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Antimicrobial Agent and Therapy

An antimicrobial agent is a substance that inhibits microorganisms and its use in may food industry for packaging materials to enhance the shelf-life of packaged foods, by preserving the foods against microbial spoilage and hazardous food-borne microorganisms whereas in therapy to get rid of the microorganisms such as bacteria, fungi, or protozoans. Therapies that kill microorganisms are called microbiocidal therapies and therapies that only inhibit the growth of microorganisms are called microbiostatic therapies.

Characteristics of Antimicrobial Agent

1. The capacity of the substance to kill or inhibit microorganisms is the first requirement. The chemical should possess a broad spectrum of antimicrobial activity at a low concentration.
2. The chemical substance must be soluble in water or other solvents to the extent necessary for effective use.
3. The substance should be stable. Changes in the substance upon standing should be minimal and should not result in significant loss of germicidal action.
4. The compound should be ideally lethal to microorganisms and non-injurious to humans and other animals, i.e., it should be nontoxic to humans and other animals.
5. While using the compound, it should not be necessary to raise the temperature beyond that normally found in the environment where it is to be used. It is important because, for convenience, a chemical reaction occurring at 37°C (body temp.) may not take place at 25°C (room temp.).
6. Unless the substance can penetrate through surfaces, its germicidal action is limited solely to the site of application. Sometimes, of course, surface action is all that is required.
7. The preparation of the chemical substance must be uniform in composition so that active ingredients are present in each application. Pure chemicals are uniform, but mixtures of materials may lack homogeneity.
8. Many chemical agents show a great affinity for proteins or other organic materials. When such disinfectants are used in situations where there is considerable organic material besides that of the microbial cells, little, if any, of the disinfectant will be available for action against the microorganisms.
9. A disinfectant which is also a detergent (cleaning agent) accomplishes two objectives, and the cleansing action improves the effectiveness of the disinfectant.
10. It should not rust or otherwise disfigure metals nor stain or damage fabrics.
11. The compound must be available in large quantities at a reasonable price.
12. The substance should possess deodorizing ability because this ability is a desirable attribute. Ideally, the disinfectant itself should either be odorless or possess a pleasant smell.

Factors Influencing Antimicrobial Agent

During selecting the most appropriate antimicrobial agent to be used for a particular practical application, certain factors are required to be assessed.

The main such factors are the following:

1. A chemical agent used to disinfect contaminated inanimate objects might be quite harmless to the skin; i.e., it might not do serious injury to the tissue cells.
2. The substance selected must be compatible with the material to which it is applied.
3. Chemical agents are not all equally effective against microorganisms like bacteria, fungi, viruses, etc. For example, gram-positive and gram-negative bacteria show differences when subjected to the same chemical agent; Escherichia coli is much more resistant to cationic disinfectants than Staphylococcus aureus. Differences in the action of the chemical agent also exist between strains of the same species. Therefore, the agent selected must be known to be effective against the type of microorganism that needs to be destroyed.
4. The factors discussed such as temperature pH, time, concentration, and presence of extraneous organic material, may all have a bearing on the rate and efficiency of antimicrobial action. The successful use of an antimicrobial agent needs a clear understanding of the influence of these conditions on the particular agent, so it can be employed under the most favorable conditions

Classification of Antimicrobial Agent

The antibacterial agent can be classified into the following five major groups and they are-

1. type of action,
2. source,
3. the spectrum of activity,
4. chemical structure,
5. and function.

Classification based on the type of action

1. Bacteriostatic and
2. Bactericidal

Bactericidal antibacterials:  They slow or inhibit the growth of bacteria are referred to as bacteriostatic. e.g. penicillin, e.g. pen V, penicillin G, procaine penicillin G, benzathine penicillin G, methicillin, oxacillin, cloxacillin, dicloxacillin, and flucloxacillin. They belong to a β‐lactams antibiotic class, Carbapenems like imipenem, meropenem, aztreonam, ticaracillinclvulnate, and piperacillin‐tazobactam; these are β‐lactam/β‐lactamase inhibitors. Some other β‐lactam inhibitors are cephalosporin, e.g. cefotaxime, ceftriaxone, ceftazidime, and cefepime, Gentamicin, tobramycin, and amikacin are aminoglycosides, Quinolones, and fluoroquinolones, such as levofloxacin, ciprofloxacin, and moxifloxacin, Vancomycin is a glycopeptide, Polymyxin B and colistin are polymyxins.

