In this article we will discuss about Microorganisms:- 1. History of Microorganisms 2. Inhibition of Microorganisms: Antimicrobial Agents 3. Summary 4. Classes of Antibiotics.
History of Microorganisms:
When it was assumed that the communicable diseases were caused by microorganisms, the methods of sterilisation, disinfection and sanitation were adopted in the daily practice. In early nineteenth century, the hospitals were dirty buildings accommodating the patients on the mats spread on the floor, overcrowded together, died and dying in the same beds.
Surgeons wore black coat with linen thread, wound around the button of his coat, which was used as suture material. His scalpel, kept attached to his breast pocket, was used periodically for the incision and was sharpened on the heel of his boot.
The doctors washed their hands when they were soiled with pus and blood. The sleeve of the coat was rolled back to prevent from getting it spoiled, as these unhygienic conditions prevailed in those days in the hospital.
During this century came, Louis Pasteur (1863) who proved that the microbes caused the diseases. Robert Koch (1865) described many microbial diseases and isolated the causative microbes in pure culture. Later, Lister (1867) advocated in surgery the use of dilute carbolic acid as disinfectant.
In obstetrics and gynecology practice, the hand washing before delivery and operation was made compulsory. The practice of antisepsis and disinfecting started from 1867. Napoleon Bonaparte ordered to serve better food for his armies.
In order to obey his order, a French scientist, Nicholas Appert developed a process which was called as “Appetizing” which involved steam, heat, and pressure-as in autoclaving-this principle is even now adopted in the modern method of sterilisation in every hospital.
Later Pasteur advocated a method to check spoilage of beer by subjecting it to a temperature of 50°-60°C for a few minutes and named it as “Pasteurization” which is used in our day to day practice, i.e., Pasteurization of milk.
Koch sterilised the culture media by a method of intermittent heat and 100°C. Tyndall modified, perfected the method and called it as “Tyndallisation“.
In 1880, Pasteur constructed a miniature early autoclave similar to the modern pressure cooker. In 1890, for the first time in the world, an autoclave was installed in Rochester City Hospital, New York. It is well-established that the microorganisms (both harmful and harmless) are ubiquitous in nature.
The pathogenic (harmful) bacteria should be killed or eliminated completely particularly in surgery, surgical environment, hospital and public health. This procedure can be accomplished by the professional trained nurse by her skillful intelligent application of her Nursing microbiology knowledge in the medical and nursing practice.
In bacteriological laboratory, mostly pure cultures grown on nutrient media are dealt with.
Therefore, the disinfection and sterilisation methods advocated for pure cultures were followed in the diagnostic laboratory, whereas in nursing practice, the situations are different unlike laboratory, so the ingenious professional nurse has to deal with mixed cultures, e.g., resistant forms and vegetative forms of microorganisms living in tissues, exudates, discharges, faeces mucus, pus, blood, Therefore the intelligent, well-trained nurse must exercise her own judgement after evaluating each situation to adopt the best methods to remove, disinfect, inhibit or destroy completely the pathogenic microorganism.
The following are the best possible procedures:
(1) Removal by:
(a) Passing the fluids through very fine filter which retains bacteria;
(b) High speed centrifugation.
(2) Inhibition by:
(a) Low temperature (dry ice);
(b) Dessication (drying process)
(c) Combination of low temperature and dessication,
(d) High osmotic pressure (brine);
(e) Micro-biostatic chemicals and drugs
(i) dyes-eosin, methylene blue, crystal violet;
(ii) Chemical agents-sulfonamide, antiseptics.
(3) Destruction by:
(a) Heat-dry (hot air oven); moist (autoclave);
(b) Chemical agents (disinfectants);
(c) Radiation (x-rays, ultraviolet);
(d) Mechanical agents (crushing).
Inhibition of Microorganisms: Antimicrobial Agents:
Antimicrobial agent is a chemical which inhibits the growth and causes the death of organisms.
The agents can be grouped accordingly:
1. If the substance merely causes a cessation of growth of microorganisms, which is reversed when the chemical removed. It is called static agent, if the substance kills the bacteria, it is called cidal agent. A static agent may become cidal if the concentration is increased. The disinfectants have cidal action, while chemotherapeutic agents are often static at the concentration used.
2. Agents acting on bacteria are bacteriostatic or bactericidal, those acting on fungi are fungi-static or fungicidal.
3. In practice, chemotherapeutic agents can be distinguished.
(a) Disinfectants are chemicals used to kill potentially infective organisms normally present on inanimate objects, surface, water etc. They are potentially toxic to man, when they come in direct contact with man.
(b) Antiseptics (mild disinfectants) are relatively non-toxic and non-irritant, antimicrobial agents which may be used for the topical application on the body surface to kill or inhibit the pathogenic microorganisms.
(c) Chemotherapeutic agents are chemicals used to inhibit or kill bacteria already established in the body tissues and are used for the therapeutic purposes in the treatment of microbial infections.
Chemotherapeutic agents should act at a concentration that can be tolerated by the tissues of the host and, therefore, they must have a selective toxicity for the microorganisms compared with the host cells.
