Classification and Intrinsic Resistance
In microbiology, two main types of resistance are distinguished: intrinsic (natural) and acquired.
Intrinsic resistance is a stable, species-specific trait inherent to certain families or genera. The primary mechanism of this defense is the absence of a drug "target" within the bacterial cell. A classic example is bacteria of the genus Mycoplasma. Because they naturally lack a cell wall, they are completely resistant to antibiotics that inhibit cell wall synthesis (such as penicillins and cephalosporins).
The barrier function of cell envelopes also plays an important role in natural resistance by restricting drug penetration. For instance, the outer membrane of Gram-negative bacteria contains small-diameter porins that physically block large-molecule compounds from entering the cell.
Genetic Basis of Acquired Resistance
Acquired resistance relies on the presence of specific genes. There are four main pathways for the emergence and transmission of resistance within a microbial population:
- Chromosomal mutations. These can be single-step (altering a single protein) or multi-step (altering an entire set of proteins). An example of a multi-step mutation is the alteration of penicillin-binding proteins in penicillin-resistant Streptococcus pneumoniae.
- Transfer of R-plasmids (transmissible plasmids). These frequently encode resistance to multiple antibiotic families simultaneously. Inter-species exchange is possible: the TEM-1 plasmid, which encodes $\beta$-lactamase, is typical of Gram-negative Enterobacteriaceae but can also be transferred to penicillin-resistant Neisseria gonorrhoeae and ampicillin-resistant Haemophilus influenzae.
- Transposon-mediated transfer. Mobile genetic elements facilitate gene migration within the cell (between the chromosome and plasmids) and promote both vertical and horizontal transmission of resistance.
- Integron activity. This involves the expression of gene cassettes containing resistance factors.
Role of the Antibiotic and Selection
A key rule of resistance biology is that the drug itself is not the cause of mutations. Genomic changes occur spontaneously and independently.
An antibiotic acts as a selection factor. Under its pressure, sensitive cells perish, while accidentally generated mutant clones gain an advantage—they survive, proliferate actively, and are subsequently transmitted to new hosts (humans or animals). This problem is particularly acute in clinical settings, where the continuous use of drugs leads to the emergence of nosocomial strains responsible for hospital-acquired infections. Their hallmark feature is multi-drug resistance (polydrug resistance).
Biochemical Defense Mechanisms
Normally, an antibiotic must remain active, cross the cell envelopes, and bind to its intracellular target. In the case of resistance, the bacterium blocks this process. There are three primary defense mechanisms:
- Target modification. The structure of the target enzyme is altered: it continues its normal function, but sharply loses its ability to bind the drug (reduced affinity). Alternatively, the microbe may activate a metabolic "shunt"—deploying an alternative enzyme that is unaffected by the drug.
- Target inaccessibility. Achieved through decreased cell wall/membrane permeability or via an efflux pump mechanism that actively expels the penetrated antibiotic out of the cell.
- Drug inactivation. Bacteria produce enzymes that destroy the drug. The genes for such enzymes are localized on both plasmids and the chromosome. Prime examples include $\beta$-lactamases, aminoglycoside-modifying enzymes, and chloramphenicol acetyltransferase.
Overcoming Resistance and Control Strategies
The most well-known group of destructive enzymes are $\beta$-lactamases, which cleave the $\beta$-lactam ring. To neutralize them, pharmacology employs $\beta$-lactamase inhibitors (clavulanic acid, sulbactam, tazobactam). While they possess low intrinsic antibacterial activity, their chemical structure contains a $\beta$-lactam ring. These inhibitors bind to bacterial enzymes, "distracting" them and thereby protecting the primary antibiotic. For example, clavulanic acid is successfully combined with penicillins (amoxicillin, ticarcillin, piperacillin).
It is impossible to halt the emergence of resistance entirely. The primary goal of medicine is to prevent its spread. To achieve this, clinicians must prescribe antibiotics strictly based on indications, rotate drugs every 10–15 days of therapy, prioritize narrow-spectrum agents, and limit antibiotic use outside of human medicine (such as banning their use as growth promoters in veterinary practice).