resist the trio "APN," Aztreonam, Polymyxin (colistin), and Nalidixic acid.

Oxygen decides two of them: anaerobes cannot take up aminoglycosides (no oxygen-driven uptake), and aerobes cannot activate metronidazole (no anaerobic reduction).

Where students actually get confused

Intrinsic vs acquired resistance. Intrinsic is natural, chromosomal, and affects the whole species (the drug never worked). Acquired is gained by a previously susceptible strain and can spread.

Intrinsic resistance is usually not "reported" as resistance. Because the organism was never susceptible, labs often do not test these drug-bug combinations at all, rather than testing and reporting them resistant.

Why vancomycin fails against gram-negatives is intrinsic, not acquired. It is a size and permeability problem (the outer membrane), present in every gram-negative from the start, not something the bacteria evolved.

An unexpected susceptible result is a warning sign. If an organism tests susceptible to a drug it should be intrinsically resistant to, suspect a misidentification or a laboratory error before believing the result.

Key exam facts

References

1. Cox G, Wright GD (2013). Intrinsic antibiotic resistance: mechanisms, origins, challenges and solutions. International Journal of Medical Microbiology. 303(6-7): 287-292.
2. Blair JMA, Webber MA, Baylay AJ, Ogbolu DO, Piddock LJV (2015). Molecular mechanisms of antibiotic resistance. Nature Reviews Microbiology. 13(1): 42-51.
3. Jacoby GA (2009). AmpC beta-lactamases. Clinical Microbiology Reviews. 22(1): 161-182.
4. Clinical and Laboratory Standards Institute (CLSI). M100: Performance Standards for Antimicrobial Susceptibility Testing (current edition). CLSI, Wayne, PA.