The most widely used chemotherapeutic agents are antibiotics which are naturally occurring antimicrobial agents produced by the microorganisms. The most common groups of antibiotic producers are actinomycetes and fungi.
Sulfonamides:
The discovery of sulfonamides in 1935, by Dmagk, German chemist, began with the finding that the red dye, Prontosil, was capable of inhibiting infection with Streptococcus pyogenes, Prontosil itself had no effect on streptococci in vitro; but to be effective it has to be broken down in the animal to sulfonamide. Sulfonamide itself is effective in non-toxic doses.
By substituting different organic groups (represented by R group in the formula below). For example, the addition of a pyridine groups gave sulphyridine, a thiazole gave sulphathiazole and so on. In this way, thousands of derivatives called sulfonamide were produced and many of them were found to be effective antimicrobial agents.
Because of the development of newer antibiotics, relatively few sulfonamides have found a practical therapeutic use. It was observed that yeast and meat extract had the property of overcoming the inhibitory effect of sulfonamides on bacteria in vitro. The sulfonamide “antagonist” in the extracts was isolated and was shown to be para (p)-amino benzoic acid (PABA).
It was also shown that PABA is an essential metabolite in any organism and was converted by them to dihydrofolic acid then to tetrahydrofolic acid, which act as a cofactor in reaction leading to the synthesis of nucleic acids and proteins required for cell growth.
It was found that the sulfonamides inhibit the first stage of the synthesis of dihydrofolic acid from para-amino benzoic acid. This drug, trimethoprim, which was later discovered and introduced into the medicine at much later date, inhibits the conversion of dihydrofolic acid into tetrahydrofolic acid.
Summary:
Since p-amino benzoic acid and sulfonamide have similar chemical structures (Fig 85.1), there is competition between sulfonamide and PABA for the active site on the surface of the enzyme initiating the conversion of PABA to dihydrofolic acid. To inhibit the conversion of one molecule of PABA into folic acid, a large number of sulfonamides molecules are required in the competitive inhibition (Fig. 85.2).
Antibiotics Inhibiting Cell Wall Synthesis:
Mucopeptide of the bacterial cell walls is responsible for their mechanical strength. If the synthesis of mucopeptide is inhibited, while synthesis of other cell components continue, then the cell can be expected to be lysed rapidly in normal osmotic environment. Penicillin inhibits the conversion of a complex nucleotide (uridine diphosphate UDP-N-acetyl muramic acid pentapeptide) into mucopeptide (Fig. 85.3).
Penicillin was discovered by Alexander Fleming in 1922.
Antibiotics Affecting Protein Synthesis:
A number of antibiotics (Streptomycin, Kanamycin, Tetracyclines, Erythromycin and Puromycin) are known to inhibit protein synthesis. Streptomycin, discovered by Waksman, was the first antibiotic to be introduced clinically after penicillin.
Antibiotics Affecting Nucleic Acid Synthesis:
Few antibiotics combine withstand, alter the functioning of nucleic acids though they are generally toxic for therapeutic use. Ribonucleic acid (RNA) synthesis is prevented by Actinomycin, because it forms a complex with double stranded Deoxyribonucleic acid (DNA), but not with single stranded DNA; however, at higher concentrations, DNA synthesis is inhibited.
On the other hand, mitomycin links complementary strands of DNA resulting into blockage in DNA synthesis.
Antibiotics Action on the Cytoplasmic Membrane:
Polymyxin acts on the cytoplasmic membrane as a cationic detergent binding to the membrane, as a result the semipermeable properties are lost and essential low molecular weight intermediates and coenzymes pass into the environment causing the death of bacteria. Polymyxin should be used with caution as it binds also with the membrane of the host cells.
Source of Antibiotics:
Unlike synthetic sulphonamides, most of the antibiotics are produced in the culture media during the growth of certain microorganisms (Streptomyces, Bacillus, Penicillium, Aspergillus). New antibiotics found in plants and animals, terrestrial and marine.
Manufacture of Antibiotics:
The desired antibiotic producing organism is cultivated in large tank of suitable liquid medium for a predetermined length of time. The culture is then centrifuged and filtered to remove the microorganisms. The filtered fluid, containing the antibiotic is then subjected to purification and concentration process.
These at length yield crystals of the pure antibiotics which are then tested for purity, sterility, then packed and distributed.
Classes of Antibiotics:
Antibiotics may be grouped as:
Group I:
(Penicillin, Streptomycin, Bacitracin, Polymyxin B) can be used together, they are often synergistic; never antagonistic. For example, when penicillin and streptomycin combined they are valuable in endocarditis. The combination is more effective than the sum of both.
Group II:
(Tetracyclines, Chloramphenicol, Erythromycin, Carbomycin, Neomycin, Oleandromycin, Novobiocin) is neither antagonistic nor synergistic. This sometimes -works well in combination with antibiotics of first group. This second group is generally of the “broad-spectrum type“.
There are many other broad-spectrum antibiotics. The usual response to antibiotic therapy and antibiotic resistance or drug fastness are two important properties of organisms.