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Bacteria are generally classified as gram positive or gram negative bacteria based on how they respond to use of dyes (Gram Stain) used in staining in the process of identifying them. Gram positive bacteria have the outer region of their cell wall made of peptidoglycan and the innermost region made up plasma membrane. Peptidoglycan is very prominent among the prokaryotic bacteria. It guarantees them the rigid structure. It is made up of a compound N-acetyl glucosamine (NAD) and N-acetyl muramic acid (NAM). This compounds interlock with each other. Peptidoglycan makes up 90% of the entire composition of the gram positive bacteria cell wall. Gram negative bacteria cell wall is made up of the outer membrane, the peptidoglycan, and the plasma membrane. Peptidoglycan in gram negative bacteria makes only 5 % of the entire composition of the cell wall. The outer membrane in gram negative bacteria is made up of Lipopolysaccharides (LPS). The outer membrane is less permeable unlike that in gram positive bacteria. Peptidoglycan in gram negative bacteria unlike in gram positive bacteria is sandwiched between the outer region of the wall and the plasma membrane. Periplasm, a space between plasma membrane and cell wall helps in controlling substances that get into and out of the cell (Cowan and Talaro, 2008, p.12). This essay seeks to critically look at how the composition of the bacterial cell composition determines the type of antimicrobial agent used against it.
Linking of the interpeptides and the peptidoglycan is gravely impaired by the activities of penicillin and cephalosporin. However, the lipopolysaccharides make it extremely difficult for the antimicrobial agents to reach the peptidoglycan of the gram negative bacteria. Penicillin and cephalosporin effectively act on gram positive bacteria because they do not have LPS, hence direct access of the antimicrobial agents to the peptidoglycan. Activities of penicillin and cephalosporin are greatly enhanced when the bacterial cell wall they are working on has weak cross-linkages of peptidoglycan. Beta-lactam antibiotics are normally used because they block the division of cyanobacteria, bacteria, glaucophytes photosynthetic organelles, and bryophyte chloroplasts. Interestingly, beta-lactam antibiotics cannot act on higher plants plastid. Beta lactam antibiotics have some structural similarity with the bacterial NAG and NAM cross linkages hence ease of mimicking. The NAG and NAM D-alanyl-D-alanine similarity to beta lactam make it possible for them to bind to penicillin binding protein (PBPs) active sites. The nucleus of the beta-lactam normally binds irreversibly to the acylates. Penicillin binding protein irreversible inhibition interferes with transpeptidation of gram positive nascent peptidoglycan layer thereby interfering with the synthesis of the cell walls. The presence of peptidoglycan precursors signals bacterial cell wall reorganization and consequently necessitates activation of hydrolases found in the autolytic cell wall. When the beta lactam inhibit cross linkages, the peptidoglycan precursors build up leading to digestion of peptidoglycan by enzyme autolytic hydrolases. This further enhances the activity of beta-lactam. However, bacteria can develop some resistance towards beta lactam by either producing enzyme beta-lactamase or enzyme penicillinase which renders the antibiotic useless by breaking the beta lactam ring.
Polymyxins normally get inserted into the cell membranes where they act as detergents. They increase the permeability of the cell leading to its death. They work best against gram negative rods. Polymyxin B is very effective against P. aeruginosa and other gram negative bacteria that have bacillus shape (Pommerville, 2010 p. 556).
Reference List
- Cowan, M. and Talaro, K. (2008). Microbiology: A systems Approach. N.Y.: McGraw-Hill
- Pommerville, J.C. (2010). Alcamo’s fundamentals of Microbiology: Body Systems. Ontario: Jones and Bartlett Publishers. Print.
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