Bacteriostatic antibacterials: They destroy bacteria by targeting the cell wall or cell membrane of the bacteria, which are termed bactericidal. e.g.  Sulphonamides, Amphenicols, chloramphenicol, Spectinomycin, Trimethoprim, Tigecycline,  macrolides, Linezolid(oxazolidinone class), Doxycycline, tetracycline, and minocycline belong to tetracyclines class.

Classification based on the source of antibacterial agent

• Natural antibiotics/antibacterials: naturally obtained from fungal sources. e.g. Cephalosporins, cefamycins, benzylpenicillin, and gentamicin
• Semi‐synthetic
• Synthetic antibiotics: e.g. Moxifloxacin and norfloxacin

Classification based on the spectrum of activity

Narrow spectrum: It can work on a narrow range of microorganisms .e.g. β‐Lactamase‐sensitive, first-generation include penicillin G, benzathine penicillin,β‐Lactamase‐resistant, Ist generation include; Cloxacillin (dicloxacillin flucloxacillin), methicillin, nafcillin, oxacillin, and temocillin are narrow‐spectrum antibacterials, Cephalosporins (first generation and second generation) antibacterials are a relatively narrow spectrum, Vancomycin, clindamycin, isoniazid, rifampin, ethambutol, pyrazinamide, bacitracin, polymixins, sulfonamides, glycopeptide and nitroimidazoles are counted in this group.

Broad-spectrum: It affects a wide range of pathogenic bacteria, including both Gram‐positive and Gram‐negative bacteria. e.g. Ampicillin and its derivative amoxicillin are broad‐spectrum antibacterials. Amoxicillin/clavulanic acid (common name co‐amoxiclav) is an antibiotic useful for the treatment of, a number of bacterial infections, Quinolones, Aminoglycosides, Cephalosporins (third, fourth, and fifth generations) are relatively extended to the broad spectrum of activity, Carbapenems, Macrolides, Tetracycline, tigecycline, Chloramphenicol, Ticarcillin, Rifamycins.

Classification based on chemical structure

Usually, on a structural basis, antibacterials have been classified into two groups: group A (β‐lactams) and group B (aminoglycosides). However, in a more elaborated way, the antibacterials can be classified into β‐lactams, β‐lactam/β‐lactamase inhibitor combinations, aminoglycoside, macrolides, quinolones, and fluoroquinolones, Streptogramin antibiotics, Sulphonamides
Tetracyclines and Nitroimidazoles.

Function‐based classification of antibacterial drugs

Inhibitory of bacterial cell wall 

1. All penicillins (Beta-lactam agents)-e.g. Ampicillin, Amoxycillin, Piperacillin, Cloxacillin, etc.
2. Cephalosporins- e.g.  ceftriaxone, ceftazidime, cefotaxim, cefoperazene
3. Vancomycin
4. Bacitracin
5.  Isoniazide and ethambutal

Inhibition of protein synthesis

1. Aminoglycosides- e.g. Gentamycin, Tobramycin, Amikacin, Streptomycin
2. Tetracyclin
3.  Chloramphenicol
4. Macrolide- e.g. erythromycin, azithromycin, clarithromycin

Inhibition of bacterial nucleic acid

1. Rifampin
2. Actinomycin
3. Novobiocin
4. Quinolones e.g. nalidixic acid, norfloxacin, ciprofloxacin, ofloxacin
5. Metronidazole

Inhibiting cytoplasmic membrane

e.g. colistin, polymyxin-B, amphotericin

Key Notes

• Synthetic antibiotics are also designed to have even greater effectiveness and less toxicity and, therefore, have an advantage over natural antibiotics in that the bacteria are not exposed to the compounds until they are released.
• Antibiotics are a broad chemical class of therapeutic agents originally derived from natural sources (molds, bacteria, etc.).

References

1. https://www.intechopen.com/books/antibacterial-agents/classification-of-anti-bacterial-agents-and-their-functions
2. https://pubmed.ncbi.nlm.nih.gov/9155168/
3. http://www.cyto.purdue.edu/cdroms/cyto2/17/chmrx/intmicr.htm
4. https://www.researchgate.net/publication/317381477_Antibiotics_Mode_of_action_and_mechanisms_of_resistance
5. https://www.tandfonline.com/doi/full/10.1080/13102818.2019.1611385
6. https://courses.lumenlearning.com/microbiology/chapter/mechanisms-of-antibacterial-drugs/
7. https://www.scielo.br/scielo.php?script=sci_arttext&pid=S1678-77572012000300002
8. https://www.sciencedirect.com/topics/chemistry/antimicrobial-agent