Cephalexin is a widely used medication employed to combat various bacterial infections throughout the body. This drug is classified as an antibiotic, meaning its purpose is to either kill bacteria or slow their growth. It will not work against infections caused by viruses, such as the common cold or flu. Known commercially by several brand names, including Keflex and Keftab, Cephalexin is frequently prescribed for common issues like skin and soft tissue infections, ear infections (otitis media), and infections of the respiratory and urinary tracts. The drug is valued for its reliable effectiveness when taken orally and its general tolerability among patients.
How Cephalexin Kills Bacteria
Cephalexin functions as a bactericidal agent, meaning it directly causes the death of the targeted bacterial cells. Its action is highly specific, disrupting the process bacteria use to construct and maintain their protective outer shell, the cell wall. The integrity of this wall is maintained by a complex polymer called peptidoglycan, which provides the cell with structural strength.
The final stage of construction is a cross-linking reaction that binds the peptidoglycan strands together, executed by specialized bacterial enzymes called Penicillin-Binding Proteins (PBPs). Cephalexin works by mimicking a component of the peptidoglycan precursor, allowing the drug to bind irreversibly to the active site of the PBPs.
Once Cephalexin is bound, it inactivates the PBPs, preventing the enzymes from completing the cross-linking. This disruption compromises the structural integrity of the cell wall, especially as the bacterium attempts to grow and divide. Without a functional cell wall, the bacterial cell becomes susceptible to osmotic pressure, leading to rapid rupture and breakdown (lysis). Human cells, which do not have peptidoglycan cell walls, are unaffected.
The Body’s Processing of the Drug
The way the body handles Cephalexin, referred to as pharmacokinetics, involves absorption, distribution, metabolism, and excretion. After a patient takes a dose orally, the drug is rapidly and almost completely absorbed from the upper gastrointestinal tract. This excellent absorption profile achieves high bioavailability in the bloodstream.
Once absorbed, the drug is distributed widely throughout the body’s tissues and fluids, allowing it to reach the sites of infection. While it distributes well to areas like the skin, bone, and respiratory tissues, Cephalexin does not effectively cross the blood-brain barrier, limiting its use for infections of the central nervous system.
Cephalexin undergoes minimal metabolism, meaning it is not broken down by the liver into inactive compounds. The primary route of elimination is through the kidneys, which rapidly filter the intact drug from the bloodstream. Approximately 90% or more of the administered dose is excreted in the urine within a few hours.
This rapid clearance results in a short elimination half-life, typically ranging from 0.5 to 1.2 hours. Because of this short half-life, Cephalexin must be dosed multiple times a day—often every six or eight hours—to maintain continuous antibacterial activity.
Understanding Bacterial Resistance
Bacterial resistance is a defense mechanism that microbes develop to survive exposure to antibiotics like Cephalexin. The most common way bacteria overcome this drug is through the production of enzymes called beta-lactamases. These enzymes attack the beta-lactam ring, the structural component of Cephalexin responsible for its antibacterial action.
Beta-lactamases hydrolyze the ring structure, rendering the antibiotic molecule chemically inactive and unable to bind to the Penicillin-Binding Proteins. This inactivation allows the bacterium to continue building its cell wall undisturbed.
A secondary mechanism of resistance involves the modification of the drug’s target itself. Bacteria acquire genetic mutations that alter the structure of their Penicillin-Binding Proteins (PBPs). These structural changes reduce the drug’s ability to bind to the PBP active site, lowering its affinity for the target.
Even with the antibiotic present, the modified PBPs can still perform the necessary cross-linking reactions for cell wall synthesis. The increasing prevalence of these resistance mechanisms is a direct result of antibiotic use, selecting for the survival of bacteria that possess these defensive traits.
Chemical Classification and Makeup
Cephalexin is a semi-synthetic antibiotic categorized as a first-generation cephalosporin. This classification reflects its earlier development and its activity profile, which is generally stronger against Gram-positive bacteria than later-generation cephalosporins.
The fundamental chemical structure is centered on the cephem nucleus, composed of a four-membered beta-lactam ring fused to a six-membered dihydrothiazine ring. The presence of the beta-lactam ring is the defining feature of the entire class.
The unique antibacterial spectrum and pharmacokinetic properties of Cephalexin are determined by specific chemical side chains attached to the cephem nucleus. The side chain at position 7 influences the drug’s ability to target different types of bacteria, while the side chain at position 3 affects its excellent oral absorption and rapid renal clearance.