Pharmaceutical Microbiology

Cephalosporins & mechanism of action

Written by MicroDok

Cephalosporins are betalactam antibiotics that are penicillinase-resistant, and with related mode of action to the penicillins. Drugs in this category are clinical substitutes for penicillins due to the development of resistance to the later by pathogenic bacteria. Cephalosporins are classified into four (4) generations based on their spectrum of activity, side chain modifications and clinical applications.


The 1st-generation cephalosporins are active against Gram positive bacteria, and they include cephalexin, cephalothin, cephapirin, cefazolin, cephadrine and cefadrox. They have a narrow-spectrum of activity against pathogenic bacteria especially the Gram negative rods (e.g. Klebsiella species and Escherichia coli). Though their activity is mainly against penicillinase-resistant Gram positive bacteria, antibiotics in this group are also active against some Enterobacteriaceae.


The 2nd-generation cephalosporins were designed to counter the resistance of Gram negative bacteria (mediated by beta-lactamase production). They have expanded antibacterial activity against pathogenic bacteria in the family Enterobacteriaceae (e.g. Klebsiella species, Haemophilus species and E. coli) and even against some anaerobic bacteria (e.g. Bacteroides species). Antibiotics that are 2nd-generation cephalosporins include cefaclor, cefotetan, cefuroxime, and cefoxitin. 2nd-generation cephalosporins generally have expanded Gram negative activity than the 1st-generation cephalosporins that are more susceptible to beta-lactamase enzymes secreted by these pathogens.    


The 3rd-generation cephalosporins include ceftazidime, cefotaxime, and ceftriaxone. Antibiotics in this category are more effective against Gram negative rods than the 2nd-generation cephalosporins; and they have antibacterial activity against a wide variety of Enterobacteriaceae and non-enteric bacteria (e.g. Pseudomonas aeruginosa). 3rd-generation cephalosporins have enhanced systemic activity against a wide variety of Gram negative rods including those that penetrate the CNS (e.g. Neisseria meningitidis); and they are the preferred antibiotic of choice in life-threatening bacterial diseases or infections of yet unknown cause (i.e. infections in which the causative agents are yet to be isolated from patients specimen in the microbiology laboratory).    


The 4th-generation cephalosporins have extended antibacterial spectrum or activity against pathogenic bacteria (especially the Gram negative rods) to which the 3rd-generation cephalosporins are least effective. They include cefepime and cefpirome. 4th-generation cephalosporins have activity against Enterobacter species, Citrobacter species, Neisseria species, Haemophilus species, Enterobacteriaceae and a wide variety of Gram negative bacteria. Antibiotics in this category have more OM-penetrating potential than the other generations of cephalosporins; and they are more stable to beta-lactamase enzymes secreted by Gram negative bacteria.       


The 5th-generation cephalosporins are usually administered intravenously (IV). And they are active against a variety of multidrug resistant bacteria pathogens including the dreaded methicillin resistant Staphylococcus aureus (MRSA). The 5th-generation cephalosporins include Ceftaroline Fosamil (IV) and Ceftobiprole (IV).


Cephalosporins are naturally sourced from the fungus Cephalosporium (e.g. C. acremonium). The cephalosporins were originally produced in the early 1940s from moulds (i.e. Cephalosporium species). However, cephalosporins can also be produced semi-synthetically by the addition of substituent groups to the side chains of the 7-aminocephalosporanic acid structure (Figure 2).


The basic structure of the cephalosporins is known as 7-aminocephalosporanic acid (Figure 2). Addition of substituent groups to this basic structure result in the production of different variants of cephalosporins.

Figure 2: Basic structure of cephalosporins (7-aminocephalosporanic acid, 7-ACA). The beta lactam ring is fused with the dihydrothiazine ring. Cephalosporins unlike the penicillins have a six-membered ring (the dihydrothiazine ring); and group substitution occurs in two places or at both sides of the 7-aminocephalosporanic acid nucleus. The cephalosporins work in the same manner as the penicillins by inhibiting the cross-linking of peptidoglycan during bacterial cell wall synthesis.


Clinically, the cephalosporins are effective against a wide variety of pathogenic bacteria especially the Gram negative rods to which the penicillins have little or no activity against. 1st-generation cephalosporins are very active against Gram positive bacteria (e.g. Staphylococcus species). 2nd-generation cephalosporins are effective against Gram negative rods (e.g. Klebsiella species and Proteus species) but not against Pseudomonas species. 3rd-generation cephalosporins are active against Pseudomonas species and a variety of Enterobacteriaceae and other Gram negative rods. 4th-generation cephalosporins are active against a variety of pathogenic bacteria including those that are resistant to the antibacterial onslaught of 3rd-generation cephalosporins.       


Cephalosporins (including the 1st, 2nd, 3rd and 4th-generations) have a broader spectrum of activity, and they are bacteriocidal in action. They have activity against both Gram positive and Gram negative bacteria.


Cephalosporins like the penicillins are cell wall synthesis inhibitors. They bind to the PBPs on bacterial cell wall, and interfere with the synthesis of peptidoglycan in growing bacteria. Thus, cephalosporins have similar mode of action like the penicillins but the former have increased stability to beta-lactamases produced by Gram negative bacteria than the later.


Though very effective against a wide variety of pathogenic bacteria, most cephalosporins are fast becoming susceptible to some antibiotic-degrading enzymes such as the ESBLs and other multidrug resistance factors produced by bacteria. The production of ESBLs aside other antibiotic-hydrolyzing enzymes by Gram negative bacteria have significantly compromised the clinical use of the cephalosporins for treating some bacterial-related infections or diseases.


Cephalosporins are generally well tolerated by the body when administered orally or otherwise. They have a wider systemic distribution in the body.


Untoward effects due to the usage of cephalosporins are attributed to the host’s allergic reactions to the drug as is applicable to the use of penicillins. But hypersensitivity is fewer compared to that produced by penicillins.


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